Hydraulic Training Wirgent Document

TRAINING DOCUMENTBasic Hydraulics © Wirtgen GmbH 2014  This training document is not a substitute for technical documentation (instruction manual, safety instruction and spare parts catalogue). It is not subject of the technical modification service. All technical instructions for description, operation, maintenance, etc. are taken by the instruction manual, the safety manual or the spare parts list. Publisher Reinhard-Wirtgen-Straße 2 53578 Windhagen Germany Phone:: +49 (0) 2645 / 131 - 0 http://www.wirtgen.de Document name Handout_Basic Hydraulics_2364339_V01_en Translation of the original training document Module HYD 1 – Basic Hydraulics Date of first issue 15.11.2012 Date of amendment 30.04.2014 Copyright © Wirtgen GmbH 2014 Distribution and copying of this document, usage and communication of its content are forbidden, unless specifically permitted. Infringements will result in the obligations to pay damages. All rights reserved in case of a patent, utility model or registration of a design. © Wirtgen GmbH 2014 WIRTGEN GROUP TRAINING 2364339_V01 Content 1 Introduction ............................................................................................................................................................ 3 2 Basic physics of hydraulics.................................................................................................................................... 4 2.1 Mass m .......................................................................................................................................................... 4 2.2 Force F .......................................................................................................................................................... 4 2.3 Pressure p ...................................................................................................................................................... 4 2.4 Energy E ........................................................................................................................................................ 5 2.5 Power P ......................................................................................................................................................... 5 2.6 Velocity v ........................................................................................................................................................ 5 3 Hydro-mechanics ................................................................................................................................................... 6 4 Hydro-kinetics ........................................................................................................................................................ 8 4.1 Flow-through theory ....................................................................................................................................... 8 4.2 Friction and pressure losses .......................................................................................................................... 9 4.3 Flow types ....................................................................................................................................................10 5 Hydraulic systems ...............................................................................................................................................11 6 Pressure fluids .....................................................................................................................................................13 6.1 Tasks of pressure fluids ...............................................................................................................................13 6.1.1 Types of pressure fluids ........................................................................................................................13 6.2 Technical data of pressure fluids .................................................................................................................14 6.2.1 Viscosity ................................................................................................................................................14 6.2.2 Lubricating capability of pressure fluids ................................................................................................14 6.2.3 Air exclusion capability ..........................................................................................................................15 6.2.4 Durability ...............................................................................................................................................15 7 Hydraulic pumps ..................................................................................................................................................16 7.1 Outer gear pump ..........................................................................................................................................17 7.2 Inner gear pump ...........................................................................................................................................19 7.3 Axial piston pumps .......................................................................................................................................20 7.3.1 Angled plate pump ................................................................................................................................20 7.3.2 Angled axle pump .................................................................................................................................20 7.3.3 Tumbler plate pump ..............................................................................................................................21 8 Hydraulic motors ..................................................................................................................................................23 8.1 Gear motor ...................................................................................................................................................27 8.2 Annular gear motor.......................................................................................................................................28 8.3 Radial piston motor (multi-stroke principle) ..................................................................................................29 9 Valves ..................................................................................................................................................................30 9.1 Check valves ................................................................................................................................................30 9.1.1 Simple check valve ...............................................................................................................................31 9.1.2 Hydraulic releasable check valves ........................................................................................................33 9.1.3 Twin check valves .................................................................................................................................37 9.2 Directional valves .........................................................................................................................................38 9.2.1 Directional gate valves ..........................................................................................................................39 9.2.2 Directional seat valves ..........................................................................................................................43 9.3 Pressure valves ............................................................................................................................................45 9.3.1 Pressure limitation valves (PLV) ...........................................................................................................45 9.3.2 Pressure limitation valves (pre-controlled) ............................................................................................48 9.4 Pressure switch-on / cut-off valves ..............................................................................................................49 9.4.1 Pressure switch-on valves ....................................................................................................................49 9.4.2 Pressure cut-off valves .........................................................................................................................50 9.5 Pressure reduction valves ............................................................................................................................51 9.5.1 Pressure reduction valves (directly controlled) .....................................................................................51 10 Flow valves ......................................................................................................................................................55 10.1 Throttle valves ..........................................................................................................................................56 10.1.1 Twin throttle check valves .....................................................................................................................57 10.2 Flow regulation valves ..............................................................................................................................59 10.2.1 2-way flow regulation valve ...................................................................................................................60 10.2.2 3-way flow regulation valve ...................................................................................................................62 11 Hydro-pneumatic reservoir...............................................................................................................................65 12 Hydraulic filters ................................................................................................................................................70 1 / 77 WIRTGEN GROUP TRAINING 2364339_V01 12.1 Suction filters ............................................................................................................................................71 12.2 Line filters / pressure filters .......................................................................................................................73 12.3 Ancillary / return flow filters .......................................................................................................................75 12.4 Filler and ventilation filters ........................................................................................................................77 2 / 77 WIRTGEN GROUP TRAINING 2364339_V01 1 Introduction The word "Hydraulic" derives from the Greek word “HYDRO“ (which means water) and, in a scientific sense, determines the theory governing stationary and moving liquids (hydrostatics and hydrodynamics). When referring to hydraulics in machines and vehicles, this involves the practical application of physics concerning power transmission as well control and regulation technology. In order to divide this vast range of applications, the terms "stationary hydraulics" and "mobile hydraulics" are commonly used. "Mobile hydraulics" propels itself on wheels or crawler units, whereas "stationary hydraulics" is located in one position. Although the equipment for these two systems differs enormously, their boundaries cannot be clearly marked. A machine based on mobile hydraulics can consist of mobile hydraulics and stationary hydraulics. Wirtgen road construction machines combine the application of mobile hydraulics and stationary hydraulics. Example 1: Road Milling machine W 2000 is propelled on crawler units and must be mobile in order to fulfil its requirements. This is a typical example of a product being driven by mobile hydraulics. W 2000 Example 2: The Cold Mixing Plant KMA 200 remains stationary at one location when operating. This machine is therefore equipped with a stationary hydraulic system. KMA 200 3 / 77 WIRTGEN GROUP TRAINING 2364339_V01 2 Basic physics of hydraulics In order to understand the sections to follow, it is of advantage to firstly look at the basic physics of hydraulics. The following is an explanation of the most important basic physics. 