Blog

April 3, 2014

Can Precision Shaft Alignment Be Too Good?

By Stan Riddle

Photo 1

A shaft alignment tolerance is simply that – an allowable, minimal amount of misalignment.  There are many different tolerance tables for shaft alignment.  At VibrAlign, we have a good alignment tolerance, based on many years of experience and engineering.  Other alignment tools have their own tolerance tables.  Many technical and trade organizations have their alignment tolerance values.  Coupling manufacturers and machinery manufacturers have tolerances for alignment as well.

Some facilities specify an alignment tolerance of “as close to zero as possible”.  With today’s extremely accurate laser alignment tools, you can get VERY close to zero.  But to do so, you will probably spend an inordinate amount of time achieving an unnecessary goal.

Many years ago, I aligned a baghouse fan for a power plant.  And quite honestly, I wanted to show off – and get “all zeroes” on the alignment.  I spent an entire day on this fan, shimming and moving very carefully, until I got repeatable “zeroes” for angularity and offset at the coupling.

We started this fan, and almost immediately heard a loud “growl” from the motor, which also started overheating.  When the inboard motor bearing got to 180 degrees, it tripped out.  I uncoupled the machines, unwired and removed the motor, and disassembled it.  The bore in the inboard end bell was 0.0005″ oversized, which allowed the inboard bearing to slip in the end bell.

So the brand new motor was re-bored, reassembled, re-wired, and re-aligned.  But this time, the alignment was left about 2 mils misaligned on the horizontal offset.  And it ran smoothly for years.

By trying (and achieving) “perfect” alignment, I can only suppose there was little to no pre-load on the bearing.  If I had left it misaligned by only a couple of mils, the bearing probably would not have slipped, and I would not have spent an additional day in repairs and additional alignment.

Modern laser shaft alignment tools are not about how close to zero you can get the alignment, but how quickly you can complete the job, while doing it accurately, repeatably, and easily.

4 Comments

  1. Neil Gillespie on April 6, 2014 at 12:01 pm

    Hey Stan,
    Thanks for all you do in advising us about alignment etc…it is a great help to all who want it.
    I just wanted you to change one word of the topic from ‘to’ to ‘too’

    Thanks mate,all the very best
    Neil



  2. Stan Riddle on April 7, 2014 at 7:29 am

    Thanks Neil. That’s why I’m a maintenance guy – not an English teacher!



  3. Kevin Jones on April 10, 2014 at 5:23 pm

    Some times some amount of misalignment is required to prevent skidding of vertical motor bearings.



  4. Syed Haris on April 22, 2014 at 1:17 pm

    well said stan ridlle…