InsaneHydraulics - Sergiy Sydorenko © 2009-2010 All Ridghts Reserved
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 setting, namely:

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 transmission's performance.

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 components.

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 tweaking.

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 will...

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.