7.4 Couplings: Types, Selection, and Installation
Key Takeaways
- A coupling transmits torque, accommodates residual misalignment, allows axial movement, and in some designs acts as a mechanical fuse.
- A rigid coupling transmits no misalignment tolerance at all, so it demands near-perfect alignment and is used mainly on vertical pumps and line shafts.
- Gear and grid couplings require lubrication, while disc and diaphragm couplings are dry and need none.
- A flexible coupling reduces the consequences of misalignment but never eliminates the need to align the machine.
- A key must fit snugly on its sides and have a small clearance at the top, because the key drives on its flanks and not on its top face.
What a coupling is for
A coupling has up to four jobs:
- Transmit torque from driver to driven equipment.
- Accommodate residual misalignment that remains after the machine is aligned as well as practical.
- Permit axial movement as shafts grow with temperature.
- Act as a fuse in some designs, failing in a controlled way before the machine does.
Notice what is missing from that list: a flexible coupling does not correct misalignment. It survives a small amount of it. Every degree of misalignment a coupling absorbs is converted into cyclic load on the bearings and seals on both sides. The Chapter 9 alignment work is what actually protects the machine.
Rigid couplings
| Type | Description |
|---|---|
| Sleeve (muff) | A cylinder over both shaft ends, keyed and pinned or clamped |
| Flanged (compression) | Two flanged hubs bolted face to face |
| Ribbed / clamp (split) | A split cylinder clamped over both shafts; removable without moving the machines |
A rigid coupling makes the two shafts into one shaft. It transmits torque with no backlash and no wear parts, but it transmits all misalignment straight into the bearings. It belongs on vertical pump shafts, line shafts, and machines that are precision bored to run as a single rotor — and nowhere else.
Flexible couplings
| Family | Examples | Lubrication | Character |
|---|---|---|---|
| Elastomeric | Jaw-and-spider, tire, sleeve (donut), pin-and-bushing | None | Damps shock and torsional vibration; element wears and is the fuse |
| Metallic, lubricated | Gear, grid (spring) | Required | Very high torque density; misalignment taken by sliding teeth or grid flexure |
| Metallic, dry | Disc, diaphragm | None | High speed, no wear parts, no backlash; low misalignment tolerance |
| Fluid | Hydraulic coupling | Fill fluid | Soft start and torque limiting |
Notes that generate items:
- A gear coupling transmits torque through mating external and internal teeth that must slide to accommodate misalignment. Loss of lubricant is the dominant failure mode; a dry gear coupling wears its teeth away rapidly and then locks up, transmitting full misalignment load to the bearings.
- A grid coupling uses a serpentine spring steel grid in tapered slots. It damps shock and is repairable in place by replacing the grid.
- A disc coupling flexes thin metallic discs. It is dry, has zero backlash, and is common on high-speed machines, but it has the least misalignment tolerance of the flexible family and is unforgiving of a sloppy alignment.
- Jaw couplings with an elastomeric spider are the workhorse on small pumps. The spider hardness determines torsional stiffness, and a spider that is crumbling is telling you about misalignment, overload, or chemical attack.
Hub fits and keys
Coupling hubs are normally an interference fit on the shaft. Install by heating the hub — induction heater or oven, within the manufacturer's temperature limit — and sliding it into position in one motion. Some hubs are taper bored and set with a hydraulic nut or a measured drive-up; the drive-up dimension is specified by the manufacturer and controls the interference.
Keys transmit the torque and must be fitted:
- The key fits snugly against the sides of the keyway in both shaft and hub, because the key drives on its flanks.
- The key must have a small clearance at the top — typically on the order of a few thousandths of an inch — so it does not bottom out in the hub keyway and lift the hub off its fit.
- The key must be shorter than the keyway in the hub so it does not protrude and interfere.
- Key ends and corners should be chamfered or radiused to match the keyway fillet and avoid a stress raiser.
- Common types: square and flat (rectangular) parallel keys; gib-head keys for removal; Woodruff keys, which are semicircular and self-aligning on tapered shafts; and taper keys driven to lock.
A hub that spins on the shaft after a key failure almost always has a fretted bore and a rolled keyway, and both shaft and hub need inspection — not just a new key.
Setting the distance between shaft ends
Every flexible coupling has a specified distance between shaft ends (DBSE) or coupling gap. It must be set correctly because:
- Too small, and thermal growth drives the hubs together and pushes a thrust load into the bearings.
- Too large, and the flexible elements are extended beyond their design and the fit of the spacer is lost.
Measure the gap with a feeler gauge or inside micrometer at several points around the circumference. Uneven readings around the circumference indicate angular misalignment, which is measured properly in Chapter 9.
Installation checklist before alignment
- Shaft ends clean, deburred, and measured; keyways free of raised edges.
- Hubs installed to the correct axial position and fully seated.
- Keys fitted with side contact and top clearance.
- Set screws torqued where used, on the flat of the key or the shaft as specified.
- Coupling gap set and verified around the full circumference.
- Guard available and correct — a rotating coupling is an exposed hazard, and a missing guard is a stop-work condition.
- Only then begin the alignment procedure.
A gear coupling on a large blower is found with its teeth worn to knife edges after 14 months. What is the most likely cause?
How should a parallel key fit in a coupling hub keyway?
Which statement about flexible couplings is correct?