There is a peculiar case of gearbox vibration in one of our plants. We have a centrifugal overhung blower connected to a gearbox and motor. The issue is high vibration (current value 138 micron and trip at 140) in gearbox DE bearing (in both X & Y probes). The vibration levels in the remaining gearbox bearings is normal at 20 micron. Motor vibration is also normal at 20 micron and blower vibration is also normal at 15 micron.
1. The vibration is high only at Gearbox DE bearing. The spectrum indicates purely 1X component.
2. The value is varying with every startup. Sometimes the vibration is 35 micron after start up and sometimes it is 90 micron and presently it is 138 micron. But whatever the value is after each start up it will remain same till its stoppage. However, there is no change in casing vibration. It is always less with maximum value of 1.5 mm/s
3. All instruments probes, cables, proximeters and complete loop check done and no abnormality found
4. Gearbox bearing inspected and clearance checked. Found within limits
5. Vibration is increasing with increasing load. Load is increased with IGV opening
6. Lube oil pressure to bearing was checked and is ok
We don't have a forward path as both instrumentation and mechanical have made their checks and found no abnormality. Also this different vibration value with each startup is baffling. Issue taken up with blower vendor Howden and they too don't have an answer at the moment and referred to the gearbox vendor Renk.