Raw mill vibration increase — Causes, Diagnosis & Operating Targets

Vibration is the mill telling you the grinding bed is no longer behaving the way the rollers and hydraulics were tuned for. The bed may be too thin, too wet, too dry, contaminated with tramp metal, or thrown off by a worn dam ring or cracked tyre. Once vibration starts trending toward the alarm band — typically around 6 mm/s on a VRM, with trip a couple of mm/s above that — small problems start cascading: hydraulic pressure has to be raised to compensate, gearbox loading rises, and the next trip is closer. Treat the trend, not the absolute number, as the leading signal: a steady climb at the same feed rate is more useful than a one-off spike.

Common Causes

1. Uneven or unstable grinding bed

Insufficient bed depth, inconsistent water injection, or material segregation prevents the rollers from maintaining stable contact. The bed loses its plasticity and the mill starts to chatter at the same feed rate.

2. Oversize or hard clinker lumps in feed

Lumps that should have been crushed pass into the mill and impact the rollers. This often follows a crusher upset or a maintenance miss on the feed-belt screening.

3. Tramp metal or refractory fragment ingress

A fragment of metal or refractory entering the mill produces a sharp vibration spike that may settle but leaves running damage on the table. A metal-detector trip should always be investigated, not bypassed.

4. Worn or cracked roller tyres

Profile loss or surface cracks create uneven loading even on a stable bed. Vibration rises without an obvious feed change, and hydraulic pressure has to be increased to compensate.

5. Dam ring height too low

A worn or under-spec dam ring cannot hold a thick enough bed. Material flows off the table before it has been properly ground, and the bed becomes unstable.

6. Grinding pressure too low for current feed

When grinding pressure is set below what the feed hardness requires, the rollers lose stable contact with the bed. Pressure has to be raised in steps until vibration settles.

How to Diagnose

1. 01
   Confirm the reading on the second sensor and rule out a transducer fault before stopping the mill.
2. 02
   Check the metal detector and feed-belt log for any recent trip — an uninvestigated metal alarm is a leading cause of sudden vibration.
3. 03
   Increase grinding pressure in 5 bar steps within design limits and watch whether vibration settles.
4. 04
   Sample feed for moisture and oversize. Adjust water injection in small steps and confirm bed stability before adding pressure.
5. 05
   Inspect roller tyres for visible cracks, flat spots, or excessive groove wear at the next safe stop.
6. 06
   Measure dam ring height against specification; restore profile during the next planned outage if low.
7. 07
   If vibration crosses the trip threshold, stop the mill and inspect before any restart attempt — do not reset and continue.

Process Impact

Vibration that creeps up at constant feed is the mill losing its operating margin. Long before the trip, gearbox bearings and roller suspension components see cyclic loads they were not designed for, and the next planned campaign on those parts is being silently consumed. Output falls because the operator has to back feed down to keep the bed stable, and specific power rises to maintain the same finished product. Sustained high vibration also damages instrumentation and can create false readings on the bed depth and hydraulic system, which then mislead the next operator. The cost of stopping early to investigate is almost always lower than the cost of running through a vibration trend until something fails.

Operating Targets