Hydraulic Power Transmission

Hydraulic Power Transmission

SPECIAL STUDY

Submitted to: Prof. Vilas M. Salokhe

By

TASLIMAN
AEA 977051

Agricultural Machinery and Management Program
Agricultural and Food Engineering Division
Asian Institute of Technology
December 1997

TABLE OF CONTENTS

I. INTRODUCTION 1

A. Hydrostatic Transmission 1
B. Term Definitions 2

II. TRANSMISSION OF POWER 4

A. Classification of Power Transmission 4

III. HYDRAULIC SYSTEM 6

A. General Types Of Hydraulic Systems 6

1. Constant Flow, Variable Pressure System 6
2. Constant Pressure System 6
3. Variable Flow, Variable Pressure System 7

B. Components of a Hydraulic System 7

IV. HYDRAULIC PUMPS 9

A. Positive-displacement pumps 9

B. Pump Classification 10

1. Gear type 12

a. External Gear Pump 13
b. Internal Gear Pump 14
c. Gerotor Pump 15

2. Vane type 15

3. Piston type 17

a. Axial piston pumps 19
b. Radial piston pump 20

V. HYDRAULIC MOTORS 21

A. Hydraulic Motor Classification 22

1. Gear type Motors 24

a. External Gear Motors 24
b. Internal Gear Motors 25
c. Gerotor Motors 26

2. Vane type Motors 27

3. Piston type Motors 28

VI. HYDRAULIC VALVES 29

A. Directional Control Valves 29

1. Spool Valves 29
2. Check valves 30

B. Pressure control valves. 31

1. Relief valves 31
2. Pressure Reducing Valves 33
3. Pressure sequence valves 33

C. Flow control valve and flow divider valve 33

VII. HYDRAULIC POWER TRANSMISSION SYSTEM 35

A. Hydrostatic Power Transmission 35

B. Circuit Types On Hydrostatic Drives System 36

1. Closed-loop System 37
2. Open-Loop Systems 39

C. Hydrostatic Drive Classification 40

1. PF-MF Connection 41
2. PF-MV Combination 42
3. PV-MF Drive 42
4. The PV-MV Circuit 44

D. Hydraulic power transmission in multifunction system. 47

VIII. HYDRAULIC POWER TRANSMISSION ON AGRICULTURAL IMPLEMENTS 54

A. Self-Propelled Combines 54
B. Self propelled windrowers 55
C. Tractor hydraulic remote motors. 56
D. Hydrostatic Transaxle 57

IX. CONCLUSIONS 61

X. REFERENCES 62

LIST OF FIGURES

Fig. IV-1 Positive-Displacement Pump 10
Fig. IV-2 Schematic diagram of a fixed-displacement axial-piston pump 11
Fig. IV-3 Schematic diagram of a variable-displacement axial-piston pump 11
Fig. IV-4 Schematic diagram of some unusual type pumps 12
Fig. IV-5 Schematic diagram of an external gear pump 13
Fig. IV-6 Cutaway view of a two-section external gear pump used for multi-circuit hydraulics 13
Fig. IV-7 Schematic diagram of an internal gear pump 14
Fig. IV-8 Schematic diagram of a gerotor pump 15
Fig. IV-9 Schematic diagram of a vane pump 16
Fig. IV-10 Cutaway view of a vane pump 16
Fig. IV-11 Cutaway of a double vane type pump 17
Fig. IV-12 Axial and radial piston pumps 18
Fig. IV-13 Fixed displacement axial piston pump 18
Fig. IV-14 Typical adjustable-displacement axial piston pump 19
Fig. IV-15 Schematic diagram of radial piston pumps 20
Fig. IV-16 Fixed-displacement radial piston pump with rotating cam 20
Fig. V-1 Hydraulic pump and motor 21
Fig. V-2 Eight types of hydrostatic motors 23
Fig. V-3 External Gear Motor 24
Fig. V-4 Schematic diagram of a balanced external gear motor 25
Fig. V-5 Schematic diagram of an internal gear motor 25
Fig. V-6 Gerotor-type hydrostatic motor 26
Fig. V-7 Vane type hydraulic motor 27
Fig. V-8 Fixed-displacement axial-piston motor 28
Fig. VI-1 Four-way valve in a typical circuit 30
Fig. VI-2 System with parallel control valves, a pilot operated bypass valve, and pilot operated cylinder check valve. 32
Fig. VI-3 Adjustable, pressure-compensated bypass or priority flow divider valve 33
Fig. VI-4 Flow control valves (including servos) add precision to a hydraulic system 34
Fig. VII-1 The pump and the motor of a hydrostatic transmission may be separate or combined into one housing. 36
Fig. VII-2 Closed-loop system with displacement-controlled pump 37
Fig. VII-3 Open-loop system with fixed-displacement pump 39
Fig. VII-4 Schematic diagram of fixed-displacement pump and fixed-displacement motor 41
Fig. VII-5 Schematic diagram of fixed-displacement pump and variable-displacement motor 42
Fig. VII-6 Schematic diagram of variable-displacement pump and fixed-displacement motor 43
Fig. VII-7 Schematic diagram of variable-displacement pump and variable-displacement motor 44
Fig. VII-8 A closed-loop hydrostatic propulsion drive system having a fixed-displacement motor 46
Fig. VII-9 Multifunction hydraulic system for a self-propelled tomato harvester. 48
Fig. VII-10 Schematic diagram of heavy-duty hydrostatic transmission with control valve in neutral position 50
Fig. VII-11 Schematic diagram of heavy-duty hydrostatic transmission with control valve in forward position 51
Fig. VII-12 Schematic diagram of heavy-duty hydrostatic transmission with control valve in reverse position 51
Fig. VII-13 Cross-sectional view of a heavy-duty hydrostatic transmission 53
Fig. VIII-1 Hydrostatic transaxle 58
Fig. VIII-2 Overall hydrostatic transaxle system mounts into small tractor 59
Fig. VIII-3 Typical heavy vehicle has the pump at the engine and the motor on an axle or wheel. 60

I. INTRODUCTION

Hydraulic power is unique in that the fluid medium, a liquid, is pumped under high pressure to perform work remotely at any location connectable by pipe or high-pressure hose. The force and power reside in the flowing fluid and can be applied directly, which is what makes hydraulics indispensable in certain applications.