2.1 Mass m All substances have a mass. One of the accepted units for determining a mass is kilogram (kg). 2.2 Force F According to the theory of Newton, a force (F) is the product of mass (m) and acceleration (a). Force = Mass * Acceleration F = m∗a If general acceleration (a) is replaced by the earth's gravitation (g), force (F) then becomes the product of mass (m) and the earth's gravity (g). The earth's gravity has a constant value of 9.81 m/s. Force = Mass ∗ Gravity F = m∗ g m F = m ∗ 9.81 s NOTE! The measuring unit for force is Newton (N) ! In everyday practise, it is acceptable to use to value of 10 m/s instead of 9.81 m/s for the earth's gravity. Therefore, one Newton is a force that is necessary to accelerate an object weighing 1 kg over a distance of 1 metre. 2.3 Pressure p A pressure occurs when a force is working at a right-angle against a surface area. A pressure (P) is the quotient of the force (F) and the surface area (A). F P= A The accepted unit for determining a pressure is Pascal (Pa). A pressure of 1 Pa is generated when a force of 1 N works on a surface area 1 m². [Pa] =  N2  m  As one Pascal is an extremely small value, the unit of bar is usually applied in general practise. 4 / 77 WIRTGEN GROUP TRAINING 2364339_V01 [bar ] =  daN2   cm  NOTE! Although the unit for pressure is Pascal, bar is commonly used. 2.4 Energy E If an object is capable of independently carrying out work, it must have work stored inside itself. This stored work is called energy. Work and energy both use the same unit; a Joule (Nm  Newton meter). 2.5 Power P Power (P) is the quotient of work (W) and time (t). W P= t 1J 1W = 1s 2.6 Velocity v Velocity (v) is the quotient of distance (s) in metres and time (t) in seconds during which a distance is overcome. s v= t 5 / 77 WIRTGEN GROUP TRAINING 2364339_V01 3 Hydro-mechanics Hydro-mechanics is the theory of the physical relationships and characteristics of liquids. The major difference between liquids and solid bodies is that the particles in liquids can be easily moved and therefore do not possess a certain shape. A liquid will always adapt its shape to the vessel in which it is contained. Contrary to gases, liquids cannot be so easily compressed. The effect of a force on a stationary fluid is distributed equally within the liquid in all directions. The size of the pressure within the liquid equals the force of weight relative to the active surface. The pressure is always active at right-angles against the surface. NOTE! A pressure is always active at right-angles against the surface of a "container". The pressure is distributed equally in all directions. Therefore, the pressure against the surface of a container is always identical at any point. As the pressure is distributed equally in all directions, the shape of the container has no importance. Despite various container shapes, the pressure at any point inside the container is always the same. A pressure will be generated when force F1 makes contact with surface A1. F1 P= A1 The pressure P is active on all points of the system; also on surface A2. The achievable force, which has the same importance as a weight to be lifted, can therefore be calculated via the pressure P and the area A2. F 2 = P ∗ A2 6 / 77 WIRTGEN GROUP TRAINING 2364339_V01 F 2 A2 NOTE! The forces inside this system have the same behaviour as the surfaces = ! F1 A1 The pressure inside such a system always behaves according to the size of the force F and the active area A. This means that the pressure inside a system will increase until the pressure is larger than the force of the resistance which is active against the pressure. NOTE! The pressure inside a hydraulic system can only be correctly measured when the counter pressure against the pressure is larger than the pressure to be measured. 7 / 77 WIRTGEN GROUP TRAINING 2364339_V01 4 Hydro-kinetics Hydro-kinetics is the theory of the rules governing the movement of liquids and the thereby generated forces. 4.1 Flow-through theory Identical volumes flow through a pipe during an identical period. This means that the flow speed must increase where the passage is narrower. The flow volume (Q) is the quotient of the fluid volume (V) and the time (t). V Q= t The flow volume Q (L / min) is identical at any point inside a pipe. If the pipe cross-sections A1 and A2, the two cross-sections will generate their own flow speeds. This results in the following equation: Q1 = Q 2 Q1 = A1 ∗ V 1 Q 2 = A2 ∗ V 2 8 / 77 WIRTGEN GROUP TRAINING 2364339_V01 4.2 Friction and pressure losses In order to observe the rules governing flowing fluids, it is assumed that the fluids can be moved friction-free against each other and against objects. Hydraulic energy cannot be transmitted without losses through pipes or hoses. Friction occurs against the inner walls and inside the fluids themselves. The friction during transmission of hydraulic energy is converted into heat. This loss of energy can be noticed as a loss of energy. The loss of pressure (pressure difference) is depicted as Delta – P (∆P). NOTE! When the friction of fluids against each other increases, the more its viscosity (fluid toughness) will increase. The size of the friction losses greatly depend on the following values: - Length of the pipes / hoses - Cross-section of the pipes / hoses - Roughness of inner wall of pipes / hoses - Quantity of bends in pipes / hoses - Flow speed - Fluid viscosity 9 / 77 WIRTGEN GROUP TRAINING 2364339_V01 4.3 Flow types The type of flow is also decisive for energy losses inside a hydraulic system. There are two different types of flow: the laminar flow and the turbulent flow. Laminar flow Turbulent flow Fluids will move up to a certain speed in layers (laminar) through a pipe or hose. The inner fluid layer moves fastest, whereby the most outer layer remains stationary against the inner wall. When increasing the flow speed beyond the critical speed, the flow characteristics of the fluid will change from laminar to turbulent. When changing to turbulent flow, the flow resistance and the resulting hydraulic energy losses will increase. For this reason, it should always be an objective to avoid a turbulent flow in as far as possible. 10 / 77 WIRTGEN GROUP TRAINING 2364339_V01 5 Hydraulic systems Inside hydraulic systems, mechanical energy is transformed into hydraulic energy, where after it is transported, controlled and regulated. Afterwards, the hydraulic energy is converted back into mechanical energy. Characteristics of hydraulic systems: - Transmission of large forces (torque) while using relatively small component dimensions - Its functions under full load are immediately available from a machine that was previously stationary - Infinitely variable regulation and control of speeds and torque - Simple overload protection - Realisation of fast and extremely slow movements - Energy storage with gases - Simple and centralised drive systems - Non-central transformation of hydraulic energy into mechanical energy is easily possible 11 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The illustration shows the components that can be used in a hydraulic system to ensure the transformation of energy. A schedule of the components are shown at the right, and at the left there are their respective symbols which are used in hydraulic diagrams according to DIN standards. 1, 2 The tank and filer components are responsible for preparing the fluid 3 The hydraulic pump is responsible for the necessary energy transformation 4 The pressure limitation valve is responsible for energy control 5 The direction valve is responsible for energy control 6 The non-return valve (check valve) is responsible for energy control 7 The throttle valve is responsible for energy control 8, 9 The cylinder or hydraulic motor is responsible for energy transformation 12 / 77 WIRTGEN GROUP TRAINING 2364339_V01 6 Pressure fluids The correct function, life-expectancy, operational reliability and economy of a hydraulic system is decisively influenced by the choice of a suitable pressure fluid. In general, the applied fluids are based on mineral oil, and are called hydraulic oils. Apart from these oils, liquids that are difficult to ignite are also used. For some time, Wirtgen introduced biological decomposable fluids in their machines to protect the environment. The table below contains a list of the hydraulic oils that are applied in Wirtgen machines: Manufacturer Type Designation DEA Astron HVLP 46 Bechem Hydrostar HEP 46 Panolin HLP SYNTH 68 6.1 Tasks of pressure fluids The tasks of pressure fluids vary and are important to ensure the correct function of hydraulic systems. These functions are: - Lubrication of moving parts, such as pistons, valves, bearings, etc. - Anti-corrosion protection of contacted surfaces - Transmission of the pump's hydraulic output - Flushing out any contamination - Conduction of frictional heat In accordance to the tasks, certain oil characteristics must be adhered to which are partly expressed by their grade designations. 6.1.1 Types of pressure fluids Mineral-based hydraulic oils: The following hydraulic oils are defined according to DIN standards. Designation Description H Hydraulic oils without additives. Their characteristics can be compared with C lubricants and today are rarely used in hydraulics HL Hydraulic oils with additive to increase anti-corrosion and life-expectancy. These oils are generally used in hydraulic systems with pressures up to approx. 