MORE WET PUMP DETAILS

1. Wet Vacuum Pumps:

All airplanes with gyro instruments have vacuum pumps driving the gyros. The early design of vacuum pump is lubricated and cooled by a flow of engine oil, thus called wet pumps. The vacuum produced on the suction side of the pump expels air on the output side, along with the oil. The oil returns to the engine, but the air, if returned with the oil could cause foaming, so it is expelled through a separator.

2. Wet Pump Separators:

Various designs of air/oil separators for wet pumps have been used throughout the history of flying, but none have ever worked very well. Small separators allow the air to escape through a pecan sized open vent fitting, others use a large canister to hold the oil as the air escapes. Both allow the escaping air to “spit” oil all over the engine compartment. This oily mess drove the industry to switch to “dry” vacuum pumps, although some owners have stayed with the wet design.

3. Dry Vacuum Pumps:

These do not use oil. The pump vanes are graphite, which is self-lubricating. It, also, wears quickly and is highly subject to breakage. But they do run very clean. Useful life is said to be fro 1 to 700 hours.


Wet pumps are still running after 35 years, so they have devotees who will not give them up.

~ Bill Sandman ~

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