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Today, another issue to be aware of with hydrostatic transmissions and a simple technique to minimize it:
When a hydrostatic transmission is subject to a sudden increase in load, the motor stalls momentarily and system pressure increases until the increased load is overcome or the high pressure relief valve opens - whichever occurs first.
While the motor is stalled, there is no return flow from the outlet of the motor to the inlet of the pump.
This means that the transmission pump will cavitate for as long as it takes to make-up the volume of fluid required to develop the pressure needed to overcome the increased load (or the high-pressure 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 pipe or hose, and the decrease in volume of the fluid.
This is called the 'accumulator effect'.
One way to minimize stalling and pressure spikes and the resulting 'accumulator effect' in applications where the load on the transmission varies - in say drill rigs for example, is to install a flywheel between the hydraulic motor and reduction box.
The stored energy in the flywheel assists the hydrostatic drive to maintain speed and torque, and minimize the magnitude of pressure fluctuations resulting from sudden increases in load.

By Brendan Casey