In your last hydraulic maintenance email,
I explained the pitfalls to look out for when
assessing the condition of a hydrostatic transmission
by measuring its case drain flow.
There's another component present in most
hydrostatic transmissions that complicates
this issue further: the flushing valve.
A closed circuit flushing valve (also called a
transmission valve or replenishing valve or purge valve)
usually comprises a pilot operated directional valve
and a low pressure relief valve.
When the hydrostatic transmission is in neutral,
the directional valve is centered and the gallery
to the low pressure relief valve is blocked.
When the transmission is operated in either forward
or reverse, the high pressure side of the loop pilots
the directional valve. This opens the low pressure
side of the loop to the relief valve gallery.
In a closed circuit, fluid from the motor outlet flows
directly to the pump inlet. This means that apart from
losses through internal leakage, which are made up by
the charge pump, the same fluid circulates continuously
between pump and motor. If the transmission is heavily
loaded, the fluid circulating in the loop can overheat.
The function of the flushing valve is to positively
exchange the fluid in the loop with that in the reservoir.
( You can watch a 2-minute simulation video
which explains how it does this, here
http://www.hydraulicsupermarket.com/hydro-tranny )
When the hydrostatic transmission is in neutral,
the flushing valve has no function and charge pressure
is controlled by the charge relief valve in the
transmission pump.
When the transmission is operated in either forward or reverse,
the flushing valve operates so that charge pressure in the low
pressure side of the loop is controlled by the relief valve
incorporated in the flushing valve. This relief valve is set
around 30 psi lower than the charge pump relief valve located
in the transmission pump.
The effect of this is that cool fluid drawn from the reservoir
by the charge pump, charges the low pressure side of the loop
through the check valve located close to the transmission pump
inlet. The volume of hot fluid leaving the motor outlet,
that is not required to maintain charge pressure in the low
pressure side of the loop, vents across the flushing valve
relief and back to tank, often via the motor and/or pump case.
So if a flushing valve is fitted to a transmission,
it acts as the charge pump relief valve once the transmission
is operated in forward or reverse. So if the flushing valve
vents into the case of the motor, then it is possible to
determine the condition of the pump by measuring its case
drain flow, but not the motor.
If the flushing valve vents into the case of pump, then
it is possible to determine the condition of the motor by
measuring its case drain flow, but not the pump.
This reinforces the point that 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 your next hydraulic maintenance email, I'll explain another
important concept to keep in mind when testing hydrostatic
transmissions. Look out for it in a few days time.
Yours for better hydraulics knowledge,
Brendan Casey
Author of 'Hydraulics Made Easy'
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