As a compare, for example, an electrical power needs conductive windings at every work point to convert current flow into force and motion. There is no way, short of extremely high currents, that electrical actuators can produce the force per unit volume easily obtainable in a high-pressure hydraulic actuator.

A. Hydrostatic Transmission

A hydrostatic transmission, one of the hydraulic power system applications, has number of advantages, among which are:

  1. Output shaft speed can be maintained at a constant speed for a variable speed input.
  2. Speed and direction of the output shaft can be controlled accurately and remotely.
  3. Constant power output can be maintained over a wide range of speeds.
  4. Automatic torque control can be maintained at the output shaft.
  5. Output shaft speed can be varied in extremely small
  6. The output shaft can be reversed quickly and without shock.
  7. Power consumption can be kept at a low level.
  8. Automatic overload protection can be maintained.
  9. No clutch operation and no gear changing are required during operation.
  10. Speed can be varied on the go while the working mechanism is independently operated at its most efficient speed.
  11. The hydrostatic transmission is a compact unit in relation to its horsepower output.
  12. There is no tendency to creep.

Hydrostatic transmission is well suited to condition requiring application of power in remote, inaccessible situation especially where multiplication of force is involved, where linear motion is required and where great flexibility and a very wide range of speeds is advantageous.

B. Term Definitions

In this section some terms related to hydraulic system is defined. However the following definitions is not clearly distinguished one and another and frequently interchange in their meanings.

Hydraulic system is total interconnecting components to manipulate fluid and its mechanical characteristics. It includes hydraulic transmission system and hydraulic control system.

Hydraulic power transmission means the process of power transmission using fluid as the transmission medium or the arrangement of hydraulic system used to transmit mechanical power.

Hydrostatic power transmission is a type of hydraulic power transmission where the power transfer between hydraulic fluid and mechanical device is by utilize its pressure rather than its kinetic energy. It is contrasted with hydrodynamic or hydrokinetic where the transfer mechanical power is by transfer of kinetic energy between mechanical device and the fluid. In hydrostatic transmissions, positive-displacement pump and motor are used.

Hydrostatic power drive or Hydrostatic propulsion drive is a special arrangement used to replace function of conventional mechanical power transmission (e.g. function of clutch-gear box-differential in a typical vehicle transmission system) especially the function of gear box, by using hydraulic coupling of a (positive-displacement) pump and motor(s).

II. TRANSMISSION OF POWER

Power is the rate of energy transfer for the motion of bodies. Therefore this term is always associated with motion. Power can be cropped by converting other form of energy to motion form. Power source is sometimes understood as the device where energy from other source is converted to a motion (that are essentially linear but usually converted to rotation). It is rarely that an energy source is converted directly to move its final working device, therefore in almost all cases an arrangement to catch the power form its source then give it to the motion-need part at some distances is needed. This arrangement is called as power transmission.

A. Classification of Power Transmission

Basically there are four categories:

  1. Mechanical power transmission
  2. Electrical power transmission
  3. Hydraulic power transmission
  4. Pneumatic power transmission

Mechanical power transmission uses solid as the medium of transmission. Some types are: chain-sprocket wheel, gears, belt-pulley, couplings and shaft.

In electrical power transmission, power is converted to an electrical form (voltage and current) by an electric generator, transmitted at this form, and converted again to motion form by electric motors or actuators.

In pneumatic power transmission, mechanical power is converted to pneumatic power form (pressure and flow) by a compressor, transmitted through the pneumatic line (hoses, pipes or tubes) and converted to mechanical power form using pneumatic motors, cylinders, or actuators

In hydraulic power transmission, mechanical power is converted to hydraulic power form (pressure and flow) by a pump, transmitted through the hydraulic line (hoses, pipes or tubes) and converted to mechanical power form using hydraulic motors, cylinders, or actuators.

III. HYDRAULIC SYSTEM

A. General Types Of Hydraulic Systems

From the relationship of flow and pressure of the system point of view, hydraulic systems can be categorized into 3 basic types:

  1. Constant flow, variable pressure
  2. Constant pressure
  3. Variable flow, variable pressure
1. Constant Flow, Variable Pressure System

In a constant flow system, a fixed displacement pump operates continuously. The output is bypassed back to the reservoir at low pressure when not needed for any function. The two usual bypass methods are through open center valves and through pilot operated bypass valves. When all the open-center valves in a circuit are in the neutral position, an open bypass from the pump back to the reservoir is provided through the valves. In a pilot-operated bypass arrangement, all control valves are of the closed-center type. When all valves are in the neutral position, the bypass valve opens to permit the pump flow to the reservoir at a low pressure.

2. Constant Pressure System

One method of maintaining an essentially constant-pressure system is by means of a variable-displacement pressure compensated pump. The pump displacement changes in response to system pressure to meet the flow demands of the system while maintaining the predetermined system pressure. Another method involves a combination of an accumulator tank with compressed gas in the upper portion, a fixed-displacement pump, and an unloading-type bypass valve for the pump. With an accumulator system, the pressure varies between predetermined maximum and minimum pressures that may differ by 10 to 15%. The change in gas volume as oil withdrawn or pumped in provides a finite storage capacity between the cut-in and cut-out pressures.

A constant-pressure system provides much greater flexibility than a constant-flow system, with simpler valving and circuits for multiple functions. The full pressure is available for any number of functions in parallel at any time, and one function does not affect the others. Response is fast , since pressure is always available. But wherever the pump is needed it must operate against the full system pressure even though the demand may be for only a low pressure. The throttling required to reduce the pressure to that needed by the actuator produced heat which must be dissipated and represents wasted energy.

3. Variable Flow, Variable Pressure System

Variable flow, variable pressure system are basically closed-loop arrangements, with oil recirculating between the pump and a motor. The flow is usually varied by changing the pump displacement but could be varied by changing the speed. The pressure adjust itself to accommodate the motor load at any particular flow rate. The hydrostatic propulsion drive is an example of a closed loop system.

