TL;DR

Trunnion bearing failure starts as elevated temperature and low lube pressure. Stop the mill, check lube flow to each bearing, and pull an oil sample for metallic contamination before the bearing seizes.

What you might see

trunnion temperature above alarm setpoint
increasing oil consumption in lube system
audible grinding from the trunnion area
oil pressure reducing on the bearing supply

Likely causes

Tap to mark as ruled out

Lubrication film breakdown from insufficient flow or contaminated oil
Trunnion bearing white metal (babbitt) overlay worn through to the steel backing
Misalignment between the trunnion and bearing housing from foundation settlement
Lube system pump worn or filter blocked reducing bearing supply flow

Required tools

Calibrated oil pressure gauge
Oil sample kits
Dial indicator
LOTO kit

Safety first

A failing trunnion bearing is a catastrophic failure mode. Maintain the high-pressure jacking system in operation until the bearing is assessed.
Lock out all drives and ensure no mill rotation before entering the area below the trunnion.
Hot oil under high pressure at the bearing supply ports. Use PPE rated for hot oil splash.

Procedure

7 steps · ~60 min

1
Reduce the mill feed rate and monitor the trunnion temperature trend. If temperature is rising faster than 2 degC per minute, stop the mill.
Warning: A seizing trunnion bearing can cause catastrophic mill collapse. Do not delay shutdown if the temperature alarm is active and rising.
2
Stop the mill and engage the high-pressure jacking oil system to lift the trunnion off the bearing.[1]
3
Check the high-pressure lube pump output pressure and flow at each bearing supply port.[1]
4
Inspect the low-pressure circulating oil return lines for a normal oil return flow. Reduced return flow indicates bearing clearance is tightening from wear.
5
Pull oil samples from the supply and return lines of each trunnion bearing. Analyze for babbitt (lead, tin, antimony) and iron particles.
6
Use a dial indicator across the trunnion to check for runout or distortion while the jacking system is active.
7
If metallic contamination is confirmed or the bearing temperature exceeded the alarm limit, schedule a bearing inspection and possible white-metal re-pour with the mill manufacturer.

Sources

Metso Outotec Metso SAG / Premier mill Grinding Mill (SAG / Ball) general technical documentation, Metso Outotec
Metso Outotec grinding mill service documentation, trunnion bearing lubrication and inspection procedures (general)

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