Power at Work: The case of the misleading grease nipple

Monday, August 9, 2010

Most vehicle wheel bearings are greased the same way. But our expert recently encountered one trailer bearing that could cause you problems if you don't approach it properly

by KEITH BERGLIND

Vehicle wheel bearings basically have always been greased the same way. You pack the bearings with grease, then install the bearings in an enclosed hub, with an inner seal and an outer cap. There is no provision to add grease. There are no grease nipples.

This is the best way. The bearings are packed with an approved wheel bearing grease and the hub is sealed to keep out normal dirt and water.

Mechanics learn early how to clean, pack and adjust wheel bearings. Home repair guys will continue to do it wrong, but that is their problem. The most common errors are using the wrong grease and using too much grease.

However, there are special cases that do warrant some exemptions. The most obvious case is boat trailer bearings, because these trailers so often get backed right into the lake, and even sit there for a while. There is a high risk of water getting into the normally sealed hub.

I have to admit that, because I am not a boat owner and have never dealt with this problem, I have no experience with it. However, because I live in a resort area, I see the boat trailers with grease nipples in the regular dust caps. Some of these are simple grease designs, while others are spring-loaded to keep the hub under pressure and keep the water out.

I don't have any concerns with this other than that I've seen guys greasing with ordinary gun grease. But, since the idea is to keep the bearing full of clean fresh grease, that can't be all bad.

An oil company engineer told me once that the best grease is new grease, and lots of it. "Flush the junk out."

Now, with all that said, I want to share a new wheel bearing experience I had recently.

I was helping a friend service the wheel bearings on a large toy trailer (RV toys) and we discovered a new design. Note that this is a large twin-axle trailer, made in the western United States. The axles appear to be imported from offshore.

We first noticed a grease nipple in the end of the spindle. The spindle was also bored all the way to the back, exiting behind the inner bearing. In Figure 1, you can see where the cross hole is bored to intersect the center hole.

So, why? Obviously, it only lubricates the face of the spindle where the bearing rests. Note: we normally do not expect the bearing to wear against this face. Most mechanics believe the bearing inner race does not turn on the spindle. However, in fact, this is a serious concern for many trailers. I will address this problem in a future story, because there are ways of rebuilding and repairing this wear.

It appears this imported axle comes from a manufacturer who is concerned about this problem. All I can say at this time is that, if you encounter such a design, you should service it carefully.

My problem with the design is that ordinary owners would see the grease nipple and think it was for greasing both wheel bearings. In fact, it does neither bearing, just the rear bearing-to-spindle wear area. Unless, of course, you pump tubes of gun grease into the rear area of the bearing.

I am concerned that an owner would occasionally add a few shots of grease, like our boat trailers, thinking he's doing the full set of bearings. I can see a sad situation developing.

My advice would be to service the bearings normally by removing the bearings yearly and re-packing. Then use the spindle-end grease nipple occasionally to add a bit of grease to the spindle wear areas.

These are easy to spot. There is a plastic plug in the center of the "dust cap." When you pry this out or remove the whole cap, you can see that the grease nipple is in the end of the spindle. If you do not have instructions from the trailer manufacturer, then take one wheel apart and see how it is built. Then use your own discretion about packing the bearings and how you will service them in the future.

Caution: The center plastic caps are known to get knocked out, allowing dirt and water into the bearing area. BF

Keith Berglind is a licensed heavy-duty mechanic