Blog

February 9, 2015

A Little Shim Can Make a Big Difference

By Stan Riddle

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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 http://thealignmentblog.com/blog/2013/12/18/alignment-triangle-3/

6 Comments

  1. Mike Keohane on February 10, 2015 at 9:26 am

    Excellent information!



  2. Andrew on February 11, 2015 at 11:23 am

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



  3. Lonnie on February 17, 2015 at 3:24 pm

    good read,



  4. Stan Riddle on February 17, 2015 at 7:10 pm

    Thanks Lonnie!



  5. Mike Hinkley on February 19, 2015 at 3:54 pm

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



  6. Stan Riddle on February 19, 2015 at 4:22 pm

    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.