Most couplings that fail early are not defective. They are doing exactly what they were bought to do - absorbing misalignment - and they wear out doing it. The coupling is the symptom; the shafts are the problem.
There are three ways two shafts can be out of line, and they do not behave the same way. One of them is easy to measure with a dial gauge, one is easy to see, and one only appears after the machine has been running for an hour.
The three types, and how each one behaves
Angular misalignment means the two shaft centrelines meet at an angle. The gap between the hub faces stays constant as you turn the shaft, but the hub faces are not parallel.
Parallel offset means the centrelines are parallel but not in line. The gap between hub faces changes as the shaft turns - widest on one side, narrowest on the opposite side, which is why it is easy to measure.
Axial misalignment means one shaft moves along its own centreline relative to the other. This is the one that appears after warm-up, when the motor and the screw grow by different amounts. It is also the one operators blame on the coupling, because the fault only shows on a machine that has been running.
| Type | How it shows up | What usually causes it | First thing to check |
|---|---|---|---|
| Angular | One tight spot per revolution when turned by hand | Motor face not true, shim stack uneven | Dial gauge on the hub face |
| Parallel offset | Hub gap varies around the circle; spider wears to one side | Mounting face machined off centre, bracket deflection | Measure the gap at four points |
| Axial | Fine cold, noisy or warm after an hour | Thermal growth along the axis, screw fixed at one end | Re-check gap when the machine is at working temperature |
How to measure it without special tooling
You do not need a laser alignment kit for a coupling on a CNC axis. A dial gauge with a magnetic base is enough, and the sequence takes about fifteen minutes.
First, mount the gauge so the stylus touches the outside diameter of one hub and rotate the shaft through a full turn. The total swing of the needle is your parallel offset plus any error in the hub itself. Second, move the stylus to the hub face and rotate again - that reading is angular misalignment.
Then do the same on the other hub. If the two hubs disagree, the error is in the assembly rather than in one component.
Two readings worth writing on the machine:
1. Total indicator reading at the coupling, cold, when the machine was first set up.
2. The same reading when the machine is at working temperature. The difference is thermal growth, and it tells you how much axial capacity the coupling needs.
Why shims and soft feet fool you
Two things make alignment readings unreliable on a real machine.
The first is a soft foot. If one corner of the motor mount is not sitting flat, tightening the bolt twists the motor housing and changes the alignment. Check each foot on its own, with the others loose - if the gap under a foot changes when you tighten it, that is the problem, not the coupling.
The second is reading the alignment hot. A machine that reads perfectly cold can be out by the time it has run for an hour, because the screw and the motor housing grow at different rates. On a long axis this is not a small effect.
Choosing a coupling for the misalignment you have
Once you know which type you have and roughly how much, the coupling choice gets much easier.
Small angular error is handled by almost anything flexible. Significant parallel offset needs a coupling designed for it - a jaw coupling with an elastomer spider copes far better than a metal-disc type. Axial movement is the one to take seriously: if the shaft distance changes through the working day, the coupling has to allow that travel or it will push the load into the bearings.
Where you cannot reduce the misalignment, choose a coupling that accepts it and treat the wear part as a consumable. That is a valid approach - it is just better to make it deliberately than to discover it when a spider fails mid-job.
We supply couplings with different misalignment capacities across the shaft coupling range - the plum coupling for combined offset and shock, the diaphragm coupling where positioning matters more.
Tell us the misalignment type and the reading you measured through this form and we will match the coupling to it.
