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Repeatability Issues

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During the “heat of the battle” to get the precision shaft alignment job finished it is easy to overlook simple items that can cause unexpected results, non-repeatability, and soft foot issues. Here are some examples;

Chain Hoists:

  1. The Issue:  While using a chain hoist to lift a motor for the vertical shim change a customer was experiencing “weird” soft foot issues after the hold down bolts were tightened.
  2. The Cause: The chain hoist was not completely slacked off.
  3. The Fix:  Easy to overlook and easy to correct by ensuring the chain hoist tension is released fully.

Vertical Jack (lifting) Bolts:

  1. The Issue:  Similar to the customer using a chain hoist to lift the motor.
  2. The Cause:  The vertical jack bolts were difficult to turn with a wrench due to rust and dirt build up, even when not in contact with the base, plus it was difficult to see if the jack bolts were completely backed off the base. Two jack bolts were still contacting the base on diagonally opposed feet.
  3. The Fix:  If the vertical jack bolt threads are rusted, dirty, etc. its best to clean them first and look with a flash light if needed to ensure they are not contacting the base.

Horizontal Jack Bolts:

  1. The Issue:  After completing a Verti-Zontal Correction the vertical position of the motor was still high by a fair amount and out of tolerance. The customer questioned the laser’s repeatability.
  2. The Cause:  Before tightening the hold down bolts the horizontal jack bolts were tightened to “maintain” the horizontal position. The jack bolts had dug into the motor feet holding it up when the hold down bolts were tightened.
  3. The Fix:  After loosening the jack bolts and re-measuring, the vertical and horizontal alignment was in tolerance. Horizontal jack bolts can just barely touch the motor feet before tightening the hold down bolts, however do not tighten them with a wrench.

By Brad Case, Vibralign


This is a subject that comes up relatively frequently with our customers and observations from the field. The purpose of today’s blog post is to illustrate what can and often does happen.  While misalignment can and does present with easily identifiable vibration signals; often it does not. Misalignment may be present in a machine, developing forces detrimental to machine and component life; while at the same time NOT producing a vibration signature easily distinguishable from unbalance or looseness. Further, depending on machine design and duty, misalignment may be present and not produce substantially elevated vibration at all.

In the case presented today, excessive vibration was identified; however the vibration did not appear initially to be caused by misalignment. Let’s take a look:

The machine tested is a 10hp overhung chilled water pump servicing a building. The machine was initially run for vibration testing and was audibly loud. Since we were onsite for an alignment class, the pump was intended to be checked for misalignment and corrected if found out of tolerance. An initial vibration test using a OneProd Hawk revealed the following:

Hawk dashboard pic

Note: misalignment was not specifically diagnosed.

The pump was shut down and a visual inspection performed, which revealed the following:

Pump pic

Installed, the coupling element is clearly distorted, and black rubber shavings are present below the coupling on the base.

Removed, the coupling element damage is even more obvious.

The motor/pump was aligned and the coupling element replaced.  Here are before and after alignment data:

alingment data 1

You should notice the very high angularity in the horizontal plane (-5.8mils/1”). This level of angular misalignment can be seen in the wear pattern of the coupling element; the pattern is angular and the entire insert is twisted or distorted in shape. The motor and pump was left in very good alignment condition as documented on the right with all green coupling icons for this 1800 RPM machine. For reference, the tolerance used is documented at the bottom of the saved results screens, 0.7mils/1” angularity and 4.0mils offset.Vibration testing following the alignment job revealed the following:

dashboard 2
dashboard 3

All levels across the entire machine were reduced to very good levels, resulting in a machine that is now in good condition for long reliable service. This particular coupling was so degraded it was producing strong unbalance and looseness indications which basically served to mask the real underlying misalignment problem. The one thing we know for sure in all cases is that misalignment will produce excessive forces that serve to prematurely wear primary components such as bearings, seals, and couplings.

By Steve Mathews, Vibralign


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