In this short series I would like to talk about one of the most basic
closed loop adjustments - the charge pressure, addressing the two
questions, that normally arise in relation to this very important
a) How high should the charge pressure be? and
b) Where to read the charge pressure?
Which will be discussed in two separate parts, starting, obviously, with the Part 1.
Indeed, before going into the adjustment itself (most of
the times simple and dull procedure, which resumes to turning an
adjustment screw and securing it with a nut) we must define its value.
The easiest way to get the answer is by consulting the machine's
technical manual - the answer will be there, and any mechanic
will be able to perform the adjustment. However not all mechanics can
answer this questions with certainty without a manual, nor can they
justify the necessity of the pressure to be set at that particular
value. Ergo the following IH elucidation, aimed to those, who prefer "knowing why" over "blind following instructions".
Let us take a look at basic closed loop layout. The
primary purpose of a charge pump is to make sure the loop stays filled
with oil - without it the oil would eventually "escape" the loop
through the pump and motor internal leakage, and our transmission would
cavitate itself to death. The charge pump is connected to the closed
loop via a couple of check-valves (single check-valve in case of
unidirectional transmissions), which make sure the loop is "charged"
always at the low pressure side. A relief valve, which is most of the
times mounted inside the pump, limits the charge pressure to a certain
level, which if set incorrectly negatively influences the
The rule of thumb is - the charge pressure of a closed loop should be set to a value high enough to ensure correct operation of the loop, and not more! The charge system is a pure heat load, therefore
setting the charge pressure too high results in unnecessary oil heating
and energy waste. Another good reason to keep the charge pressure as
low as possible is the fact that the motor's output torque is defined
by the lines delta P, and not the absolute pressure at
the high pressure loop side, therefore the same output torque can be
achieved at a lower absolute pressure with a lower charge pressure
setting. Lowering the absolute high pressure of a system reduces
internal leakage (decreasing losses), as well as the high
pressure load on pipes, hoses, and other high pressure exposed
Another very important issue about the charge
pressure is the relief valve operation. Unlike a common relief valve,
whose function is to stay closed and open only during pressure surges,
the charge pressure relief valve stays open most of the time, metering
oil to tank to maintain the desired pressure level. In some designs
these valves can demonstrate significant pressure override (difference
between cracking pressure and full flow pressure), which causes the
charge pressure to drop when the flow through the valve decreases
(either due to lowering the pump speed, or due to the system pressure
rise and consequent leakage increase). It is important to take this
under consideration, and that is why the charge pressure adjustment
should be performed at normal operating temperature and speed.
OK, setting the charge pressure too high is bad, but what about setting it too low?
Absolute majority of closed loop pumps uses
the charge pressure to supply servo pressure. As a rule, closed loop
pumps are designed in a way that the internal high pressure induced
forces act to destroke the pump, which means that the servo-pressure
can become a pressure-limiting factor. For example, in automotive
closed loop transmissions that use high valve plate carry-over
angles and strong springs in the swashplate centering
mechanism, the servo-cylinder may require 25-30 bars to allow the high
pressure reach 420-440 bar. Setting the charge pressure at a 20 bar
level will effectively limit the maximum attainable pressure of such
transmission to an unacceptably low level.
It is also very important to consider closed loop
circuits equipped with loop flushing. Most modern designs use flushing
valves with relatively low force springs and fixed orifices in the
poppet, which are a rough analogue of flow limiters, and aren't
adjustable. However in systems that use pressure limiters for
loop flushing purposes, setting the pump charge pressure below the
flushing valve setting will render the flushing circuit inoperable -
something that can easily result in instant overheating and lead to a
major failure. Therefore it is important to distinguish between
different flushing valve types, and to understand that when a pressure
limiter valve is used for flushing, it is this valve that limits the
charge pressure when the pump is on stroke.
Resuming all the above - the charge pressure should be set to a "golden middle" value - as low as possible without compromising the
correct transmission operation. Set it too high - and you will have
unnecessary losses, set it too low - and you may limit your system
maximum pressure or jeopardize the loop flushing. The best way to
determine this value is through experiment and observation - something
the guys at the factory already did before putting the recommended
value in that manual. If you have it - good - use it as a reference. If
you don't have it - use your head and the above mentioned factors
to determine the best charge pressure value for the loop you are
It is very depressing to find closed loops with
charge pressure set to 35 bars when 18 bars is enough, - it is like
watching them rich old folks burn their money on slot machines -
doesn't cause you any harm at all, but still makes you wonder why
throwing away all that money...
And, to all the craftsmen out there:
If you believe that tuning the charge pressure up a
little can boost performance - It can't! (unless, of course, the charge
pressure had been previously set too low).
If you believe that when a closed loop starts
loosing charge pressure, turning the charge pressure adjustment screw
in a couple of turns can help - it can't! Repairing the components
If you believe that scoring of a charge pressure
relief valve seat and poppet is a sufficient reason to replace it -
most of the times it's not! The valve wasn't built to stay closed and
leak-proof - it is leaking all the time! Does the poppet move freely?
It does? Then leave it be...
Now, with no doubts left as for the correct charge
pressure setting, what needs to be discussed is the charge pressure
reading (where, how and what for), which will be done in Part 2.