September 2, 2012
After Shaft Alignment, Back Off on Jacking Bolts to Reduce Vibration
By Stan Riddle
Jacking bolts provide an easy and accurate way to horizontally position a motor during a shaft alignment. However, it they are left tight against the feet, some unexpected problems may occur.
During a recent training class at an Ohio steel mill, we went out to check the alignment on a blower fan that had been experiencing higher than wanted vibration. The shaft alignment was acceptable, but it was noticed that the jacking bolts had been left tight against the motor. The VibrAlign trainer recommended measuring the vibration, then backing off the jacking bolts, and rechecking.
The following reductions in vibration (in inches per second) resulted:
Motor Outboard H- 0.1665 0.1235
Motor Outboard V- 0.0642 0.0667
Motor Inboard H- 0.1574 0.1218
Motor Inboard V- 0.1218 0.082
Motor Inboard A- 0.0934 0.051
While not earth-shattering, it does represent about a 25% reduction in vibration, especially in the horizontal and axial positions, which is the same direction jacking bolts were used–and they were not overly tight to begin with.
If your company’s policy is to keep jacking bolts tight, here’s a tip:
- Mount a jam nut on the jacking bolt.
- Use a 5 mil shim between the bolt and motor foot, run the bolt up until it lightly pinches the shim.
- Set the jam nut, and remove the shim.
This will prevent jacking bolt induced soft foot and possibly deforming the stator of the motor, but will still prevent the motor from moving over 5 mils out of the aligned condition.
We always remove jacking screws (jack bolts) after grout curing,a s recommended by API RP 686 para 4.20.9. If you have an alignment problem, there is a very good chance that you also have a grout problem, so combining the regrout and alignment always make the job easier, and longer lasting.