Should Oil Bath bearing be right full

[shunra]

I looked on UFP's website and found they say:

"The hub level should be completely full of oil"

"It is important that the hub cavity is completely filled with oil. Air and oil thermally expand at different rates. Air in the hub cavity will expand at a higher rate than the oil causing the pressure in the hub to increase above 6 psi, possibly leading to seal damage or premature seal wear and water intrusion."

I have always been under the impression that they should only be filled to roughly the 3/4 mark on the site glass. Is this just a difference between UFP and Reliable?

Also they recommend 50 weight motor oil rather than gear oil.

[TMCNo1]

I don't think all the axle/bearing/oil bath manufacturers have the same exact owner instructions or design criteria. The the type of lubricant, amount of fill, air remaining and the cap design may vary from one manufacturer to the other. If you have a UFP axles/bearing/oil bath, then I would follow their recommendations.

[KHall]

Wonder if they missed the word not as in " not completely full with oil". Air is compressible relatively speaking oil is not. I would want some air space for getting hot. i could compare the rates of expansion of the steel hub and the oil, but that would require effort.

[ttu]

i have a 07 mc trailer with oil bath bearings. dealer told me they should be 3/4 full.

hope they are correct

[shunra]

I am starting to think that the difference is that most of you guys with the newer boats have the Reliable hubs and maybe those ones need to be 3/4 full.

I see that the UFP hubs have a "pressure compensating piston". You can see it in the picture below. Maybe that makes the difference. The piston moves outward effectively expanding the internal volume of the hub. In normal operation the piston is held in the retracted position by the spring shown. Any heat expansion in the oil is taken up by outward movement in the piston assembly. Even though air is much more compressable than oil, it will expand much more with heat, so maybe they are suggesting to keep air out of the system so that the piston doesn't reach the end of its travel, and internal pressures don't start to rise.

In the design for the Reliable hub, I don't think they have this movable piston. In that case maybe they are counting on the compressability of the 1/4 air in the hub to keep the internal pressure in check. And that is why they need to be only 3/4 full.

Sound beleivable?

[sk8salomon]

it might also depend on "full" when checked hot or cold. kinda like a transmission or power steering, the level fluctuates with temperature.

[cf_koch]

My 2007 trailer manuel states that the oil should be to the center of the "Reliable" logo on the sight window.

[Thrall]

Filled mine 3/4 + full. No problems. Used 75W140 gear oil in them too, not sure why lighter weight oil is specified. 2006 model.

[TOO-TALL]

I run my hubs at 3/4 full and use 75-90 weight gear oil.
With no problems..

[Craig]

Not all Mastercraft trailers have the same hub manufacturers.

UFP Hubs are supposed to be completely full. They recommend any 50W synthetic oil, but regular 50W oil will work as well. I had these hubs put on my trailer when it was refurbished by BoatMate Trailers, this was confirmed by them as well.

I am of the opinion that you should follow the instructions of the particular manufacturer.