March 28, 2018
Installation Errors Part 1- Coupling Care and Balance
By Tom Shelton
Observing a training class of new mechanics can sometimes be a stressful proposition for a former field Millwright. Especially when said millwright has control issues. As part of our Precision Shaft Alignment Training, the trainees move from the classroom setting into the field to practice what they have learned. Transferring the knowledge gained in a class setting to a field setting can be difficult for the trainees. They can revert to past practices forgetting all they learned and demonstrated understanding of in previous day’s training. After a mechanic performs a task in the field, things make more sense and are practiced as such in perpetuity.
As a mechanic, I can tell you that adding technology to a process can sometimes cause confusion in mechanics mind. We will forget to look for the simplest, most basic “best practices” in the alignment process.
As an observer accustomed to laser alignment, I see the most common installation errors as they occur during training classes. Most of the following observations have little to do with the actual alignment, but more to do with precision practices when aligning basic close coupled pump and motor combinations. Some are procedural, some relate to other aspects of reliability, but all relate to the prolonged operation of both man and machine!
Safety First! Lock out- Don’t start a job before the lock out procedure is complete and verified!
NO HAMMERS, especially on the coupling! Hitting any part of a motor or pump assembly will damage bearings. Failure is a direct result of this action. Maybe not today, but it will cause failure in time. To learn more, look up False Brinelling. When searching for alternative means of moving a motor, see our blog “Use Your Head Not Your Hammer”
- Read the coupling manufacturer supplied directions- When installing a new coupling or replacing an insert, it has specific installation requirements. These specs are typically included in the box with the parts. One overlooked aspect is the gap between the coupling hubs. This gap is critical in proper torque and power transfer from the motor to the pump or driven equipment. Gap is also a factor in axial movement of the shafts. Incorrect gap will result in high vibration, premature failure of the coupling. Incorrect lubrication, too much or too little, is also detrimental. Below is an example taken from typical grid coupling instructions for setting the proper hub gap during installation.
“Move the equipment into the operating position ensuring the gap between the shaft ends matches the ‘G’, or Gap dimension in Table-X.”
- Set Screws- Set screw holes should be filled completely to replace metal that was removed. This is for proper balancing of the coupling. Do not stack multiple set screws in the hole, that is certain to cause issue later when the hub is needed to be removed.
Figure 1- Incorrect
It should be noted that this is a very small amount of weight. It would more critical to replace this set screw in machines that can tolerate no imbalance induced vibration.
- Proper Key Length- Another facet of balancing your coupling is to replace the metal removed in the cutting of keyways in the shaft and hub. There is an easy formula to determine the proper key length.
The formula for determining the proper key length is; Length of keyway on hub + length of keyway on shaft; divided by 2. Then multiply by .95. (hub key length + shaft key length = X, X ÷ 2 = Y, Y x .95 = proper key length) For more info on This see “Balance Your Couplings” in our Alignment Blog.
Stay tuned for additional episodes of “Installation Errors”.
Installation Errors Part 2- Shim handling and Shims under pump?
Installation Errors Part 3- Pump bowl torque and Torqueing of hold down bolts. Torqueing Bolts in General!
Installation Errors Part 4- Base bolts and Jack bolts.