B. Components of a Hydraulic System

The basic components of a hydraulic system include:

1. Reservoir
2. Pump(s)
3. Actuator(s)
4. Control valve(s)
5. Plumbing

Another components are:

1. Accumulator
2. Pressure relief valve(s)
3. Filter(s)
4. Heat exchanger (cooler)

IV. HYDRAULIC PUMPS

A hydraulic pump is the source of the hydraulic power supply for the hydraulic system. A pump converts mechanical energy into fluid energy. There are two basic types of pumps: positive displacement type pumps and dynamic type pumps. For pressurized hydraulic power and control system, positive displacement pumps are chosen most often. Centrifugal and other dynamic types usually are reserved for fluid handling rather than pressure and power.

A. Positive-displacement pumps

Positive-displacement pump is a pump where the liquid is sucked at the inlet, trapped by pump displacing device and forced into the outlet, without any slippage between the pump and the liquid (except due to leakage). Positive-displacement pumps seal the flow path between the pump inlet and outlet ports and displace a specific volume of fluid during each cycle (Fig. IV.1).

Positive-displacement pumps generally have good volumetric efficiency. They can develop high pressure, produce flow that is directly proportional to speed and prime themselves, even at very low inlet pressure. On this Chapter (and in other Chapters, whenever there is no additional attributes) the term hydraulic pump refers to positive-displacement type pump.

As it internal mechanism starts through its cycle, the hydraulic pump creates a partial vacuum on the intake side. Then the atmosphere pressure acting on the liquid in the reservoir forces the fluid into the pump. As the cycle progress the pump traps this liquid and forces it through the pump outlet under pressure.

Fig. IV-1 Positive-Displacement Pump

B. Pump Classification

Hydraulic pumps may be classified into two basic types determined by their performance:

1. Fixed-displacement pumps.
2. Variable-displacement pumps.

Fixed-displacement or fixed-volume pumps move the same volume of fluid with each operating cycle. Hydraulic fluid flow can only increase as pump operation speed increases and decrease as speed slows.

Variable-displacement or variable volume or adjustable-displacement pumps permit the discharge volume to change without changing pump operating speed. The change in the output of the pump is usually controlled by some external means such as handwheel controls, mechanical controls, servo-valve controls, etc. Variable-displacement pumps are more costly than fixed-displacement types and are employed on some sophisticated systems. Many hydrostatic transmissions make use of variable-displacement pumps.

Fig. IV-2 Schematic diagram of a fixed-displacement axial-piston pump

Fig. IV-3 Schematic diagram of a variable-displacement axial-piston pump

Fig. IV-4 Schematic diagram of some unusual type pumps

There are three major design categories for hydraulic pumps: gear type, vane type, and piston type. Other categories that is not widely used are: screw pump, lobe pump, diaphragm pump, and squeegee pump.

1. Gear type

A gear type pump is a fixed-displacement rotary pump in which the gear elements rotate and cause a pumping action. The liquid enters the pump at its inlet port, is carried between the teeth and is forced through the outlet into the hydraulic system. As the unmeshing of the gears forms a vacuum, the atmospheric pressure on the surface of the liquid in the reservoir causes the liquid to fill up the spaces between the teeth. The liquid is then carried between the teeth to the opposite side of the pump. As the gears mesh, the liquid is forced into the outlet port of the pump.

There are some sub-types: external gear pump, internal gear pump, and progressing-tooth gear pump or gerotor.

a. External Gear Pump

External gear pump carries trapped liquid between pairs of outer teeth, forces it against existing outlet pressure. Outlet-to-inlet leakage is kept small by close meshing teeth at center.

Fig. IV-5 Schematic diagram of an external gear pump

Fig. IV-6 Cutaway view of a two-section external gear pump used for multi-circuit hydraulics

The spur is the most common design of the external gear type pumps but other gear designs, i.e. herringbone, helical, and spiral patterns are in use too

In external type, only one gear is connected to the pump input power shaft and it is known as the driving gear. The other gear is known as the driven gear.

b. Internal Gear Pump

Internal gear pump is similar to vane pump in operating principle. Inlet liquid fills expanding pumping cavities, is carried around inner and outer path as shown in Fig IV-7. Outlet-to-inlet leakage is kept small by close meshing spider teeth and vanes.

Fig. IV-7 Schematic diagram of an internal gear pump

c. Gerotor Pump

Internal and external gear shaped elements are combined in the gerotor pump mechanism. This type of pump also known as the progressing-tooth gear pump. This type of pump has one less tooth in rotor than in stator, thereby moving trapped volume one tooth with each revolution. The tooth shape of the inner gerotor is generated from the tooth shape of the outer, providing continuous fluid-tight engagement. As the teeth disengage, the space between them increases in size, creating a partial vacuum, and the liquid flows from the suction port into this chamber. When the chamber reaches its maximum volume, it is exposed to the discharge port and diminishes in size due to the meshing of the teeth, thus forcing the liquid out of the pump at the outlet port.

Fig. IV-8 Schematic diagram of a gerotor pump

2. Vane type

A vane type hydraulic pump operates on the principle of increasing the size of the cavity to form a partial vacuum, allowing the liquid to fill up the space, and then decreasing the cavity size to force the liquid through the outlet port under pressure.

Vane type gear pumps are available in either fixed displacement or adjustable displacement.

Fig. IV-9 Schematic diagram of a vane pump

Vanes in rotor version is most common: vanes move radially to follow pump contour. Leakage from outlet to inlet across top is kept small because vane is completely retracted, rotor-to-stator clearance is practically zero. The vanes are flat rectangular segments of hardened steel that are held to exacting tolerances. Some designs use one vane per rotor slot while other designs use two vanes per rotor slot. The vanes and the rotor work within a cam ring , and when these parts become worn they can be replaced as a cartridge assembly, without disconnecting the piping. The vanes may be held against the cam ring by small springs placed in the rotor slots or by hydraulic pressure acting on the bottom of the vanes.

Fig. IV-10 Cutaway view of a vane pump

Vanes in stator pump has hollow shaft inlet. Pumping occurs as rotor turns because trapped volume of liquid is squeezed out by vanes. Outlet is through rotor-flow leaves rotor through slip rings.

