Friday, January 23, 2015

Coupling Peak Torque Failure at Keyway

Gear Coupling Hub Burst Due to Over Torque
Gear Coupling Hub Burst Due to Over Torque
Coupling hubs failing, cracking, or bursting over the keyway corners is a common failure mode that clearly indicates the coupling hit a peak torque that it was not designed or built to take. The reason the coupling hub breaks at this point is because it is generally the weakest point on the hub. (There is less material between the corner of the keyway and outside diameter of the hub than any other point on the inside diameter of the hub.)

The root cause for this over-torque failure may be that the coupling was undersized for the given application (perhaps the wrong factor of safety was used when calculating required torque), that the application saw a massive unexpected peak load (and perhaps the coupling failing may have saved more expensive equipment from failing), the coupling bore and/or keyway were oversized relative to the maximum bore and keyway combination specified from the manufacturer for the given product size, or the coupling had excessive misalignment.

Jaw Coupling Hub Burst Due to Over Torque
Jaw Coupling Hub Burst Due to Over Torque
Identifying the peak torque load and either eliminating it from re-occurring and/or ensuring the replacement coupling is designed to handle the peak torque is critical to ensuring this failure mode is not seen again. (Unless there was a material defect in the hub, which, at least for Lovejoy manufactured couplings, is extremely unlikely... simply replacing the hub will likely result in another hub failure.)

It is also important to note that, while undersized/underrated couplings are not good and can cause failures... it is also a mistake to significantly oversize a coupling. There are a number of drawbacks to this approach, including excessive overhung loads, potentially reduced performance, and cost.... and should be avoided.

For further information on sizing a coupling, please consider reading  "Coupling Service Factors - Best Practices" and "Coupling Sizing Torque - How to Quick Calculate". For further information on misalignment, please see "Top Reason for a Coupling Failure"

To learn more on coupling failure analysis, go to:
Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)
Gear Coupling Tutorial - Part V: Failure Analysis (with photos)
Grid Coupling Failure Analysis (includes photos) 
Top Reason for a Coupling Failure

And, if you still have questions or concerns... please feel free to contact a Lovejoy application engineer or specialist.

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