200 bar and fulfil the general thermal load requirements. HLP Hydraulic oils with special high-pressure additives to offer high protection against wear. These oils are used in hydraulic systems with pressures exceeding 200 bar. HV Hydraulic oils of particularly low viscosity and temperature dependency. Other characteristics are like HLP oils. 13 / 77 WIRTGEN GROUP TRAINING 2364339_V01 6.2 Technical data of pressure fluids 6.2.1 Viscosity Viscosity defines the toughness of a pressure fluid. Example: Using hydraulic oil DEA – Astron HLVP 46 The stated figure of 46 designates the viscosity grade of the oil. The unit used for the viscosity of hydraulic oil is mm² /s. The higher the unit value, the higher the viscosity. The following table includes a few viscosity grades and the respective applications. This table offers a rough idea of the resulting effects when filling a hydraulic system with unsuitable hydraulic oil. The viscosity of hydraulic oil increases with increasing pressure; a fact that must be observed with pressures exceeding 200 bar. At around 400 bar, the viscosity could be doubled. 6.2.2 Lubricating capability of pressure fluids A pressure liquid must be capable of sufficiently lubricating and creating surface adhesion on moving parts of a hydraulic system; in particular the inner components pumps and motors. If the lubrication film should collapse due to insufficient viscosity or excessive surface pressure, the moving parts will come into contact with each other. This can cause extreme wear on the contoured surfaces and then result in a function fault of hydraulic components. 14 / 77 WIRTGEN GROUP TRAINING 2364339_V01 6.2.3 Air exclusion capability Under normal atmospheric conditions, hydraulic oils contain approximately 90% depleted air. This is not detrimental for a hydraulic system. The depletion rate will increase when the pressure and temperature increases. When a pressure decrease occurs, for example behind throttles or with a vacuum inside a suction hose, the air saturation limit will drop rapidly and the depleted air inside the hydraulic oil will be released as air bubbles. Air can also enter a hydraulic system through leaking suction lines or shaft seals. This released air, which is visible as foam, is a particular hazard for hydraulic systems because it can cause the dreaded CAVITATIONS. As cavitations can lead to material abrasions, pressure impacts and noise, the development of foam inside a hydraulic oil tank must be avoided. IMPORTANT! Development of foam must be avoided! This can be achieved by a sufficiently dimensioned hydraulic oil tank from which any released air can escape. In addition, the hydraulic oil must be capable of allowing the air bubbles inside the tank to float to the surface and then explode. 6.2.4 Durability Durability of a pressure fluid means its resistance to chemical changes under high temperatures and to catalytic (non-ferrous) metals. In general, its durability at a temperature above 70° Celsius will be halved by every further 10 °C increase of temperature. This ageing processing is also accelerated by contamination due to abrasions and rust, etc. NOTE! Old hydraulic oil will darken and appears resinous, which can lead to malfunction of hydraulic valves. 15 / 77 WIRTGEN GROUP TRAINING 2364339_V01 7 Hydraulic pumps As already described in a previous section, hydraulic pumps have the task of transforming mechanical energy into hydraulic energy. This involves transforming torque (mechanical) into flow volume (hydraulic). The following criteria must be considered when choosing hydraulic pumps: - Operational media - Required operational pressure - Expected speed range - Minimum and maximum operational temperatures - Installation situation - Drive type (coupling) - Expected durability - Maximum noise level - Service convenience - Costs This list could be extended. The multitude of requirements clearly shows that a single pump cannot perfectly fulfil every criteria. This is why various pump designs are available. All designs have one thing in common; they function according to the displacement principle. They are equipped with mechanically sealed chambers in which the fluid is transported from the input side (suction connection) to the output side (pressure connection). As there is no direct connection between these two connections, pumps of displacement principle are suitable for high system pressures and are therefore ideal for hydraulic applications. The illustration below contains various types of hydraulic pumps which function according to the displacement principle. From this vast range of hydraulic pumps, Wirtgen uses only a few types. These types will be explained later in more detail. Principle Type Design Volume Outer gear pump Constant Gear pump Inner gear pump Constant Gear ring pump Constant Gear Spiral pump Spindle pump Constant Single stroke Constant / Variable Wing Wing cell pump Double stroke Constant Outer piston support Constant / Variable Radial piston pump Inner piston support Constant / Variable Piston Angled plate pump Constant / Variable Axial piston pump Angled axle pump Constant / Variable 16 / 77 WIRTGEN GROUP TRAINING 2364339_V01 7.1 Outer gear pump Outer gear pumps function according to the displacement principle just like all hydraulic pumps. The flow volume (displacement volume) of outer gear pumps is constant because the same volume of oil is displaced by its chambers per pump rotation. Under high pressure, internal oil leakages will occur at the seals and the flow volume will decrease. The pump consists of two gear wheels (1) which are supported by four bearings (2) with an axial field seal (7), as well as a housing (3) with a front and rear lid (4.1 and 4.2). The drive shaft passes through the front lid where it is sealed by a shaft seal (5). The bearing forces are absorbed by so-called DU-bushes equipped with Teflon inlays. Outer gear pumps are commonly used in hydraulics. The characteristics of these pumps are: - Relatively high pressure of approx. 300 bar with small installation dimensions - Low price - Large speed range (500 – 6000 rpm) - Large temperature and viscosity range Inside the displacement chambers, the air which is inside the suction line is firstly transported from the suction side to the pressure side. This will generate a vacuum inside the suction line. When the vacuum increases, the fluid will be extracted from the tank into the suction line until it reaches the pump. The gear chambers will fill with oil and then displace the oil around the outside to the pressure side. The combing of the gear teeth will prevent the oil from flowing back. 17 / 77 WIRTGEN GROUP TRAINING 2364339_V01 This type of outer gear pump is used in almost every Wirtgen machine. Some examples: W 600 DC – Cylinder functions W 2000 – Fan drive WR 2500 – Oil feed pump SP 250 – Screw conveyor drive 18 / 77 WIRTGEN GROUP TRAINING 2364339_V01 7.2 Inner gear pump This type of gear pump is frequently used as an oil feed pump in addition to an axial piston pump. The displacement volume is generated between the gear flanks and the walls of the housing. The flow volume is constant. Here are some examples of inner gear pumps as auxiliary pumps for axial piston pumps: - W 2000 Advance drive pump - W 500 Milling drum drive pump - WR 2500 Advance drive pump 19 / 77 WIRTGEN GROUP TRAINING 2364339_V01 7.3 Axial piston pumps Axial piston pumps are displacement pumps inside which the pistons are configured parallel to the rotary axis of a cylinder drum. The drive movement is generated according to three basic principles as illustrated below: 7.3.1 Angled plate pump This type of pump is used more frequently in Wirtgen products. The cylinder drum (1) is driven so that the pistons (2), which are guided inside it, are also driven. The axial movements of the pistons are determined by an angled plate (3) inside the pump housing which can be tilted away from right-angle position to the drive axis. The pistons move along an elliptical orbit against the stationary angled plate (deflection plate). The generated friction is controlled by a friction disk (4) and the axial bearing. During the suction phase, the pistons move outwards while being held against the deflection plate by retainer devices. During the pressure phase, the pistons are forced inwards by the plate. The direction of oil flow for each piston, meaning either to a pressure or suction port, is governed by control slots. They are located in a rigid control plate (5) against which the free end of the cylinder drum will rotate. The subject of axial piston pumps will be looked at in more detail later in this document. 7.3.2 Angled axle pump With this type of pump, the cylinder drum (1) is driven by the pistons (5) which are themselves driven via a drive flange (2). The cylinder drum is guided either by a central trunnion or by a needle bearing around its circumference and it tilted away from the axis of the drive shaft. The displacement volume varies according to the deflection angle. This principle enables these pumps to run in reverse direction. 20 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The connection between the piston and drive flange is carried out by a ball-joint (6) which pulls the piston inside its cylinder during the suction phase and pushes the piston during the pressure phase. A further joint is necessary between the actual piston and ball-joint in order to equalise any circular or elliptical orbits. The suction and pressure side of the pump are divided, just like the angled plate principle, by a slotted plate. This flat or spherical control plate (7) deflects with the cylinder drum meaning that the pressure port must pass through the deflection bearing or be connected to the control plate via a sealed friction guide. 7.3.3 Tumbler plate pump With this pump, the shaft (1) drives a tumbler plate (2) which transmits its axial movement to a non-rotating pistons. The pistons are pressed via springs against the tumbler plate. The forces between the piston and the tumbler plate when pressing against each other are transmitted by an axial bearing (3). The flow direction from each piston is governed either by control valves (6) or via control slots in each piston. The angle of the rotating tumbler plate cannot be changed, which means that the displacement volume of this type of pump remains constant. 