Air in hydraulic systems can have very serious consequences. Drill rig masts and mast raise cylinders have been destroyed due to the fact that a mast raise cylinder was partially filled with air. Loads have being dropped due to failure of winch motors because of air being entrained into the hydraulic system.
A dropped mast incident is described in the following safety alert: mast raise cylinder failure. Another incident is described here: telescoping cylinder failure. One drill rig manufacturer recommends that a drill mast should be raised and lowered gradually from the mast rest. The manufacturer recommends that the mast is initially raised approximately five degrees from the mast rest and lowered. This is repeated at ever increasing angles until the mast is fully raised. This ensures that if air is present it will gradually be removed. A better solution would be to install a bleed valve at the highest point in the cylinder and bleed air from the cylinder prior to lifting the mast.
In the example in the above link, the technician thought he was working “by the book”, but was unaware that the pump was sucking air through it’s suction and constantly supplying air into the system. He would have noticed the problem before anything serious happened if the cylinder was connected to the mast and if he followed a procedure similar to that described above.
Another serious failure occurred when a motor in a closed loop winch system failed catastrophically due to cavitation. See a picture of the failed motor below. This occurred during load testing of an offshore crane to its maximum rated capacity. The load fell to the deck and luckily no one was injured. During the test, cavitation occurred in the suction of the motor due to an incorrect hose size fitted. As air was being entrained into the system, the motor lost its load holding capability and rotated freely.