This is the second part of the short IH series devoted to its majesty charge pressure (read part 1 here). In this bit I would like to talk about peculiarities of closed loop charge pressure measuring, namely how, where and what for.
As a rule, a technician may require to read charge
pressure of a closed loop in one of the two cases - either he is
adjusting its level, or he is troubleshooting a malfunctioning
transmission. Despite of the charge circuit being relatively simple,
there are more than one ways, or better, places to measure charge
pressure in a "standard" closed loop. A good technician should
understand the differences between the measuring points, because the
information these readings can provide varies greatly with the point you
are reading it at.
Let us take a look at a typical closed loop charging circuit.
Here we have the charge pump ( I put the suction filter on the
schematics, as well as the suction line pressure measuring port just to
show that this option can exist, and that this port can be used for
verifying suction line condition), the charge pressure relief valve, oil
conditioning element/elements (a filter, a cooler, or
both), and the check-valve. Note that on the diagram
I put two relief valves. The actual position of the relief valve (whether
it is before or after the oil conditioning elements) will depend on the
manufacturer, some brands have, actually, both of the relief valves installed, one of
them being a safety valve (installed before the filter or cooler to protect them during cold starts, often called a "cold start valve", and is normally set around 5-10 bars above the "normal" relief valve setting), and the other one being
the charge pressure relief valve.
On the diagram you can see that there are three
points in the charge system where we can install our pressure gauge and
take a reading - directly at the charge pump outlet (before the oil
conditioning elements), after the oil conditioning elements (before the
check valve), and after the check vale (in the low pressure side of the loop). Let
us consider them one at a time.
1. Directly at the charge pump outlet, before
the filter. This is a good place to measure charge pressure for
"adjusting" purposes, and often there will be a
conveniently placed test port, but there's a catch - in pumps that have
charge pressure relief valves installed before the filter
(cooler, or both) - clogging of the filter can result in insufficient oil
supply to the loop and the consequent cavitation, which will occur without the charge pressure drop
at this measuring point - a situation that can lead a technician to
wrong conclusions. Sauer Danfoss series 90 is a good example of such
pump design.
2. After the oil conditioning elements. Again
a good place to measure charge pressure, without the above described
drawback of leading into an error in case of clogging of the oil
conditioning system, and most of the pumps have a test port in this
point. There's still a drawback , however, and quite similar to the
above one - in case of a check valve malfunction (blocked valve) the
loop will cavitate, without the charge pressure drop at this measuring port - once again making it possible to come to an incorrect conclusion.
Despite of what some might think, it is possible for
such a simple thing as a check valve to malfunction. For example, Sauer
Danfoss series 90 - multifunction valves can easily fail due to particle
contamination. In some small pumps, the check valves are actually small
ball valves, that can through time damage the housing and literally
fall into the oil passage blocking the loop feeding gallery (example). Want another example? - Take a look at these two check valve/relief valve
assemblies from different size Rexroth A4VG pumps, as you can see - the
threads are equal, while the travel of the poppets is not. If the
smaller pump gets the "bigger pump valve", the poppet will have very
little travel, resulting in partial blockage of the charge oil passage
and the consequent loop cavitation.
It must also be noted that the charge pressure at
this measuring point will be affected by the oil conditioning system
pressure drop (will be lower than at the point 1) - something that can
be used for filter condition monitoring, for example, or for verifying
if the cooler is obstructed.
3. And finally - the reading taken after the check valve,
or directly in the low pressure side of the loop. This is the
best place to see if the loop is cavitating, because any volumetric or
charge feeding problem will be immediately indicated by the
pressure drop. One thing you don't want to do is to use a low scale
pressure gauge at this point, like the one you used in the above
described points (unless, of course, you are fond of gambling or are
collecting burst pressure gauges). The best way.to take the reading at
this point is with a high range digital pressure gauge. An analogue one
will also do, but it may be difficult to read with precision low
pressure values on a high range scale.
Although this point is the best spot to check for
cavitation symptoms, it's not always the best place to read the charge
pressure for adjustment purposes, especially on old equipment,
which can have lines' pressure fluctuations due to the drifting null
condition (IH articles on hydrostatic transmission null adjustment techniques are here).
So, if you are simply adjusting charge
pressure of an operational closed loop, the most convenient points to
read it are points one or two. Reading and comparing pressures at both
points will give you an idea of the pressure drop, caused by the oil
conditioning system, and will help you come to a conclusion whether the
filter/radiator needs to be changed/cleaned. In this case it will be OK
to use a low scale pressure gauge.
If you are troubleshooting a closed loop and are
suspecting excessive leakage - you should first measure the pressure in
the low pressure side of the loop during its operation, and then, if
the cavitation condition (charge pressure drop) is confirmed, you
should measure the charge pressure at the point one, to see if it drops
too. If it's not dropping, the malfunction is being caused by the charge
oil flow interruption either due to the filter/cooler clogging or due
to a check valve malfunction.
Understanding these simple differences is very
important for correct charge pressure adjustment and efficient closed
loop troubleshooting, and can literally save you many an hour of
downtime.
