Couplings

Couplings join two shafts to transmit torque. Learn rigid (flange, sleeve) and flexible couplings (jaw, disc, gear, bellows, Oldham, universal joints), misalignment, keys and bolt sizing, service factors and selection.

Motors rarely line up perfectly with the pumps, gearboxes or fans they drive. Couplings connect the shafts, transmit torque, and — if flexible — tolerate small misalignments, damp shocks and protect equipment.

Choosing and sizing a coupling uses torque, speed, misalignment, and the service conditions of the driven machine.

An analogy

A coupling is like the joint between two train carriages: it transmits the pull but lets the carriages move slightly relative to each other.

Rigid vs flexible

Rigid couplings (sleeve, flange, clamp) need accurate alignment and are used where shafts are well supported, like line shafts. Flexible couplings accommodate angular, parallel and axial misalignment and are the default for motor-driven machines.

Flexible coupling types

Jaw (elastomer spider, damps shock), tyre/elastomeric (large misalignment), disc/diaphragm (no lubrication, torsionally stiff — pumps, compressors), gear (very high torque), bellows and beam (servo motion, zero backlash), Oldham (parallel offset), universal/Cardan joints (large angles — drive shafts; use pairs to cancel speed fluctuation).

Service factors

Catalogue ratings are multiplied by service factors for the driven load: ≈ 1.0 for uniform (fans, centrifugal pumps), 1.5 for moderate shock (conveyors), 2–3 for heavy shock (crushers, reciprocating compressors).

Alignment and torsional dynamics

Laser alignment to within ~0.05 mm greatly extends bearing and seal life. Coupling torsional stiffness shifts the drivetrain's torsional natural frequencies; soft couplings can detune resonances with engines, while stiff ones are needed for servo accuracy.

Formulas

• Design torque: T_design = SF · T_nominal
• Flange bolts (shear): F_bolt = 2T / (n · D_b), τ = F_bolt / A_bolt
• Key in shear: L = 2T / (d · b · τ_allow)
• Key crushing (bearing): L = 4T / (d · h · σ_c,allow)

Worked example: Flange coupling bolts

Given: T = 500 N·m; 4 bolts on 100 mm PCD; τ_allow = 60 MPa

Find: Bolt force and size.

• F = 2 × 500 000/(4 × 100) = 2500 N
• A = 2500/60 = 41.7 mm² → d = 7.3 mm
• Choose M10 fitted bolts

Answer: 2.5 kN per bolt → M10

Worked example: Key length

Given: T = 500 N·m, shaft 40 mm; Key 12 × 8 mm, τ_allow = 60, σ_c = 120 MPa

Find: Required length.

• Shear: L = 2 × 500 000/(40 × 12 × 60) = 34.7 mm
• Crushing: L = 4 × 500 000/(40 × 8 × 120) = 52.1 mm (governs)

Answer: Key ≥ 52 mm → use 56 mm

Common mistakes

• Wrong: Relying on a flexible coupling to fix bad alignment. — Right: Align properly — misalignment still loads bearings and wears the element.
• Wrong: Ignoring shock loads. — Right: Apply the appropriate service factor.

Applications

• Pumps & fans: Jaw, tyre and disc couplings with spacers for seal maintenance.
• Servo axes: Bellows and disc couplings with zero backlash.
• Vehicle drivelines: Universal joints and CV joints in propeller and drive shafts.

Key takeaways

• Rigid for aligned shafts, flexible for motor drives.
• Size with design torque = SF × nominal.
• Check keys for shear and crushing; bolts for shear.
• Align carefully; couplings don't cure misalignment.

Going further

• Fluid couplings: Hydrodynamic couplings give soft starts and overload protection for conveyors and crushers.
• Torque limiters: Slip or ball-detent couplings disconnect on overload to protect machinery.
• Spline connections: Involute splines carry higher torque than keys and allow axial sliding.