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Hydraulic » Hydraulic Article Resources » Hydrostatic Pumps



There is a wide range of available hydrostatic pumps and motors from the market and the purpose of the article is to describe the operating principles and features of the most commonly used types. The formulate that are used for determining the performance of pumps are presented and some of the major parameters that can be used as a basis for comparison are outlined as a background for the selection process. However, because of the wide variety of the types of units that are available it is impossible to generalise on the selection process in any given application.

External Gear Pump MotorCommonly machine builders and users have preferences for particular types of pumps that are based on experience with particular applications which are determined by factors such as the system function, its control, servicing aspects, enviornmental features, life expectancy, duty cycle and type of fluid to be used. The designer needs to be aware of the relative performance of the difference types and how this knowledge can be utilised in the selection process to suit a particular application.

1. Introduction

Internal Gear PumpPower transmission pumps in fluid power systems are usually hydrostatic or positive displacement units, which convert mechanical power into fluid power, the most common types being gear pumps, vane pumps and piston pumps. In these pumps fluid is transfered through the machine in discrete volumes e.g. a gear tooth cavity. The pump size and speed determines the fluid flow rate.

Hydrostatic pumps are sources of flow so that when they are connected to a hydraulic motor, the outlet pressure will rise so that the flow can cause the motor to rotate against the load torque. Hydrostatic motors convert fluid power into mechanical power so that rotation of the output shaft can take place against an opposing torque load. Generally speaking pumps can be run as motors but a number of factors influence this possibility, some of which are:

  • Not all pumps are reversible in direction of rotation because of their internal and external sealing arrangements.
  • Pumps are designed to operate at relatively high speeds and can be inefficient at low speeds particularly during starting.
  • Motor applications often require significant shaft side load capacity. Pump rotating components are generally not designed to carry such shaft side loads and consequently cannot be directly coupled to the output drive where side loading exists.
Ballance Vane Pump
This chapter is concerned with describing the operating principles of hydrostatic units, some aspects involved in their selection and the determination and presentation of their performance characteristics.

2. Major aspects in the selection of pumps and motors

The selection of pumps can be determined by a number of factors, which need to be considered by the user. These factors include:

  • Cost
  • Pressure ripple and noise
  • Suction performance
  • Contaminant sensitivity
  • Speed
  • Weight
  • Fixed or variablea displacement
  • Maximum pressure and flow, or power
  • Fluid type.
3. Types of pumps and motors

The mechanical principle that is chosen in the design of high-pressure positive displacement pumps and motors, which includes those using pistons, vanes and various gear arrangements depends on a number of factors. These include the operating speed and pressure, the type of fluid and the requirement for providing variable displacement control.

Pumps normally operate at constant speed (e.g. driven by electric motor) although in some situations (e.g. those driven by an internal combustion engine as found typically in mobile applications) the speed will vary over a small range. However, for motors it is normally required to operate at varying speeds including starting from rest (e.g. winch drives) and this aspect is reflected in the design of some available types.

Variable Displacement Vane Pump
Positive displacement machines are quite distinct from those using rotodynamic principles, which are often used for the transfers of fluid at relatively high flow rate and low pressure. Positive displacement units operate relatively low flow rates and high-pressure and normally can only be used with fluids having good lubricating properties. However, there are machines that can be used with fire resistant fluids and pure water.

3.1 Fixed displacement units

3.1.1 External gear pumps/motors

In many applications, for operation at pressure up to 250 bar, external gear pumps/motors are used extensively because of their simplicity, low cost, good suction performance, low contamination sensitivity and relatively low weight. In applications requiring low noise, vane or internal gear pumps are often used.

Essentially the unit consists of two meshing gear pinions, mounted in bearings and contained in a housing or body as shown in Figure 1. As the pinions are rotated, oil is trapped in the spaces between the gear teeth and the housing and carried round from the pump inlet to its outlet port when the trapped volume is discharged by the action of the gears meshing together.

Torque is required at the input shaft at a level that is dependent on the outlet pressure acting on the gear teeth. When supplied with high-pressure flow the unit acts as a motor by providing torque to drive the load on the output shaft.

