In your previous two hydraulic maintenance emails,
I explained how to correctly determine the condition
of a hydrostatic transmission using case drain flow,
and discussed the role and influence of a flushing
valve when doing this.
Today I want to clarify one last source of confusion
about the influence of the charge pump when assessing
case drain leakage from a hydrostatic transmission.
It is sometimes assumed if the charge pump relief vents
into the case of the pump, transmission pump leakage
can be determined by subtracting charge pump flow from the
total flow from the pump case.
For example, if total charge pump flow is 10 GPM and the
flow-meter in the pump case drain line is reading 15 GPM
then transmission pump leakage is 5 GPM (15 - 10 = 5).
This is incorrect because it suggests that a hydrostatic
transmission can leak more than the total available flow
from its charge pump. It cannot.
That is, it is impossible for the flow meter in the pump
case drain line to read 15 GPM when the total available
flow from the charge pump is only 10 GPM, as in the
above example.
The reason is simple. Because the function of the charge
pump is to make up losses from the loop through internal
leakage, if total losses exceed available charge pump flow,
the transmission will cavitate.
If in the above example, the transmission was leaking 5 GPM
more than the total available flow from the charge pump,
there would be a serious deficit of fluid in the loop.
In practice, the transmission would destroy itself through
cavitation before it got to this point.
Let me explain this another way. Let's assume we have a
transmission that has a volumetric efficiency of 100%,
that is, the pump and motor have no internal leakage.
The loop has a total volume of two gallons and is full
of fluid. Because there is no internal leakage there
is no need for a charge pump.
The pump is stroked to maximum displacement, which circulates
the two gallons of fluid in the loop at a rate of 50 GPM.
Because it's a closed loop, with no leakage, the flow from
pump to motor is 50 GPM and the flow from motor to pump
is 50 GPM.
Now let's introduce internal leakage of 0.5 GPM in both
pump and motor. The result is that, with no charge pump
to replenish the loop, after one minute there will only
be one gallon of fluid left in the loop (the other gallon
will have leaked back to tank). Within a second of the
transmission starting to leak, the transmission pump
will start to cavitate and the severity of this cavitation
will increase with each passing second until
the transmission destroys itself.
Now let's install a charge pump with a flow rate of 1 GPM
in the circuit. Problem solved, temporarily at least.
With 1 GPM leaking out of the loop and 1 GPM being
replenished by the charge pump the status quo is maintained...
until wear causes the internal leakage of the transmission
to exceed 1 GPM.
As you can see, it's not possible for the internal leakage
of a hydrostatic transmission to exceed the flow rate of
its charge pump. Charge pump flow rate is typically 20%
of transmission pump flow rate.
This means that volumetric efficiency can drop to 80%
before the transmission will cavitate and destroy itself.
The trick is to overhaul the transmission
BEFORE this point is reached.
Yours for better hydraulics knowledge,
Brendan Casey
P.S. If you've struggled with any of the concepts
in these past three bulletins on hydrostatic transmissions,
you'll find the simulation videos
in 'Hydraulics Made Easy' a BIG help:
http://www.hydraulicsupermarket.com/hydro-tranny
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