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Why do Gearboxes Fail?

Steve Koen
7th February 2009

Introduction

Gearboxes, or speed reducers as they are often called, may fail for many reasons. While the majority of these failures have some origins in rolling bearing failures, there may well be other causes which either result in, or indeed generate bearing failure. In a catastrophic failure, the precise initiator may well be shrouded in the subsequent damage and therefore make it difficult to identify the primary cause.

Causes of failure

Bearing Types

Most speed reducers will use rolling element bearings, unless the rotational speed exceeds the rolling bearing capacity. Different types of bearings will be sensitive to different effects, although most will be susceptible to shock and vibration, unexpected loads, lubrication deficiencies, lubricant contaminants, heat and possibly even misalignment for some bearing styles.

Shock and Vibration

Vibration of a ship's impeller or the road shocks of heavy transport during long trips can cause brinelling of the bearings, generating premature failure. Storage adjacent to rail lines or reciprocating machinery can generate the same bearing failure.

Unexpected Loads

Normal operation of gear couplings can cause both bending moments and radial forces, and worn couplings will generate axial forces. Cardan shafts operating at an angle of 5° or greater will cause a pulsation at 2 x shaft rpm, which in turn cause resonances through the drive system. High starting torques can also change the system resonances, as well as overload the gearing. This overload precipitates high gear separation forces, which can cause high axial and radial loads at the bearings.

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