My discussion of closed loop charge pressure related stuff (started in part1 and continued in part2)
would be incomplete without addressing the loop flushing subject, which
is , therefore, the prime topic of the following article.
The main purpose of a closed loop flushing
system is loop oil renewal during transmission operation. You will find
many closed loops out there that aren't equipped with loop flushing and
apparently work fine and problem free, so naturally a question pops up
- why bother? - Well, the main reason loop flushing exists and is
widely used is very simple - transmissions equipped with loop flushing last longer, because the oil inside the loop runs cooler and cleaner.
Any loop flushing system can be divided in two parts
- the directional part, which serves to always pick low pressure side
of the loop to work with, and the purging part, which serves to
divert oil from the loop. As a rule, in vehicle propulsion
closed circuits the flushing system is incorporated into the drive
motor. In industrial applications, however, it is not uncommon to find
external loop flushing manifolds.
The classic and time proven layout of the directional part consists of a dumbbell shaped spool (picture, and a cut-view), which functions in the following manner.
It is possible to come up with a different technical solution, like for
example two touching spools instead of a single one, but the final
purpose will always be the same - choosing the low side of the loop to
purge. In some cases system designers adopt the use of solenoid
valve/valves instead of a shuttle spool and in this case the loop
flushing is operated electrically (for the record - I don't like this
solution). And in some motors the complete flushing system is
integrated into a special valve plate (like Rexroth AAFM, A2FE motors,
etc..). I don't have pictures of such a valve plate at the moment, but
as soon as one turns up I'll update.
The purging part of the loop flushing arrangement is the
part that should interest us the most, because it is directly connected
to the loop charge system, and therefore can influence the charge
pressure. Let us take a look now at different ways the purging can be
1) Classic loop purging, done by means of a relief valve. In this arrangement it is very important to adjust the purging valve below
the setting of the pump's charge pressure relief valve (2-4 bars
lower), as well as to understand that it is this valve that defines the
charge pressure level during transmission operation. In this system the
flushing flow equals charge pump flow minus the leakage, and it is very
easy to confirm its correct operation by monitoring the charge pressure
drop when the pump is on stroke.
2) Orificed relief valve purging, done by means of a relief valve combined with an orifice (schematics).
A relief valve is set to a relatively low threshold, and the purge flow
is then determined by the orifice size. Most of the times the spring
setting is fixed, although some designs will allow you to exchange
orifices. Depending on the size of the orifice, the charge pressure
level may and may not drop during the operation. Very often the charge
pressure will exhibit momentary drop during the initial phase of the
pump stroke, and then will return to its normal value - a behavior
which can be easily misinterpreted as a malfunction or incorrect
adjustment if the technician isn't familiar with peculiarities of this
system. The best way to confirm its correct function is through
flushing flow (motor case flow) measurement - an important verification
- because like any orifice, this one can get blocked and cause flushing
In this system flushing flow is a percentage of the charge pump flow,
and is defined by the orifice size and the charge pressure setting.
3) Simple orifice as a purge valve - by far the most
compact and the simplest system you could possibly imagine, often used
for flushing solutions when small size is mandatory. Its main drawback
is the fact that it doesn't have an opening threshold like the above
system (defining minimum charge pressure level), which can become an
issue during low speed and low charge flow operation,
causing the charge pressure to drop beyond acceptable level. The integrated
flushing valve built into motor valve plate is one example of such a
system - there's no place for a relief valve inside the valve plate,
therefore the purge flow limitation is done by restriction, performed
either by an orifice, or by a slotted shim mounted in the center of the
4) Flow limiter as a purge valve - a solution which
is rarely used in closed loops (no need to go that complicated) but can
be occasionally found in open loop transmissions. When such a purge
valve is used in a closed loop, there also will be no noticeable charge
pressure drop during the operation.
As you can see, it is the type of the flushing
system that defines if and how much the charge pressure drops when the
pump is on stroke, and consequently it may and may not influence the
charge pressure setting. For that reason it is important to identify
which type of flushing system is used in a closed loop transmission
before adjusting the charge pressure.
Serving the same purpose, closed loop flushing
systems elaborated by different brands have their design and
performance peculiarities, therefore the most perfect loop flushing
learning material is technical catalogues, which will also provide you
with characteristic curves and technical data. That is why I am
concluding this article with these files, which are a good place to
start for those who like going into detail.
Another important loop flushing issue to consider is
the fact that, like any high pressure exposed component, the
directional part of a loop flushing system (the shuttle spool) can
become the point of high pressure leakage and cause the transmission to
malfunction when damaged or worn out, that is why it is important to
check it for excessive clearances every time such a motor is overhauled
. A good example of this type of wear is in this short video, which shows a loop flushing block from a Sauer Danfoss series 20 motor worn beyond repair.