21 / 77 WIRTGEN GROUP TRAINING 2364339_V01 22 / 77 WIRTGEN GROUP TRAINING 2364339_V01 8 Hydraulic motors Hydraulic motors transform hydraulic energy into mechanical energy. Just like hydraulic pumps, there are various types and design available on the market. As no single hydraulic motor can perfectly fulfil all requirements, an optimum compromise must be found for every application when selecting a hydraulic motor. Only very few hydraulic motors can cover a speed range from very slow to over 1000 rpm. Therefore, hydraulic motors can be divided into fast-running types (n = 500 to 10000 rpm) and slow-running types (n = 0.5 to 1000 rpm). The torque produced by a hydraulic motor depends on its consumption volume and the pressure difference inside the motor. Slow-running hydraulic motors are generally designed so that they produce high torque when running at slow speeds. The performance of a hydraulic motor depends on its flow volume and the pressure difference inside the motor. As its performance is proportional to its rotation speed, fast-running motors are highly suitable for applications that require a high performance. Not all types are used in Wirtgen machines. For this reason, this training documentation - in particular in this section - will only deal with hydraulic motors in more detail that are used in Wirtgen machines. Here are some examples of hydraulic motors: Gear motor Gear ring or planetary motor 23 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Wing cell motor Radial piston motor with inner piston support Axial piston motor with angled plate 24 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Multi-stroke radial piston motor with outer piston support Multi-stroke axial piston motor with static shaft Multi-stroke axial piston motor with static housing 25 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Axial piston motor with angled axis 26 / 77 WIRTGEN GROUP TRAINING 2364339_V01 8.1 Gear motor A gear motor is designed very similar to a gear pump. The differences can be found in the axial pressure field as well as the fact that gear motors are designed for changing its sense of direction and therefore is equipped with a leakage oil connection. The pressurised fluid flowing into hydraulic motor is active on the gear wheels. This generates a torque which is then transmitted to the motor output shaft. Gear motors are frequently installed in mobile hydraulic systems and agricultural machinery to drive conveyor belts, cooling fans, screw conveyors or air blower fans. Important nominal values for gear motors are its consumption volume (approx. 1 to 200 cm³), its maximum operational pressure and its speed range (500 to 10000 rpm). Gear motors and axial piston motors belong to the fast-running family. Fast-running hydraulic motors operate as speeds higher than 500 rpm. 27 / 77 WIRTGEN GROUP TRAINING 2364339_V01 8.2 Annular gear motor The main components of this motor consist of a free-standing displacement ring (1), its inner teeth made up of 7 rollers and a moving gear wheel (3) equipped with 6 outer teeth. When the tooth cavities are subjected to pressure, the gear wheel will rotate and simultaneously circulate in an eccentric orbit around the centre of the gear wheel. These two movements take place in opposite directions. With a tooth ratio of 7:6, this will result in a transmission ratio of 6:1, which means that every time the shaft rotates, each chamber is pressurised and relieved six times. This system produces a down-gearing function. The motor is classified as a slow-running motor. The rotary movement of the gear wheel is guided by a jointed shaft (4) in order to equalise the eccentric movements before the drive shaft (5). The input and output flow is governed by control slots which are located on a rotating control bush (6). It is then dog-driven by the drive shaft via a pin (7). Two non-return valves inside the motor housing release the flow of leakage oil. 28 / 77 WIRTGEN GROUP TRAINING 2364339_V01 8.3 Radial piston motor (multi-stroke principle) With his type of motor, the that radial configured pistons (3) are supported on the stroke radius (4) by rollers (8). The cylinder chamber is supplied with fluid via axial bore holes in the control unit (5). Per rotation, every piston is pressurised and then relieved according to the quantity of actuation cams. The resulting torque generated by the shape of the stroke ring is then transmitted via teeth (6) from the rotor / piston group (3) to the drive shaft. A conical roller bearing is integrated inside the housing (1) to absorb the high axial and radial forces. It is technically possible to install a multi-disk brake (9) on an extending drive shaft passing through the housing (2). If the air pressure inside the ring chamber (10) falls below a certain value, the spring plate (11) will force the disk pack (12) together and the brake will be actuated. If the air pressure exceeds a certain value, the brake piston (13) will be forced against the spring plate. The disk pack will then be relieved and the brake will be released. This type of hydraulic motor is used, for example, in the following Wirtgen machines: - W 500 Milling drum drive - SP 250 and SP 500 Trimmer drive 29 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9 Valves In the previous sections of this document, it was clearly stated that a hydraulic system is designed to transform mechanical energy into hydraulic energy, and then later to transform the hydraulic energy back into mechanical energy. Between the components that are necessary to carry out these transformations, other components are necessary to govern and control the transformed energy. One of these component types is the hydraulic valve. As Wirtgen machines use valves of various types and designs, it is necessary to divide this large valve group into various sub groups. - Check valves - Directional valves - Pressure valves - Flow valves 9.1 Check valves The function of check valves in a hydraulic system is to stop the flow of hydraulic fluid in one direction and to allow its flow in the other direction. This is why check valves are also known as non-return valves. Check valves are flushly seated and therefore function without causing a flow of leakage oil. Their shut-off elements can consist of either balls, plates, cones or cones with shaft seals. A ball as shut-off element has the advantage that it can be economically produced. The disadvantage of a ball is that it can slightly deform during operation. The ball seat will press itself into the ball. As the ball will not always take on the same position, this can lead to leakages over the course of time. The ball must also be guided so that it will not become worn by the spring pressure or flow pressure. A cone is guided and will therefore always take on the same position. After a short operational time, the cone will adapt to the cone seat and form a perfect seal. Although the production expenditure for a cone is higher than for a ball, the cone system is preferably used in Wirtgen machines due to its higher reliability. Check valves can be divided into different groups according to their applications: Check valves Simple Hydr. releaseable Filler valves check valves check valves 30 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.1.1 Simple check valve Check valve as pipe fitting This valve consists of a housing (1) and a hardened piston (2) which is forced by a spring (3) against a seat (4). When fluid flows in the required direction, the fluid pressure will force the piston cone away from its seat to allow the fluid to pass through. If the fluid flows in the other direction, the spring and fluid pressure will force the piston cone against its seat and stop the flow of fluid. The opening pressure of the check valve depends on the force of the spring and therefore cannot be influenced externally. Depending on their applications, the pre-tension of these springs can be between 0.5 bar and 5.0 bar. If the check valve is not equipped with a spring, it must always be installed vertically so that the weight of its internal closing element determines its opening pressure. Important nominal values for check valves are the nominal size (6 - 150), flow volume (up to 15000 l/min), operational pressure (up to 315 bar) and the opening pressure without a spring (0.5, 1.5, 3.0 or 5.0 bar). Check valves are mainly installed in Wirtgen machines: - to bypass a throttle - to stop flow direction - as bypass valve to bypass a return flow filter after a certain back-pressure is achieved to contamination 31 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Example for the application of a check valve in the hydraulic circuit diagram of Cold Milling Machine W 1000 F to protect the hydraulic oil cooler (1) and to protect the hydraulic oil filter (2) against excessive back-pressure. Check valve 2 Check valve 1 32 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.1.2 Hydraulic releasable check valves Contrary to simple check valves, the non-flow direction of releasable check valves can be activated. These valve are used: - to hold the pressure inside operational circuits - to secure raised components against lowering in the event of a pipe fracture - to prevent creeping movements of involved consumers There are two different types of releasable check valves: - Releasable check valves without leakage oil connection - Releasable check valves with leakage oil connection Releasable check valves without leakage oil connection (valve type SV) Design without leakage oil connection and without pre-opening of main cone (including the respective symbol used in hydraulic diagrams). B A X In the above illustration, the free volume flows from A to B. The fluid is active on surface A1 of the main cone (1) and forces it against the spring (3) away from its seat. With a volume flow from B to A, the flow is interrupted according to a normal check valve. The flow release in direction B to A is carried via the control piston (4). The necessary control oil pressure coming from connection X will move the control piston to the right to open the main cone (1). 33 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The necessary control pressure equals the surface area ratio between surface A1 and the opening control piston. This ratio is usually 1:3. The total cross-section of surface A is instantly opened during the opening function. This could result in pressure relief impacts, particularly when large volumes under pressure are released. Such relief impacts not only produce noise, they also create peak loads in the total hydraulic system; in particular in hydraulic connectors and moving parts. In applications where these relief impacts are not compensated, the valve goes through a pre-opening phase (see illustration and symbol below). B A X When control connection X is subjected to pressure, the control piston (4) firstly forces the pre-opening ball (2) away before forcing the main cone (1) from its seat. The pre-opening ball uncovers only a small cross-section. This enables the cylinder to extend slowly before the main cones moves and uncovers the total cross-section. Such designs enables dampened pressure relief of the pressurised fluid. 34 / 77 WIRTGEN GROUP TRAINING 2364339_V01 This is an example of a releasable check valve which is frequently used in Wirtgen machines. Here it is in the hydraulic system of the Cold Milling Machine W 1000 F The check valve used here has the task of holding the laden conveyor at the selected height and preventing it from lowering when its height regulation system UP/DOWN has not been adjusted. Design with leakage oil connection (valve type SL) Releasable check valve with pre-opening of main cone and leakage oil connection. 