There is a very simple, but crucial step in the machine installation process that oftentimes gets missed. When this step is missed, it leads to a world of problems, from excessive vibration to premature failure of bearings, seals, and especially couplings.
Let me establish the problem first, then I will explain how it negatively impacts the machine.
The problem
If some types of couplings are tightened before alignment, they ultimately end up in a bind. This bound condition can also be referred to as a “preloaded” condition. When this happens, an undesirable axial and radial load is introduced into the machines even though the shafts are aligned within tolerances.
Be aware of this potential problem and remember that some couplings should not be tightened until the shaft alignment is completed.
What types of couplings are susceptible to this issue?
This issue only affects coupling that have elements that bolt the two hubs together. These couplings typically have pieces that bolt to the outer or radial face of each hub. Although there are certainly others, the main types of couplings that are extremely susceptible to this are Omegas, Raptors (New Omega), and Rubber Tire couplings.
Some couplings are less or not affected by this problem, depending on how bad the initial misalignment is when the couplings are tightened. These might include gear couplings, grid couplings, and insert-type couplings. Shim pack type couplings are flexible couplings that are particularly rigid and can be affected by preload. Leave the final torque for the end!
Summary
Omega, Raptor, Rubber Tire, and other types of couplings that bolt up, easily bind up if you move the machine after they are tightened.
Gears, grids and inserts move more freely when they are aligned. One main reason for this is that these couplings have the ability for the connecting component to freely move axially.
What does coupling preload look like?
Let’s say you are a technician that is called out to troubleshoot a major vibration issue on a brand-new fan that you did not install. Let’s also assume that the millwright that did the install, tightened an Omega coupling and then aligned the motor to the fan.
If vibration data is collected it is likely to indicate misalignment with elevated 1×, 2×, and sometimes 3× in the spectrum. When the time waveform is analyzed, it will likely indicate binding that looks like misalignment.
Sometimes this preload condition can produce only an elevated 1× which would seem to indicate unbalance. The problem with preloaded coupling issues is that a laser alignment may look completely fine, and a balance will be unsuccessful and unnecessary. The only solution is to remove the preload condition of the coupling.
How to remove coupling Preload
Here’s the easy part. This problem is extremely easy to correct and even easier to avoid. To correct, simply loosen the coupling and retighten. This should relieve the preloaded condition. It is always a good idea to collect vibration data before and after, to document the improvement.
To avoid the issue, when dealing with preload-prone couplings, bolt the pieces of the coupling together but leave them loose. We need the hubs to rotate together in most cases for laser alignment. Align the shafts to tolerances, then tighten the coupling. When this happens in the proper sequence, everything is happy.
Conclusion
Be aware of the potential for coupling preload to affect vibration readings and machine health and strive to avoid this conditions from occurring in the first place, using the guidelines described above.
Download our Misalignment & Vibration infographic a reference guide to misalignment condition.
Filed under:
Vibration Analysis by Diana Pereda