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Shop Talk

June 2014 Issue




Maintenance Matters

Wheel bearing service—easy add-on to a brake job.

Wheel bearing service should be part of your yearly condition inspection and should be very high on your to-do list if one or more of your wheels become submerged in water for any reason, (unless your whole plane becomes submerged in water, which is an entirely different matter). If your plane spends a lot of time outside in rainy weather, you also need to keep especially close watch on the condition of your wheel bearings. In any case, water and wheel bearings don't mix well. If you are a particularly active flyer, you should service wheel bearings every 200 flight hours if that comes sooner than yearly.

This is an exploded view of a typical Cleveland wheel and brake rotor assembly. The Grove wheel is very similar, but lacks the center O-ring of the Cleveland wheel. If you can't remember where all your parts go, this could come in handy.

First—The Wheels Come Off

To service your wheel bearings you will need to remove your wheels. This should be done with an eye towards safety at all times. Remember that you should never leave your plane supported only by a hydraulic jack. Always jack the plane up, remove the wheel, and then lower the plane back onto a solid support. If you don't have proper jacking equipment and supports, wait until you do. This is no place to cut corners or get in a hurry.

This drawing shows support blocks under the hub as the bearing race is pressed out and a press bushing that just fits the bearing. By using this setup and a hydraulic press, you will avoid damaging the wheel.

A hydraulic press set up to push an old bearing race out of a wheel. A large socket proved to be just the right size to clear the hole in the hub and press on the bearing. Note the slight gap under the wheel flange.

To remove your wheels you will first need to disassemble the brake calipers. Be sure to release your parking brake first if you have one. It is also a good idea to deflate the tire before you remove the axle nut. For more details on brake disassembly, you can refer to the "Maintenance Matters" article in the April 2014 KITPLANES® and/or the wheel manufacturer's service literature, which is available online. With the wheels off, remove the retaining clips that hold the inside and outside bearings and their related plates and seals in place. Be sure not to mix up parts from the inner and outer bearings or between right and left wheels. Clean each bearing and its related parts one at a time to be safe. Avgas will work well for this, but Stoddard solvent is a better choice for cleaning and degreasing parts since it is less volatile and has lower toxicity. Any well-stocked hardware store should carry it. Consult the MSDS for safety considerations. These should be available from a retailer selling the product or online. In any case use the minimum amount practical and dispose of it properly after use. Wear some disposable nitrile gloves to reduce skin exposure to the solvent, and use safety glasses to prevent getting solvent into your eyes.

Clean and Inspect

Thoroughly clean all grease and dirt from the bearing and the race, and inspect the race and each roller for pitting, surface deterioration, or any other signs of impending failure. Any discoloration that won't rub off is a cause for concern, and any damage to the wearing surface of any part that you can feel is cause for replacement. After cleaning the bearing parts, dry them off with compressed air by blowing parallel to the rollers only. Resist the temptation to spin the bearing with compressed air. Spinning the bearing when it is dry can damage it.

Note the pitting and discoloration on this bearing race. This needs to be replaced.

Removing and installing the roller part of the bearing (also called the cone) is pretty straightforward. Just get a new replacement part with the same part number, clean it up, pack it with grease, and put it in. Do not install a new bearing without cleaning it thoroughly and packing it with the proper grease. The packing grease that comes with the bearing is not suitable for your use. Replacing a bearing race is somewhat more challenging, but not terribly difficult. That said, there are definitely better and worse ways to go about it.

This bearing cone is beginning to show signs of deterioration from corrosion. It needs to be retired, along with the race.

To the Races

To replace a bearing race, you first need to remove the tire and completely disassemble the wheel. Take care to fully deflate the tire and remove the valve stem before you remove them from the plane or attempt to disassemble the wheel. Safety dictates that the tire should be deflated before the axle nut is removed. If there is any issue with the wheel or wheel bolts, you certainly do not want to find out about it by having the wheel come apart on you when you remove it.

Be sure to keep track of and keep clean all your bearing parts as you disassemble them. This wheel will need to be disassembled if a bearing needs to be replaced.

With the wheel taken apart, you have two ways to go to remove the race. By far the preferred way to remove the race is with a hydraulic press. A large socket may work or you might need to find a piece of round steel stock that is large enough to engage the race, but small enough to pass through the hole in the wheel and push the race out. The exact size may vary with the size of the wheel. The other thing you will need is a solid object on which to rest the wheel hub when you are pressing the race out. Do not attempt to press out the race while supporting the wheel on its outer flange. The wheel is not designed to take this stress. As a distant second choice, you may use a drift punch and hammer to tap the race out of the wheel hub, also while fully supporting the outer portion of the hub during the process. When using a punch and hammer, it is very important to keep the race moving outward evenly and to avoid damaging the hub with the punch or the hammer.

