Fluid Power Journal

Strange Noise When Starting a Hydraulic Motor


A steel mill reported a strange noise at one of several hydraulic motors driving a roll on a rolling mill. The hydraulic power supply was in the basement of the steel mill.

The hydraulic motors on all the driven rolls were bent-axis piston motors with a manifold block mounted on the motor containing two cross post reliefs. The control valves were mounted on valve stands in the basement along with the main pumps. The noise was only on one unit at the roll.

Maintenance told me that one hydraulic motor was “gurgling” on start up. After a few minutes it would quite down and seem fine. However, each time the mill was stopped for more than a few minutes, it would happen again. They replaced the motor but still had the same problem. One mechanic thought it might be caused by a new hose they had installed before the noise problem, but it looked like a good assembly. There was no sign of oil leaking from any plumbing connections to the motor, which could allow air to enter when the unit was shut down.

Any idea what was causing the noise?

See the Solution

When you normally experience noise in a system at start up, it usually is an air problem.

A few months earlier, I was troubleshooting a power unit that was noisier than two identical units nearby. The units had hinged tops on the reservoirs, and the noisy one had a bad problem of aeriated oil. But there were no areas with a small amount of oil on the floor, indicating where air could inter when the unit was running. I smeared #2 grease on each of the joints on the intake line while it was running. I found that one joint with grease quieted down for 15 to 30 seconds each time I greased it. Resealing that joint solved the problem. I did the same thing to the new hose on the rolling mill and found that one hose fitting was causing the problem. Air can get in where oil will not leak out.

If you have any questions about the details, please  contact Robert Sheaf at  rjsheaf@cfc-solar.com.

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One thought on “Strange Noise When Starting a Hydraulic Motor”

  1. David Van Sickle says:

    1st would be the crimp on the new hose assembly or the assembly itself. It seals well with no leaks under pressure but when not under pressure it allows air in the hose line, especially if a vacuum is pulled.
    2nd Could be the check valve in the return line side. If the head pressure is enough to flow past the check it will drain the return line creating a vacuum and with cross port relief it will pull oil from the supply side line.

    Look forward to the answer.
    Thank you

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