Hidden deep in the recesses of every airplane wheel assembly are the wheel bearings, technically known as cups and cones. Aircraft owners are permitted to service these in accordance with FAR Part 43, Appendix A through processes such as cleaning and greasing. This task isn't difficult, but if it's poorly done the servicing will harm more than help.
Important concerns revolve around bearing cleaning, bearing inspection, bearing and cone matching, proper lubricants, lubrication practices, and ensuring that proper axle nut torques are applied during wheel re-installation.
Almost all light-airplane wheels consist of two halves that are held together with through-bolts. Each half has a center section and a flange. Lugs on the inner edge of each wheel half fit together when the halves are aligned correctly. The two halves are held together by three or five AN-style bolts. The torque applied to these bolts is critical.
Almost every wheel assembly used on GA airplanes has been manufactured by either Cleveland or Goodyear. These wheels have provided excellent service, but there is one troublesome exception.
Cessna installed McCauley wheels that are commonly known as "three piece" or "five piece" wheels on its single-engine airplanes from 1974 through mid-1977. The five pieces consisted of a hub, an inner- and outer- flange, and two phenolic (thin plastic-like material) spacers that prevented dissimilar corrosion between the hub and the two flanges. These wheels are easily identifiable since the flanges are distinctly separate pieces from the hub. There are five service bulletins and an airworthiness directive (AD74-13-06) that apply to these wheels because of hub cracking, broken bolts, and loose flanges. The McCauley three- and five-piece wheels do not have a good service history.
Common to all light-airplane wheels are the bearings and bearing races. These are often listed in parts manuals as the bearing cup (the race) and the bearing cone (the bearing), and these terms help in the understanding of how they work.
The bearing cone has a series of extremely hard steel rollers that are held in position by a steel cage. The inner surface of the cone assembly and the outer surface of the cup are hardened and tapered at the same angle so that the rollers are captured between the inner and outer cones when the axle nut is tightened.
There is an inner and an outer race in each wheel. Races are circular and are shrunk-fit into recesses machined in each of the wheel halves. The exposed surface of the race (cup) is tapered — the innermost portion being closer to the axle than the outer portion. Races are firmly held in position in the wheel halves, while the bearing cones are held loosely in position relative to the race by washers and a snap ring that seats in a groove in the wheel. When wheels are installed, the inner bearing cone seats up against the axle shoulder while the outer bearing cone is pushed into position as the installer snugs up the axle nut. The installed wheel/tire/bearing assembly is similar to a sandwich, which consists of an inner bearing cone snug up against the tapered cup in the inner wheel half. The outer bearing cone is snug up against the tapered cup in the outer wheel half and the axle nut, if properly tightened, permits free rotation of the wheel on the bearings without allowing in-and-out movement of the wheel on the axle. Again, it is legal for an owner or operator to clean and lubricate the cups and cones on both the nose and main wheels of his airplane under the preventive maintenance paragraph of Part 43. But there is scant information contained in the average light-airplane service manual on this task.
Preventive maintenance rules grant some limited freedom to owners wishing to do unsupervised maintenance on their own aircraft. But owners wishing to do preventive maintenance are bound by the same performance rules that apply to certificated technicians or repair station employees, and must perform the work in accordance with appropriate maintenance manuals (FAR 43.13). Finally, maintenance-record entries must be generated for this work (FAR 43.9).
Owners wishing to perform any of the many minor tasks permitted under the preventive maintenance appendix of FAR Part 43 always should be willing to take the time to arrange for instruction in each task from an experienced maintenance technician. It may seem as if lubricating wheel bearings is an easy task. Easy yes, simple no. The following will outline the approximate steps of the procedure.
Start with jacking up the airplane so the tire is clear of the ground and free to rotate. Again, like everything else in aviation maintenance, this takes some know-how. Jacking may require a wing jack, or a bottle jack or wheel jack with an axle or gear-leg adapter. Service manuals often detail these special tools. Other sources of information on jacking may include the type club for your airplane. There's a list of type clubs on the AOPA Web site.
After the wheel is off the ground and before removing the hubcap, or dust cover, that covers the main axle nut, remove the two or four bolts that hold the brake-pad carrier in position (see " Airframe and Powerplant: Gimme a Brake," February 2006 Pilot). The brake assembly can be left in place with the guide pins resting in the torque plate bushings.
The next step is a safety step. Before removing the axle nut, deflate the tire. Cracked wheel flanges, stripped hub threads, broken through-bolts, and damaged wheels are not unusual and any of these faults can result in a situation where the axle nut is the only force holding the wheel halves together. Deflating the tire will eliminate the possibility of personal injury. The cotter pin, or other locking device, securing the axle nut can then be removed and the wheel can be slid off the axle. It is a good practice to then lower the axle onto some wood blocks. This prevents airplane damage if the jack fails, and stabilizes the airplane so other work can be done while servicing the wheels and bearings.
