Repairing a Damaged Crankshaft Pulley and Keyway

[MikePhua]

Understanding the Failure
Crankshaft pulleys are critical components that transfer rotational force from the engine to auxiliary systems like the alternator, hydraulic pump, and cooling fan. In the case of a John Deere 310A backhoe loader, a seized hydraulic pump led to a catastrophic failure: the crankshaft pulley cracked, and the keyway—the machined slot that aligns and locks the pulley to the crankshaft—was severely damaged. Interestingly, the key itself did not shear, which is often the intended failure point. Instead, the keyway in the crankshaft deformed, making it difficult to seat a new pulley securely.
Why the Keyway Matters
The keyway ensures that the pulley rotates in perfect sync with the crankshaft. If the keyway is damaged, the pulley may slip under load, leading to misalignment, vibration, or complete loss of drive to critical systems. A bunged-up keyway also prevents proper torque transfer, which can cause premature wear or failure of the new pulley.
Conventional Repair Options
There are several approaches to repairing a damaged crankshaft keyway, each with trade-offs:

• Widening the Keyway: If the damage is localized, the keyway can be machined wider to accept a larger key. This requires broaching the new pulley to match. It’s a precise operation and may require removing the crankshaft, depending on access.
  
• Epoxy-Based Repair: High-strength metal-filled epoxies like JB Weld, Devcon, or Belzona can be used to rebuild the damaged keyway. The process involves cleaning the crankshaft slot thoroughly, applying the epoxy, inserting a new key, and allowing it to cure fully before installing the pulley. This method is surprisingly durable when done correctly and has been used in both agricultural and industrial settings.
  
• Welding and Re-machining: In extreme cases, the keyway can be welded shut and re-machined to original dimensions. This requires removing the crankshaft and precision machining, making it the most expensive and time-consuming option.
  

Field-Proven Epoxy Techniques
One practical method involves filling the damaged keyway with JB Weld, inserting a new key while the epoxy is still pliable, and installing the pulley to hold everything in alignment. After curing for 24–48 hours, the bond is strong enough to resist rotational forces. Key steps include:

• Degreasing the crankshaft and pulley thoroughly
  
• Removing all burrs and corrosion from the mating surfaces
  
• Using a release agent on the key and pulley if future disassembly is desired
  
• Applying uniform clamping pressure during curing
  

This method has been used successfully in remote locations where machine downtime is costly and machine shops are inaccessible.
Preventing Future Failures
To avoid similar failures:

• Inspect driven components like hydraulic pumps for signs of internal binding or shaft misalignment
  
• Use torque wrenches to ensure pulleys are installed to spec
  
• Replace keys and pulleys at the first sign of fretting or movement
  
• Consider using anti-seize or thread locker where appropriate
  

In one documented case, a contractor in Alberta experienced repeated pulley failures due to a misaligned pump shaft. After correcting the alignment and switching to a keyed hub with a taper-lock bushing, the issue was resolved permanently.
Conclusion
A broken crankshaft pulley and damaged keyway may seem like a death sentence for an older machine, but practical repair options exist. Whether through precision machining or field-applied epoxy, the key is to restore alignment and torque transfer integrity. With careful preparation and the right materials, even a severely damaged crankshaft can be brought back into service, extending the life of valuable equipment without the need for a full engine teardown.