Couplings

A coupling connects two shafts to transmit torque. Depending on its construction, it can also compensate for limited misalignment, damp torsional vibration, permit axial motion, protect against overload, or engage in only one direction.

Design torque

Nominal torque follows from power and speed:

$$T=\frac{9550P}{n}.$$

The coupling is selected for design torque

$$T_d=K_AT,$$

where \(K_A\) accounts for duty, starts, shocks, and driven-machine behavior. Peak and reversing torques must be checked separately.

Main types

• Rigid: sleeve and flange couplings; high stiffness but strict alignment requirements.
• Flexible: elastomeric, jaw, pin-bush, gear, grid, disc, bellows, and Oldham couplings.
• Safety: friction or positive-release torque limiters.
• Overrunning and engagement: clutches that transmit conditionally or in one direction.

Alignment and dynamics

Misalignment may be radial, angular, or axial. Catalog limits usually apply individually; combined misalignment reduces capacity. A flexible coupling does not eliminate alignment work—it limits the reaction forces and fatigue damage caused by residual error.

Check keys or splines, hub and shaft strength, bolts, contact pressure, flexible-element fatigue, heat, balance, torsional natural frequencies, and guarding. Select from torque, speed, bore, stiffness, permissible misalignment, environment, maintenance, and fail-safe behavior.

Rigid couplings

Sleeve and flange couplings provide a nearly rigid connection and are appropriate when shafts are accurately aligned and the supporting structure is sufficiently stiff. Flange bolts may transmit torque by fitted shear, bolt shear and bearing, or friction from preload, depending on the design concept.

Elastomeric couplings

Elastomer elements accommodate limited misalignment, reduce shock, and add torsional damping. Their stiffness and allowable torque depend strongly on temperature, frequency, aging, and chemical exposure. Heat generated by cyclic deformation can limit continuous speed or misalignment.

Metallic flexible couplings

Disc and bellows couplings transmit torque through elastic bending of metallic elements and can provide high torsional accuracy with no lubrication. Gear and grid couplings carry high torque and tolerate displacement but require lubrication and inspection. Oldham couplings accommodate parallel offset through sliding and may generate reaction forces and wear.

Misalignment forces

Catalog misalignment limits are not targets for installation. Radial offset, angular error, and axial displacement act together; using one at its maximum often leaves no capacity for the others. The reaction may be estimated from coupling stiffness and included in bearing loads and shaft bending.

Torsional dynamics

The coupling stiffness \(k_t\) and connected inertias determine torsional natural frequencies. For a two-inertia idealization,

$$\omega_n=\sqrt{k_t\left(\frac{1}{J_1}+\frac{1}{J_2}\right)}.$$

A softer coupling can move resonance and reduce shock, but may increase twist and control error. Damping, backlash, nonlinear stiffness, and motor torque pulsation should be considered in critical drives.

Safety and overrunning functions

Torque limiters release or slip at a set torque to protect machinery. Their setting must exceed normal transients yet remain below the weakest protected component. Overrunning clutches transmit torque in one direction or above a relative-speed condition and require checks for engagement energy, wear, heat, and lubrication.

Hub connection and balance

The coupling rating does not cover an inadequate shaft–hub joint. Check keys, splines, interference fits, taper-lock bushes, clamping hubs, hub bursting, shaft fatigue, and axial retention. High-speed couplings require an appropriate balance grade and concentric mounting surfaces.

Selection sequence

1. Determine nominal, peak, reversing, and emergency torque plus speed and duty.
2. Define required torsional stiffness, damping, and permissible positioning error.
3. Estimate residual radial, angular, and axial misalignment.
4. Select coupling type and size and verify combined misalignment and temperature.
5. Check hub joints, fasteners, fatigue, balance, and bearing reactions.
6. Specify alignment procedure, guarding, lubrication, inspection, and replacement criteria.