In your last hydraulic maintenance email,
I talked about the leakage characteristics
of spool-type and poppet-type valves.
Still on the topic of leakage, today I'm going to
explain the potential pitfalls when measuring
case leakage from a hydrostatic transmission.
A hydrostatic transmission consists of a
variable-displacement pump and a fixed or
variable displacement motor, operating together
in a closed circuit.
In a closed circuit, fluid from the motor outlet
flows directly to the pump inlet, without returning
to the tank.
Because the pump and motor leak internally, which allows
fluid to escape from the loop and drain back to the tank,
a fixed-displacement pump called a charge pump is used to
ensure that the loop remains full of fluid during normal
operation.
In practice, the charge pump not only keeps the loop full
of fluid, it pressurizes the loop to between 110 and 360 PSI,
depending on the transmission manufacturer.
( If I've lost you at this point, you'll find the
hydrostatic transmission, simulation videos
in 'Hydraulics Made Easy' extremely helpful:
http://www.hydraulicsupermarket.com/hydro-tranny )
When a pump or motor is worn or damaged, internal leakage
increases and therefore the flow available to do useful work
decreases.
This means that the condition of a pump or motor can be
determined by measuring the flow from its case drain line
(internal leakage) and expressing it as a percentage of
its design flow.
However, using case drain flows to determine the condition
of the components of a hydrostatic transmission, without
a thorough understanding of the circuit in question, can
result in incorrect conclusions and the costly change-out
of serviceable components.
In most transmissions, the charge pump relief valve vents
into the case of either the pump or the motor.
This means if the motor case drain flushes through the
transmission pump case to tank, you would expect to see
the flow meter in the transmission pump case drain line
reading design charge pump flow. Here's why:
Say charge pump flow is 10 GPM, of which 4 GPM is leaking
out of the loop through the motor's internals (case drain)
and 2 GPM is leaking out of the loop through the pump's
internals. The balance of 4 GPM must therefore be going over
the charge relief - but still ends up in either the pump or
motor case, depending on the location of the charge relief
valve.
In a circuit where the motor case drain flushes through the
transmission pump case to tank, you would expect to see the
flow meter in the transmission pump case drain line reading
the sum of these three flows (10 GPM).
So before any meaningful conclusions can be drawn, the case
in which the charge pump relief is venting (motor or pump)
must be determined and the two case drain lines (motor and pump)
must be isolated from each other.
In your next hydraulic maintenance email in a few days time,
I'll reveal another component of most hydrostatic transmissions
that complicates this issue further.
Yours for better hydraulics knowledge,
Brendan Casey
Author of 'Hydraulics Made Easy'
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