Fitting a coupling takes twenty minutes. Doing it so the coupling is still there in two years takes a bit longer, and almost all of the difference is in the order things are done.
The mistake that causes most repeat failures is simple: the coupling gets fitted, the motor bolts get tightened, and the alignment is checked afterwards. By then the motor has already moved, and what you are measuring is the result rather than the setting.
Before anything is fitted
Clean the shaft ends and check them. Burrs, dried threadlocker and paint all change the fit, and a hub that has been pressed on over a burr will never come off straight.
Measure both shafts. Not from the drawing - with a micrometer, at the point where the hub will sit. A shaft that has been running with a slipping setscrew is often undersize, and a clamping hub will not grip it properly.
Check the hub bore against those measurements with the hub cold. If a bore needs to be warmed to slide on, you get one attempt at positioning it, so know where it has to end up first.
The order that avoids rework
| Step | What to do | Why it matters |
|---|---|---|
| 1 | Check each motor foot sits flat with the other bolts loose | A soft foot twists the housing when tightened and moves the shaft |
| 2 | Fit the hubs to both shafts, leave the coupling element out | You cannot align a shaft you can only reach through a coupling |
| 3 | Align the shafts and tighten the motor bolts | Alignment is set with the machine in its final assembly state |
| 4 | Re-check alignment after tightening | Tightening moves things; this is the reading that counts |
| 5 | Fit the element and set the hub gap | Too little gap leaves no room for axial movement |
| 6 | Tighten the hub fixings to the specified torque, evenly | A single over-tightened screw tilts a clamped hub |
| 7 | Turn the axis by hand through several revolutions | A tight spot once per turn means the assembly is wrong, not the coupling |
The hub gap, specifically:
Measure it with feeler gauges at four points around the circle, not at one. An even gap means the shafts are parallel; a varying one is parallel offset and it will wear the element unevenly.
Set the gap slightly wider than the minimum in the coupling's instructions. On a long axis, thermal growth will close it.
Clamping hubs, done properly
A clamped hub that will not grip is nearly always one of three things, and none of them is a faulty coupling.
The bore is oversize because the shaft is worn. The screws are tightened one at a time to full torque, which tilts the slit. Or there is grease between shaft and bore, which halves the friction the clamp relies on.
Clean both surfaces, tighten the screws progressively in small steps rather than one screw fully, and measure the shaft first. That fixes most grip problems without changing the coupling.
After the first day of running
Come back and check three things once the machine has run a full shift.
Re-torque the hub fixings. Check the hub gap again, at working temperature - if it has closed to nearly nothing, the axis needs a coupling with more axial capacity. And look at the element or disc for a wear pattern: an even one is normal, one worn to a single side is parallel offset that was there from the start.
That five-minute check after the first shift catches almost everything that would otherwise become a warranty conversation six months later.
We supply jaw, diaphragm, flexible and rigid couplings across the shaft coupling range, bored and keyed to your shafts so the fit is right when the coupling arrives.
If you are replacing a coupling that failed early, send us the old one's dimensions and how it failed through this form - the failure pattern usually points at the cause.