Fig. IV-11 Cutaway of a double vane type pump

3. Piston type

In a piston single pump, a piston move back and forth in a cylinder bore. This reciprocating action allows inlet liquid to fill cavity on suction stroke, then forces it out on pumping stroke. Because of the reciprocating piston movement, piston pump flow pulsates. Pulsing is corrected by using multiple pistons that act in sequence.

Although there are single cylinder and multiple cylinder arrangement of piston pumps, in the hydraulic power transmission, only multiple cylinder is used. Multiple cylinder piston pumps are identified by the way the pistons are arranged around the drive mechanism. They come into two categories: axial arrangement and radial arrangement. In the radial design, the pistons are arranged radially around a rotor hub while in the axial design the pistons are arranged parallel to the shaft of the pump rotor.

Fig. IV-12 Axial and radial piston pumps

Of the positive-displacement pumps, the piston type has the highest volumetric efficiency because the leakage path between the piston and cylinder can be closely controlled in manufacture. Although piston pumps are more complex and expensive than the other types, they operate well under high speeds and high pressures and relatively easy to service in the field.

Fig. IV-13 Fixed displacement axial piston pump

a. Axial piston pumps

Axial piston pumps have pistons arranged parallel to the pump rotor shaft (Fig. IV-12, IV-13, IV-14). They can be either fixed or variable displacement units. Most axial piston pumps (in-line types) have pistons parallel with the driving member and an angled drive called swashplate to move the pistons. Another variation of the axial piston pump has the pump housing slanted in relation to the drive. This is called a bent axis pump.

Fig. IV-14 Typical adjustable-displacement axial piston pump

A fixed-displacement axial piston pump has the swashplate fixed so it can not be tilted (Fig IV-13). In the variable-displacement axial piston pumps (Fig. IV-14), the angle of the swashplate can be changed, thus changes the displacement of the pump. The swashplate is tilted manually or by a hydraulic servo piston which moves the control rods. The angle of the swashplate controls the distance the pistons move back and forth in their bores, and therefore the volume of fluid being pumped. When the swashplate is parallel to the cylinder block surface, there is no fluid flow. If the swashplate angle is reversed, fluid flow is reversed even though the drive shaft continuous to turn in the same direction.

b. Radial piston pump

Fig. IV-15 Schematic diagram of radial piston pumps

In a radial piston pump the pistons are moved in their cylinder bores by a rotating cam on the drive shaft or rotating cylinder which holds the pistons. Radial piston pumps are available either fixed-displacement or variable-displacement. In a variable-displacement radial piston pump, an arrangement that puts pressurized fluid in the crankcase controls the piston stroke by holding the pistons away from the cam when less fluid flow is needed.

Fig. IV-16 Fixed-displacement radial piston pump with rotating cam

V. HYDRAULIC MOTORS

Hydraulic motors convert fluid power to a rotary mechanical output. Basically there are two means to convert fluid power to mechanical power, by hydrostatic (positive-displacement) motor and by dynamic type motors. In a positive-displacement motor, the fluid can only exit the motor after turning the output shaft. The flow of a certain quantity of fluid result in a certain rotation speed of the motor output shaft. In this Chapter we only discuss about the hydrostatic type motors.

Hydraulic motors (of hydrostatic types) are similar in design to hydraulic pump and the same general types are available. Some units can be used interchangeably as motor or pump.

Fig. V-1 Hydraulic pump and motor

Hydraulic motors have many advantages over electric motors in that they can be stalled without harm, can be rapidly reversed, have a wide range of speed, and can be installed in limited space. Hydraulic motors are much more compact than electric motor of the same horsepower output.

Hydraulic motor may be either fixed-displacement or variable-displacement-type. The variable-displacement type is generally more costly but usually has a higher overall efficiency.

In hydraulic motor, the major emphasis is placed upon mechanical efficiency. Mechanical efficiency is based upon the ratio between the actual output power and the hydraulic power of the geometric displacement at the speed. The total overall efficiency of a hydraulic motor is the product of volumetric and mechanical efficiency. Some hydraulic motor have a very high overall efficiency, as much as 96 to 98 %.

A. Hydraulic Motor Classification

Hydraulic motor are available in three major general categories:

1. Gear type
2. Vane type
3. Piston type

Fig. V-2 Eight types of hydrostatic motors

1. Gear type Motors

Gear type motors are available in external design, internal design and gerotor. The construction of gear type hydraulic motors in general are the same with of gear type pumps. Most gear motors can rotate in either direction. Reversing fluid flow through the motor reverses the direction of output shaft rotation.

a. External Gear Motors

External gear motors have two equal size gears which rotate in opposite directions, one gear is connected to the motor shaft while the other gear acts as an idler. In order to keep internal leakage minimum, it is necessary to hold close tolerances between the gears and the housing. Some external gear motors are balanced to equalize pressure on the gears and gear shaft. The two additional passage for each gear provide opposing high and low pressure areas on each gear to balance the loads and reduce wear.

Fig. V-3 External Gear Motor

Fig. V-4 Schematic diagram of a balanced external gear motor

b. Internal Gear Motors

Internal gear type motors have the design similar with internal gear pumps. Fig. V-5 shows a schematic diagram of internal gear motor.

Fig. V-5 Schematic diagram of an internal gear motor

c. Gerotor Motors

The gerotor type motor is a lobed motor with inner rotor that has one less tooth than the outer stator. For every orbit of the inner rotor (full sequence of pressurization) the output shaft rotates only one lobe. Thus there is a mechanical advantage of 6:1 for six lobe inner and seven lobe outer set.

In operation, oil pressure and flow against the lobes causes the rotor to orbit. A splined dogleg shaft takes that rotation and delivers it as a very slow high torque output. The dogleg also rotates internal valving to synchronize the oil flow.

Fig. V-6 Gerotor-type hydrostatic motor

Some gear type hydraulic motors are manufactured with a built in gear reduction unit so that the output rpm can be in the low range while the motor is running at high rpm to produce its highest efficiency.

