In a recent training class, a mechanic asked, “Why should I worry about aligning a flexible coupling? I thought the reason we used flexible couplings is because they didn’t have to be aligned.” That’s a great question, and one that deserves an honest answer.
- Cost – about $40.00 for the sleeve, or insert.
- Maximum RPM – 4500
- Parallel misalignment – 0.020 in.
- Angular misalignment – 0.094in. (about 0.012”/inch)
Based on the manufacturer’s recommendations, this coupling can tolerate a fairly substantial amount of misalignment. And, since the insert is relatively inexpensive, why go to all the trouble of precision alignment?
Because we don’t align couplings – we align shafts! The coupling sleeve, or insert, can tolerate that amount of misalignment; but, the bearings, seals, shafts, gears, and so on, can not!
The coupling insert will bend or flex, but it also resists bending. It would prefer not to be flexed at all. And when it is flexed, it requires a good amount of energy to counteract that force. That energy manifests itself in:
- increased heat due to friction
- increased radial and axial loading on the bearings, seals, gears, and shafts.
- Premature wear, and eventual failure, of these components, including the insert.
Think of it this way. Take a wide rubber band, and place it across the ends of your index fingers. Stretch it until it just begins to straighten out. You could hold it there for a long time. Now, stretch the rubber band until just before it breaks. It won’t take long until your knuckles start hurting. It’s still the same rubber band, but it requires much more effort to keep it stretched.
So to answer the question, “Why should I worry about aligning a flexible coupling?” – you shouldn’t worry about it at all. But aligning the shafts to a precision alignment value is extremely important. And it you align the shafts, the coupling will “go along for the ride”.
You’ll increase the life of your machines, and your couplings!