Categories: Shaft Alignment

A Little Shim Can Make a Big Difference

| | |

By on February 9, 2015

To the casual observer, precision shaft alignment of large machinery may look like “grunt work” a task that only requires a strong back. But those of us who perform shaft alignment know it is moving big heavy machines to within very small tolerances. As such, small movements can often make big differences in alignment quality.

As an example, I recently received an email from a VibrAlign client, with the following problem:

A 400hp, 1800 rpm motor, driving a gearbox. The motor could not be lowered further, due to being base bound. The customer stated, “If I were to go back, and add 4 mils to the front, would it make a substantial change?”

My answer, “a little, but not enough”, however adding 10 mils to the front would make the alignment substantially better.

photo 1

How could a change of 10 mils, roughly the thickness of 3-4 human hairs, make such a change? By making the front feet number positive. In doing so, the shaft centerlines are much closer at the coupling.

photo 2 photo 3

The customer made this one small move, and made the alignment MUCH BETTER! Actually better than the 3600 RPM tolerance.

Remember this rule of thumb. You want your foot values like this:
• The back, or outboard feet values should be the bigger number.
• The front, or inboard feet values should be the smaller number.
• Both values should have the same sign, such as both positive, or both negative.

For more information, see VibrAlign’s blog post on this topic at

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.

6 responses to “A Little Shim Can Make a Big Difference”

  1. Mike Keohane says:

    Excellent information!

  2. Andrew says:

    awesome, simple, explanation that can save a lot of extra work.

  3. Lonnie says:

    good read,

  4. Mike Hinkley says:

    I’ve always been told to try and “split the difference”.

  5. Stan Riddle says:

    Mike, so was I. While splitting the difference might get you close, and it is “moving in the right direction”, it’s not always right. Splitting the difference assumes that the distance between feet is the same as the distance from the inboard feet to the coupling center. But, if the distance between feet is twice as much as the distance from inboard foot to center, then at coupling center you will be halfway to zero. But if you had a long motor, and a short shaft, splitting the difference won’t quite get you there.