Can anyone explain the basic idea of using a bypass or flush/boost valve ( for the purpose of a hotoil bypass, back to tank) in a closed hydra circuit?
In a closed loop system, particularly in flow reversing loops, you don't want a cooler, or resivoir inline. As the loop is closed, you need to deal with heat buildup. An excellent approach is to divert a quantity of oil out of the loop so that it can flow through your coolers and to resivoir. This loss of loop volume must be made up with a charge/boost pump which takes from your resivoir and adds it back to the closed loop. In our drilling systems, we divert oil out of the loop with a combination of sequence and flow control valves, so that diverging only takes place above a certain pressure, and only so much quantity. The quantity is dependant on the charge pump capacity, ensuring that it will always deliver sufficient pressure to charge the loop.
Automatic2's reply makes a lot of sense and matches what we do in position control systems driven with variable displacement pumps (we tap a bit of fluid from whichever line happens to be "return" at the time and dump it to tank via the case drain and the cooler, making up the fluid with the boost pump).
With a constant displacement pump/distributor valve circuit, you need a bypass valve across the flow and return lines to prevent the pump deadheading. Keying this to demanded flow rather than just using a pressure relief valve massively reduces the amount of heat generated when the system is idling.
In mobile equipment, usually the shuttle valve (to select the low pressure side of the loop) and the relief valve (to hold minimum charge pressure at say 350 or 400 psi) are in the same block/manifold. Sometimes there is an orifice or flow control to limit flow out any given pump or motor, to balance the cooling flows more evenly, but usually that is done by proper sizing of the relief valve. If the RV is small enough, the pressure rise/flow curve self limts the flow.
If the motor flushing falve is set slightly lower than the pump, there will be some cooling through motor, some remaining charge flow at pump.
Other important functions of the charge pump are to provide pressure for the servo stroker control, to provide positive force holding the pistons against the swash plate as they go past top center and back down, and to provide enough positive pressure (typically 250 to 400 psi) at the inlet side to prevent cavitation.
Check the Sauer, Rexroth, Linde, or other sites for 'how hydrostatic transmissions works' type of circuit pages.
Yes kjc, good comment on the additional use of charge pressure. Our systems as well utilize the charge pressure to stroke pump swash plates, via an electro proportioning valve.
All of you have written very correct answers. Just to stress a point from my side that there should be 45 psi pressure difference between flushing valve downstream relief valve & charge pump relief valve. I mean to say setting of charge pump relief valve should be 45 psi more than flushing valve relief valve. This will make sure enough hot oil is taken out of loop & at the same time keeping the neccessary pressure in the loop.
Best Regards
Rajan Bedi