Try to minimize the amount of solvent you use when you clean your bearings. It will make for less hazardous waste to dispose of later. Nitrile gloves will help protect your hands.

To install a new race, place it into position and then use the old race placed directly on top of it to drive in the new race. This may be done with a hammer or, better yet, with a hydraulic press. The old race is the perfect size for doing this job. Again, support the opposite side of the wheel hub and do not rest the wheel on the machined mating surface where the two halves bolt together. This keeps all the stress in the hub part of the wheel where it is supposed to be. Make sure the new race is solidly seated into position.

Replacement Parts

The question often arises, if I replace part of the bearing do I have to replace the other part, too? Unfortunately, the answer is yes. If the race is bad, buy a new race and a new cone (the part with the rollers), even though they are sold separately. How about using bearings from a non-aviation source? If you are an Experimental owner, there is no restriction as to where you buy your bearings. I would not hesitate to use a bearing of the same number if it came from an auto parts store or a bearing retailer, rather than an aviation parts supplier. But it is your decision, so do what makes you comfortable.

And Now—the Grease

With a new race in place, it's time to grease the wheel bearing. This calls for the question, which grease? There are at least two possible choices to consider just among AeroShell products. AeroShell Grease 5 is a good choice for wheel bearings and is well-priced at $6.85 per tube for the budget-minded mechanic who still wants a good product. AeroShell Grease 22 is a synthetic grease that Shell specifically recommends for wheel bearings, but it is more expensive at $11.85 per tube. Exxon Mobil also makes a suitable synthetic grease called Mobilgrease 28. It runs about one dollar more per tube than AeroShell 22. Note: Grove specifically recommends AeroShell 22, but Cleveland recommends Mobil. If you decide to switch grease from one type to another, you must thoroughly clean off all of the old grease and replace the felt seals before doing so.

Place about two tablespoons of grease in one hand and use the other to work the grease into the bearing cone.

One thing to consider when buying grease is the recommendation of your propeller manufacturer, assuming you have a constant-speed prop. It makes good sense to buy one grease that will serve both purposes. It will also save you from mixing different types of grease in your prop, something that Hartzell for one strongly recommends against.

The actual greasing process can be messy and a little tedious, but it isn't too bad once you get the hang of it. Take a glob of grease, say about two tablespoons, in one hand and grasp the bearing cone in the other with the narrow end facing up. Place your thumb on top of the bearing and press it repeatedly into the grease until it begins to squirt up through the rollers. Then rotate the bearing and do another section until you have completely gone around the bearing. While you are at it, apply a thin coat of grease to the race. It is a little easier to say than do, but with a little practice you will get good at it. Low-cost bearing packers have not worked well for me, and higher priced ones are just not worth it for once-a-year use. The hand method has the advantages of being well-proven and extremely cheap. A pair of disposable nitrile gloves makes personal clean-up much easier.

Before installing or removing a bearing race, be sure to set the wheel up so that it rests on the hub and not the flange or center of the rim. Here a block of steel will be used to bear on the hub.

Reassemble—and Enjoy!

With the bearings well-greased, reassemble the parts, replacing felt seals and plates as needed to keep the bearing protected from dirt and water. After cleaning, felt seals should be lightly oiled before being reinstalled. These seals have the very important job of keeping moisture out of your bearings, so give them the attention they deserve, and replace them if they show signs of wear.

If you must, you can use a hammer to carefully drive the new race into the hub. Use the old race as a driver. The old race also makes a great driver if you are using a hydraulic press, which is the preferred method.

Needless to say, be sure to carefully inspect your wheels and brakes before reassembling everything. Replace any worn or damaged parts. Especially look for any cracks in your wheels. These can lead to catastrophic failure and send your plane careening into the weeds or worse.

After assembling your newly serviced bearings, be sure to clean up any excess grease, especially off of the brake rotors. Reinstall each wheel by tightening the nut back to its original position. Be sure to test each wheel to make sure it doesn't drag or feel loose. When you are sure the nut is in the right position, replace the cotter pin. You are ready to roll for another year.

Put all the bearings and seals back together the same way they came apart. Keeping things as clean as you can will minimize clean-up later.

For more information about maintaining wheels and brakes, see the manufacturers' publications available online. And don't be shy about calling if you have any questions. They want to help you do the job correctly and get long and trouble-free use out of their products.

For more information, visit Grove Aircraft Landing Gear Systems: www.groveaircraft.com or phone 619-562-1268. Cleveland Wheels and Brakes: www.parker.com or phone 800-272-5464.


Dave Prizio is a Southern California native who has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed three—a GlaStar, a Glasair Sportsman, and a Texas Sport Cub—and he is helping a friend build a fourth, an RV-8. When he isn't building something, he likes to share his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also a member of the EAA Homebuilt Aircraft Council.

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