TL;DR

Hammer mill rotor bearing failure shows as high vibration and elevated bearing temperature. Measure vibration and temperature at both bearing housings and replace bearings at the first sign of wear.

What you might see

vibration increase at the hammer mill rotor bearings
bearing temperature rising above 80 deg C
grinding or rumbling noise from the rotor end
shaft seal leaking lubrication

Likely causes

Tap to mark as ruled out

Bearing contamination from grain dust or moisture ingress through the shaft seal
Insufficient re-greasing interval for the operating environment
Rotor imbalance from uneven hammer wear creating excessive bearing load
Bearing electrical erosion from static discharge through the rotor shaft

Required tools

Vibration meter
IR thermometer
Grease gun with correct grease grade
LOTO kit
Replacement bearing and shaft seal

Safety first

Hammer mill rotors are heavy and the hammers are sharp. Lock out fully before any rotor or bearing work.
Grain dust at explosive concentration is present near hammer mills during operation. No open flame or spark-generating tools near the mill.

Procedure

7 steps · ~120 min

1
Measure vibration velocity at both bearing housings with a vibration meter at the current operating speed. Record dominant frequency and amplitude.[1]
2
Measure bearing temperature with an IR thermometer. Readings above 80 deg C confirm a bearing problem.
3
Check the bearing grease fittings. Add grease with the mill running to purge contaminated grease per the re-greasing schedule.
Warning: Re-greasing a rotating hammer mill requires keeping hands and tools away from the rotating coupling. Only approach the grease fitting from a safe angle.
4
If vibration is above 0.5 in/s, stop the mill and lock out. Rotate the rotor shaft by hand to feel for roughness or heavy drag.[1]
5
Remove and inspect the shaft seal. A failed seal allows grain dust and moisture into the bearing housing.
6
Replace bearing and seal as a unit. Match bearing number from the housing nameplate exactly.
7
After reinstallation, check rotor balance by measuring vibration at no-load. If vibration is still elevated, the rotor hammers should be inspected for uneven wear.

Sources

Bliss Industries Bliss Eliminator Hammer Mill general technical documentation, Bliss Industries
Hammer mill rotor bearing maintenance and vibration monitoring, general feed industry rotating equipment practice (general)

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