When I worked in an auto parts store, every so often a guy who had just bought a used boat would bring in a shop towel filled with pieces of exploded trailer bearings and chunks of what looked like hard plastic.
The story was always the same: He’d gotten a great deal on the boat and couldn’t wait to take his family out. Too bad the previous owner had never bothered to service the trailer bearings. Believe it or not, even bearing grease wears out over time. One cause for its degradation is oxidation, the same chemical process that causes fuel to spoil. The other cause has to do with how grease functions as a lubricant.
“Grease works by releasing its oil, so in a bearing application you get a little bit of oil released by the grease, and that oil is what lubricates the balls or rollers in a bearing,” said Bryant Hardy, principal scientist in lubricants R&D for ConocoPhillips. “So you release this oil and pretty soon you have a reduction in oil content, and you only have to reduce that oil content by about 15 to 20 percent and now it’s so hard that it looks like a brick. So you either oxidize it or you use for so long that you’ve actually sucked enough of the oil out of the grease that it will no longer release oil, and it no longer lubricates.”
Grease is actually about 90-percent oil, Hardy explained. The other 10 percent is comprised of thickening agents, performance additives, oxidation inhibitors, components to help prevent wear, and rust-inhibiting compounds. Those chunks of what looked like hard plastic spilled on my parts counter so many years ago were all that was left of the grease.
Hardy recommends that boaters in cold climates make cleaning and repacking bearings part of their winterizing maintenance routine. As a precaution, use new grease seals each time you clean and repack your bearings. Typically, the seals get mangled when you pull them out anyway, and seals aren’t expensive. Maintained properly, wheel bearings will last several years.
A problem unique to boat trailers is water intrusion. Water dilutes grease, but it causes even more problems for the carbon steel components in a typical wheel bearing. Here’s what happens.
As you tow your boat, heat builds up in the bearings and hub. When you back the trailer into the water, which can be 100 degrees colder than your hub and bearings, that rapid cooling causes a quick vacuum effect that can actually draw water—and fine sand—into your hub assemblies. Now the moisture is in place to dilute your grease and begin corroding metal parts—especially if you’re boating in salt water.
“That’s one of the dangers, because now what you’ve done is actually sucked water in, and now you’re going to drive back … and mix that water with the grease,” Hardy said, adding that Bearing Buddies are highly effective but still need to be opened up and checked periodically. “When you do that, you will thin that grease out.”
After a long drive, Hardy said, wait a while before backing your trailer into the water. Once you get to the ramp, let the bearings cool down a bit. Use the time to load the boat and be sure you haven’t forgotten anything. “If you just let the trailer sit for a little while until the hubs cool off, then you have less chance of sucking water past the seals—it probably won’t at all. It doesn’t take very long. You’re only talking about 15, 20 minutes, maybe a half hour.”
In warm-weather climates, where it’s possible to use your boat year-round, Hardy recommends servicing your trailer bearings a couple of times a year, if not more often. He also recommends synthetic grease, if you can justify its price, which can be three times as much as conventional greases. At a minimum, you should use a grease with a GC-LB rating from the National Lubricating Grease Institute. The GC means it’s rated for chassis components, such as tie rods and ball joints, and the LB rating means it’s good for high-speed applications such as wheel bearings. And if you can find a grease rated for saltwater environments, so much the better. From the ConocoPhillips line, Hardy recommended Kendall SHP, which is rated for salt water.
“We’ve got a number of GC-LB greases,” he concluded. “We’ve got three or four in the line, but this is the only one that really is good for salt water. I suspect all the other majors probably have one too.”