Loss of torque on hydraulic motor

I'm working on a machine with a Vickers angular piston hydraulic motor that turns a lead screw and moves part of the machine. This is a fixed displacement motor.

The system runs at 600psi. Apparently it was working fine and then suddenly it stopped turning the screw. I turned the system pressure up to 800 and it seems turn fine.

So, what can cause a sudden loss of torque in a hydraulic motor? It's supposed to have 1 ft-lb / 100 psi, so it should have 6 ft-lbs at normal pressure, but I can easily stop the motor by grabbing the hand wheel on the lead screw. I pulled the line off the case drain and there's barely a drip.

Any thoughts?

Elementary question, but I'll ask it- are you sure the system is still delivering 600psi? Maybe a relief valve is on the fritz?

Face seals in the piston block tend to wear and oil just flows from the input to the output, but if your getting a full 800 psi on the acutal line to the motor its probably not this, (system pressure just proves you have this before the valve to the motor, again, dont make assumptions, valves often wear there operating mechanisms wear and depending on the valve just a little less travel can really hold back the flow to the actual motor. Helps to tee in a few more pressure gauges, one on each side of the motor and then retest, a blocked return line or similar can cause similar issues and old hoses can fall apart internally and jam stuff up nicely!

Other thing can be the pistons in the motor picking up and galling there bores and simply jamming, though normally that increases the case drainage. But its not unheard of for some of the pistons to jam so you only end up with only a few working. The non excessive case drain and the sudden onset though to me means you need to consider far more than just the motor, hence why i recommend gauges in more places and retest, even if your using the same gauge - tee and moving it around, treat it much as a electronic fault and take all the measurements first and let them tell you what’s wrong.

I don't think it's a system pressure issue because all the other hydraulic stuff works fine. I don't think it's the valves because I manually actuated them in both directions and had the same result.

I pulled the motor out and the shaft turns freely.

Jammed pistons, pull it apart you have little to lose at this point, may just need em tapped out and cleaned.

ewlsey said:

I don't think it's a system pressure issue because all the other hydraulic stuff works fine. I don't think it's the valves because I manually actuated them in both directions and had the same result.

I pulled the motor out and the shaft turns freely.

Click to expand...

The method you used only verifies that the motor turns easily under no hydraulic pressure.

The angular piston motors use a drive pin in a ball socket to connect the piston rotating element to the output shaft. I have seen the drive pin break in half and cause a sporadic jamming problem.

This may be the operation and repair manual you are looking for:

http://www.fizner.com/3RDP/eaton/site/pdfs/i3230s.pdf

The previous posts listed the same items that the Vickers manual listed. Its either the pressure relief valve, the four way control valve or damaged pistons in the motor. One test would be to reinstall the pressure inlet line, hold the output shaft fixed and verify that no fluid comes out the motor outlet port. If fluid is flowing through the positive displacement motor with a locked shaft you know that the motor needs repair.

Robert

Disconnect the motor and see if there is drag in the mechanicals.

Ed.

http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_2230.pdf

I imagine you've already read the troubleshooting guide there, but it suggests the most common cause of trouble is the port plate gets scored. a scored plate may leak input to output through the annular groves without much leakage to drain because the grooves arent radial. I have repaired a few hystat lawn mowers that quit being able to go up any hill but otherwise seemed to run fine on flat ground.

I bet it leaks more when stalled but fairly little when actually running. the scoring increases the surface area and upsets the balance of the hydraulic pressure pushing the barrel off of the port plate / vs the pistons and spring pushing it together.

Well. I pulled one line off and stalled the motor. Sure enough, oil was pissing out the outlet.

I don't know why I didn't think of that. I thought for sure it would leak out the case drain.