Hydraulic motors - how dry starts damage them

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I was asked recently to conduct failure analysis on a hydraulic motor that was the subject of a warranty claim. The motor had failed after only 500 hours in service, some 7,000 hours short of its expected service life.

Inspection revealed that the motor's bearings had failed through inadequate lubrication, as a result of the hydraulic motor being started with insufficient fluid in its case (housing).

A common misconception among maintenance personnel with limited training in hydraulics, is that because oil circulates through hydraulic components in operation, no special attention is required during installation, beyond fitting the component and connecting its hoses. Nothing could be further from the truth.

After this hydraulic motor was installed, its case should have been filled with clean hydraulic oil prior to start-up. Starting a piston-type motor or pump without doing so, is similar to starting an internal combustion engine with no oil in the crankcase - premature failure is pretty much guaranteed.

Some of you may be thinking that the case should fill with hydraulic fluid through internal leakage. In most cases it will, but not before the motor or pump has been damaged. In many cases, this damage may not show itself until the component fails prematurely, hundreds or even thousands of service hours after the event.

In this particular example the warranty claim was rejected on the basis of improper commissioning and the customer was lumbered with an expensive repair bill.

How can this type of failure be prevented?

This example highlights the importance of following proper commissioning procedures when installing hydraulic components. As in this example, if the case of the hydraulic motor had been filled with oil prior to start-up, the failure of this motor and the significant expense of its repair could have been prevented.

Editor's note: for more information on hydraulic failures and how to prevent them, read Preventing Hydraulic Failures.

Related articles:

How to prevent 'dry' starting of hydraulic pumps
The anatomy of hydraulic vane pump failure
Hydraulic cylinder failure caused by the 'diesel effect'

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