Couplings in Mechanical Engineering: Types and Applications

Couplings are essential components for connecting two shafts in drive systems. They reliably transmit torque while allowing compensation for manufacturing and assembly tolerances. In mechanical engineering, different coupling types are used depending on the application – from rigid couplings for maximum precision to switchable couplings for variable torque limiting.

This guide introduces the most important coupling types, explains their technical properties, and shows which coupling is suitable for which application.

Torque Sizing: Rated Torque and Service Factor

A coupling is selected based on the torque to be transmitted. The rated torque follows from power and speed:

For sizing purposes the rated torque is multiplied by a service/shock factor S_B, which covers peak loads, start-ups, and shocks:

Rigid Couplings

Rigid couplings connect two shafts without play or elasticity. They require the highest alignment accuracy, but offer maximum stiffness and minimal wear when properly aligned.

Flange Coupling

Flange couplings consist of two flange halves connected by bolts. They offer maximum stiffness; rigid flange couplings are standardised according to DIN 116 (disc couplings) — DIN 740 applies exclusively to flexible (elastic) shaft couplings. Flange couplings are used in high-performance drives (pumps, compressors) and transmit torque rigidly without misalignment compensation. They are more cost-effective in mass production and enable high torques of up to several kNm.

Flexible Couplings

Flexible couplings combine torque transmission with misalignment compensation and vibration damping. They use elastic elements (springs, elastomers) and are therefore better suited for dynamically loaded drives than rigid couplings.

Jaw Coupling (Elastomer Coupling)

The jaw coupling consists of two opposing coupling halves with elastomeric inserts (usually polyurethane or rubber). The elastomer layer allows limited misalignment compensation in the radial and axial directions. Due to their robustness and low price, jaw couplings are widely used in conveyor technology, valve drives, and general mechanical engineering. They can accommodate shaft deflections of up to approx. 0.2 mm but are not suitable for zero-backlash applications.

Bellows Coupling

The bellows coupling offers zero-backlash or reduced-backlash torque transmission with torsional backlash of less than 1 arcmin. It consists of two coupling halves and a flexible bellows that compensates for radial, axial, and angular misalignment. Bellows couplings are ideal for precision drives, robot wrist axes, and positioning drives. The high torsional stiffness makes them the standard in automation technology. Disadvantages: They are more expensive than jaw couplings and require a clean environment.

Jaw Coupling with Elastomer Insert

This variant combines the robustness of the jaw coupling with elastomeric inserts for misalignment compensation. The elastomeric material absorbs shock loads and reduces vibrations. It is more cost-effective than pure bellows couplings but offers only limited backlash tolerance (approx. 3–10 arcmin) and misalignment compensation. Application: General mechanical engineering drives with moderate zero-backlash requirements.

Spring Coupling

Spring couplings use disc springs or coil springs for torque transmission. They offer high flexibility in misalignment compensation, but are not zero-backlash and have lower torsional stiffness. They are used in drives with larger misalignments and shock loads (e.g., combustion engines, coupling between gearbox and drive motor).

Torsionally Stiff Couplings

Torsionally stiff couplings are characterized by their stiffness against torsion. High torsional stiffness is critical for precise positioning and control in automation systems.

Metal Bellows Coupling

Metal bellows couplings consist of a folded metal bellows (usually stainless steel) that enables torsional backlash of less than 0.5 arcmin. They are zero-backlash, highly stiff, and function even at extreme temperatures. The metal bellows is maintenance-free and chemical-resistant. Application: High-precision robots, medical technology, measuring instruments.

Disc Spring Coupling

Disc spring couplings use several stacked steel discs to elastically transmit torque. They offer excellent damping of torque fluctuations and are less sensitive to contamination than bellows couplings. The torsional stiffness is higher than with elastomeric couplings. They are used in high-load drives (pumps, generators).

Switchable Couplings

Switchable couplings can be engaged or disengaged under load and often serve for torque limiting and safety.

Magnetic Coupling

An important distinction applies here: Electromagnetic clutches are switched by a control current and allow remote engagement and disengagement, for example in presses or elevators. Permanent magnet couplings, by contrast, operate entirely without control energy: permanent magnets transmit torque contactlessly across an air gap — hermetically sealed with a containment shell, for example in pumps handling aggressive media. Under overload, the coupling slips or breaks away, acting as a wear-free overload protection. Learn more: PMKC permanent magnet couplings from TEA.

Overrunning Clutch (Freewheel)

An overrunning clutch is an automatic switching device that transmits torque in one direction only. When the direction of rotation reverses or the motor runs faster than the output, the overrunning clutch disengages automatically. Application: Safety applications, elevators, hoists.

Coupling Types Comparison Table

Selection Criteria for the Right Coupling

1. Torque (M)

The transmittable torque is the primary selection criterion. For jaw couplings, the maximum torque is typically between 10 and 500 Nm, for flange couplings up to 1,000 Nm and above. Size the coupling to 1.3 to 1.5 times the continuous drive torque to account for shock loads.

2. Misalignment Tolerances

Manufacturing and assembly tolerances lead to radial (0.1–1.0 mm), axial (0.2–2.0 mm), and angular misalignments (0.5–2.0°). Flexible couplings compensate for these misalignments and reduce bearing loads. Rigid couplings require precise alignment.

3. Speed and Centrifugal Forces

At high speeds (n > 5,000 rpm), centrifugal forces become relevant. More massive couplings must be designed for larger diameters. The dynamic load Ω² × D increases quadratically with speed – therefore, light and compact designs should be preferred at high speeds (e.g., bellows couplings instead of jaw couplings).

4. Zero-Backlash Requirements

For positioning drives and robots (requirement: torsional backlash <1 arcmin), bellows couplings are required. For valve drives and conveyor technology, jaw couplings or simple elastomeric couplings are sufficient. Torsional backlash is measured in arcminutes or degrees: 1° = 60 arcmin. High-precision applications require metal bellows couplings with <0.5 arcmin.

5. Temperature Range

Elastomeric couplings are usually limited to -20 to +80 °C. At extreme temperatures or near open flames, metal bellows couplings or flange couplings should be used. Check the material certificates and operating instructions of the manufacturer.