2. Vane type Motors

The most parts of vane type motors are the same general construction as vane pumps. The cam ring is machined with a circular contour in the unbalanced rotor design and with an elliptical contour in the balanced rotor design. A rotor, turning inside the cam ring, is provided with vanes that track inside the cam ring. Liquid is forced into the motor, pushing against the vane, and the vane begin to extend itself as it tracks inside the cam ring and turns the rotor. Timing of fluid injection and fluid exhaust is extremely important to control leakage paths (with resultant motor slip) and to make full use of the pressure acting again the vane areas for developing torque. A shaft attached to the rotor delivers the output power.

Fig. V-7 Vane type hydraulic motor

3. Piston type Motors

Rotary type piston motors always have multiple cylinders. Like piston pumps, piston motor are available in either axial or radial piston design.

Fig. V-8 Fixed-displacement axial-piston motor

Piston type motors have a high volumetric efficiency. The friction in these motors is usually quite low which helps to provide high overall efficiencies. The weight-space- power rations and inertia of the moving assemblies within pitons motors are low, making the motors ideal for many mobile application.

Piston motors are available in many sizes and pressure ratings, and for many different shaft speed. Design are available to meet most any requirements.

VI. HYDRAULIC VALVES

Hydraulic valves control the pressure, direction and volume of fluid flow in hydraulic systems. There are three kinds of hydraulic valves:

1. Directional control valves
2. Pressure control valves
3. Flow control valves

A. Directional Control Valves

Directional control valves start, stop, and direct fluid flow to cylinders, motors and other actuators. There are two types of directional control valves: check valves and spool valves.

1. Spool Valves

Spool-type valves are widely used, primarily because of the great variety of valving arrangement which they offer and because they can be hydraulically balanced. Fluid flow between annular ports is controlled by lands on a sliding spool that cover and uncover the ports. The land edges can be notched, tapered or chamfered to permit feathered control at partial flow rates.

Spool valves must be accurately machined and fitted to minimize leakage past the spool. In multifunction system, several valves may be stacked in a compact control package. Valves may be actuated manually, mechanically, hydraulically, electrically, or pneumatically.

Spool valves are classified by the number of control positions and the number of hydraulic circuits. The three most popular classes are: two-way, three-way, and four-way.

Fig. VI-1 Four-way valve in a typical circuit

2. Check valves

Check valves pass flow freely in one direction but do not permit flow in the opposite direction. Spring loaded ball type or poppet type check valves are used extensively in hydraulic systems. Pilot operated check valves prevent reverse flow until the ball or poppet is unseated by pilot pressure from another part of the circuit, acting through a piston and push rod.

B. Pressure control valves.

There are three classes of pressure control valves: relief valves, pressure reducing valves, and pressure sequence valves.

1. Relief valves

Relief valves limit the maximum pressure that can be developed in a hydraulic circuit. Direct-acting relief valves are satisfactory for relatively low flow rates and infrequent operation. But with high flow rates the full flow pressure is considerably higher than the cracking pressure because of the increased spring force as the valve opens wider. This difference is known as pressure override and reduces the allowable system operating pressure for given full-flow relief pressure.

A pilot-operated relief valve relieves over a wide range of flow with very little pressure override, although the response is slower than with a direct-acting valve. The pilot section is a small, spring loaded relief valve that control the main valve. When the pilot valve is closed, the main valve is held closed by the light spring and by system pressure acting on the larger piston area at the spring end. When the system pressure rises enough to overcome the force of the pilot-valve spring and open the pilot valve, flow through the orifice creates a pressure drop across the piston. This pressure differential opens the main valve and holds it open until the system pressure drops and allows the pilot valve to close.

An unloading valve is used to direct pump output back to the reservoir at low pressure when the system pressure is satisfied. The valve opens when the system pressure, transmitted to the pilot piston through the pilot port, is sufficient to overcome the force of the adjusting spring. A check valve holds the system pressure while the pump is bypassing. Accumulator system employ a variation of this type of valve, designed to provide the differential between cut-out and cut-in pressures that is needed for accumulator reserve capacity.

A pilot-operated unloading or bypass valve designed to respond to low pilot pressure is shown in Fig. VI-2. This type of arrangement is suitable for constant-volume systems, whereas unloading valves of the type described in preceding paragraph are for constant-pressure applications.

Fig. VI-2 System with parallel control valves, a pilot operated bypass valve, and pilot operated cylinder check valve.

2. Pressure Reducing Valves

When one part of a circuits is to be maintained at a constant-pressure that is below the operating pressure in the main circuit, a direct-acting or pilot-operated pressure reducing valve usually is used. A pressure reducing valve stay open when pressure is normal. When pressure begin s rising in the secondary circuit, the valve closes partially.

3. Pressure sequence valves

Pressure sequence valves are used to deliver fluid to one actuator in series before another. These valves stop the fluid from entering one branch of the system until the pressure in the main system reaches a preselected level. Flow is directed then to a second branch while full pressure is maintained on the first.

C. Flow control valve and flow divider valve

Flow control devices are used to control actuator speeds by either restricting the flow or diverting the excess flow. Either method causes a pressure drop that is converted into heat and represents wasted energy. Manual valves and orifices are non-pressure-compensated restrictive devices, since the flow rate is related to the pressure drop. Nevertheless, needle valves and orifices are useful in many hydraulic-circuit situations.

Fig. VI-3 Adjustable, pressure-compensated bypass or priority flow divider valve

Figure VI.3 shows a pressure-compensated priority flow divider valve. It maintains a constant flow rate from the priority outlet bypassing the excess to other functions in the system or to the reservoir, pressure compensation is achieved by automatic operation of the balanced spool to maintain a constant pressure drop across the metering orifice.

Fig. VI-4 Flow control valves (including servos) add precision to a hydraulic system

VII. HYDRAULIC POWER TRANSMISSION SYSTEM

In hydraulic power transmission, mechanical power is converted to hydraulic power form (pressure and flow) by a pump, transmitted through the hydraulic line (hoses, pipes or tubes) and converted to mechanical using hydraulic motors, cylinders, or actuators.

