Sizing a coupling is usually described as picking a torque rating off a table. In practice most sizing mistakes are not in the table at all - they are in deciding which torque to look up, and in assuming the bore range on the drawing is the bore range you can actually use.
Three numbers settle almost every coupling sizing question: the torque you size on, the bore you can really bore to, and the speed the coupling will see. Here is how each one is worked out.
Which torque you actually size on
A coupling has a continuous torque rating and a peak rating, and the difference between them matters more than the numbers themselves.
Continuous is what the coupling can carry all day. Peak is what it survives briefly - a hard reversal, a rapid deceleration, a jam. Sizing on the motor's continuous torque alone leaves nothing for those moments, and on a machining centre they happen on every tool change.
The usual method is to take the peak torque the axis will produce and apply a service factor. A machine running one direction under steady load needs a small factor. An axis that reverses hard, or drives a load with backlash and shock in it, needs a larger one.
| Duty | Character of the load | Size on |
|---|---|---|
| Steady, one direction | Pump, fan, conveyor | Running torque with a modest margin |
| Frequent reversing | CNC positioning axis, tapping cycles | Peak torque plus a service factor |
| Shock loading | Cutting with interrupted engagement | Peak torque with a larger factor again |
| Behind a reducer | Motor drives a gearbox, coupling on the output | The reducer's output torque, not the motor's |
That last row catches people out regularly. A small motor through a large reduction produces a large output torque, and a coupling sized from the motor nameplate will be badly undersized for the shaft it is actually connected to.
Bore: the range is the outer limit, not the target
A coupling datasheet lists a bore range, which is the smallest and largest hole the manufacturer will put in it. Both ends of that range come with a caveat.
At the large end, boring close to the maximum leaves a thin hub wall. That is where cracks begin, and it is also where the clamping screw has less material to work against. If your shaft only fits the coupling at its maximum bore, take the next size up.
At the small end, a coupling bored far below its maximum has more hub material than it needs, which is harmless for strength but wastes mass and inertia on a servo axis. On a high-speed axis that mass is a real cost.
Before ordering a bored coupling, confirm:
1. Both shaft diameters measured with a micrometer, not assumed from the drawing - a worn shaft is smaller than its nominal size.
2. Whether either shaft is keyed. A keyway has to be cut before the hub is hardened, not after.
3. The distance between the shaft ends, so the coupling's overall length is right for the gap.
Speed limits and balance
Every coupling has a maximum speed, and it is usually set by one of two things: the material being thrown outward, or balance.
On a jaw coupling the limit is normally the elastomer - above a certain speed it starts to lift. On a metal-disc coupling the limit is balance, because the discs and bolts add mass away from the centreline.
For most CNC axes running a ball screw at a few hundred to a couple of thousand rpm, this is not the constraint. It becomes the constraint on spindle drives and on any axis that has been geared to run fast. If the axis runs fast, ask for the balance grade as well as the speed rating.
Inertia, the number nobody asks about
On a servo axis, the coupling's inertia adds to the load the motor has to accelerate. It is small compared with the screw and the table, but on a short, fast axis with a small motor it stops being negligible.
The practical rule is to keep coupling inertia low relative to the motor's own inertia, and to prefer a smaller, stiffer coupling over a larger, more forgiving one when the axis is fast and lightly loaded. A big coupling chosen for its torque capacity can make a fast axis harder to tune.
This is also the reason aluminium couplings are common on servo axes and steel ones on heavier drives: less mass at the same torque, at the cost of some strength.
The four numbers to send
Shaft diameter each side, the torque you are sizing on, the top speed, and the misalignment you measured. With those, the sizing stops being a guess - and if the coupling that fits the torque does not fit the bore, we will say so rather than bore it past its limit.
We bore couplings to your shaft sizes in aluminium and steel across the shaft coupling range, including rigid and steel types for heavier drives on the steel shaft coupler page.
Send the four numbers through this form and we will come back with a size rather than a range.
