How gear couplings fail

The typical failure mode of gear couplings is that the teeth wear away from improper lubrication and/or excessive misalignment until there is excessive backlash and tooth tip clearance leading to high vibration, or they become so thin they can no longer support the torque and roll over, an example which is a hub spinning inside a sleeve.

While failures caused by a combination of factors occur, a primary cause can usually be identified, and the most common gear coupling failures involves improper lubrication (approximately 75% of known failures).

This article is extracted from the paper, "Coupling credible failure modes and owner options to intervene," presented at the 2013 Turbomachinery Symposium by Stephen R. Locke, Thomas D. Hess, Michael J. Burgess of DuPont and Joseph Corcoran of Kop-Flex.

In one scenario, the additives and impurities present in the lubricant are separated and retained in the coupling due to centrifugal forces, creating sludge. As the quantity of sludge increases over time, it can impair axial float, corrode the teeth thereby accelerating their wear rate, or reduce the circulation of the lubricant. In extreme cases, sludge can even lock-up the coupling and prevent the movement necessary to accommodate misalignment in per Calistrat and Webb (1972).

Another common scenario is caused by inadequate supply of lubrication. For example, in a continuously oil lubricated special purpose gear coupling, if the oil flow rate is too low, or

the oil flow nozzle is pointed in the wrong direction, increased friction will accelerate tooth wear. As the wear increases the cross sectional area of the tooth will decrease, and can lead to

premature failure.

In addition to lubrication, excessive misalignment causes failures due to an increased bending stress towards the end of the gear teeth, or end loading. Under extremely high misalignment, tremendous forces are transmitted through the couplings to the connected shafts and bearings. As the bearings wear and the misalignment increases,bending stress at the end of the gear teeth is increased and wear accelerated. Misalignment failures account for about 20% of known gear coupling failures.

It’s important to note that tooth wear is not the only result of inadequate lubrication and excessive misalignment. If a gear coupling locks up from excess sludge or just from high misalignment the bending loads on the coupling are severe and other coupling components and shaft and flanges can fatigue and fail catastrophically