For the parts customer, it's nice to be able to get the part that's needed. But better yet to be able to know why the part failed in the first place.
In a work titled “Treatise on the Unreliability of Tapered Roller Bearings,” engineer G Allan Hagelthorn takes a look at how bearings work and how they can fail.
“Probably the least understood component in the wheel assembly of highway tractors and trailers is the tapered roller bearing,” Hagelthorn says. “It's necessary to understand what happens within the bearings as they rotate in conjunction with the wheels.”
The load that individual rollers must withstand varies greatly, depending on where in the bearing it is located. The greatest load is placed on the roller closest to the ground. Loads diminish as the rollers move toward the upper half of the bearing, with those rollers at the 90° position from the bottom of the bearing having virtually no load to carry.
Loads also vary according to the diameter of each individual roller. The smaller the diameter of the roller, the greater the stress that is concentrated on the rollers at the bottom of the bearing. For example, if the width of the surface of a roller is 1/32", it may have to carry 36,200 psi. With the same amount of weight on the bearing, a 1/64" roller would be subjected to 72,400 psi.
Roller bearings can show several types of damage, including:
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Fatigue spalling. This is a geometric stress concentration caused by misalignment, deflections, or heavy loading.
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Inclusion origin. This type of spalling is the result of oxides or other hard inclusion in the bearing steel.
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Excessive preload or overload. Rapid and deep spalling can be produced by unusually high stresses.
“Spalling is a condition of the hardened surface of the tapered roller bearing that shows separation, flaking, or peeling of the surface of the bearing cup that is in contact with tapered rollers,” Hagelthorn says. “The spalling condition continues to worsen. It is the first stage of bearing deterioration that culminates in total bearing failure and the separation of the dual wheel from the axle.”
Here are some examples of bearing failure that Hagelthorn has seen: