July 1, 2012
Troubleshooting Looseness During Shaft Alignment
By Patrick Lawrence
Everyone has had shaft alignments that seemed to be possessed. Whether you’re using a laser alignment system or dial indicators, one set of readings tells you to go in one direction, the next set of readings tells you something completely different. If you’re having repeatability problems, something is loose. Is it that simple? Yes!
So what can be loose? When troubleshooting looseness look for unintentional movement–movement you are not controlling. Let’s break it down to three different areas: sensor movement, base/foot movement, and torsional movement. Ask yourself these questions as you are stepping through the options.
- Are the chain brackets tight to the shaft or hub?
- Are the extension rods tight to the brackets?
- Are the sensors tight to the rods?
- If using magnetic brackets, do the magnets have full contact to prevent them from slipping?
- If mounted to a hub instead of the shaft, is the hub tight to the shaft?
- Is the sensor rubbing on something when positioning the shaft?
- For sleeve bearings, has the shaft come to rest so you know for sure it’s in the same spot for measuring?
- Did you bump something out of place?
- Are all the hold down bolts on all pieces of equipment tight?
- For all the above, are you sure?
- Are the motor feet moving relative to the base?
- Is the base moving relative to the feet?
- Is there debris in/under/around the shims and base?
- Can multiple shims be consolidated into fewer shims?
- Does this sound like soft foot?
- Is there looseness in the coupling?
- Are you controlling it in a consistent manner?
- Are you controlling the backlash in the gearing?
- Are the chain/magnetic brackets tight to the hub/shaft (see above)?
I’m sure you can think of some more. It would be great to hear your suggestions. Remember that consistent readings will produce consistent results. Something loose is the number one cause of shaft alignments gone bad.
While fixing the chains on the shaft with keys ensure that the bracket is 180 degree opposite to the key position so that the bracket can be fixed tight on the shaft. This will ensure there is no looseness in the bracket fixture.
Thanks, Rao. Good point. We like to get the brackets hand tight and then another half turn or so with the little tightening tool.
That is a great sharing of experience Patrick. i think i have really experience lots of this especially during Precision alignment of a Boiler FD fan of recent. The fan bearings were replaced with brand new refurbished ones. Alignment readings were not closely repeatable. a vibration survey immediately revealed loose bearings.
I really like this post. please keep it up
Mike, thank you very much for the contribution and encouragement! My cohorts and I see a lot of stuff out there week in and week out. Passing it along is a pleasure.
We are using Fixtur laser laser alignment for the last couple of years of all aligment purposes. While aligning machines with sleeve bearings how we can eliminate axial float of the rotor effect on the alignment reading ? Does this come under looseness effect while taking reading? Is there any specific procedure for taking alignment with such machines? Please clarify.
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