Trailer hubs shedding grease caps

[Silvic]

I recently replaced the brakes, wheel bearings and races on my equipment trailer. Took it out for test run of about 30 miles and everything went fine.
Left on a 1000 mile round trip. About half way through I noticed that one of the grease/dust caps on the hubs had departed the trailer. Of course there was grease all over the wheel.
When I got to my destination I purchased some spare grease/dust caps, cleaned up the wheel put more grease in the bearings and reset the cap. It was very tight, more so than the used ones that I had re-installed after the earlier mx. Had to use a big mallet to get it to seat properly and was happy. Somewhere between 30 miles and 170 miles later there was no grease/dust cap on the same hub. Put another cap on the hub and more grease in the bearings. After 49 miles it was missing again. Three caps in 750 miles and they fit really tight.

Called etrailer.com as I got the bearings and brakes from them to ask if they had heard of the issue. Pretty much they said no. After that I googled the issue and found several Q&A on etrailer.com site where the question had been asked before. In every case the answer was the cap must not have been fully seated in the hub.

I KNOW that the cap was seated in the hub fully. There is no reason to suspect that the hub ID has been compromised due to the tight fit of the cap on installation but is a possibility.
I have considered using RTV to try to seal and hold the cap in the hub but don't really think it will work.
The hub is fine otherwise and don't really want to get a new hub but may have to do so.

Any ideas as to how to stop this. It is not good and makes one heck of a mess.

 

[RandyT]

Are we talking about the metal caps coming out or just the rubber dust covers that are used on greaseable axle trailers. I suspect that you have too much grease behind the caps and when the grease gets warm it expands to the point that it is pushing the rubber caps out if your trailer has those.

 

[nisaacs]

Are we talking about the metal caps coming out or just the rubber dust covers that are used on greaseable axle trailers. I suspect that you have too much grease behind the caps and when the grease gets warm it expands to the point that it is pushing the rubber caps out if your trailer has those.

He used a mallet to seat the caps.

Op, I had this issue on a trailer several years ago, they were the all metal caps, no rubber plug, so no vent. I ended up drilling a 1/16 hole for a vent and no more issues. I suspect they sealed too tight and the heat/pressure buildup, pushed them off.

 

[rScotty]

He used a mallet to seat the caps.

Op, I had this issue on a trailer several years ago, they were the all metal caps, no rubber plug, so no vent. I ended up drilling a 1/16 hole for a vent and no more issues. I suspect they sealed too tight and the heat/pressure buildup, pushed them off.

I agree. Sounds like the new caps are not vented or the grease is plugging the vent.

 

[jonsstihl]

If he replaced the cap on the road with the axle already warm it shouldn't be a vent issue but anything is possible.

 

[Midniteoyl]

If he replaced the cap on the road with the axle already warm it shouldn't be a vent issue but anything is possible.

The grease is still cool when applied.

 

[ruffdog]

Something is wrong with the bearings on that hub causing a spike in temperature to pop the caps over and over.

 

[WinterDeere]

I think ruffdog could be onto something here. Check bearing temperature after several miles, and see if it's abnormally high. If not, the only explanation I can imagine is that there's just not enough air space (eg. too much grease), causing abnormally high pressure for the small expected temperature rise.

OP didn't state if these were EZ Lube, Bearing Buddy, or any other specialized system, so guessing just regular old hand-packed bearings. EZ Lube usually has rubber caps that fit inside the stamped steel hammer-ons, and bearing buddies usually have rubber condoms that go over a machined steel hammer-on.

Regular old stamped steel grease caps aren't vented. Usually not an issue.

 

[rScotty]

If he replaced the cap on the road with the axle already warm it shouldn't be a vent issue but anything is possible.

The grease is still cool when applied.

Applied hot or cold, the hydrocarbons in the grease continually form gases. It's just normal "hydrocarbon cracking" and happens as long chain molecules break into shorter and shorter ones - plus gases. It speeds up when things get hot. That's how a cracking tower works.... and incidentally how grease forms oils for lubrication. .
A lot of old axles didn't have inner grease seals, but modern ones do. They seal better, too. Either the hubs or the caps need to be vented. A one way vent will keep water out.
rScotty

 

[MMC]

I think this is a bit more common than some think. While it is possible to get bearings too tight and experience heat build-up, lubricants do expand as they get warmer and with no place to go except around seals and the cap, they can push the cap off. And like others in this thread, I have simply drilled a small hole in the cap to relieve pressure. Perhaps a short test drive, reach down and feel the hubs on both sides. If one is much hotter than the other, pull it apart and see what the real problem is. If the same, try the 1/16" hole approach. Cheap solution.