Couplings and Shaft Couplings - Selection Procedure by Motor, Torque, Torsion and Assembly Procedure

Shaft couplings are central components in drive technology and mechanical engineering because they connect shafts and enable efficient power transmission. The selection of the appropriate shaft coupling, such as e.g., a linear coupling as a non-rotary coupling, can, however, be difficult due to the wide variety of coupling types. This article introduces various selection criteria, such as motor power, required torque, and torsional characteristics, and provides guidance on designing the perfect shaft for your application.

Explanation of shaft couplings

Mechanical couplings are used for the permanent or temporary transmission of torque. The transfer is usually between two aligned shaft ends. In addition to their main function, shaft couplings also compensate for shaft offsets depending on the variant. Couplings can be categorized into switchable and non-switchable couplings. Switchable couplings can be opened and closed in a targeted manner. This is different for non-switchable couplings. Once installed, the shaft ends connected by the coupling cannot be easily separated again. These non-switchable couplings also include shaft couplings. This creates a permanent connection between the shafts.

Non-switchable shaft couplings are further divided into rigid couplings and compliant couplings, such as those with and without compensation. Some compliant shaft couplings allow only slippage, while compliant compensating couplings allow an elastic or torsionally rigid connection depending on the design. Depending on the design, flexible shaft couplings can compensate for radial offset, axial offset, and/or angular offset.

The following graphic provides a detailed overview of non-switchable shaft couplings:

You can also read more about the basics and applications of shaft couplings in our article Shaft couplings – Basics and application areas.

What is important when selecting shaft couplings?

Various technical criteria must be taken into account for the ideal use of a shaft coupling. For a better understanding, the most important terms are explained beforehand.

Definitions

In the context of shaft couplings, some terms are used repeatedly. For a better understanding, here is an explanation of the key terms. A (rotational) moment (M) results from a force (F), which acts perpendicular to the connecting straight line at a distance from the pivot point (r). If this moment acts on a rigid body, it tends to rotate as a result. If the body is prevented from rotating, it is subjected to torsion.

The allowable torque is the maximum possible torque that the shaft coupling can transmit without causing damage. The speed indicates how many revolutions a rotating shaft performs per minute. Similar to the permissible torque, the maximum speed of a shaft coupling must not be exceeded, otherwise damage may occur.

Other terms include:

• Torsional stiffness: Also referred to as a torsion spring constant. It specifies the torque required to twist an object by a certain angle. The torsion spring constant is specified in N*m/rad (Nm per radian). Radian (rad) is an angular unit related to the arc length of a circular section, wherein 1 rad × 180/π corresponds to an angle of approx. 57.3°. Since such a strong twist is not possible in most materials, it is a theoretical value. A value of e.g. 550 Nm/rad means that a torque of 550 Nm is necessary to turn a body by theoretically 1 rad. So the higher the torsional stiffness of a shaft coupling, the less twisting under load it allows.
• Moment of inertia: The moment of inertia reflects the resistance of a body and its mass to a change in its motion. When rotating a stationary shaft coupling or when changing the direction of rotation, the mass of the shaft coupling creates a resistance or an inertia, which is indicated by the moment of inertia. The required torque of the drive increases accordingly with the value of the moment of inertia.
• Tightening torque (especially for clamping screws): The tightening torque describes the force on the screw head that is used to tighten a screw. The tightening torque must not be too low, but also not too high, to ensure that the shaft coupling is firmly seated on the shaft and to prevent damage to the components. Appropriate tables provide information on the ideal tightening torque per screw. You can also find more information in our article Calculation of tightening torques for screws - What role does compliance play?.

Technical Selection Criteria

Technical selection criteria for selecting a shaft coupling may include, for example, the power function or the torque to be transferred, a balancing function, or a required rotational compliance.

If two connected shafts are not ideally aligned with each other, this is called a misalignment. Depending on the type and severity of the deviation, this shaft misalignment can be reliably compensated by a shaft coupling with a compensation function. Non-compensable and undetected misalignments can lead to increased wear and may impair functionality.

There are three types of misalignment:

• Angular offset (Figure 1): The rotation axes of the 2 shafts are not the same, but are at a different angle (a) to each other.
• Radial offset (Figure 2): The axes of rotation are parallel to each other but offset in the radial direction (b) (perpendicular to the axis).
• Axial offset / axial play (Fig. 3):  The shafts shift along their axis; a change in distance (c) between the shaft ends, e.g., due to thermal expansion, is possible.

The use of flexible couplings can compensate for many misalignments, see also our article about flexible shaft couplings and shaft joints.

The rotational compliance of a shaft coupling affects the dynamic properties of a drive system, such as in a multiphase motor shaft coupling, where precise positioning is required. There are rotationally rigid shaft couplings with low rotational compliance, but also elastic shaft couplings with high rotational compliance. Torques can be transmitted almost losslessly with low rotational compliance. High positioning accuracy is achieved. This is relevant, for example, in CNC applications. The situation is different for couplings with high rotational compliance: Here, the focus is on damping and shock mitigation; positioning accuracy is lower. This can be useful, for example, in conveyor systems or in machinery with high dynamics and changing loads. To reduce stress peaks, these elastic couplings often also have additional elastic elements, see also our article Details of elastomer inserts for couplings.