Some of the outlet fluid is transferred back to the low pressure side by way of small leakage flows through the:
Piston Pump

  • Clearance space between the teeth and the housing
  • Shaft bearing clearances.
  • Clearance between the gear faces and the side plates in the housing. Most gear units have pressure loaded side plates to minimise this leakage.
The design of the unit is such as to minimise flow losses as they reduce its efficiency, particularly when using fluids of low viscosity such as with some water based fluids. The geometric capacity, or displacement, cannot be varied so their displacement is fixed. For a given gear form the manufacturer can produce pumps of different displacements by using different gear widths.

Standard types operate at speeds of 1000 to 3000RPM and at pressures up to 250 bar but higher speeds and pressure are available. Powers range from 1 to over 100kW. The efficiency of gear units has been raised during recent years, with peak overall efficiencies of 90% above.

3.1.2 Internal gear pumps

Internal gear pumps, as shown in Figure 2, have an internal gear driven by the input shaft and an external gear which rotates around its own centre and driven by the internal gear. By means of the spearator element, both gears transmit fluid from the pump inlet to the outlet. This pump creates a low noise level that favours it for some applications although its pressure capability is about the same as that of the external gear pump.

3.1.3 Vane pumps/motors
Piston Motors
The vane pump/motor consists of a rotor, carrying a number of sliding vanes, rotating in a circular housing. With the rotor being eccentric to the casing, oil is transmitted in the vane spaces across the pump from the suction to the discharge port.

The vanes are acted on the centrifugal force when the units is rotating, but in order to reduce leakage ata the tips it is common practice to pressure load them(by supplying discharge pressure to the base of the vane slots) and sometimes to spring load them against the track. As with the gear unit, control of the clearances at the sides of the rotor assembly is most important.

The balanced design in Figure 3 eliminates pressure loading on the bearings and uses and ‘elliptical’ vane track with the vanes moving in and out twice each revolution. There are diameterically opposed suction ports and discharge ports as shown in Figure 3 and these are connected together in the cast body. This pump is only available as fixed displacement.

Vane pumps are inherently more complex than gear pumps, they contain a greater number of components and are, therefore, more expensive. However, vane pumps operate at much lower noise levels than gear pumps and their cost can be offset against their good serviceability, which is not available with gear type pumps.

4. Variable displacement units

4.1 Vane pumps

Variable displacement vane pumps are available as shown in Figure 4 where the centre of the rotating vane block can be moved in relation to the centre of the housing. Unlike the balanced vane unit of Figure 3, these are single acting and, as a consequence, there is a unbalanced pressure force on the rotor so that the bearing size has to be increased in order to obtain adequate life.

4.2 Piston pumps/motors
Piston units operate at higher efficiencies than gear and vane units and are used for high-pressure applications with hydraulic oil or fire resistant fluids. Several types of piston pump are available that use different design approaches and these include those having axial and radial piston arrangements.

Pump Performance
The majority of piston pumps/motors are aof the axial variety, in which several cylinders area grouped in a block around a main axis with their axes parallel as shown in Figure 5 which has variable displacement capability. The pressure force from the pistons is transferred to the angled swash plate lubricated slippers that are mounted onto the pistons with a ball coupling. Rotation of the cylinder block causes the pistons to oscillate in their cylinders by the action of the swash plate, which provides the coversion between the piston pressure force and shaft torque.

The piston cylinders are alternately connected to the high and low -pressure connections by a plate valve between the cylinder block and the port connection housing. Varying the swash plate angle allow the displacement to be changed over the full range from zero to maximum. The swash plate angular position can be arranged to vary either side of the zero displacement position so that flow reversal is obtained. This is referred to as over centre control.

Figure 6 shows a fixed displacement bent axis type of axial piston unit whereby the ball ended pistons are located in the output shaft. During rotation of the shaft there will be a rotating sliding action in the ball joint, and possibly, between the piston and the cylinder. Each cylinder is connected successively to the high and low-pressure ports by a similar valve to that used in the swash plate units.

In variable displacement units a mechanism is used to vary the tilt angle of the cylinder block from zero to maximum which, if required, can provide over centre operation to give reverse flow.

There are two types of radial piston pump: those in which the cylinder block rotates about a stationary pintle valve, and those with a stationary cylinder block in which the pistons are operated by a rotating eccentric or cam.

Many standard piston unit of recent design operate at pressures of up to 450 bar. A wide range of types are available up to powers of 100 kW, although a number of manufacturers provide units having powers up to 300kW with some available at powers of 1000 kW. Peak overall efficiencies in excess of 90% are usually obtained. The price of piston units varies from manufacturer to manufacturer but may be as much as ten times the price of a gear pump of similar power.

