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.
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