How Improper Mounting Can Cause Premature Bearing Failure

July 16, 2019

Generally, the inner ring of a rotating bearing locks tightly against a shaft so that the two mechanical parts can spin as one. In order to accommodate this operation, massive amounts of pressure force a slightly smaller ring opening onto a slightly larger shaft circumference. That’s how press fit fastening works. Regrettably, this high-tolerance press or interference fit process doesn’t always last, and here’s why.

Bearing-to-Shaft Installation Aberrations

Inside a bearing mounting facility, a clean room environment must be established. There can’t be any dirt in here, for contaminants could get between the two races where the rolling elements reside. Just as bad, the particles could end up stuck to the shaft. After the bearing has been mounted onto that shaft, those dirty particles will prevent the two components from locking tightly together. As for the mechanism used to achieve this press fit, high hydraulic pressures and induction-heated inner rings combine to generate linear force and alloy expansion. As the hot inner ring expands, the pressing mechanism pushes the bearing onto the shaft. As it cools, it locks into place. However, problems occur when this pressing action goes awry. During the installation phase, dirt is one possible mounting impairment factor, then there’s the less likely prospect of a press fit error in the assembly equipment.

Beware of Operational Impairments

Of course, the above assembly floor scenario assumes the application of state-of-the-art equipment. That’s not always the case, unfortunately. When replacing bearings, which are innately high-tolerance devices, some less than professional repair shops use hammers and mallets, plus heat-inducing blowtorches. These aren’t the recommended shop tools for mounting bearings. At any rate, assuming the mounting work has been done properly, there are forces in play inside machines. High temperature can introduce creep. As this action worsens, the interference fit loosens until the shaft and bearing no longer align. A sudden blow to the system or a shock load, those are also potential misalignment producers here. Then there’s an opportunity for angular error, perhaps because a maintenance technician hasn’t tightened down a pillow mount after an alignment correction has been attempted.

Whatever the reason, press fit error or operational over temperature defect, bearing misalignments and improper mounting procedures combine to increase failure episodes. They’re a true recipe for disaster. Mounted sub-optimally, the loose fit produces inner ring friction. Lateral vibrational energies peak and produce more heat. Brinelling comes next, with dents cropping up along the inner raceways. And what’s the end result here? With stress and material fatigue ongoing and increasing, premature bearing failure is at hand. To avoid mounting problems, use established press fit guidelines, not brute force mounting methods.

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