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Couplings - Rigid couplings
Rigid, non-disengageable couplings are robust, compact, and torsionally rigid. Additionally, their design allows two shafts to be aligned exactly axially with one another. However, rigid couplings do not allow for flexibility, movement, or compensation. If the two shafts cannot be perfectly aligned, the use of a rigid coupling can lead to unwanted bending stresses in the system. In this article, you can read about when it is worthwhile to use a rigid shaft coupling, what advantages they offer, and what different types are available.
Basic information on rigid shaft couplings
Rigid shaft couplings connect two shafts or axes rigidly and securely. They do not compensate for misalignment but ensure precise alignment. Thanks to their robust design, they can transmit high torques and withstand heavy loads despite their compact form. Read more about the basic options for torque transmission in our article Transmission of rotational movements – Basics of couplings. Rigid shaft couplings are among the simplest designs for centering and connecting two shaft ends. A distinction must be made between rigid couplings and flexible shaft couplings, also known as compensating shaft couplings. Depending on the design, these offer the ability to compensate for axial, radial, or angular misalignment. Some types of flexible shaft couplings, such as elastic shaft couplings, also allow for angular misalignment compensation.
Power transmission in a rigid shaft coupling can be achieved through positive locking, friction, or a combination of both.
In a positive-lock rigid shaft coupling, the coupling parts interlock like teeth or claws due to their geometric shape, thereby preventing relative movement. Power is transmitted from the drive shaft to the driven shaft via direct mechanical connections between the two shaft coupling parts mounted on the shaft ends. An example of a positive-lock shaft coupling is a spur-tooth coupling. In these tooth couplings, the teeth of the two coupling halves interlock, ensuring self-centering and a secure fit. Spur-tooth couplings require axial preload for proper operation.
In a friction-type rigid shaft coupling, power is transmitted from one shaft to the other via friction. This can occur between the shaft and the shaft coupling or through components within the coupling itself that are connected by frictional engagement. The connection is established solely by an applied force and the resulting friction. The frictional force is generated, for example, by clamping screws or tension sleeves. This type of connection is suitable for high power transmission but less so for temperature changes, as these can affect the friction characteristics. A typical example is a shell coupling.
Advantages of rigid shaft couplings
Rigid shaft couplings are characterized above all by high torsional stiffness, zero backlash, and excellent torque transmission.
Torsional stiffness means that torque can be transmitted by the shaft coupling without causing any twisting in the coupling itself. This is essential for precise motion control, which is particularly important in robotics and CNC machines. Thanks to their simple and straightforward design, they are usually easy to install. Rigid shaft couplings prevent any compensatory movement, which increases system stability but requires high precision in terms of running stability and alignment of the shafts relative to each other.
For more information on torsion, visit our blog: Torsion: How to understand torsion.
Requirements for the use of rigid shaft couplings
The basic requirement for using rigid shaft couplings is therefore that the connected shafts are aligned with sufficient precision. If the misalignment is too great, it can lead to issues such as imbalance and bending stresses, which have a particularly significant impact at higher speeds. Even minor misalignments can lead to increased wear and premature failure. If the shafts to be connected cannot be properly aligned, flexible shaft couplings should be used. For more information on this topic, please refer to our article "Couplings – Flexible Shaft Couplings and Shaft Joints".
Variations and clamping options
A variant of the rigid shaft coupling is the flanged shaft coupling, also known as a disc coupling. These feature two flanges that are bolted together to ensure a particularly stable connection. Flanged shaft couplings also require two precisely aligned shafts. Flanged shaft couplings are used in applications involving particularly high loads.
Another variant is the two-piece coupling, also known as a shell coupling. Shell couplings are split lengthwise down the middle. Both coupling halves are connected to each other using clamping screws. Two-piece couplings are easy to install and remove, as they are mounted onto the shaft from the outside and do not need to be slid over the shaft. Here, too, aligned shafts are essential.
Couplings can also be equipped with shrink sleeves. These shrink sleeves can be tapered or conical and are positioned between the shaft and the coupling. Axial tightening creates pressure, resulting in a high holding force. The shaft itself is not damaged. This type of coupling is frequently used in applications involving particularly high torques.
Another special type of rigid coupling is the spur gear coupling. In this coupling, form-fitting teeth mesh with one another; these teeth are located on two opposing faces at the ends of the coupling and are connected using screws or bolts. The teeth of the two coupling halves mesh like two gears, thereby creating a positive connection. This variant is also suitable for high torques.
Metal bellows couplings are not typical rigid couplings, but rather torsionally rigid shaft couplings capable of compensating for axial, radial, and angular shaft misalignments. At the same time, they offer high torsional stiffness.
Applications and key selection criteria
The first question to ask before making a selection is, of course, whether misalignment compensation is required or not. Rigid shaft couplings can only be used if no compensation is needed.
Other parameters do not differ significantly from the selection of couplings in general:
- Shaft dimensions: Shaft couplings come in different diameters. The shaft diameter and bore design must match the shaft.
- Torque transmission: The maximum torque of the application, including any load peaks, must be transmitted.
- Speed range: The coupling must be suitable for the maximum speed. Otherwise, overheating and instability will result.
- Operating environment: Additional stress factors such as temperature, humidity, or dust accumulation must be taken into account.
- Maintenance requirements: Rigid couplings are generally low-maintenance. However, regular inspection for damage and looseness is still necessary.
- Design and weight: Weight influences the moment of inertia and should be selected accordingly. The design must fit the existing installation environment.
The shaft ends must also be supported to ensure correct shaft alignment and prevent bending stresses. For an introduction to the selection of shaft couplings in general, we also refer you to our article Couplings and Shaft Couplings - Selection Procedure by Motor, Torque, Torsion and Assembly Procedure.
Limitations of rigid shaft couplings
Rigid shaft couplings generally cannot compensate for shaft misalignments, such as radial or axial misalignment. If the shaft is not aligned within the tolerance requirements, this can lead to bending stresses and increased wear. The rigid connection also means that this type of shaft coupling offers no protection against peak loads or dynamic loads. Impact loads and dynamic load cycles are transmitted directly from one shaft to the other. If additional flexibility, compensation, or mobility is required, rigid shaft couplings are usually unsuitable. In such cases, a flexible coupling with an elastomer insert may be the solution. For more information on elastomer inserts, visit our blog Details of elastomer inserts for couplings.
Installation of rigid shaft couplings
Since rigid shaft couplings can only be used with precisely aligned shafts, it is crucial that the shafts are aligned with the appropriate degree of accuracy. Even minor misalignments can lead to significant additional loads that affect both the coupling and the adjacent shaft sections. Shaft alignment is usually performed using a ruler, dial gauge or a laser. While alignment using a ruler and dial gauge is usually complicated and time-consuming, laser-assisted shaft alignment provides accurate and fast results. A laser beam projects a line along the shaft axis, which can be used to align the shafts. Deviations are detected directly by sensors and can be readjusted. This method is also used for condition monitoring and has the advantage of detecting even minor changes in alignment or vibrations at an early stage.
