Basic knowledge

The coupling - at the heart of the powertrain

| Author / Editor: Andreas Mühlbauer / Theresa Knell

Special couplings and clutches are available for a wide variety of applications. Their common feature is the transmission of rotary movements.
Special couplings and clutches are available for a wide variety of applications. Their common feature is the transmission of rotary movements. (Source: SKF Group)

Couplings as construction elements: Couplings and clutches are components for the transmission of rotary movements. Amongst others, the compensate for shaft misalignment and dampen vibrations.

Couplings are generally used when rotations are transmitted from one component to another within the drive train. The task of shaft couplings is to connect the motor to a gear or a machining element and in some cases to transfer the movement to another shaft. On the one hand, they transmit the rotary motion and the torque.

Tasks of the coupling - technical requirements and design variants

  • Protection of individual components: The coupling limits or interrupts the maximum permissible torque
  • Damping of vibrations: Couplings dampen vibrations between individual machines
  • Electrical insulator: In generator systems, the machine element acts as an electrical insulator
  • Mounting of attachments: Couplings are used to accommodate attachments such as a brake drum

Couplings for compensating shaft misalignment

If the reset forces are not too high, the shaft coupling compensates for a minor shaft misalignment - an important feature for the installation of a plant and for error avoidance. The shaft misalignment is caused by assembly or operation and leads to an unnecessarily high stress exerted on the shaft bearing when two components are rigidly connected. The precise alignment of components is often not sufficient or simply not possible to take the load of these bearings. For example, the thermal deformation of the machine during operation causes the alignment of the shafts to differ from that during the switched-off state. When high loads are exerted on the bearings during operation, the resulting reset forces must also be taken into account when using couplings.

The difference between clutches and couplings

There are many different types of couplings and clutches. They are divided into different categories. A basic distinction is made between clutches that can be engaged or disengaged and couplings that cannot be engaged or disengaged.

Examples of switchable clutches

Safety clutches separate both sides of the drive train in the event of overload and thus protect the machines from damage.
Safety clutches separate both sides of the drive train in the event of overload and thus protect the machines from damage. (Source: Wittenstein)

Switchable clutches are used to interrupt or engage a drive. Depending on the type, these clutches can be switched during operation or at standstill. Switchable clutches include the safety clutches, which serve to protect people or machines:

  • Torque-dependent slip clutch: They limit the maximum torque transmitted, for example by overcoming static friction. Another variant is to disconnect the drive train when a certain torque is exceeded.
  • One-way clutch (dependent on the direction of rotation): Freewheel runs directionally, i.e. it transmits torque in one direction of rotation.
  • Speed-dependent centrifugal clutch: The centrifugal clutch engages depending on the speed. At a certain, safety-relevant speed, the rotation is interrupted.

Examples of non-switchable couplings

Non-switchable couplings differ between rigid, torsionally rigid and flexible couplings. They play the most important role in industrial applications:

Rigid couplings without compensation: They connect two shafts with each other. Accordingly, they are not suitable for compensating radial or angular misalignment of the shafts. These are, for example, disc or clamp couplings.

The characteristic feature of multi-plate couplings is the parallel arrangement of several friction linings. The same contact force is applied to all friction pairs. These couplings are frequently used because they are compact and inexpensive.
The characteristic feature of multi-plate couplings is the parallel arrangement of several friction linings. The same contact force is applied to all friction pairs. These couplings are frequently used because they are compact and inexpensive. (Source: SKF)

Torsionally rigid couplings with compensation: They are suitable for angular synchronous transmission of the rotary motion. Depending on the design, torsionally rigid couplings are equipped with one or two gimbal joints to compensate for radial misalignment. Examples of double-gimbal couplings are multi-plate or spring disc couplings.

Metal bellows couplings enable the axially and angularly offset connection of two shafts. They are backlash and wear-free. In automation, for example, they are used as connecting elements for servo motors and stepper motors.
Metal bellows couplings enable the axially and angularly offset connection of two shafts. They are backlash and wear-free. In automation, for example, they are used as connecting elements for servo motors and stepper motors. (Source: Wittenstein)

A metal bellows coupling also meets these requirements. An elastic metal bellows compensates for the difference between the shaft ends. Due to the special design of the flexible metal bellows, this coupling can compensate for a small angular offset. Each of the coupling types mentioned has different characteristics with regard to nominal torque, maximum speed and possible compensation of radial and angular misalignment.