35 / 77 WIRTGEN GROUP TRAINING 2364339_V01 B Y A X The difference to valve type SV is the additional leakage oil connection Y. The ring surface of the control piston is separated from connection A. Any pressure at connection A is only active on surface A4 of the control piston. Important nominal sizes of SV and SL check valves - Nominal size 6 – 150 - Flow volume up to approx. 6400 l/min. - Operational pressure up to 315 bar 36 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.1.3 Twin check valves Twin or double check valves are created by assembling two releasable check valves (1 and 2) inside one valve housing. In flow direction A1 to A2 or B1 to B2 the fluid flow is unrestricted, and in flow direction A2 to A1 or B2 to B1 the fluid flow is interrupted. When, for examples, the fluid flows through the valve from A1 to A2, the control piston (3) is move by the pressure to the right and pushes the cone (2) of the check valve against its seat. Unrestricted flow is now possible from B1 to B2. This procedure functions in the same manner when fluid flows from B1 to B2. 37 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.2 Directional valves The term "directional valves" includes all valves that can start, stop or alter the flow direction of a hydraulic pressure medium. Directional valves are designated according to the quantity of their switching positions and connections, whereby the control connections are disregarded. A directional valve with four consumer connections and three switching positions is therefore designated as a 4/3 directional valve. The switching positions and the respective actuation elements are designated with the small letter "a" and "b". a b When a valve has three positions, the central position is "neutral" and is usually designated with figure "0". a 0 b "Neutral" is the position where moving components remain in non-active condition (e.g. by the force of a spring, pressure or flow, etc.) When in horizontal position, the sequence of descriptions (switching positions) is always carried out according to the alphabet from left to right. A B a b a b P T Directional valves are divided into the various groups and are described in more detail in the next section of this document. Directional valves Directional gat valves Directional seat valves Direct control Pre-control Direct control Pre-control 38 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.2.1 Directional gate valves Direction gate valves are valves with a gate piston inside the bore hole of their housing. Depending on the quantity of channels to be controlled, their housings of cast graphite, steel or other suitable materials are equipped with two or more cast or screw-in ring channels. These channels run concentric or eccentric around a bore hole. This will result in control edges inside the housing which function together with the edges of the control piston. The ring cavities inside the housing are separated or connected by displacing the control piston. Sealing is carried out along a gap between the moving piston and the housing. The sealing effect depends on the size of the gap, the viscosity of the fluid and, in particular, the prevailing hydraulic pressure. Directional gate valves can be controlled directly or be pre-controlled. Whether a directional gate valve is directly or pre-controlled initially depends on the size of the necessary actuation force an therefore on the component size (nominal size) of the valve. Directly controlled directional gate valves The term "directly controlled directional gate valve" means a directional gate valve in which its control piston is actuated either by a solenoid, pneumatic, hydraulic or via any other mechanical device without assistance of an amplifier. Due to the static and dynamic forces generated inside directional gate valves as a result of pressure and fluid flow, directly controlled directional gate valves are normally available up to nominal size 10. This limit equals a performance of approximately 120 l/min with an operational pressure of 2350 bar, and applied in particular to directional gate valves that are actuated by solenoids. Electro-magnetic (solenoid) actuation is available in numerous designs. Due to the numerous automatic functions, this type of actuation is most common. Solenoids are usually available in four basic designs: - Direct current solenoids inside a chamber containing air. Also known as a dry solenoid. - Direct current solenoid inside a chamber containing oil. Also known as wet or pressure-sealed solenoid. The solenoid anchor moves in oil while the anchor chamber is connected to the T-channel. - Alternating current solenoid inside a chamber containing air. - Alternating current solenoid inside a chamber containing oil. Direct current solenoids are very reliable and enable soft switching function. They will not burn out if, for example, the piston jams when switching. They are suitable for precise switching requirements. Alternating current solenoids have a short switching phase. If the solenoid cannot fully switch to end position, the valve will burn out after approximately 1 to 1.5 hours. REMARKS! Wirtgen machines are only equipped with direct current solenoids. 39 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The above illustration shows a directional gate valve with three switching positions, and is equipped with an oil chamber direct current solenoid at the left (4), and an oil chamber alternating current solenoid at the right (5). The anchor chamber of the each valve is connected to the tank inside the valve housing. These valves are therefore called 3-chamber valves. The springs are supported by the housing and align the piston over a disk (8) at central position. The illustrated solenoids are equipped with manual emergency actuation (7) so that the control piston can be actuated externally. The channels P, A and B are separated from each other by webs inside the housing. The T-channel does not have a separating web. It is connected by a bypass channel inside the valve to the two tank chambers which are sealed by installing actuating elements or lids. With type equipped with five chambers, as illustrated above, the T-channel (1)is formed just like P, A and B as webs at both sides of the housing and designed as chambers. The two end chambers (2) are connected to each other via a bore hole. When the control piston is moved, the hydraulic fluid is displaced from one chamber to the other. 40 / 77 WIRTGEN GROUP TRAINING 2364339_V01 When installing a nozzle or adjustable throttle (3) in this connection bore hole, the switching duration can be influenced according to the nozzle design or throttle setting. Example of a directional gate valve with mechanical actuation: The valve is actuated via the control lever (1). The control piston (2) is connected to the actuation mechanism and will therefore follow its movements. The piston is returned to starting position at the centre via springs (3) after the actuating movement, meaning the movement of the control lever, has finished. Example of a directional gate valve with hydraulic actuation: This type of directional gate valve is actuated by pressure in the connections at the sides. The actuation cylinder will be forced against the spring and force the control piston (1) in the required direction. When the control pressure reduces, the control piston (1) will be moved by the centralising spring back to central (starting) position. 41 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Pre-controlled directional gate valves It is necessary to use directional gate valves of pre-controlled design when controlling large hydraulic fluid volumes. The reason for this is the higher force that is necessary to move the control piston. Directional gate valves with a nominal size up to 10 are directly controlled. Above the value, pre-control is used. An exception to this rule is manually (mechanically) actuated directional gate valves that need not be pre-controlled up to a nominal size of 32. A pre-controlled directional gate valve is normally equipped with a solenoid valve as pre-control valve, and a main valve. When the pre-control valve is actuated, the main valve is subjected to pressure so that the control piston of the main valve can be moved. The pre-control valve in the above illustrated is marked as (1). The main valve, which carried out the main work, is marked as (2). 42 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.2.2 Directional seat valves Directional seat valves are directional valves with one or more exactly fitting but moving control pistons inside the bore(s) of the valve housing. These control pistons can be designed as balls, cones or plates. With this type, an increase of pressure will in increased tightness. Directional seat valves have the following features: - No leakages - High durability because there are no leakage oil channels or throttle gaps that can become clogged - Locking function without additional locking elements - Suitable for high hydraulic pressures because no hydraulic jamming (deformations under pressure) or leakages can occur - High pressure losses due to short strokes - Performance losses due to insufficient pressure equalisation Directional seat valves are directly or indirectly (pre-control) actuated. Whether a valve is actuated directly or with pre-control mainly depends on the nominal size of the valve and, therefore, on the actuation force that is necessary to operate the valve. Directly controlled directional seat valves These are directional seat valves that are actuated by mechanically operated devices. An example is an electrically actuated 3/2-way seat valve (in ball valve design). 43 / 77 WIRTGEN GROUP TRAINING 2364339_V01 A P T As directional seat valves are rarely used In Wirtgen machines, this type of directional valve will not be explained in more detail. 44 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.3 Pressure valves "Pressure valves" is a general term for ALL valves that directly or indirectly influence a part of or the total system pressure inside a hydraulic system. This is carried out exclusively by changing the throttle cross-section by means of either mechanical, hydraulic or electrical control elements. When the throttle cross-section is closed, all pressure valves can be divided into gate valves and seat valves in a similar way to directional valves. Pressure valves can be divided into sub-groups according to their functions: - Pressure limitation valves (PLV) - Pressure switch-in valves - Pressure reduction valves (PRV) - Pressure cut-off valves These valves can be either directly or pre-controlled 9.3.1 Pressure limitation valves (PLV) Inside a hydraulic system, a PLV has the task of limiting a pressure to a certain value. If the required pressure is exceeded, the PLV will be actuated and directs the excessive flow volume out of the hydraulic circuit back to the oil tank. Consumer Directional valves Pressure Flow valve Limitation valves Check valve Pump The above illustration shows a simple circuit of a pressure limitation valve. It is always located in a bypass line. According to its task, a pressure limitation valves can also be called a safety valve. The principle common to all pressure limitation valves is that the input pressure is guided onto a surface which is subjected to a force. 