I read this text in Sauer Danfoss “transmission circuit recommendation” and would appreciate if anyone could enlighten me more about what the writer tries to say:
For systems with integral charge pumps, the charge pressure relief valve is referenced to case pressure, and the relevant charge pressure is actually the differential above case pressure. When case pressure elevated, particularly during transient peaks, the differential charge pressure must be maintained. Failure to maintain charge pressure differential with respect to case pressure may result in damage to hydraulic unit.
thanks

That was nice of them to tell you about a problem without suggesting ways to overcome it.

Maybe KevinJ can enlighten you/us?

quote:

Originally posted by power hydraulics:
I read this text in Sauer Danfoss “transmission circuit recommendation” and would appreciate if anyone could enlighten me more about what the writer tries to say:
For systems with integral charge pumps, the charge pressure relief valve is referenced to case pressure, and the relevant charge pressure is actually the differential above case pressure. When case pressure elevated, particularly during transient peaks, the differential charge pressure must be maintained. Failure to maintain charge pressure differential with respect to case pressure may result in damage to hydraulic unit.
thanks

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In Sauer Hydrostatic Pumps the pistons are forced against the Swash Plate by the charge pressure.The case pressure works against the force generated by the charge pressure. If the difference will not be maintained the pistons will be disconnected from the swash plate and damaged immediatly.
Have a look at the attached picture.

Nahum;

You show Case Pressure pushing against the Bottom of the Shoe only. Is'nt Case Pressure also pushing against the opposite side of the Show except for the Middle Portion occupied by the Ball End of tte Piston? Tnat would mean the only force trying to lift the Shoe would be possibly up to 50 PSI Case Pressure on that Small Differentila Area.

Since Charge Pressure is usually 100-300 PSI the Case Pressure should have minimal effect on raising the Shoe off the Swash Plate.

Just a thought.

Bud,
In any case , the different between charge pressure and case pressure effects the force
of the pistons , ball and sockets against
the swash plate.

the case pressure is applied against (almost) the entire piston area .

For an open loop pump, with suction on the working side of piston, it doesn't take much case pressure to cause the pistons to float on the swash plate, hence case presure maximum limits of a few psi are common.

For hst, with positive pressure in the working piston (charge pressure) the case pressure can be somewhat higher. Limited by seal lip gripping shaft and overheating the very lip of seal, hence short seal life .

Anyway I think what they are referring to is that the internal charge relief valve references the case of the pump, so the charge pressure is always say 300 psi over case. Case at 10, charge at 310. Case at 75, charge at 375. Constant and self correcting. Charge pressure is also used for stroking the pump, so having a constant value above case helps the controls.

However, if you have another charge relief at the motor, or external in line flushing/charge relief, the reference point is not pump case but some other place. Thus, if pump case varies, the charge pressure differential isn't constant in the pump. May not be a big deal, but can be a problem.

Actually at work this month I am trying to sort out a circuit that has 3 large pumps in parallel. They all have internal charge reliefs, plus 8 motor charge reliefs, plus a 100 gpm external charge pump that has 2 additional in line reliefs. Some reference cases of pumps or motors, some reference the return line before a thermal valve and cooler circuit, then a return filter. Some reference the filter only. And other open loop things go into the returns. The actual charge pressure varies all over, up to 100 psi, as open loop things cycle into return, as oil warms up, as thermal valve routes through cooler or not, etc. etc. Poor design, and I am trying to fit some externally drained RV and manifolds into very restricted space......

kcj