Before adjusting the charge pressure in a closed-loop transmission we must define its value - and that's where doubts may arise. How high should it be? 18 bar? 22? Maybe 28 bar? How about 30? The easiest way to get the answer is to consult the machine's technical manual, but what if you don't have one? Can you say with certainty what pressure should it be set at? And even if you have a figure in mind, can you justify the necessity of the pressure to be set at that particular level?
To give a definite answer to these questions, we must look into the functions of a charge pump in a closed-loop transmission, and apply the "relief valve rule of thumb", which states that pressure should be set to a value high enough to ensure the correct operation of the system and not more!
The primary purpose of a charge pump is to make sure the loop stays filled with oil. Without the charging system, the oil would eventually "escape" the loop through the internal leakage of components, and the transmission would "cavitate to death". Almost always a charge pump is a fixed displacement pump that "charges" the closed-loop legs via a couple of check-valves, and a relief valve, often located in the pump, is what limits the charge pressure to a certain level (the very level that we are trying to define).
And whenever you put a fixed displacement pump constantly relieving to tank, what you get is a pure heat load, therefore setting the charge pressure too high is a waste of energy. Energy waste is never a good thing, don't you agree?
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 and high-pressure load on components.
Now let us consider the charge pressure relief valve operation. Unlike an "ordinary" relief valve, whose function is to stay closed and open only during pressure surges, the charge pressure relief valve stays open all the time, metering oil to the tank to maintain the desired pressure level. In some designs, these valves can demonstrate significant pressure override (the 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 is another reason 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?
The absolute majority of closed-loop pumps use the charge pressure for servo pressure. As a rule, closed-loop pumps are designed in a way that the internal high-pressure-induced forces act to de-stroke the pump, which means that the servo-pressure can become a pressure-limiting factor if set too low. 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 25-35 bar to allow the high-pressure reach 420-440 bar. Setting the charge pressure at 20 bar in such a pump will effectively limit the maximum attainable pressure of this transmission to an unacceptably low level.
It is also very important to consider closed loops equipped with loop flushing. Most modern designs use flushing valves with weak springs and fixed orifices in the poppet, which are a rough analog of flow limiters, and aren't adjustable. However, in systems that use relief valves 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 relief valve is used for flushing, it is this valve that defines the charge pressure when the pump is on stroke.
Resuming all the above - the charge pressure should be set to the "golden middle" value - i.e. it should be as low as possible without compromising the correct operation of the transmission. 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 the manual - 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 come about a closed-loop with charge pressure set to 35 bar when 18 bar would have been enough - it is like watching someone burn his money on a slot machine - it doesn't cause you any harm at all, but still makes you wonder why...
Now, a message to all the "hydraulic artists" 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 losing 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 wear of a charge pressure relief valve seat and poppet is a sufficient reason to sound the "stop everything" alarm - relax, 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...