45 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Seat valve Gate valve As long as the spring force is higher than the force of pressure, the control element will remain on its seat. If the force of the pressure exceeds the spring force, the excessive hydraulic fluid will be directed back to the tank. When flowing back to the tank, the hydraulic energy inside the fluid will be transformed into heat. The illustrated PLVs are directly controlled PLVs according as can be seen by their functions. This type of pressure limitation valve is frequently used in Wirtgen machines. Examples are as twin pressure limitation valve for the conveyor UP/DOWN function in Cold Milling Machine W 2200, or as a safety valve inside the advance drive pump of the same machine (see extract of hydraulic circuit diagram). 46 / 77 WIRTGEN GROUP TRAINING 2364339_V01 47 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.3.2 Pressure limitation valves (pre-controlled) The possibility of constructing directly controlled valves for large flow volumes is limited, just like directional valves, by the space requirement of the control spring. Large flow volumes require larger valve seats or gate diameters. The surface area and therefore the spring forces increase in square of the diameter. In order to remain within acceptable design dimensions, these valves are pre-controlled. They are used for pressure limitation or for pressure limitation and pressure relief via solenoids of the operational pressure. The illustrated pre-controlled pressure limitation valve consists of a main (1) and a pre-control stage (2), whereby the latter involves a simple pressure limitation valve of seat design. It is a measuring unit within a system because the setting of its spring (3) is decisive for the actuation pressure of the total valve. The input pressure reaches the lower end of the valve and, via a throttle (4), to the upper end. From here, there is a connection to the pre-control valve. As long as this valve is not actuated, the pressure is balanced and its closed switching position can be maintained by the relatively weak spring (5). When the opening pressure is achieved at the input of the of the valve, a small control pressure will flow through the throttle and the pre-control valve. This will generate a pressure drop at the throttle - therefore a force difference between the upper and lower ends of the valve - which will cause the throttle to be forced upwards against its spring and open a connection between the input and the output. The main and pre-control valves can be illustrated separately or combined in hydraulic circuit diagrams. In general, the preferred method of illustration is the simple method shown at the right. 48 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.4 Pressure switch-on / cut-off valves Pressure switch-on and cut-off valves are installed in the main flow of a hydraulic system and are actuated when a certain pressure is achieved to switch on or switch off another hydraulic system. 9.4.1 Pressure switch-on valves It is generally possible to use pressure limitation valves as a replacement for pressure switch-on valves. A requirement for this is that the pressure inside channel T (with directly controlled PLV) or inside channel B (with pre-controlled PLV) cannot change the pre-selected pressure setting. This can be achieved when the leakage oil of a directly controlled pressure limitation valve, or the control oil of a pre-controlled pressure limitation valve, is returned externally to the hydraulic tank. Directly controlled pressure switch-on valve Directly controlled pressure switch- Pre-controlled pressure cut-off valve on valve with internal control oil input with internal control oil input and and external control oil output external control oil output Settings of the switch-on pressure are carried out on the adjuster element (4). The compression spring (3) will hold the control piston (2) at starting position. The valve is closed. The pressure inside channel P passes through the control line (6) and is active on surface (8) of control piston (2), therefore against the spring force (3). If the pressure in channel P exceeds the value of the spring, the control piston (2) will be forced away against the spring (2). The connection between channel P and channel A is opened. The hydraulic system connected to channel A will be switched in without any pressure loss in channel P. The control signal is received internally via the control line (6) and the nozzle (7) from channel P, or externally via connection B (X). Depending on the application, the leakage oil is returned via connection T (Y), or internally via connection A. In order to ensure an unrestricted return flow from channel A to channel P, it is possible to install a check valve. The manometer connection (1) is designed to check the switch-on pressure. 49 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.4.2 Pressure cut-off valves Pressure cut-off valves, also known as reservoir charging valves, are mainly used in hydraulic systems equipped with a pressure reservoir. Their task is to switch over the flow volume to pressure-free circulation when the pressure reservoir has achieved its nominal pressure. Pressure cut-off valves are also used in hydraulic systems equipped with high-pressure and low-pressure pumps (twin circuit systems). In this case, the low-pressure is switched over to pressure-free circulation when the pressure range of the high-pressure has been achieved. Pre-controlled pressure cut-off valve A pressure cut-off valve mainly consists of a main valve (1) with a main piston unit (3), a pre-controlled valve (2) with a pressure adjuster element (16) and a check valve (4). With valves of nominal size 10, the check valve is installed in the main valve. With valves exceeding nominal size 10, the check is located in a separate intermediate plate. The hydraulic pumps feed the flow volume via the check valve (4) in the hydraulic system. The pressure in channel A passes through control line (5) to the control piston (6). At the same time, the pressure is active in channel P via the nozzles (7) and (8) on the spring-loaded side of the main piston (3) and the ball (9) inside the pre-control valve. As soon the pre-selected cut-off pressure of the pre-controlled valve is achieved, the ball will move from its seat against the spring (10). The fluid will then flow via the nozzles (7) and (8) into the spring chamber (11). From here, the fluid flows internally or externally via the control line (12) and channel T back to the tank. 50 / 77 WIRTGEN GROUP TRAINING 2364339_V01 9.5 Pressure reduction valves Contrary to pressure limitation valves, pressure reduction valves have the task of reducing the input pressure in certain sections of a hydraulic system. The reduction of the input pressure (primary pressure) and maintaining the output pressure (secondary pressure) is carried out at a value which is lower than the value of the pressure inside the main hydraulic circuit. It is therefore possible to use a pressure reduction valve to reduce the pressure inside a certain section of a hydraulic system. This is frequently the case in Wirtgen machines; as explained by the application examples that follow more. In order to reduce and maintain the output pressure at a certain level, the pressure input pressure works against the end of the control valve (piston or cone) where it is compared with the force of the regulation spring. If the hydraulic force pA * AK exceeds the spring force, the piston will move upwards towards closing position. While regulating, the control gate is in a balance of pressure. The average cross-section that is necessary to hold pA at a constant value is regulated according to the flow volume Q and input pressure pE. 9.5.1 Pressure reduction valves (directly controlled) 51 / 77 WIRTGEN GROUP TRAINING 2364339_V01 In principle, directly controlled pressure reduction valves are produced in 3-way design, meaning that the pressure safeguarding of the secondary circuit is carried out via the adjuster element. The design of the adjuster element can vary according to the customer or application requirements. The valve is open at starting position; meaning that the flow volume can flow unrestricted from channel P to channel A. The pressure in channel A is simultaneously active via the control line (2) and the piston surface against the spring (3). When the pressure inside channel A increases above the pressure value of the spring (3), the control piston (4) will move to regulation position in order to hold a constant pressure inside channel A. The signal and control flow volume is monitored internally via the control line (2) from channel A. If the pressure increases in channel A due to external influences on a consumer, the control piston (4) will adjust even further against the spring (3). Channel A will then be connected to the tank via the control edge (5) of the control piston (4). The necessary quantity of fluid will flow back to the tank to prevent the pressure from increasing. The leakage oil will flow out of the spring chamber (6) via channel T(Y). If required, a check valve can be installed to allow the fluid to flow freely back from channel A to channel P. The manometer connection (8) is designed to monitor the reduced pressure. Directly controlled pressure reduction valves (left with and right without check valve) Important nominal values: - Nominal size NG 5,6 and 10 - Max. input pressure 315 bar - Max. output pressure 210 bar - Flow volume up to 80 l/min. 52 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The following are some examples in which Wirtgen machines and for which functions pressure reduction valves are used: Example 1: Pressure reduction valve in SP 500. The valve has the task of reducing the pressure to the set value of 70 bar in order to release the hydrostatic brakes of the advance drive gearbox. If the system pressure of 190 bar was used for this purpose, the gearbox would be damaged. 53 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Example 2: Pressure reduction valves in Cold Milling Machine W 2200. As can be seen here, different consumers require different output pressures. 