Fluid Power Journal

Oil Rig Top Drive Knocking Noise in the Pump

figureitout-logoA drill rig operator was complaining about a knocking noise coming from his top-drive HPU. It controlled a hydraulic motor that drove a winch attached to the drive. The noise reminded me of my first car that lost a rod bearing.

I felt the noise was coming from the pump, so I had them remove and disassemble the pump. We found one piston’s brass shoe worn down approximately 1/16 of an inch compared to the other apparently good pistons. We noticed the small lubrication hole was plugged with brass, possibly from the shoe wearing down on this piston. All the other piston lubrication holes were clear, and their shoes looked fine except for quite a few scratches on their face. The scratches were probably caused by the one piston’s wear material.

We checked the filters and found everything in good working order with the elements having been recently changed before the noise developed. There was a nick in the housing O-ring, but it didn’t leak when re-assembled. The HPU was a simple tank, pressure-compensated piston pump, safety relief, proportional valve, brake valve, and motor.

Why would only one piston fail while the others seem OK?

See the Solution

The lubrication holes on piston shoes allow high-pressure oil to float the piston shoe slightly off the shoe plate. If this hole gets plugged, the piston shoe loses the pressure that hydrostatically floats the shoe. The shoe loses its film of oil that would reduce any wear between the shoe and shoe plate.

When we looked down the hole in the center of the piston from the opposite side of the shoe, we could see a piece of O-ring plugging the hole from the topside of the piston’s shoe. We examined the new filter housing O-ring and found it had a small nick that cut a piece of the O-ring off, and as luck would have it, getting caught in the small piston lubrication holes.

We installed a new piston barrel assembly from a spare pump, and they were up and running again.


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