January 10, 2013
Flexible Couplings and Flexible Shafts
By Stan Riddle
Most mechanics are pretty familiar with flexible couplings. They are designed with an elastomer, or flexible element, which compensates for slight amounts of misalignment through a sliding motion between the coupling hubs and the insert. However, it is very important to have some idea as to how much the coupling will flex before it begins imparting the forces of misalignment to the shafts, seals, and bearings. This is not always easy to determine simply from engineering specifications by the coupling manufacturer.
I recently fielded a call from a customer of ours who was having problems aligning a refrigerant centrifugal compressor. He stated that he was an experienced aligner, but his shim corrections did not produce the outcome he expected. After some conversation, he stated to me that the machine had a disc-type coupling, and that the compressor shaft diameter was reduced quite a bit just outside of the compressor.
I mentioned to him that I had the same problem with the same type of chiller about 20 years prior, and what I had done was to align the compressor and motor shafts with the disc pack removed. He called me the next day to let me know that after removing the disc pack, he completed the shaft alignment quickly, accurately, and in a short amount of time.
The cause of the alignment problem was not with the measuring tool, but in the fact that, in this particular case, the disc-type coupling was stronger than the small diameter shaft. The coupling would partially bend the shaft instead of the other way around. This in no way lays blame on the coupling or the chiller design. Disc-type couplings work well, and transmit large amounts of torque for their size. But they are relatively stiff.
When it comes to aligning machines with either a disc-type or elastomer “tire” style coupling, pay special attention to your alignment values. If you make vertical and horizontal corrections per the amounts specified by the alignment tool, and your alignment is not close to what you expected it to be, remove the elastomer or shim pack element then align the shafts with the flexible element(s) removed. If you are unsure as to how to do this, please contact us at VibrAlign. We’ll be glad to help.