Hydraulic power transmission system are more expensive than mechanical systems. Hence, they are used primarily in situations where some of their characteristics are important enough to warrant the extra expense or where the annual use is relatively high. Hydraulic pumps and motors usually have efficiencies of 75 to 90%, but multiple function system can be extremely inefficient if the components are not properly balanced in regard to pressure and flow requirements. Low efficiencies are not serious for intermittent operation but they are of concern for continuous operation because of heat dissipation requirements and the waste of engine power.

A. Hydrostatic Power Transmission

A hydraulic displacement type transmission or hydrostatic transmission is a combination of two interconnected positive displacement units, i.e. a pump and motors. The rotary hydraulic pump transform mechanical energy into fluid pressure energy. In the motor, the fluid pressure energy is transformed into mechanical energy in the form of torque.

Hydrostatic rotary drives provide steeples change of output speed under load, easy to reversal of rotation, automatic overload protection with a simple pressure relief valve and no damage from stalling the hydraulic motor. Hydraulic drives simplify the transmission of power to locations remote from the power source and are especially advantageous on machines with complex drive requirements. They are better than mechanical systems if the power requiring unit must be movable through a wide range of positions with respect to the power source.

Fig. VII-1 The pump and the motor of a hydrostatic transmission may be separate or combined into one housing.

There are actually only four basic components involved in hydrostatic transmission: fixed-displacement pumps, fixed-displacement motors, variable displacement pumps, and variable displacement motors.

B. Circuit Types On Hydrostatic Drives System

Basically there are two types of circuit used on hydrostatic drives systems: closed-loop system and open-loop system.

1. Closed-loop System

Closed-loop hydraulic system are usually used for hydro¬static drives where efficiency is of prime importance. Fig. VII-2 shows a typical closed-loop system having a displace¬ment-controlled pump driving a hydraulic motor. The name closed-loop is derived from the fact that fluid flows in a closed flow path between the pump and the motor without passing through the reservoirs. Low side pressure is main¬tained by a charging pump that also bypass a small percentage of the main oil flow through a cooler and a sup¬ply reservoir.

Fig. VII-2 Closed-loop system with displacement-con¬trolled pump

The basic components of a closed-loop system are the pump, the actuator and the charge pump. The pump must be over center variable-displacement to provide control of both load speed and direction by control of pump displacement. The pump in Fig. VII-2 is displacement-controlled, meaning that the machine operator controls command to the pump con¬trol.

A major advantage of closed-loop system is that they are inherently efficient, since all of the hydraulic power from the pump is delivered to the motor. No hydraulic power is lost between the pump and motor simply because the flow loop contains nothing to bypass or restrict fluid flow. Dissipation of hydraulic power can occur only when operat¬ing conditions are such that they cause fluid flow through the system overpressure relieve valves.

Another advantage of closed-loop system is the ability to dynamically brake overrunning loads. An overrunning load is one which drives the hydraulic motor, rather than being driven by the motor. An example of overrunning load is the condition encountered when a vehicle with hydrostatic pro¬pulsion drive is driven downhill. Under this condition, the transmission must retard the vehicle to prevent down hill runaway. When an overrunning load occurs in a closed-loop system, the motor becomes a pump driven by the load, and the pump becomes a motor which drives the system prime mover. Thus, the energy contained in the overrunning load can be reclaimed and used to drive other loads on the same prime mover or to overcome internal losses of the prime mover.

A disadvantage of closed-loop system is their requirement that each load must have a separate drive pump. When several loads must be driven, the use of closed-loop systems can be costly because of the number of pumps required.

Another disadvantage is the special features usually needed to allow the use of differential-area cylinders. When using differential-area cylinders, either a large charge pump or a charge pressure accumulator is needed to make up the dif¬ference between cylinder input and output flow rates.

2. Open-Loop Systems

Open-loop hydraulic systems are widely used for hydrostatic drives where many loads may be driven. Fig. VII-3 shows a typical open-loop system using a fixed-displacement pump driving two loads: a hydraulic motor and a hydraulic cylin¬der. The Name open-loop is derived from the fact that fluid flow occurs in an unclosed flow path which has inlet and outlet ends at the system reservoir. The basic components of an open-loop system are the pump, the directional con¬trol valves and the actuators.

Fig. VII-3 Open-loop system with fixed-displacement pump

One advantage of open-loop systems is their compatibility with differential-area cylinders. Another advantage is the ability to allow multiple pump loads. Since the directional control valves direct and meter flow to the actuators, the pump can drive any number of loads by supplying flow to a corresponding number of directional valves. Also, the pump need not be variable-displacement since the directional valves control the actuator flow rates.

One of the major disadvantages of open-loop systems is load interference. Load interference occurs whenever two or more actuators requiring different driving pressures are operated simultaneously. Since the pump supplies flow to all of the actuators, its outlet pressure is determined by the actuator requiring the highest driving pressure. Therefore, the flow supplied to actuators requiring less pressure must be restricted as the directional valves to reduce its pres¬sure. This flow restriction causes a hydraulic power loss which reduces system efficiency.

Another disadvantage is the inability to dynamically brake overrunning loads. When an overrunning load is encountered, back pressure must be created on the outlet side of the actuator to prevent runaway. In an open-loop system, this back pressure can be created only by restricting actuator outlet flow at the directional valve. This flow restriction dissipates the energy contained in the overrunning load. Therefore, the energy of the overrunning load can not be reclaimed and is dissipated by heating the hydraulic fluid.

C. Hydrostatic Drive Classification

With the four basic components involved in hydrostatic transmission, four basic circuits can be set up. They are:

1. Fixed displacement pump and fixed displacement mo¬tor (PF-MF).
2. Fixed displacement pump and variable displacement motor (PF-MV).
3. Variable displacement pump and fixed displacement motor (PV-MF).
4. Variable displacement pump and variable displacement motor (PV-MV).

1. PF-MF Connection

This combination is the hydraulic equivalent of a mechanical shaft and gear drive. It can be used to transmit power without altering the speed or power between the engine and the load. This type of transmission would be convenient if the power source is remote from the load.

