Re: Do I have to fill the entire hub w/ grease if using bearing buddy?
The whole idea of the Bearing Buddy system is for grease to have a slight positive pressure against the rear seals. If you don't fill the entire cavity and you dunk the hub into the water, the temp drops, a vaccum is created and water can be sucked into the hub.
So your argument is now down to you have to fill it with grease because it will suck air if the housing is pressurized with air.
Ok, lets look at that theory.
Air pressure rises linearly with temperature. Its very easy to calculate air pressure in a fixed volume at varying temperatures. So let put your vacuum argument to the test.
Atmospheric pressure is roughly 15 psi. If you add in the 3 psi positive pressure that bearing buddy claims we're at roughly 18 psi inside the hub.
Thermal expansion of air is roughly 0.3% per 1 degree C.
So lets say you filled the hub with grease when it was 20 degrees C outside. You drive down the road for a while and your bearings are just fine. They're running at a temperate of say 40 degrees C. So the pressure inside your hub is now 20 x 0.3 = 6%. So we add 6% (1.1 psi) to our 18 psi inside the hub at 20 degrees C and you're now up to 9.1 PSI.
So now you back your trailer into the water. The water is cold, it's only 15 degrees C. We neglect the fact that the thermal mass of the hub, grease, bearings, etc all play into the Delta T of the air inside the hub. Lets just assume that the air inside hub temps drops to 15 degrees C in a matter of lets say 180 seconds
So, -15 degrees C x 0.3 = -4.5% . So we take the 19.1 PSI and we subtract the 4.5% (0.85 psi) from the 19.1 PSI you end up with 18.2 PSI in the hub. We're still above the vacuum point since we're still above atmospheric pressure.
See where this is going?
The 3 psi is not an arbitrary number. It was used because the pressures swing inside the hub, at normal operating temps, is within that pressure range. This design would negate any vacum issues.