How do you solve your loose coupling bolt issue? Here's 2 easy ways.
As a critical component to machine operation across a variety of industries, flexible coupling requires consistent tension to dampen vibration and protect against torque overloads. That tension is maintained by bolts that naturally loosen over time due to a combination of frequent maintenance, improper installation, differential thermal effects, and applied forces to the joint. The result is a weakened coupling between shafts, causing excessive vibration, machine downtime, damaged equipment, and even potential catastrophic accidents.
What is the loose coupling bolt issue?
Considering the normal wear and tear that occurs with flexible coupling bolts, they are often checked, tightened, and replaced as a normal maintenance routine. Each time the bolt is removed, reinstalled, or re-torqued, the nut lengthens the bolt slightly, kind of like a metal spring that is frequently stretched. This results in eventually reducing the bolt’s clamping load, and as a result, the strength of the coupling.
Another problem occurs when replacing bolts. Since there is a wide range of bolt kits (also known as fastener sets) that are available on the market, they have a range of specs related to tightening torques. For example, a common size used is the 3-1/2EB AGMA bolt kit, but across the four major brands offered, the tightening torque varies between 175 ft-lbs. to 250 ft lbs. This could cause confusion for a technician who assumes the torque would be consistent across kits without closely checking that particular manufacturer’s specs.
Added to these issues, in paper machine applications, the OEM mating rigid hubs used feature fine, threaded holes, instead of the typical through holes. This creates a difference in shank length of the bolt being used. The routine fix for this is the use of socket head cap screws, which often results in under-torqueing the bolt. The most recommended work-around in this scenario is to replace the OEM rigid hubs with a standard rigid hub for the fastener set.
How to solve it
1. Replace all fasters
If you are dealing with loose bolts in a flexible coupling setup, an easy solution would be to replace all the fasteners with new ones while ensuring that the manufacturer’s torque specifications are used. This will at least skirt the issue of have over-torqued or under-torqued bolts, which are more likely to fail. However, this still doesn’t solve the problem of knowing how long the bolts can be used and still maintain their integrity and clamping load after multiple maintenance routines, vibration, and differential thermal effects.
2. Use specialty fasteners
That problem has an easy solution, as well. Specialty fasteners like this one are available that offer a built-in indicator showing the degree of tension with each use. The indicator darkens from bright red to black as the fastener is tightened, offering a simple, visual cue of torque value. The best part is, a quick visual inspection indicates if the bolt can be reused, reducing time and potential damage due to maintenance routines. If the indicator does not return to red after removal it should be replaced. It’s a painless, cost-saving solution to a common hassle.