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

PlantServices.com

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	Excessive torque loading due to improper coupling selection or system changes
Cracked flexible element	Hardened rubber due to chemical contamination
Split hub	Improper installation
Key roll in seat	Possibly due to improper hub size selection
Burst hub	Improper size selection coupled with overload
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	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	Improper coupling alignment
Degradation	Sludge contamination reduces lubricant flow, which can cause premature failure
Internal cracks	Sludge contamination reduced coupling flexibility, leading to fatigue failure
Discoloration	Improper lubrication
Fretting corrosion	Improper lubrication or lubricant contamination