Tracking the causes of coupling failure

Explore coupling maintenance and the telltale signs of failure to maximize coupling life and ensure reliable system operations.

By Bob Boyle

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Shaft couplings are critical parts of any power transmission system, providing the smooth transfer of power from one component to another. As with any other part of the system, maintenance maximizes coupling life and ensures reliable system operations.

Coupling maintenance is generally a simple matter, requiring a regularly scheduled inspection of each coupling. It consists of:

  • Performing visual inspections, checking for signs of wear or fatigue, and cleaning couplings regularly.
  • Checking and changing lubricant regularly if the coupling is lubricated. This maintenance is required annually for most couplings and more frequently for couplings in adverse environments or in demanding operating conditions.
  • Documenting the maintenance performed on each coupling, along with the date.

In most cases, these maintenance steps should be sufficient to keep couplings working smoothly and to enable them to reach their full service life. Even with proper maintenance, however, couplings can fail. Underlying reasons for failure, other than maintenance, include:

  • Improper installation
  • Poor coupling selection
  • Operation beyond design capabilities

Telltale signs of failure

The only way to improve coupling life is to understand what caused the failure and to correct it prior to installing a new coupling. Every failed coupling provides some evidence about what caused the failure. Some failures can be seen readily, while others require some investigation.

Some external signs that indicate potential coupling failure include:

  • Abnormal noise, such as screeching, squealing or chattering
  • Excessive vibration or wobble
  • Failed seals indicated by lubricant leakage or contamination

In addition, examination of damaged parts helps provide an explanation of what caused the failure:

 

Failed part

Cause

 

Ruptured flexible element
Ruptured flexible element

 

Excessive torque loading due to improper coupling selection or system changes

 

Cracked flexible element

Cracked flexible element

 

Hardened rubber due to chemical contamination

 

 

Split hub

Split hub

 

Improper installation

 

 

Key roll in seat

Key roll in seat

 

Possibly due to improper hub size selection

 

 

Burst hub

Burst hub

 

Improper size selection coupled with overload

 

 

Elongated bolt holes

Elongated bolt holes

 

Excessive torsional vibration or loading. In properly installed couplings, torque is carried by contact friction between coupling surfaces rather than by the bolts. Excessive vibration can loosen bolts, causing torque to be applied to bolts.

 

 

Accelerated wear and early failure

Accelerated wear & early failure

 

Lack of maintenance

 

 

Internal signs of failure must be investigated when the equipment simply stops running and there is no external evidence.

 

Failed part

Cause

 

Tooth damage

Tooth Damage

 

Improper coupling alignment

 

 

Degradation

Degradation

 

Sludge contamination reduces lubricant flow, which can cause premature failure

 

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