Gearbox Oil Filter Types

Hi Everyone,

I want to take a minute to talk about the filtering of PSRU Gearbox oil.

The gearbox that I purchased for my RV-10 from Geared Drives came to me with a Fram oil filter installed on it. It turns out that the selection of an automotive engine oil filter designed for 10W-30 oil isn’t a good choice at all when trying to filter gear oil.

Testing has shown that the gear oil doesn’t want to go through the paper filter medium very easily and as a result, pressure builds up ahead of the filter which forces open the bypass valve. This results in unfiltered oil being circulated through the bearings and the addition of air to the oil stream at the bypass valve.

This ends up in a couple of more things happening. The oil going to the bearings with air in it or being foamed causes a temperature rise in the bearings which in turn heats the oil. We feel that this was a major contributing factor in the loss of the input shaft seal. As the temperature increased at the input bearing the aluminum casing around the bearing would expand.

We believe that his expansion in addition to a gearbox internal pressure increase caused by a poorly located gearbox breather causing cyclical pressure build ups in behind the input shaft seal resulted in the input shaft seal working itself loose and allowing the gear oil to escape the gearbox.

As soon as the gearbox lost its oil a catastrophic bearing failure ensued which resulted in the main output shaft twisting into two pieces.

This brings me back to the filter which we feel is the initial culprit. Stuart Davis who is the new owner of Geared Drives under the new name Auto PSRUs LLC, has done some very comprehensive testing of the filter and oil distribution system on the BW350 PSRU. He is currently testing my new gearbox with a number of synthetic oils. He is also testing new oil filters and screens in comparison to the old style automotive engine oil filter previously used. These results will be available in a couple of weeks but we are already seeing good results using synthetics. We have not seen any foaming of the oil in testing so far.

I will post more as I receive information from Stuart.

Thanks and Blue Skies



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