Fig. VII-4 Schematic diagram of fixed-displacement pump and fixed-displacement motor

If a flow control is placed in the line to vary the rate of fluid flow to the motor and if the load on the output shaft is constant, the torque is constant and the power can be varied with the speed. In this form, where a fixed-displacement pump driving a fixed-displacement motor in an open loop circuit, it is the lowest cost of all hydrostatic drives. Efficiency is relatively low because control is by throttling, but there are many important uses for this basically simple drives.

Speed is controlled by means of an adjustable bypass flow divider or with an adjustable flow regulator valve in parallel with the motor. Speed control is not as smooth or positive as with a closed-loop system. The motor can overrun the pump unless a suitable restriction or pressure control is provided between the motor outlet and the reservoir.

2. PF-MV Combination

This circuit is a constant power, variable speed, variable torque transmission. The output speed of motor is controlled by changing the motor’s displacement, i.e. by changing the effective stroke of the piston or vanes. In the PF – MV drive, the output torque decreases as the speed of motor shaft increases. Such a system has no neutral because of the fixed displacement in the pump.

Fig. VII-5 Schematic diagram of fixed-displacement pump and variable-displacement motor

Motor displacement is gradually increased or decreased by the control. If the motor displacement increases, the motor speed decreases. The speed drops because the motor takes more fluid per revolution. Motor torque rises because the effective moment is increased. The power remains relatively constant over the entire speed range.

3. PV-MF Drive

This type of transmission is the simplest system providing infinite control, and is the most frequently used system. In this system motor speed is adjusted by varying pump displacement to vary pump flow. Torque remains constant over the entire speed range because motor displacement is fixed and the pump is able to maintain full pressure at all flows. Motor speed is proportional to pump delivery rate, and pump power is proportional to motor speed. Pump speed is constant even though the delivery rate is adjustable.

Fig. VII-6 Schematic diagram of variable-displacement pump and fixed-displacement motor

The PV-MF circuit has the characteristics: the maximum efficiency occurs near the top input speed; the output speed is controlled by the pump displacement and the input speed; and for optimum hydraulic sizing, the pump size should be equal to the motor size.

The most common type of hydrostatic propulsion drive found on agricultural equipment consists of a variable displacement axial piston pump driving a fixed displacement axial piston motor in a closed-loop circuit. Axial piston pumps permit simple, positive control of the variable-displacement. Axial piston motors provide good reversibility and if desired may have variable-displacement control. Piston-type pumps and motors have higher volumetric and overall efficiencies than other types.

Figure VII-8 show a hydrostatic drive system having a variable-displacement pump and a fixed-displacement motor. The charge pump output enters the closed loop at whichever port of the variable-displacement pump is the inlet (depending on the direction of motor rotation) and the excess leaves the loop at the motor through the charge-pressure control valve. Free-flow oil from the charge-pressure control valve circulates through the motor and pump housing to cool them, then through a heat exchanger to cool the oil, and finally into the reservoir.

Two servo cylinders change the swashplate angle to increase or decrease the pump displacement or reverse the flow, in response to changes in the control lever position. The servo control has a built in centering device for the neutral position with some degree of free movement to eliminate the need for precise positioning of the control lever for zero speed. Oil supplying the energy for the servo control comes from the charge pump through a four-way control valve.

4. The PV-MV Circuit

In this system, motor speed is adjusted by a combination of adjustments of pump and motor displacements. By the system, the maximum efficiency occurs near the mid speed. The speed range is greatly extended. For optimum hydraulic sizing, the pump should be about one half the displacement size of the motor.

When two machines or two separate parts of the same machine must be driven at different speeds and one of the speeds varies in a different manner from the other, the PV-MV drive with two motors is the most suitable answer.

Fig. VII-7 Schematic diagram of variable-displacement pump and variable-displacement motor

Fig. VII-8 A closed-loop hydrostatic propulsion drive system having a fixed-displacement motor

D. Hydraulic power transmission in multifunction system.

If a hydraulic system includes both motors and cylinders but requires only intermittent operation of the motors and only one function at any one time, a simple open-center circuit with a series-parallel valves is satisfactory. But if one or more motors are to be operated continuously, or if two or more functions must be operated simultaneously, flow dividers or flow regulators are needed. If no more than two or three functions are involved, these might be operated in series, provided their flow requirements were the same and the sum of their pressure requirements did not exceed the rated pressure for the system.

The pump for a system having several functions in parallel must have enough capacity to meet the total needs for all components that might be operated at any one time. The components should be selected such that their required operating pressures match as closely as possible, because the pump must always deliver its full flow capacity against the highest pressure required by any function that is being operated. Excess pressure not needed for other functions operated at the same time must be absorbed by throttling.

When a system includes continuously operating motors, it is often advantageous to divide it into two or more independent circuits with a separate pump for each. Tandem gear or vane pumped, with two or more pumping units in the same housing and on the same shaft are available for this purpose. Single pumps may also be used. Each motor is connected to a separate pump which automatically adjust its discharge pressure to the motor needs, Motor speed can be controlled by a bypass flow divider without affecting any other components of the system.

Two or more motors loaded in proportion to their ratings might be operated in parallel from the same pump, provided the loads maintain a reasonably constant relationship and moderate speed variations can be tolerated. In most cases all cylinders in a system would be served by one pump.

Hydraulic motors are sometimes connected in series for driving conveyors that operate in sequence. This arrangement ensures approximate synchronization of speed even though the loads vary. If one motor stalls because of an overload or stop for any other reason, all conveyors in the system are automatically stopped. To produce a given torque at a given speed, two series-connected motors would need to be twice as large as two parallel motors because of the smaller pressure differential across each motor.

Fig. VII-9 Multifunction hydraulic system for a self-propelled tomato harvester.

Figure VII-9 shows the circuit diagram for a multifunction system having three pumps. One pump is for power steering and the lift cylinders, one is for the propulsion drive and the third supplies all other motors. The output from the third pump can be directed to the propulsion motor for increased road travel speeds.

