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Shaft Couplings and Joints - Compensation of Angular and Radial Offset
In drive technology, shaft couplings and shaft joints are essential for transferring torque between two shafts. If the shafts are not exactly aligned with each other or if vibrations and sudden load changes occur in the system, compliant shaft couplings or shaft joints are used. This article explains the difference between shaft joints and flexible or compliant shaft couplings, what each component can be used for, and what variants there are.
Basic information on compliant shaft couplings
Compliant shaft couplings, often referred to as compensation couplings, are used as reliably operating, torque-transmitting elements with a compensating function. Unlike rigid connections that do not tolerate shaft offsets, compliant couplings can partially compensate for axial, radial, and angular variations. The possibilities of compensation here depend on their design and construction. With the elastic coupling, this is done by means of various technical solutions, such as damping shocks and vibrations using elastic or movable elements. At the same time, reliable torque transmission is guaranteed. Flexible shaft couplings often rely on elastic elements for functional performance.
Definition of terms: Flexible shaft coupling and shaft joint
Both shaft couplings and shaft joints are used to transfer torques and rotational movements between shafts. So what is the difference? Shaft joints, unlike flexible shaft couplings, can also compensate for larger angular deviations in shafts that are not in line. The angle deviations can be above 45°. Many compliant shaft couplings can also compensate for angular deviations, but depending on the design of the flexible coupling, the compensation is only possible up to a maximum of 5°. The following use cases arise:
MISUMI shaft joints differentiate between permissible torque and static failure torque, as elastic materials wear out faster under shock-like loads and vibrations. The allowable torque is the maximum torque that a shaft joint can safely transfer under normal conditions, i.e. without damage and excessive wear. The static failure torque, on the other hand, is the load limit at which the shaft coupling fails (e.g. by deforming or breaking). Shaft couplings and shaft joints should only be used within the permissible torque range. The failure moment is to be understood only as an additional indication and for determining mechanical safety.
Variants of flexible shaft couplings and shaft joints and their applications
Depending on the design, compliant couplings can also take on compensation functions such as axial offset, radial offset, or angular offset. These compensation couplings are also commonly referred to as flexible shaft couplings. A key difference between resilient, elastic shaft couplings and rigid shaft couplings is the use of elastic elements. These are often made of polyurethane or rubber of varying hardness. An elastomeric insert is a damping element and provides shock and vibration dampening torque transmission. It is suitable for use with low dynamic loads, yet high torque transmission. In our article Details of Elastomer Inserts for Couplings, you can learn about the many forms of elastomer inserts. However, elastic shaft couplings do not have to be fitted with elastic elements. But what variants of flexible shaft couplings are available? The following is an overview of various shaft couplings available from MISUMI:
The star tooth coupling has a modular design and consists of two hubs with claws and a star-shaped elastomer insert. The example CPB is characterized by a maximum 2° angular offset and 0.2 mm radial offset. Thanks to its modular design, the star tooth coupling can withstand high loads. Possible applications are conveyor systems and compressors.
Metal bellows couplings are among the torsionally rigid shaft couplings. They transmit torque accurately and virtually without twisting. Unlike rigid couplings, they can reliably compensate for axial, radial, and angular shaft offsets to some extent. They are ideal for applications with high positioning accuracy requirements. Their high torsional rigidity ensures that no significant twisting occurs even during dynamic load changes. The connection to the shaft is usually made via backlash-free clamping hubs, which ensures easy assembly and a secure hold.
In the spring coupling, two hubs are connected by a spring. The spring allows for use at particularly high angles of rotation. The example FKSZS 1225 has a maximum radial offset of 0.5 mm and a maximum angular offset of 5°. Spring couplings are highly vibration-absorbing and resistant to stress peaks. Spring couplings are highly elastic. Possible applications are printing machines and automation technology.
The bolt coupling connects two flanges together. On one flange there are bolts with rubber elements that engage in holes in the second flange. With a size of 110 mm and a speed of 250 rpm, the coupling in our example has a radial offset of up to 0.5 mm; the angular offset is up to 0.2° and also depends on size and speed. Potential applications include large pumps, conveyor systems, and power drives.
A loop coupling is a flexible shaft coupling consisting of a loop-shaped plastic element and two integrated steel hubs. Its mode of operation is based on the ability of the elastic loop material to compensate for axial, radial, and angular shaft offsets, allowing vibration- and shock-damping torque transmission. The example DKPS 2928 has a radial offset of max. 2 mm and an angular offset of max. 10°. Possible applications are: Control technology, measurement technology.
