ACCUMULATOR EFFECT

Hello again ABDUL AZIZ,

In you last hydraulics email, I explained why
the charge pump in a hydrostatic transmission is
pure heat load and how one hydraulic equipment
user's failure to understand this cost him dearly.

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.


Yours for better hydraulics knowledge,


Brendan Casey
Author of 'Insider Secrets to Hydraulics';
'Preventing Hydraulic Failures' and
'Advanced Hydraulic Control'.
http://www.hydraulicsupermarket.com/books.html