54 / 77 WIRTGEN GROUP TRAINING 2364339_V01 10 Flow valves Flow valves are designed to alter (increase or decrease) their throttle cross-sections and thereby influence movement speeds of consumers. A flow distributor has a special function in that is divides an incoming flow volume into two or more outgoing flow volumes. Flow valves can be divided into four groups according to their operational characteristics. Flow valves Throttle valves Flow control valves Pressure ratio dependent Pressure ratio independent Direct control Pre-control Direct control Pre-Control 55 / 77 WIRTGEN GROUP TRAINING 2364339_V01 10.1 Throttle valves The flow volume of throttle valves depends on the difference of pressure; which means that a larger pressure difference will result in a larger flow volume. In numerous control systems where a constant speed is not decisive, throttles are used because flow regulation valves would be too expensive for such purposes. Throttle valves are used when: - A constant working resistance is available. - A speed change with a fluctuating load is not important or can be disregarded. As can be seen in the illustration on the previous page, the viscosity is the influencing factor. When the throttle distance is shorter, a change of viscosity will have less influence. It should be noted that the flow volume will increase as the fluid viscosity decreases. Whether a valve is dependent or not dependent on the fluid viscosity, this depends on the design of the throttle distance. Aperture and throttle valves An ideal throttle, meaning it is less dependent on the viscosity, is a circular aperture (1) with a short throttling distance. This design will offer the maximum ratio of surface area and circumference. Long throttle distances (2) are highly dependent on fluid viscosity. Adjustable throttle cross-sections are virtually impossible to produce as infinitely adjustable circular areas. Adjustable needle throttles (3), particularly with small adjustment ranges, have a poor ratio of surface area and circumference. They are therefore highly dependent on viscosity and also have poor resolution at small settings. An acceptable compromise is a triangle with equal sides. This type of cross-section can be achieved with piston throttles (4). Throttles and flow control valves are often required for only one flow direction. They are then combined with check valves to produce throttle check valves (5). In the event of flow direction B > A, the bushing in the illustrated type will lift and enable the full flow volume. Throttle check valves of intermediate plate design are frequently used in Wirtgen machines because they can be easily integrated in the modular valve blocks. Their task is to limit the main and control flow of one or two consumer connections. In the illustration below, two symmetric throttle check valves are installed to limit the flow volume in one direction and to enable unrestricted return flow in the other direction. 56 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The fluid in channel A reaches consumer A2 via the throttle unit (1) consisting of the valve seat (2) and the throttle piston (3). The throttle piston (3) can be axially adjusted via the adjuster screw (4) in order to adjust the throttle cross-section (1). The fluid returning from consumer B2 will force the valve seat against the spring (5) in the direction of the throttle piston (3) to enable an unrestricted return flow. Depending on its installation configuration, the throttling effect will take place in the feed or return flow direction. 10.1.1 Twin throttle check valves In order to alter the speed of a consumer (main flow limitation), a twin throttle check valve is installed between the directional valve and the connection plate. With pre-controlled directional valves, the twin throttle check valve can be utilised to set the switching time (control flow limitation). It is then installed between the pre-control valve and main valve. 57 / 77 WIRTGEN GROUP TRAINING 2364339_V01 This is an example for the application of twin throttle check valves in Wirtgen machines. The illustration is an extract from the hydraulic circuit diagram of Cold Milling Machine W 2000. The twin throttle check valve is installed here in the function "Rear height adjustment". It has the task of limiting the return flow and enabling it to be adjusted. This valve will enable the speed of the rear height adjustment cylinder and front height adjustment cylinder to be adapted to each other. 58 / 77 WIRTGEN GROUP TRAINING 2364339_V01 10.2 Flow regulation valves The task of a flow regulation valve is to maintain a constant selected flow volume independent of any pressure fluctuations in the system. Apart from the adjustable measuring throttle (1), this task is achieved by installing an additional moving throttle which works as a regulation throttle (pressure balance) and is therefore has a comparison function in a circuit (see illustration below). When these two throttles work together, the pressure load is separated by the varying pressure difference p 1 – p3 into two branches: - The inner and constant pressure difference p1 – p3 at the adjustable measuring throttle, and - the outer and varying pressure difference p1 – p3 The flow regulation valve is designed as a regulator with the following main components: - Measuring throttle (1) - Pressure balance (2) with spring (3) When temperature or viscosity fluctuations occur, the adjustable measuring throttle (1) will monitor a change of pressure difference of p1 – p2. The shape of the throttling point can counteract this influence. The configuration of the pressure balance determines the type of flow regulation valve. If it is positioned in series with the measuring throttle, the result is a 2-way flow regulation valve. If it is positioned parallel to the measuring throttle, the result is a 3-way flow regulation valve. 59 / 77 WIRTGEN GROUP TRAINING 2364339_V01 10.2.1 2-way flow regulation valve With 2-way valves, the measuring aperture and pressure balance are configured in series, whereby the pressure balance can be at preceding or following side. Whether the pressure balance in a 2-way flow regulation valve precedes or follows behind is a matter of design and has no influence in practical use. There are three basic installation possibilities: - Input control (primary control) - Output control (secondary control) - Input ancillary regulation (bypass) Input control The flow regulation takes place between the hydraulic pump and the respective consumer. This type of regulation is recommended for hydraulic systems in which the consumer generates a positive resistance (counter force) against the regulated flow volume. The advantage of this switching system is that only the pressure is active between the flow regulation valve (1) and the pressure cylinder (2) which results from the resistance of the pressure cylinder. As the pressure against the cylinder seals is lower, the friction on the sleeves in the cylinder is also lower. A disadvantage is that the pressure limitation valve (3) in front of the flow regulation valve must be adjusted according to the largest consumer. The hydraulic pump (4) therefore always generates the maximum selected pressure even at times when the consumer requires less pressure. The generated throttling heat is also transmitted to the consumer, which can have a negative influence, for example, on hydraulic motors. Input control Output control With this type, the flow regulation valve (1) is installed in the line between the consumer (2) and the tank. This type of regulation is recommended for hydraulic systems with negative or precipitating (pulling) working loads, which have the tendency to move the cylinder faster than is possible, by the displaced flow volume. The advantage is that a counter valve is not necessary. The generated throttle heat is also directed back to the hydraulic tank. A disadvantage of this control type is the pressure limitation valve (3) must be set according to largest consumer pressure (heat development). Even when idling, all components of the cylinder are subjected to the maximum operational pressure (high friction). 60 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Output control Input auxiliary control (bypass) With this type, the flow regulation valve (1) is installed parallel to the consumer. The flow regulation valve can only regulate the flow volume to the consumer in that a selectable amount of the flow volume is returned back to the hydraulic tank. The advantage is that only the necessary pressure is generated to move the load during the working stroke. Therefore, less energy is transformed into heat. Only when the cylinder rests against the buffer will the set pressure of the pressure limitation valve be achieved. The throttling heat of this control type is also directed back to the hydraulic tank. 61 / 77 WIRTGEN GROUP TRAINING 2364339_V01 10.2.2 3-way flow regulation valve Contrary to a 2-way flow regulation valve, the measuring aperture A1 and the regulation aperture A2 of a 3-way flow regulation valve are installed parallel and not in series. The pressure balance returns the excess flow volume via an additional line back to the tank. In order to safeguard the maximum pressure, a pressure limitation valve must be installed in the hydraulic circuit. This pressure limitation valve is frequently included inside a flow regulation valve. As the excess flow volume QR is directed back to the tank, 3-way flow regulation valves can only be installed in the feed or leading lines. This type of valve also enables a relief connection X for virtually free circulation. The working pressure of the hydraulic pump is only larger than the consumer pressure according to the reduction by the measuring aperture, whereby the hydraulic pump with 2-way flow regulation valves must always generate the pressure, which has been set on the pressure limitation valve. A 3-way flow regulation valve therefore causes less losses in the lines, offers a preferable system efficiency and has lower heat development. 62 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The hydraulic circuit diagram below shows the conveyor belt drive in machine Cold Milling Machine W 1000 which is controlled by a flow regulation valve. This valve enables the conveyor belt speed to be infinitely adjusted. 63 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The hydraulic circuit diagram below of the SP 1600 shows various applications of flow regulation. As can be seen by the ident numbers, different consumers requires different designs of flow control regulators. 