April 12, 2018
Channel Strut as a Motor Sub-base?
By Brad Case
At first thought it may sound like a good idea as the specialized fasteners in the channel make it easy to tighten the motor hold down bolts, however in reality it’s a cause for precision shaft alignment issues. Here’s why.
A good pump/motor base is engineered to meet specific standards (IE: ANSI, API, etc.) with steel of sufficient thickness (0.500” or thicker) to properly support the mass of the equipment mounted on the base.
Channel strut is usually formed from mild steel sheet with a typical thickness of 0.105″ (12 gauge) or 0.075″ (14 gauge). The mild steel does not properly support the motor and flexes when tightening the hold bolts leading to unintended movement resulting in “alignment frustration” as the motor moves out of tolerance. Also, the “sides” of the channel are not flat causing various angled soft foot issues.
A field alignment for a recent training class at a chiller plant was on a motor/pump set with the electric motor sitting on channel strut.
The initial alignment results revealed the vertical angular misalignment more than 2x tolerance, vertical offset 4 times tolerance, and the horizontal angular misalignment 5 times tolerance. The horizontal offset was just slightly out of tolerance.
The class inventoried the total amount of shim thickness under the electric motor feet and found a 100mil difference in the shims under the left and right front feet and a 70mil difference between the left and right rear feet. Plus, there were several small pieces of shim stock under various corners of several feet apparently to correct angular soft foot.
The class “backed up” to the beginning and roughed in the motor to straightened up the shims and checked for soft foot. The “not flat” sides of the channel strut caused angular soft foot which was corrected.
Using a shim as a feeler gauge checking for angular soft foot, which was correct by cutting the shim in half.
After completing all the pre-alignment steps and conducting the precision alignment we discovered the horizontal offset was pulling to the right 10 to 11mils as the hold down bolts were tightened. We went through the check list of items that can cause unintended movement and no other issues were found. Repeated attempts had the same results and it was determined the mild steel of the channel strut was flexing.
It should be noted that a local service company had corrected several other motor/pump sets in the chiller plant and replaced the channel strut with machined blocks under the motor feet and performed precision shaft alignments.
After witnessing the issues created by the use of the channel strut, the chiller plant maintenance manager said they would replace the channel strut with machined blocks on all their other motor/pump sets.