Couplings: backlash, runout and stiffness
The coupling sits between your two shafts and your sensors. If it has play, runout or high stiffness, the numbers you collect describe the coupling as much as the machine.
Backlash
Backlash is the free movement between coupling halves. Gear, jaw, grid and chain couplings all have some. A reading taken while one shaft rotates and the other stays put can include that play, so the values wander.
- Rotate both shafts together when you can, with the coupling bolts or spacer tight or the halves pinned. The play is then locked out of the reading.
- If only one shaft can turn, always turn it the same way and take the reading from the same side of the play.
- Sweep a small mark across the coupling and watch for a "jump" as the shaft rolls over its play.
- Where your tool allows it, use a reading method that works from a part of a turn. Some tools accept as little as about 60 degrees of rotation, which suits a coupling you cannot turn fully.
Runout
Runout is an uneven rotation of a hub, a sleeve or a shaft surface. It tells a dial indicator or a laser sensor that the shaft is moving when it is not. Typical causes:
- A bent shaft or a bent coupling hub.
- A hub that is not square to the bore, or a hub that is loose on a worn key.
- Dirt, rust or burrs where the bracket or the indicator contacts the surface.
- A rolled or dented bracket surface.
Measure runout on a dial indicator on the shaft (not on the coupling) before you align. If it is larger than about a third of the tolerance, fix it before trusting the alignment: the readings will carry an error you cannot remove with shims.
Measuring methods in plain terms
| Method | Good for | Watch for |
|---|---|---|
| Rim and face (one shaft turned) | Cheap and quick, when the coupling is a short spacer | Sag of the bracket, face runout, backlash |
| Reverse dial (both shafts, two rim readings) | Long spans and large couplings. The best manual method | Sag again; do the maths for bracket sag |
| Laser, both shafts | Fast, repeatable, handles soft foot checks and live moves | Dirty lenses, ambient light, a loose bracket |
| Laser, partial turn | Couplings you cannot rotate a full turn | Still needs three well-spaced positions |
Reverse-dial readings need correction for bracket sag, the small droop of the indicator bar under its own weight. Measure it by mounting the bracket on a stiff tube, setting the dial to zero at the top, then reading at the bottom. Enter that value as a correction.
Stiff couplings
Disc, diaphragm, rigid and gear-type couplings, and spacer shafts, are stiffer than elastomer ones. Tolerances may be tighter and what the coupling can absorb is smaller. Some stiff couplings behave like a rigid link: a small error means a large shaft load. Two practical consequences:
- Align them with the coupling off the machine if at all possible, then rotate each shaft separately.
- Check the maker value for the angular limit of each disc pack. Often it is a fraction of a degree.
- Re-check after the coupling is bolted. If a stiff coupling pulls a shaft out, the machine has soft foot or a strained pipe, not an alignment error.
Coupling hub fit
A loose hub or a worn key looks like looseness in the readings. Mark the shaft and hub with a paint line and rotate. A worn key shows as a small step. Bolted or taper-lock hubs need the correct torque and a clean taper.
Offset couplings and spacer shafts
With a spacer, the spacer length lets you hit the measuring distance more easily, but the effect on angular error is higher because there are two flexible points. Align the machines on both sides of the spacer to their own tolerances.
Questions
What is backlash in a coupling?
The free movement between coupling halves, common in gear, jaw and chain couplings. It makes readings wander if only one shaft turns.
How do I measure runout?
Mount a dial indicator on the shaft surface or hub, rotate slowly and note the total swing. If it exceeds about a third of the alignment tolerance, fix it first.
Do stiff couplings need tighter alignment?
Often yes. They cannot absorb much misalignment, so the load goes straight into the shafts and bearings. Use the maker limit and check after bolting up.
Can I align with only part of a shaft rotation?
Some laser tools can, with as little as about 60 degrees. It is useful when a guard or a pipe stops a full turn, but you need good spread between the three positions.
General good practice for horizontal machines driven through a coupling. The machine maker's manual, the coupling data and your site standard come first. Lock out and isolate the machine before any alignment work.