Some pumps cannot draw the inlet fluid directly from the reservoir and may require boosting from a separate pump, often of the external gear type, that can accept low inlet pressures. However, for open loop circuits, variants are available that do not require separate boosting of the inlet which can be connected directly to the reservoir. These aspects also apply to motors that operate as pumps in hydrostatic systems when regeneration occurs (e.g. winch and vehicle drive applications).

Variable displacement pumps provide a range of control methods, which include pressure compensation, load sensing and torque, or power, limiting devices. Pumps displacement controls incorporating elector-hydraulic valves are also available.

In addition to their use to control outlet flow, variable displacement pumps provide considerable increase in overall system efficiency with the additional benefits of reduced heat generation and operating cost. This reduction in operating cost can show an overall reduction in the total lifetime cost of the machine.

5. Equation for pumps and motors

5.1 Flow and speed relationship

For the ideal machine with no leakage, the displacement of the machine and its speed of rotation determine the flow rate Q.


Q = Dw

Where D is volumetric displacement [m’rad’]
        W is the rotational speed [rad sec.]

For pumps that are driven by electric motors the speed is often constant. However for motors, the speed depends on the level of the supplied flow:


w = Q/ D

5.1.1 Volumetric efficiency

The internal flow leakage in pumps and motors affects the relationship between flow and speed and is taken into account by the use of the volumetric efficiency (n).

Thus for pumps equation 1 becomes
Q = ncwD’
And for motors equation 2 becomes

w = n v Q /D

The volumetric efficiency varies with the fluid viscosit, pressure and rotating speed as discussed in more detail in chapter 8. Manufacturers will usually give valves for the volumetric efficiency for operation at specified conditions.

5.2 Torque and pressure relationship

For the ideal machine, the mechanical power is entirely converted to fluid power.

Power = T w = P Q
Where T is torque (Nm)
P is the differential pressure [Nm¯²]

From equation 5 we get:

T = QP/W which from equation 2 gives T = PD

Thus the ideal torque is proportional to the pressure for a given displacement. In a pump this is the input torque required from the prime mover and for a motor, it is the output torque available from the motor shaft.

5.2.1 Mechanical efficiency

The presence of friction between the moving parts creates mechanical losses that are represented by the mechanical efficiency (nm). Thus:

For pumps the required input torque is given by T = PD /Nm

And for motors the output torque is given by T = nmPD

The mechanical efficiency, as for the volumetric efficiency, will vary with the fluid viscosity, pressure and rotating speed as discussed in more detail in chapter 8.

The power input,H, to a pump is:

H = PQ/nmnv

The power outpur from a motor is:

H = nmnvPQ

The total efficiency of both units is therefore:

nT = nmnv

Figure 7 and 8 show how the measured performance of pumps and motors are presented for use with a particular fluid at a particular viscosity. For the pump it can be seen that the flow output reduces with the output pressure at constant speed because of the effect of the increasing leakage flow loss.

For the motor, the output torque varies with increasing speed at constant pressure as a result of the variation in the mechanical efficiency. The theoretical analysis chapter 8 shows how the efficiencies are related to the system parameters which enables the performance for operation at other conditions to be predicted.

5.3 Pump selection parameters

The process involved in the selection of a suitable pump for a given application depends on many parameters, some of which were summarised in section 2. As a consequence a generalisation is not possible but some major features can be identified as shown in Table

Gear pump (Fixed displacement
External type
Internal type
Other features
  • Low cost
  • Low contaminant sensitivity
  • Compact, low weight
  • Good suction performance
  • 250cm³/rev, 250bar
  • Low noise
  • Low contaminant sensitivity
  • 250cm³/rev, 250bar
In-line assembly for multi-pump units
Fixed displacement
Variabale displacement
  • Low noise
  • Good serviceability
  • 200cm³/rev, 280bar
  • Low noise
  • Low cost
  • Good serviceability
  • Displacement controls
  • 100cm³/rev, 160bar
In-line assembly for multi-pump units
Fixed and variable displacement
  • High efficiency
  • Good serviceability
  • Wide range of displacement controls
  • Up to 1000 cm³/rev., 350/400bar
  • Integral boost pump and multipump assemblies (not bent axis0
  • Can use most types in hydrostatic transmissions