Fig. VII-10 Schematic diagram of heavy-duty hydrostatic transmission with control valve in neutral position

Fig. VII-11 Schematic diagram of heavy-duty hydrostatic transmission with control valve in forward position

Fig. VII-12 Schematic diagram of heavy-duty hydrostatic transmission with control valve in reverse position

Fig. VII-13 Cross-sectional view of a heavy-duty hydrostatic transmission

VIII. HYDRAULIC POWER TRANSMISSION ON AGRICULTURAL IMPLEMENTS

A. Self-Propelled Combines

A major development in agricultural equipment during the 1960s was the application of hydrostatic propulsion drives to self-propelled machines and tractors and the increasing use of hydrostatic drives for certain types of functional components on self-propelled harvesting equipment and on some pull-type implement.

Hydrostatic propulsion drives first become commercially available on self-propelled combines in 1965. Within less than 5 years nearly all combine manufacturers in the United States had hydraulic drives available on one or more models and hydrostatic drives were common on self-propelled windrowers.

Some advantages of hydrostatic propulsion drives in comparison with other types of drives are:

  1. They provide infinitely variable speed control from full reverse to full forward under load with a single control lever. No separate clutch is needed.
  2. The closed-loop feature provides positive speed control under all conditions. The ground speed at any control-lever setting is essentially constant, regardless of positive or negative propulsion power requirements.
  3. They provide dynamic braking when moving and static braking in neutral.
  4. The control is position-responsive (speed is related to lever position).

Hydrostatic drives are less efficient than full mechanical power transmission system, specially under reduced loading. This characteristic, however, is not of great consequence in combines, because usually the major portion of the engine output is required to drive functional components. Most other self-propelled machines have relatively low propulsion power requirements in comparison with tractors providing drawbar power. Hydrostatic drives are more expensive than other types, but costs are being reduced by improved technology and by increased production as their use expands.

Whereas the torque available from a variable-speed mechanical drive increases as the output speed is reduced, the maximum torque from a hydraulic motor is limited by the relief pressure and remains essentially constant as the speed is reduced. For this reason, a multispeed, gear type transmission is usually used in conjunction with the hydraulic motor on combine drives. Although the full speed ranges down to zero can be obtained in the highest transmission gear, the use of a lower gear for low forward speed increases the available torque. For a given low speed maximum torque requirement, the use of a multispeed mechanical transmission permits the use of a smaller pump and a smaller motor.

B. Self propelled windrowers

Self propelled windrowers need to be able to reverse one drive wheel. While the other one is still moving forward, in order to make square turn. This is accomplished by having two hydrostatic drive systems, one for each wheel. Control for the two variable-displacement pumps are interconnected to perform changing both speeds either equally or differentially. The motors also have variable displacement, with the minimum displacement being employed to obtain road speeds.

When a hydrostatic drive has a variable-displacement motor, it is desirable to have the controls arranged so the motor displacement cannot be reduced until the pump displacement is at its maximum adjustment. A hydrostatic drive can have 2 parallel-connected motors at the drive wheels, driven from a single pump. This arrangement eliminates the differential gears and drive axles.

C. Tractor hydraulic remote motors.

For many years, the principles function of hydraulic system on agricultural tractors was to provide energy for tractor control functions and implement controls. But as hydraulic control functions have expanded and system capacities increased, tractor hydraulic systems have become more important as a significant potential means of utilizing engine power. Since most tractors have outlets for connecting remote cylinders, it is only logical that consideration be given to providing remote motors that can be connected to these same outlet.

As a first step, an ASAE standard for remote hydraulic motors was adopted in 1968. The motor is considered to be part of the tractor. Hose lengths are to permit mounting the motor anywhere within a specified spherical radius from the tractor drawbar hitch point. Mounting dimensions and shaft sizes are specified. The rated operating speed is 1000 rpm, corresponding to the higher standard speed for mechanical power-take-off drives. Motors are to be reversible and have variable speed control.

For certain implement application that are within the limited power capabilities of tractor hydraulic remote motors, this type of drive offers significant advantages over mechanical PTO drives or small engines in regard to versatility, compactness, and ease to variable speed control. More heat-dissipation capability is needed for continuous motor control functions. Although many tractor hydraulic systems are now designed to permit remote motor operation, others do not have adequate oil-cooling capacity for continuous operation of motors.

D. Hydrostatic Transaxle

The typical small hydrostatic tractor drive is juggled together mixture of pumps, motors, gears, and plumbing. But a transaxle comes as a complete package, including main pump, charge pump, hydrostatic motors, plumbing, filters, manifolds, axles, bearings, and mounting points. The whole assembly mounts as a unit to the chassis with just five or six bolts. The only connections needed are inlet line from the reservoir, a pressure line to the auxiliary circuit for remote hydraulic power, a drain line, and a mechanical coupling from the engine. No gears are needed because the motors transmit the torque directly to the wheels. The assembly is a marriage of components proved separately over a period of years, and modified to create a modular, off-the-shelf transaxle that can be ordered in many configurations. For example, there are choices of axle length, motor displacement, mounting locations, and orientation of the pump. The final option is to split apart the assembly and mount the motors and axle separately from the pump. The idea is to squeeze the assembly into the minimum space possible with no loss of performance.

Fig. VIII-1 Hydrostatic transaxle

Fig. VIII-2 Overall hydrostatic transaxle system mounts into small tractor

Fig. VIII-3 Typical heavy vehicle has the pump at the engine and the motor on an axle or wheel.

The pump is an adjustable-displacement, radial ball piston pump. The motors are gerotor type with disk valve. The acceleration pedal under the driver’s foot mechanically turns on the stroke lever on the pump.

IX. CONCLUSIONS

X. REFERENCES

KEPNER, R.A., et al (1978), Principles of Farm Machinery, AVI Publishing Company Inc., Wesport, USA.

STEWART, HARRY L. (1978), Hydraulic for Off-the-road Equipment, Theodore Auder & CO., Indianapolis, USA.

HADEKEL, R (1954), Hydraulic System and Equipment, The Syndics of the Cambridge University Press, London, UK.

Fundamental of Services (FOS) (1982): Hydraulics Compact equipment, John Deere Services Training, Moline, USA.

YEAPLE, FRANK (1996), Fluid Power Design Handbook, Third Edition, Marcel Dekker Inc., New York, USA.

LILJEDAHL, JOHN B. ET AL (1979), Tractors and Their Power Units, Third Edition, John Willey & Sons, New York, USA.

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