How Grease Caps Work

A wheel hub with a grease cap
Grease caps, like the one at the center of this wheel hub, keep wheel bearing grease free from contamination.
Altrendo Images/Getty Images

If you're relatively new to the world of towing and all of the terms, tools, equipment and accessories that come along with it, it's easy to be a bit overwhelmed. Things like gross vehicle weight ratings, tongue weight, trailer brakes, drawbar pull, tie-down hangers, trailer roof vents and tow bars (among countless others) are often the focus of discussions online and at the local trailer supply parts counter. You'll find that sometimes it's easy to overlook the simplest of parts in these bigger discussions. Grease caps are a good example. These very small, yet very important, components typically get very little attention -- that is, until something goes wrong and a cracked grease cap is the reason you're sitting on the shoulder of the highway waiting for the roadside assistance truck to show up.

But what exactly are grease caps? What are they used for? Are they simply a decorative accessory for your trailer, or do they serve a real purpose? Well, you can be assured that grease caps are not decorative. Although a shiny chrome grease cap can be a nice touch on a rather plain trailer wheel, they serve a very important purpose. Grease caps are the small, round, metal covers you see at the very center of a wheel. Sometimes they're called dust caps, but for clarity, we'll call them grease caps from this point forward. Grease caps protect your wheel bearings from the dirt, dust, sand, water, salt and road grime that can (and will) destroy bearings. Without grease caps, your wheel bearings simply wouldn't survive for very long out on the road.


Bearings require constant lubrication to remain effective. Thick bearing grease is packed all around the wheel bearings every time they're replaced or even when they're removed for wheel hub or axle service. Grease caps protect the bearing grease from contamination and also hold the grease that surrounds the wheel bearings in place.

In fact, the caps themselves are also packed with grease. This serves two purposes. It provides a continuous supply of grease for the bearings, and it also keeps the grease from migrating away. If the cap were empty, the grease might eventually work its way into the cap, filling the cap but leaving the bearings dry. That's why it's important to not only pack the bearings with grease but to also completely fill the caps with grease.

Now that you have a good idea of what grease caps are, read the next page to find out how to use them.



Using Grease Caps

Water -- particularly saltwater -- can destroy wheel bearings
Wheel bearing grease can easily be contaminated, resulting in bearing failure. Grease caps protect the bearings (and the grease) from the stuff that destroys bearings.
Loretta Hostettler/iStockphoto

On any vehicle, protecting your wheel bearings from dirt, dust, water, salt, sand and other contaminants is critical, so when it comes to using grease caps, you don't have much of a choice. That's their job. You either use them and your wheel bearings will remain useful, or you don't, and your wheel bearings wear out in a hurry.

Part of proper trailer maintenance involves wheel bearing maintenance. The trailer wheel bearings should be removed, cleaned and repacked with grease at least once each year. If you're pulling a boat trailer, especially a trailer that's exposed to saltwater, it's a good idea to do this at least twice each year [source: Trailer Parts Warehouse]. This means that you'll be removing the grease caps in order to get to the bearings, so don't forget to completely fill the grease caps, too.


Grease caps come in a wide variety of sizes, shapes and finishes to custom fit every application. Sometimes they are the press-on type, but typically they are threaded in place -- sort of like a lid on a jar. Knowing this is critical when you're removing the grease cap. If you force a grease cap and somehow bend, crack or puncture it in the process, you'll need to buy a new cap. The good news is that they're not expensive -- they're usually only a few dollars each. The bad news is that you'll need to get a new cap before you go anywhere. If you allow the bearings to be exposed for any amount of time, you're risking contamination that can destroy the bearings.

One way to prevent damage to your grease caps (aside from knowing if the caps are the press-on or screw on-type) is to buy the proper tool to remove them. A lot of grease caps are damaged by people prying the caps away from the wheel hub with a screwdriver or similar tool. Several companies make tools designed specifically for this purpose, so if you're going to be removing and replacing your grease caps frequently, it's a good idea to purchase one. You'll find that it will save you a lot of time, money and frustration when you're servicing the wheel bearings.

To read more about towing equipment, towing accessories and other related topics, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links


  • eHow Cars Editor. "How to Remove Wheel Bearing Dust Caps." (Oct. 10, 2008)
  • "How to Replace Bearings, Races, and Seals on a Trailer Hub Assembly." (Oct. 12, 2008)
  • Johnson, Benjamen. "Save Your Dust Caps." Toolmonger. June 5, 2008. (Oct. 10, 2008)
  • Trailer Parts Warehouse. "Trailer Suspension - Grease Caps & Hub Bearings." (Oct. 12, 2008)