Fleets that use semi-fluid grease on their non-drive wheel ends rarely do so because they have a problem with oil, the more traditional lubricant. Rather, the higher viscosity of the grease means no leaky seals — and the resulting spray of oil all over the brakes.
But semi-fluid grease is hardly a pannacea for lube-related wheel-end problems. Indeed, it’s associated with a different type of failure: burned up bearings, the result of poor lubrication of the outer bearing in a tapered-spindle application (look for heat discoloration of the bearing elements and spinning of the inner bearing race or the bearing cup in their respective journals). Fleets also have reported rust on the outer end of the axle spindle, on the bearing-adjustment nuts, and even on the outer bearing itself.
Rust, heat, or both can cause the elements of the outer bearing to seize together. Not good. Again, there’s strong evidence that the semi-fluid lube itself isn’t the problem.
Recommended fill levels usually refer to the “percentage” of fill, a concept that can be hard to communicate to service personnel used to dealing with static fill-level indicators like those used for oil. It’s tough to monitor the amount of grease you’ve installed and where it is in the hub. You have some segregation of areas in the hub that can mean the outer bearing is blocked off from the centre portion of the hub, or the grease cavity. And then there’s a region near the seal. A semi-fluid lube, when it’s hot, can migrate between these bearings but it doesn’t do so readily.
If fill levels are to be determined as a percentage of the volume of space within the hub, then technicians need to know more. If they don’t know the cavity’s volume, they’ll either work blind or ask, reasonably, “percentage of what?” A fill-quantity recommendation would be more useful, either by weight or by volume, but this often requires taking into account a residual quantity of lubricant remaining in the hub — a tough call.
Because of the potential for confusion, follow up with your lube and wheel-end component suppliers. Make sure your techs understand fill procedures and the signs of potential bearing failure.