64 / 77 WIRTGEN GROUP TRAINING 2364339_V01 11 Hydro-pneumatic reservoir The task of a hydro-pneumatic reservoir is to store hydraulic energy coming from the hydraulic pump and then release this energy later when it is required. Volume equalisation and the respective storage of energy inside a hydro-pneumatic reservoir is achieved by subjecting the fluid to the pressure of a weight, spring or gas. The pressure of the fluid and the respective weight, spring or gas is always in balance. Weight-loaded or spring-loaded reservoirs are only used for very special industrial applications and are therefore of minor importance. Gas pressurised reservoirs without a separating diaphragm are rarely used because hydraulic fluid will absorb gas. The hydro-pneumatic reservoirs (gas-spring reservoir) in the majority of hydraulic systems are equipped with separating elements. These reservoir designs, which are explained in this section in more detail, are divided into bubble, piston and diaphragm reservoirs according to the type of separating elements used. As previously explained, hydro-pneumatic reservoirs fulfil various tasks in hydraulic systems. These could be: - Storing energy - Reserve fluid - Emergency actuation - Force equalisation - Damping mechanical impacts - Damping hydraulic impacts - Leakage oil compensation - Impact and oscillation damping - Pulsation damping - Vehicle suspension springing - Recycling braking energy - Maintaining a constant pressure - Flow volume compensation (expansion vessel) 65 / 77 WIRTGEN GROUP TRAINING 2364339_V01 Hydro-pneumatic reservoirs as listed above are installed in various Wirtgen machines and carry out a wide range of functions. The application of hydro-pneumatic reservoirs enable the pump performance to be reduced to medium output. This reduced flow volume will quickly fill the reservoir because the required flow volume during a working cycle us mostly smaller than the flow volume generated by the pump. When the maximum flow volume is briefly required, the hydraulic system will compensate the insufficient flow volume by extracting the missing amount from the reservoir. Important characteristics are: - Application of smaller hydraulic pumps - Lower installed output - Reduced heat generation - Simple maintenance and installation The above advantages are also enhanced by damping pressure impacts and pulses, thereby increasing the life- expectancy of the hydraulic system. The application of hydro-pneumatic reservoirs also saves energy. Hydraulic systems with short operating cycles, or those that require large flow volumes over brief periods, can only be economically realised by using hydro- pneumatic reservoirs. Example 1: Application of hydro-pneumatic reservoir in Wirtgen machine SP 1600. A reservoir (bubble reservoir) is used in order to release an extremely large flow volume to quickly extend the hydraulic cylinder to its final position. This cylinder is used to implant a metal rod (tie-bar) in the freshly paved concrete surface. As this operation is carried out while the machine is moving forwards, the insertion function must be carried out as fast as possible. 66 / 77 WIRTGEN GROUP TRAINING 2364339_V01 If the reservoir is used as a safety element, it will not be applied as an energy source during normal machine operation. It, nevertheless, is always directly connected to the hydraulic pump. Due to the application of high- quality separation elements, the stored energy can be stored for virtually unlimited time and is immediately available when required. Safety elements equipped with reservoirs are used as emergency actuators in hydraulic systems so that they can immediately take over certain functions in the event of a fault or irregularity. These functions could be: - Closing bulkheads, flaps, bypasses, etc. - Actuating gate valves - Actuating high-performance switches - Actuating rapid shut-down systems Example 2: This hydraulic circuit diagram of Cold Milling Machine W 1000 demonstrates the use of a diaphragm reservoir in the advance drive system In the event of an emergency, such as loss of electrical power, the energy stored in the reservoir will be utilised to finalise a working cycle. This can become necessary if the hydraulic pump or the surrounding hydraulic system (oil feed) develops a fault. Emergency functions of hydraulic reservoirs are characterised by: - Immediately available - Unlimited durability - No power fatigue - No inertia problem - High reliability and low maintenance requirement 67 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The most frequently used hydraulic reservoir in Wirtgen machines is the diaphragm reservoir. An elastic diaphragm (2) is used inside the pressurised reservoir (1) to separate the hydraulic fluid from the nitrogen filling. A shut-off button (3) is installed in the base of the diaphragm which will cover the connection port when the diaphragm is fully expanded and thereby prevent the expanded diaphragm from being forced into the connection port. At the gas side of the diaphragm, a plug-screw (4) enables the filling pressure of the reservoir to be checked or re-filled via a testing and filling device. The reservoir technical data is V0=0.075 to 2.8 l and Pmax =160 to 250 bar. Pressurising and relieving a diaphragm reservoir 68 / 77 WIRTGEN GROUP TRAINING 2364339_V01 As can be seen by the hydraulic circuit diagram of the SP 1600, bubble reservoirs are also used Wirtgen machines. The elastic separating wall between the pressurised fluid and nitrogen, which develops a bubble (3), is affixed inside the reservoir (1) via the vulcanised gas valve element (4) and can be removed or installed inside the reservoir through the opening in the fluid valve (2). The fluid valve has the task of closing the input port when the bubble is fully expanded, thereby preventing the bubble from being pressed into the opening. A damping device protects the valve against hydraulic impacts when it is opened quickly. 69 / 77 WIRTGEN GROUP TRAINING 2364339_V01 12 Hydraulic filters In hydraulic systems, large volumes of hydraulic oil flow under high pressure through narrow gaps. The result is that such systems are more sensitive to contamination in the oil, particularly solid particles, than other drive systems. Experience has shown that more than half of the faults that occur in hydraulic systems are caused by contaminated fluids. The task of a hydraulic oil filter is to reduce the contamination in the oil to an acceptable minimum in order to protect the hydraulic components against excessive wear. These solid particles can consist of sand, dust, metal or rust. They accelerate the wear rate on the surfaces of metal components that must move against each other, and on hydraulic seals. Contamination causes negative effects on bearings, guides, rotors, pistons and the flanks of gear wheels in hydraulic pumps and motors, as well as the pistons, piston rods and bushes of hydraulic cylinders. The wear on friction surfaces increases their gaps and can lead to leakages. Larger solid particles (< 0,050 mm²) will frequently cause sudden failure of machine components, whereas smaller solid particles (> 0,010 mm²) generally cause slowly developing damage and component faults, The damaging effect of solid contamination particles depends on the particle hardness, size and concentration, as well as the sensitivity of the hydraulic components Extensive wear is caused by particles that are of similar size as the gaps between moving components. 70 / 77 WIRTGEN GROUP TRAINING 2364339_V01 12.1 Suction filters Hydraulic systems must be equipped with a suction filter when there is a high risk of damage to a pump by large contamination particles. This is particularly the case when: - Numerous hydraulic circuits use the same hydraulic fluid - Hydraulic tanks cannot be cleaned due to their shape Pressure differences must not be too great because pumps are particularly sensitive to vacuums. This is why filters with large filtering surfaces must often be installed. A bypass valve and/or a contamination indicator on the filter is also necessary. A suction filter will only safeguard the function of the pump. The necessary protection against wear must be achieved by other filters. The illustration below is a filter that is installed in Wirtgen machine RX 4500. Adjacent to it is the respective section of the hydraulic circuit diagram. 71 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The switching symbol designates the return flow filter which is equipped with a bypass. Its flow direction must always be upwards away from the tank. 72 / 77 WIRTGEN GROUP TRAINING 2364339_V01 12.2 Line filters / pressure filters This type of filter has the task of safeguarding all components that follow. For this reason, the filter must be installed as close as possible to the components that require protection. The following criteria are decisive for the application of line filters: - Components are particularly sensitive to contamination, or are particularly important for the function of the hydraulic system - Components are particularly expensive and are decisive for the reliability of the hydraulic system - Down-time costs are particularly high with this type of hydraulic system - Line filters can be installed as safety filters or as operational filters Filters have the following tasks: Operational filters: Protection of components against wear. Maintaining the required grade of fluid cleanliness Safety filters: Safeguarding the component function. Safety filters are only installed together with operational filters This type of line filter must be able to withstand maximum system pressure. 73 / 77 WIRTGEN GROUP TRAINING 2364339_V01 The filter (illustrated below) consists mainly of an upper section (1), a screw-in lower section (2) and the filter element (3). The standard design is without a bypass valve and without a pressure differential switch to safeguard the filter. 74 / 77 WIRTGEN GROUP TRAINING 2364339_V01 12.3 Ancillary / return flow filters These filters are installed at the end of the return flow line and are generally designed as tank ancillary filters. This means that all contamination that enter the system or are generated inside the system are filtered out of the fluid before it flows back into the tank. The size of the filter mainly depends on the size of the flow volume. In order to prevent the production of foam inside the tank, it must always be ensured that the return flow of fluid always enters the tank below the fluid level. This is achieved in Wirtgen machines by a pipe inside the tanks that extends below the fluid level. When such a pipe is installed, it must be ensured that the end of the pipe is 2-3 times its own diameter above the bottom of the tank. Illustration of return flow filters in RX 4500 The return flow filter illustrated below is affixed by a retainer flange (1) to the tank lid. The housing (2) with its filter connection protrudes directly into the tank. An enormous advantage of this filter type is that it is easily accessible and therefore easy to service. Its filter element (5) can be quickly extracted after removing the tank lid (3). It is important that the filter element is surrounded by a dirt collection basket (4). This basket is extracted together with the filter element so that the filtered particles cannot flow out of the filter back into the hydraulic tank. A connection (6) for a contamination indicator is generally available. 75 / 77 WIRTGEN GROUP TRAINING 2364339_V01 76 / 77 WIRTGEN GROUP TRAINING 2364339_V01 12.4 Filler and ventilation filters In the past, not much attention was paid to this type of filter. In the meantime, it is proven that it is one of the most important filters in a hydraulic system. A large amount of contamination in a hydraulic system enters via the surrounding air. Due to continuous fluctuations of the fluid level inside the tank, a vacuum or pressure is generated against the air inside. It must therefore be ensured that no contaminated air can enter the tank. 77 / 77