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CAVITATION AND ITS CAUSES IN HYDROSTATIC TRANSMISSIONS

Cavitation occurs when the absolute pressure at the intake side of a pump falls below the vapour pressure of the

hydraulic fluid. This results in the formation of vapour bubbles within the fluid, which implode when compressed on

the outlet side of the pump. Cavitation causes metal erosion, which results in damage to the pump and contamination

of the hydraulic fluid. In extreme cases, cavitation can cause mechanical damage to the pump, such as buckling of

the piston retaining plate on axial piston pumps, which results in major failure. Common causes of cavitation

include:

clogged suction strainer or filter

restricted suction line

excessively high fluid viscosity

over-speeding the pump

rapid increase in pump displacement (variable displacement pumps)

Closed loop pumps

Cavitation occurs in closed loop transmission pumps when charge (boost) pressure falls below the vapour pressure of

the hydraulic fluid. This occurs when the volume of fluid returning to the pump from the motor, combined with the

volume of fluid delivered by the charge pump, is less than the volume of oil demanded by the pump. Common

causes of this condition include:

restricted charge pump suction line or filter

worn or damaged charge pump

excessive internal leakage (worn transmission pump and/or motor)

excessive external leakage (blown hose or pipe)

An often overlooked cause of cavitation in hydrostatic transmissions is the combined effect of fluid compressibility

and the accumulator effect of conductors (the increase in volume of a hose or pipe as pressure increases).

When a hydrostatic transmission is subject to a sudden increase in load, the motor stalls instantaneously and the

working pressure of the transmission pump increases until the increased load is overcome or the cross line relief

valve opens whichever occurs first. The transmission pump will cavitate for as long as it takes to make-up the

volume of oil required to develop the pressure needed to overcome either the increased load or the system relief

valve. How long the pump cavitates depends on the output of the charge pump, the magnitude of the pressure

increase, and its influence on the increase in volume of the conductor and the decrease in volume of the fluid.

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2

This is illustrated in the following example:

A hydrostatic transmission operating the drill head on a drill rig is delivering a flow of 35 gpm (132 l/min) at a

pressure of 1000 psi (69 bar). A sudden increase in load on the drill bit instantaneously stalls the motor until

sufficient operating pressure is developed to overcome the increase in load, which for the purposes of this example is

3000 psi (207 bar). In order to increase the operating pressure from 1000 psi to 3000 psi the transmission pump must

make-up additional volume due to the compression of the hydraulic fluid and volumetric expansion of the high-

pressure hose between the pump and the motor. But because the motor is momentarily stalled, there is no flow from

the outlet of the motor to the inlet of the pump. The only fluid available at the inlet of the transmission pump is 7

gpm (26 l/min) from the charge pump, which is around 80% less than required!

In this example, the high-pressure hose between the pump and motor is SAE 100R9AT-16, 36 feet (11 m) long. The

volumetric expansion of this hose due to the increase in pressure is 9.7 in (158 cm) and the additional volume

required as a result of the compression of the hydraulic fluid within this hose is 2.8 in (46 cm). The total additional

volume required to achieve 3000 psi (207 bar) is 9.7 + 2.8 = 12.5 in (204 cm). To calculate the time taken for the

operating pressure to reach 3000 psi, which is equivalent to the length of time the transmission pump will cavitate,

we divide the required make-up volume by the volume available from the charge pump (per second). In this case, the

pump cavitates for 0.47 seconds every time a sudden increase in load demands an increase in system pressure from

1000 to 3000 psi.

This problem must be considered in applications where the load on the transmission can fluctuate in a dramatic and

instantaneous manner, such as drill rigs, boring machines, and cutter wheels on dredges. For further information on

the volumetric expansion of hydraulic hose, refer to the hose manufacturers technical data. For further information

on the compressibility (bulk modulus) of hydraulic fluids refer to our Technical Library document titled Hydraulic

Fluids. For advice on how to get the most service life from your hydraulic equipment refer to our Technical Library

document titled Prevention of Premature Hydraulic Component Failures.

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Closed loop pumps