Anyone come across this;

I have 2 parallel sauer-danfoss pumps in a hydrostatic drive. In forward flow all is well, meaning full flow/pressure. In reverse flow I can go up to about 2000psi, then my forward line starts to build pressure. Gauging on the suction (forward) line shows one pump at 400psi (charge), whilst the other is showing the rising pressure. Is this latter pump limiting out on the multifunction valve while the other pump continues to stroke?

Not much to go on, however I would be looking for some sort of bypass in Reverse.

Could be a Cross Port Relief valve not blocking and allowing the bypass into the downstream line?????

How's that for a SWAG?

What are the complete model numbers of the two pumps? Motor(s) to if you have them.

I take this to mean two pumps parallel into one motor.
Are they 90 series, or small ones?

When you build reverse to 2000 psi, what flow is this? Are you stalled against load, building pressure only, or putting pump on stroke creating flow which in turn builds pressure due to the load?

When you say suction (forward) are you measuring in the loop (the -4 or -6 port right next to the main A or B port) or are you measuring charge pressure?

Does this pressure change stay continuous, or just a short blip until it reaches steady state back close to normal?

Charge pressure should always be 300-400, no matter what the loops does, as the charge relief and charging checks limit that circuit. Any leakage from high pressure loop back into charge should either be limited by charge relief to 400 psi, or should self correct because the source of the oil for the main loop is the charge pump. There is capacitance of stored energy and volume in the hoses and circuit, but it can’t get oil and pressure from nowhere indefinitely, not like an open loop suction circuit.

If you are measuring the low side of the loop, not charge, and it starts to rise up with flow, there may be a restriction in the low side loop.

I have very vivid memories of a similar puzzle many years back. See if any of this sounds familiar or applicable to what you are seeing. Looking back, it was a very powerful learning lesson at the time, and I have applied the lesson to other situations since. Here are the clues and process we went through.

I had a machine with Forward and Reverse and charge gauges. 8 pumps paralleled into 4 motors, but the same thing would apply with 2P and 1M.
-Charge pressure normal always, at all speeds and loads, in both directions.
-In F, at any speed, or any load, pressures and speeds and charge pressure all normal.
-In R, up to 21 mph, at any load, pressures and speeds and charge pressure all normal.
-In R, at 21 mph to 23 mph, charge stayed normal. Forward pressure would start climbing abruptly (up to about 1000 psi by 25 mph) and R (driving) pressure would increase by the same amount. i.e. F increasing from 400 to 1000, then R would increase from its load of 2000 to 2600 psi for same speed and load.

-By applying brake loading, we could drive in Reverse up to 5000 psi, under 21 mph, and have no problem. This told me the gremlin was not leakage or pressure related.

-By running across the 21 mph barrier at light load, medium load, heavy load, full pressure, we determined it happened at same speed every time, regardless of load. Again, indicates not pressure related.

-By consistently occurring at the same speed, it pointed to flow (or motor speed, but other tests eliminated that as an unlikely cause).
-Since the charge pressure didn’t change, but low side (F) in reverse did, not likely any high pressure cross leakage.

I wish I could make a simple sketch. There was a loose adaptor fitting dropped inside the Forward pipe and moving back and forth in the pipe with flow. It acted like a check free flow in one direction (F). In Reverse, it laid harmlessly there with low flow, but at a certain flow for 21 mph the flow velocity was enough to move it uphill against an elbow where it caused a significant restriction. This particular situation caused a very abrupt increase in back pressure. If it was a simple restriction it would have increased more gradually with flow.

Coincidentally, and luckily, the F side pressure gauge connection happened to be on the motor side of this restriction, so it read the restricted backpressure and the gauge would increase. Since the load to move machine remained constant, the high side R gauge increased the same 600 psi to give the same net differential pressure across motor.

If the F gauge would have been on the pump side of this restriction, it would have read charge pressure forever when in Reverse. The machine would have felt sluggish, the R driving gauge would have increased suddenly above 21 mph, and it would have felt like a sudden increase in mechanical load was causomg the drive to work harder. (Yes, went down that path also.)

Not sure what if any applies to you, but the concept may: Can you determine if the occurrence is flow or pressure related, is there a restriction, and where in the circuit it may be?


Frustrating, but the most challenging problems are the most rewarding to solve.

kcj

Forgot to note, you said multifunction valve so assuming 90 series.

Mark the MFV, keep notes, and exchange one pair at a time. Exchange F & R in one pump, then test. Exchange F&R in second pump, test. Etc.

I don't think these are likely causes, but always do the easiest and cleanest tests first!

Hello Winged 1to give you a correct answer for your problem i need the exact model no of the pumps and an information about the application.
is it always like this or happened suddenly?
do you see changes in motor speed?

The pumps are a 90 series. Don't currently have full number. In this case, they drive a single rineer motor, but we will drive one or both stators for torque/speed selection. And yes, when this condition occurs, a reduction in motor speed accompanies. However, the condition does happen at all speeds, including stall. Gauging is done on the pump outlet(inlet) test ports.
We use a seperate charge pump. Both pumps show charge pressure on the suction (forward line in reverse), until the condition occurs ( above 2000psi), at which time one of the pumps suction port begins to climb, not suddengly but increasingly along with higher reverse pressure. Pumps are seperated by check valves on both forward/reverse. For test purposed the hot oil shuttle is disabled.

In our application, we limit reverse pressures to 1000psi. Because this condition occured beyond that range, the skid was deemed acceptable and delivered to the customer.