Pulsing Hydraulic Motor

Posted on 07/01/2025 by Fluid Power Journal in Faces

By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training

A customer rebuilt one of his old machines and upgraded it to provide variable motor speeds. He added a SUN proportional valve with electronic controls for the variable speeds. The valve model was a FPBF-XDN pilot-operated throttle screw-in cartridge. This was the only item added to the original machine, and the circuit drawing shown. The circuit symbols for the pressure controls, the pilot-operated directional valve, and the pressure compensated (PC) pump are how they were shown when the machine was built many decades ago.

He complained that his machine was pulsating at certain feed rates. He was advised to add a Hydrostat, a pressure-reducing valve internally drained to the throttle’s outlet, to the circuit. This provided pressure compensation to the throttling valve, but it did not ultimately solve the problem.

He disconnected the motor from the machine. With it just lying on the floor, it pulsed drastically at most RPMs. At low speed, it would stop and start off and on. Lowering the safety relief below the pump compensator setting and running the motor eliminated the pump compensator as the pulsation problem. The 2 MPa (300 PSI) pilot supply to the directional valve was constant, as well as the mA signal to the throttle valve coil. When a slight force was applied to the motor shaft by creating friction from a shoe, the pulsing slowed and then stopped after about 10 seconds. From there, the motor ran smoothly at the expected RPM.

Any thoughts on what caused the problem?

See the Solution

When CFC Industrial Training was asked to help, they realized that this pilot-operated valve required slight backpressure to operate properly. For troubleshooting, they installed a manually adjusted flow control after the proportional valve. This provided back pressure and was not intended to control volume; it dampened the pulsing and eventually eliminated it. So, why didn’t the motor’s load provide this back pressure? It turns out that, because the machine’s gears had such a high gear reduction, and the rebuild had just replaced all the bearings, there was little resistance, and any time the motor was started, it over-spun and caused cavitation. The cavitation caused the proportional throttling valve to close. As the downstream flow slowed drastically or stopped, the valve would open again. This on-off was repeating, causing the pulsing. The manual throttle was removed, and the problem was corrected using a 207 kPa (30-psi) backpressure check installed on the tank line of the directional valve.

Robert Sheaf has more than 45 years troubleshooting, training, and consulting in the fluid power field. Email rsheaf@cfcind.com or visit his website at www.cfcindustrialtraining.com. Visit fluidpowerjournal.com/figure-it-out to view previous problems.