In a chain clutch, two single-row sprockets are mounted on the shafts and connected by a double chain. In the event of maintenance, the shafts can be easily separated by loosening the chains. Chain couplings are less suitable for shock loading. For more information on chains and sprockets, see the article Chains and sprockets: How they work together. The permissible misalignment of the chain coupling selected here as an example is 0.5° or less for the alpha angle and 1% or less of the chain pitch for the lateral omega angle. Possible applications are mixing systems and conveyor belts.
Other variants of flexible and also rigid couplings, such as the jaw coupling, slit coupling, bellows coupling and disc coupling, are presented in our article Shaft couplings - Basics and application areas. An overview of the selection of shaft couplings in general can also be found in our article "Shaft Couplings - Selection Procedure by Motor, Torque, Torsion and Mounting Method".
Overview of Shaft Joints
Double shaft joints and also so-called Drive shafts transfer torque between spatially offset drives and outputs. MISUMI shaft joints are rated for operating angles up to 30°. Single joints are not suitable for use with shaft offsets. The difference between shaft joints and drive shafts is the application: Shaft joints are used at lower to medium torques, drive shafts at very high torques and severe conditions.
This article is limited to viewing shaft joints as an alternative to shaft couplings. For more information on different types of shafts, see the article Shaft types in mechanical engineering – Linear shafts, rotary shafts, rods and how to make the right choice.
There are also different versions of shaft joints. Single shaft joints are suitable for angles up to 45°, whereas specially designed double shaft joints can be used for angles up to 90°. In the event that shaft ends are axially fixed or the length of the shaft changes due to the influence of fluctuating temperatures, extendable shaft joints may also be considered.
Shaft joints are available in many designs. Among other things, they are standardized according to DIN 808 (shaft joints with plain bearings). The standard specifies, for example, tolerance ranges, load capacities, or suitable materials.
There are different options for mounting on the shaft.
- Shaft joint with grub screw: Fastening using continuous set screw (requires continuous cross hole on shaft and additional ring spring clamp)
- Shaft joint with keyway: Fastening with key and locking screws (no cross hole required for the set screw)
- Shaft joint with fixing screw: Fastens directly to the shaft with locking screw
Determine pre-selection and operating characteristics
Several points must be examined in order to check the suitability of a shaft joint for the intended use.
They include:
- the permissible speed in revolutions per minute (1 rpm)
- the allowable operating angle in degrees (°)
- the permissible torque (Nm)s
For simplicity, an operating parameter is calculated here and compared with the maximum permissible operating parameter of the manufacturer. The calculated operating characteristic must be lower than the maximum value provided by the manufacturer.
Operating\:characteristic = Speed \times Angle(^{\circ}) \times Torque
Operating\:characteristic_{permissible} > Operating\:characteristic
The speed provided by the drive must also be considered and compared to the maximum permissible speed, taking into account the operating angle. The speed determined with the angle factor must be less than the permissible speed.
Speed_{Permissible} > Speed \times Angle\:factor
The following table provides an overview of the various angle factors for MISUMI UNCA, UNCW, UNKA and UNKW series joints depending on the angle:
| Angle | > 0° ≤ 5° |
> 5° ≤ 10° |
> 10° ≤ 15° |
> 15° ≤ 20° |
> 20° ≤ 25° |
> 25° ≤ 30° |
|---|---|---|---|---|---|---|
| Angle factor | 1.00 | 1.05 | 1.18 | 1.43 | 1.82 | 2.50 |
Benefits of Elastic Couplings and Shaft Joints
The key advantage of shaft joints and flexible shaft couplings is that they compensate for offsets between two shafts. Flexible shaft couplings also dampen vibration and shock by using elastic elements. However, torque capacity is limited by the use of flexible materials, making them less suitable for high-performance applications. Shaft joints are different: They are usually mechanically robust and can withstand high loads. However, without compensating mechanisms, they sometimes cause unequal rotational movements, which in turn cause vibration and increased wear. For more information on damping elements, see the article Damping materials in mechanics and construction: ideas, applications and advantages.
Assembly Notes
When installing shaft joints and flexible shaft couplings, the permissible offset tolerances must be observed. In addition to general requirements for couplings, DIN 740-2 also defines permissible tolerances for the various types of offsets axial, radial and angular. DIN 808 provides possible angular offsets for shaft joints. Exceeding the offset values can also lead to increased wear on shaft joints and flexible shaft couplings, therefore the correspondingly defined limits should be taken into account.
