Question
During inspection the rubber element of an elastic coupling shows fine cracks, but the machine still runs normally. Does it have to be stopped and replaced at once?
What a Crack Means
The elastomer carries torque in compression and shear, so cracks indicate fatigue or ageing of the material. Cracks reduce torque capacity and the ability to compensate misalignment, and they allow the metal jaws of the two hubs to contact each other, producing shock loads and noise. Continued operation accelerates jaw wear and can end in loss of drive.
Cases Requiring Immediate Action
- Cracks running through a lobe, or chipped and missing sections.
- Hardened or deformed element that no longer seats against the jaws.
- Periodic vibration, abnormal noise or loss of positioning accuracy during operation.
Cases That Can Be Monitored
Where there is only fine surface crazing, no missing material and no change in vibration or noise, the element can be placed under close monitoring with a shorter inspection interval while spares are prepared, but it should not be left indefinitely. High temperature and oil contamination accelerate ageing, so check for lubricant seepage or excessive ambient temperature at the same time.
Replacement and Re-check
On a split coupling the element can be renewed by removing the intermediate bolts without moving the motor. After renewal, re-check shaft alignment and tighten bolts diagonally and evenly to avoid introducing eccentricity. When replacing spider-type blocks, use the same size and hardness within one coupling.
Prevention
Keep alignment within the design range so that the element is not asked to compensate beyond its capacity; control ambient temperature and oil contact; inspect at the scheduled interval and record replacement dates so that maintenance history is traceable.
Differences Between Elastomer Materials
NBR offers good oil resistance for applications with lubricant contact. Polyurethane gives higher abrasion resistance and load capacity for high torque and dusty duty, but is less resistant to hydrolysis. FKM provides better temperature and chemical resistance at higher cost.
- Check media contact first, then torque and speed requirements.
- Confirm the allowable temperature to avoid softening and deformation.
- Do not mix materials or hardness within one coupling.
Judging Whether Re-alignment Is Needed
If vibration persists after renewal, re-check alignment with a dial gauge or laser tool, measuring radial and face deviation against the coupling allowance, and confirm foundation bolts are tight and the base has not settled.
Misalignment beyond the compensation capability will quickly destroy the new element. Correct it by adjusting shims and feet rather than repeatedly renewing elastomers.
Practical Advice
Record vibration readings, alignment data and load conditions alongside the replacement date. After several records the real service life of the element becomes visible, and spares and inspection frequency can be planned from data instead of a fixed calendar.
References
- GB/T 3852-2017 Couplings — Types and dimensions of shaft holes and connections
- GB/T 12458-2017 Couplings — Classification
- GB/T 9239.1-2006 Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state
- GB/T 3480.1-2019 Calculation of load capacity of spur and helical gears — Part 1: Basic principles, introduction and general influence factors