The bearing cones are held in position by small snap rings and should stay installed in the wheel when it's removed. If the bearing cones pop out of the wheel, stop before going further. The cones and cups wear together and always should be regarded as a matched set, so the cone always should be re-installed in the same cup as it was when the wheel was removed. The washers that are part of each bearing, snap ring, and grease seal felt package are important too — make sure that the position of each washer in the stack is noted and that each is re-installed in the same position.
Next, bearing cones and their matched cups are cleaned to spotlessness with mineral spirits. Always wear gloves to avoid skin contact with mineral spirits, oils, and greases. Cleaning can be done by brushing or by blowing spirits through the bearing with a compressed-air sprayer. Never spin the cage of a tapered roller bearing with compressed air. Clean the bearing cone and corresponding cup until they sparkle.
Each bearing cone and cup are visually inspected — a magnifying glass and a strong light are helpful tools — for discoloration, corrosion pits, and excessive wear. If the parts don't look clean and shiny, then they should be replaced. It's OK if they appear slightly gray. The retaining rings, snap rings, and felt seals also should be inspected. Often the felt seals are reused. This is OK unless the felts are dried out, compressed, or ragged looking.
If the cups are pitted or the cones have to be changed, the cups can be removed by heating the entire wheel to 212 degrees Fahrenheit for 15 minutes. This can be done by putting the wheel half into an oven or into a pot of boiling water. This task is not part of preventive maintenance and will have to be done under the supervision of a certificated airframe and powerplant mechanic. Heating the wheel loosens the shrink fit of the steel cup in the wheel. The Cleveland wheel and brake Web site (www.parker.com) has a very detailed description of how to make the proper tools to support the wheel and how to remove and install cups without damaging the wheels. One field method for installing new cups is to put the cup into dry ice for 15 minutes while heating the wheel half. Remember that cups and cones are replaced in pairs.
Unlike almost every other parts supplier in the light-airplane industry, the international bearing industry has standardized its part numbers. This ensures that each 13889 — a common light-airplane cone — will have the same outside diameter, bore, width, and cup angle. Each cup and cone has an easy-to-read part number etched or molded into the part. And that part number is a good part number at any bearing supply store in the country. Some airplane manufacturers' parts manuals include the universal bearing number, but this reference is not automatic throughout the industry.
Bearings are parts that are regularly purchased over the counter at bearing supply houses rather than through the airplane manufacturer parts supply system. According to Timken, a large supplier of bearing cups and cones, these parts are shipped with an oil-like coating that is compatible with all greases — company officials contend that it does not need to be washed off.
Nevertheless, owners who take on preventive maintenance tasks must realize that unless they consult with a certificated technician they are totally responsible for the condition of the parts they are installing. It's always wise to thoroughly inspect all parts, even new parts, before installing them.
Wheel bearings may be the most over-lubricated parts on light aircraft. If the felt seals are in good shape, and the airplane is hangared and only flown in normal conditions, bearings do not have to be lubed at every annual. One airplane manufacturer suggests that the bearings be lubed once at the first 100-hour inspection, then at 500-hour intervals thereafter. Another manufacturer suggests 400-hour intervals, and a third, 250-hour intervals. Yet most bearings are lubed at every annual. Here are some guidelines for wheel-bearing lubrication. The wheel bearings must be inspected and lubricated immediately after an airplane is painted — paint strippers and high-pressure water washes invariably dilute and wash away lubricants. Owners and maintenance shops should clean, lube, and inspect all wheel bearings when they first see any airplane. Then the lubrication intervals can be adjusted according to where, and under what conditions, the airplane is operated.
The wheel bearings of airplanes operated near saltwater, in dirty or dusty conditions, or off unimproved airstrips need shorter inspection and lubrication intervals. Cleanliness, good felt seals, and the proper grease are players in maintaining bearing airworthiness.
The most common wheel-bearing grease is Aeroshell 5, a mineral-based high-temperature grease with a temperature range of minus 23 degrees to plus 177 degrees Celsius. Many owners elect to spend a little more and use synthetic grease. Synthetic greases promise better corrosion protection, and wider operating temperatures. An additional reason to use synthetic grease is because the synthetics can double as general airframe lubrication grease for moving parts such as landing-gear retraction mechanisms. Common synthetic greases that can handle both wheel-bearing and general lubricating chores are Mobilgrease 28, Royco 22CF, and AeroShell 22.
Grease can be packed into bearings manually or with a bearing packing tool.
After both bearings are lubricated and reinstalled in the wheel, slide the wheel onto the axle, followed by the washer and the axle nut. The nut should be tightened enough to ensure that both bearings are seated against the cups, but not too tight. Here's what one airplane manufacturer advises: "Tighten axle nut to allow wheel to turn freely yet not fit loose on the axle." Most mechanics tighten the axle nut with a pair of slip joint pliers until the bearing binds slightly, then they back the nut off until the nearest castellation in the nut and the cotter-pin hole in the axle line up.
A proper workmanlike approach, a full understanding of the process, cleanliness, and attention to detail are the requirements for all owner-performed maintenance. Cleaning and lubricating wheel bearings is easy if these requirements are adhered to.
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