Coupling Types
Spacer-type coupling
A spacer adds nothing to torque capacity and nothing to angular capacity per mesh. What it adds is distance between the two meshes — and offset capacity is angular capacity multiplied by that distance. It is the cheapest way to buy alignment tolerance, and it is charged for in overhung moment.
- Author
- Priyansh Thummar, Editor
- Dates
- Published · Last updated
- Reading time
- 3 minutes
1. What a spacer actually changes
A spacer coupling is a double-engagement gear coupling with distance between its two meshes. That is the whole of the difference — worth saying plainly, because the name suggests a different product rather than the same product with one dimension changed.
It does not change torque capacity. Torque passes through the same two meshes, the same tooth count and the same geometry, so the rating comes from the size exactly as it does on a compact coupling. Angular capacity per mesh does not improve either.
2. The offset arithmetic
Parallel offset is not an independent property of a coupling. It is what two angular deflections in opposite directions produce when the points they act about are separated by a distance — so the offset available is the per-mesh angle multiplied by the separation.
emax ≈ L · tan αmax
3. BSE is a dimension you must supply
Between shaft ends: the distance from the face of one shaft to the face of the other. It has to be on the order because it cannot be derived from anything else — two identical machines on two different baseplates present entirely different separations, and the drawing is frequently not what got built.
Measure it on the installation rather than taking it from a general arrangement. The installed position is the one the coupling has to fit.
4. The other reason: withdrawing the pump
On pump trains this is usually the dominant reason, and it has nothing to do with alignment. A spacer long enough to be removed lets the pump rotating element be withdrawn without disturbing the driver or breaking the pipework — the back pull-out arrangement process pump standards are built around.
Where that is the requirement, the length is set by the maintenance envelope rather than by any misalignment calculation, and the offset capacity that comes with it is a side benefit.
5. What length charges for
| Property | Effect of more length | Why |
|---|---|---|
| Torque rating | Unchanged | Same meshes, same tooth geometry |
| Angular capacity per mesh | Unchanged | Set by size and speed, not by span |
| Parallel offset capacity | Increases, roughly in proportion | Offset = angle × separation |
| Overhung moment on the bearings | Increases — the real bill | Mass held further from the bearing |
| Rotational inertia | Increases | More mass for the driver to accelerate |
| Torsional natural frequency | Falls | Can move a resonance into the operating range, not out |
| Lateral critical speed of the span | Falls | Beyond a point, use a floating shaft instead |
Beyond a certain span the centre section has its own lateral critical speed, which is why long distances move to a floating shaft with a coupling at each end rather than to an ever-longer spacer.
Have the BSE and need sizes? Super Mech Industries' spacer coupling range — this page is the dimension and the arithmetic behind it.
Frequently asked
- Does a spacer coupling transmit less torque?
- No. Torque still passes through the same two meshes and the same tooth geometry, so the rating is set by the size rather than by the length between them. What changes with length is the bending the centre section imposes on the adjacent bearings, and the torsional behaviour of the train — not the torque the teeth can carry.
- What is BSE and why does the supplier keep asking for it?
- Between shaft ends: the distance from the face of one shaft to the face of the other. It is not derivable from anything else on the order — two identical machines on two different baseplates can have entirely different shaft separations — so a coupling cannot be built without it. Measure it, do not calculate it from drawings, because the installed position is what matters.
- How much more misalignment does a spacer allow?
- Parallel offset scales roughly with the distance between the two mesh centres, because offset is what two opposed angular deflections produce across a span. Doubling the separation roughly doubles the offset available at the same per-mesh angle. Angular capacity itself does not improve — each mesh still articulates by whatever its size and speed permit.
- Can I fit a longer spacer to fix an alignment problem?
- It will absorb more offset, and on a train that has settled it is sometimes the pragmatic answer. It is not a repair. The extra length increases the overhung moment on the bearings either side and shifts the torsional natural frequency of the train, so a spacer fitted to mask a foundation or pipe-strain problem moves the load somewhere less visible rather than removing it.
References
- AGMA 9008-B00, Flexible Couplings — Gear Type — Flange Dimensions, Inch Series, for the flange interface the centre section bolts to.
- ISO 14691:2008, Petroleum, petrochemical and natural gas industries — Flexible couplings for mechanical power transmission — General-purpose applications.
- API 610, Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries, for the back pull-out arrangement that sets spacer length on process pump trains.
- Editorial note: no spacer lengths, offset capacities or overhung moment limits are published here. Each is a property of a specific size and a specific bearing arrangement, and a length copied from another installation is the error §3 exists to prevent. Standard BSE options and per-size capacities are to be added from Super Mech Industries catalogue data and confirmed at technical review.
Related articles
Selection & Sizing
Misalignment capacity
Where the per-mesh angular figure comes from in the first place.
Coupling Types
Double engagement in section
The two meshes a spacer is separating.
Maintenance
Shaft alignment
The tolerance a spacer is buying you room against.