Categories: Shaft Alignment

After Shaft Alignment, Back Off on Jacking Bolts to Reduce Vibration

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By on September 2, 2012

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:

Before             After

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.

About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

One response to “After Shaft Alignment, Back Off on Jacking Bolts to Reduce Vibration”

  1. 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.