Application examples for torsionally rigid couplings are:

  • Transmission of torque from a servo motor to a machine
  • Connection of rotary encoders
  • Compensation for minor angular misalignments, depending on the design

Flexible couplings with rotation angle compensation: They compensate for large angular or radial misalignments. The flexible coupling is also of great importance as a vibration damping element. The basic structure consists of two hubs connected by an elastic element. Depending on its elasticity, this coupling type can only transmit lower torques. Roughly speaking, the higher the elasticity, the lower the transmittable torque.

Examples of flexible couplings:

Spring coupling: It consists of two hubs connected by a spring. Thus it has a very high maximum angle of rotation between the two shafts. The spring can compensate for large misalignments and is very effective as a vibration damping element.

Elastomer couplings are used in applications where vibrations and shocks have to be absorbed. An elastomer ring between the two hubs provides the damping properties.
Elastomer couplings are used in applications where vibrations and shocks have to be absorbed. An elastomer ring between the two hubs provides the damping properties. (Source: Wittenstein)

Claw coupling or elastomer coupling: There is an elastomeric gear ring located between two clawed hubs. The opposite claws mesh with each other, with the elastic gear ring filling the gaps. Depending on the coupling requirements, the elastomer ring is available in different hardnesses. A claw coupling can transmit significantly higher torques than a spring coupling. However, it is less elastic, which means that it only compensates for smaller offsets.

Choosing the right coupling

The selection of the coupling type depends strongly on the application - shaft misalignment, damping, torque or several of these factors determine the ideal coupling. The manufacturers offer support at any time.

Where can I find the appropriate coupling for my (industrial) application?

A selection of coupling manufacturers:

  • Jakob Antriebstechnik: Shaft couplings (metal bellows and elastomer couplings), safety couplings
  • KBK Antriebstechnik: Metal bellows couplings, elastomer couplings, safety couplings, rigid couplings, spacer couplings
  • KTR Systems: Flexible claw and pin couplings, tooth couplings, shaft couplings, multi-disc couplings, flange couplings, magnetic couplings
  • Mayr Antriebstechnik: Shaft couplings, safety couplings, electromagnetic couplings
  • Orbit Antriebstechnik: rigid couplings, compensating couplings, safety couplings
  • Reich Kupplungen: Shaft couplings, claw couplings, tooth couplings, flange couplings; rigid and flexible couplings
  • Ringspann: Shaft couplings (rigid couplings, compensating couplings), overload couplings
  • Ruland: rigid and flexible couplings, elastomer couplings, bellows couplings, oldham couplings, multi-plate couplings
  • R+W Antriebselemente: Elastomer couplings, metal bellows couplings, safety couplings, cardan shafts
  • Siemens (Flender): rigid couplings (tooth couplings), flexible couplings (claw, cam, pin couplings), hydrodynamic couplings, elastomer couplings, metal bellows couplings
  • VMA Verbindungs- Meß- und Antriebstechnik: Shaft couplings, safety couplings

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Further literature:

  • 1. Wittel, H., Muhs, D and others (2015): Roloff/Matek Maschinenelemente: Normung, Berechnung, Gestaltung. Springer Vieweg, ISBN: 978-3658090814
  • 2. Wolf, T, Miner, P and others (2014): Sicherheits- und Überlastkupplungen. Süddeutscher Verlag onpact, ISBN: 978-3862360673, Die Bibliothek der Technik Band 312
  • 3. Wolf, T, Crimson, A. and others (2006): Präzisionskupplungen und Gelenkwellen. Süddeutscher Verlag onpact, ISBN: 978-3937889481, Die Bibliothek der Technik Band 297

This article was first published by MaschinenMarkt.

Original article by Stefanie Michel

Translation by Alexander Stark

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