Starting torque and coupling torque to be transmitted

Starting torque is required when starting the motor to overcome inertia and set the machine in motion. The torque transmitted results from the starting torque of the drive, the load torque of the work machine, and other influencing variables such as impacts or inertia. The shaft coupling must transmit this between the drive and the work machine. When operating in reverse, or when starting and stopping frequently, the coupling is subjected to more stress. In such cases, a shaft coupling designed for dynamic load changes is recommended. In this context, the torque-speed characteristic curve of the drive motor also helps in the selection of the appropriate shaft couplings. This characteristic curve shows the dependence of torque on the speed of a motor. As a rule, different characteristic curves are shown, e.g. different limit characteristic curves for cold and hot motors. Overload couplings can also be installed as a safety device. These disconnect the output from the drive once a set limit torque is reached. The overload coupling provides reliable protection against overload damage and failure.

Design of flexible shaft couplings

The easiest way to preselect the appropriate shaft coupling is based on the characteristics of the motor. The following steps and considerations should be taken into account when selecting:

Preselection based on motor type

For simplified selection, the coupling is typically selected based on motor power, torque, and maximum speed, considering a safety factor. However, the suitability must always be checked based on the technical data of the manufacturer and by calculating the forces occurring for the specific application conditions.

The following table provides an overview of possible selection criteria and the characteristics of these criteria for the shaft couplings listed:

Design based on the expected load

Flexible shaft couplings are used particularly if, in addition to transmitting torques, offsets and vibrations or impacts are to be compensated. In practice, however, the required operating data and influencing variables can hardly or only with difficulty be determined by calculation. Therefore, the design is generally performed using a simplified method. For the design, there is a variant with application factor or the design according to the worst-case load type.

The selection using application factors is one of the simplified variants for carrying out a design based on load. In this procedure, which is based on experience, the design of the shaft coupling is carried out using the safety factors (application factors) provided by the manufacturer. The application factor specified by the manufacturer takes into account the type of drive, operation and any additional forces that may occur, such as shock loads or frequent fluctuating loads initiated from the outside. The determined application factor may then be multiplied by the maximum torque of the shaft coupling. The manufacturer often specifies a range for the selection of the respective factor. Here, the amount of the selected factor and the resulting safety reserve must be weighed.

The following applies here:

The more rigid the connection, the more likely and stronger the load peaks are because the coupling does not elastically compensate for them. The factor must therefore become higher to prevent the coupling from failing. The application factor calculation allows for easy calculation in standard applications, but often results in overly high safety factors. Even the torsional vibrations that may occur with a highly uneven drive may not be sufficiently taken into account. Therefore, in these cases, a calculation according to worst-case load type (DIN 740-2) should be used.

DIN 740-2 is used for the variant according to the worst-case load type. The standard provides calculation principles for compliant shaft couplings and takes into account various load types and influences such as frequency, temperature, start-up frequency, etc., to ensure a safe and reliable coupling design. Unfavorable load types result, for example, from increased shock, vibration or temperature load. Extreme temperatures can negatively affect the material properties of the shaft coupling. Excessive shock and vibration loads cause sudden and high peak loads that can lead to damage.

Further steps for the design of shaft couplings

Further steps in the design of shaft couplings are:

• Check the coupling tolerance: Tolerances (angular and radial offsets, and maximum speed) and the specified moment of inertia must meet the requirements of the device.
• Select shaft bore hole: The outer diameter of the connecting shaft must be within the range of the inner diameter of the coupling. If not, select the next higher size.
• Select the shaft connection method: The selection is made according to the clamp used, hub clamping, etc.
• Finally, check: Is the coupling compatible with the device according to the dimensional table?

Installation instructions using the example of a spring washer servo coupling with hub clamping

Careful preparation and precise alignment are essential for the installation of a shaft coupling to ensure reliable operation. However, before assembly can begin, all required parts should be checked for completeness and correct dimensions. Switch off the machine and secure it from unexpected start-up.

• Before installing the coupling, first completely loosen the hub clamping screws. Then, thoroughly clean the coupling inner bore and shaft surfaces with a cloth to remove dust, dirt, and oil.
• Place a mark on the shafts for the minimum shaft insertion depth specified by the shaft coupling manufacturer.
• Slide the shaft coupling onto the non-slidable shaft. Be careful not to apply excessive compressive or tensile forces to the spring washers.
• Tighten the clamp screw of the attached coupling side slightly, but do not secure the coupling permanently.
• Align the shaft of the movable unit concentrically with the fixed shaft.
• Carefully insert the shaft end into the shaft coupling without tilting and push it axially into the coupling until the shaft end insertion depth specified by the manufacturer is reached.
• Quickly check the angular and radial offset with the coupling as the base and correct the alignment if necessary.