Corrosion and loss of bearing lubrication are two of the major causes of failure or rejection of aircraft wheels. It is extremely important that all organi-zational maintenance activities take precautions to protect aircraft wheels/bearings from water, particularly salt water. Wheel bearing lubrication gets contaminated and/or breaks down, from excessive heat and water, more often than it is lost. When wheels are exposed to a stream of water (such as a hose), it will usually penetrate the hub area, contaminating the bearing lubricant. This contributes to corrosion in the bearing area. All wheel bearings should be lubricated at every tire change, and as required by the applicable maintenance requirement cards (MRCs). All wheel and bearing assemblies should be removed according to the applicable maintenance instruction manual (MIMs) for that specific aircraft.

When a wheel is to be removed from an aircraft the nitrogen or dry air must be removed from the tire prior to removing the wheel. This should be done with the Palmer Safe-Core valve tool (P/N 968RB), which traps the valve core in the body of the Palmer Safe-Core valve tool. See figure 11-4. This precaution must be taken because of the possibility that the bolts in split wheels might have been sheared and cause the wheel halves to separate when the axle nut is removed. A tire deflated (valve core removed) metal tag should be installed on the valve stem prior to removing the wheel from the axle. See figure 11-5. Several people have been killed because they failed to remove the air from the tire before removing the axle nut.

You should clean bearings, bearing cups, wheel bores, and grease retainers with P-D-680, type II, solvent, in accordance with NA 04-10-1, to remove all traces of the grease, preservative compounds, and contamination. Treat bearings with fingerprint neu-tralism (MIL-C-15074) by immersing and agitating for 2 to 3 minutes. Dry the bearings and the hub area with compressed air. Be careful not to spin the unlubricated bearings. You should perform a visual inspection of the bearings, bearing retainers, and bearing cups with a 10X magnifier. Replace all excessively worn, dented, scored, or pitted bearing cups. Most bearing cups will display some wear. This is not cause for replacement as long as no step can be felt and there are no dents, scores, or definite corrosion pits. Some cups will have a light gum or surface corrosion deposit that can be removed by lightly polishing with abrasive webbing (MIL-A-9962). Do not use a coarse abrasive and do not remove the base material. After polishing the bearing cup, you should thoroughly clean the bearing cup and wheel bore to remove all deposits. Reinspect the polished bearing cups for defects, and replace them if necessary. Any obvious defects on bearing cone and roller assemblies, including cracks in the bearing retainer, are cause for replacement.

Figure 11-4.Safe-core valve tool.

Figure 11-5.(A) Deflated tire flag, (B) Storage of valve core and cap using alternate deflated tire flag.