Choosing the perfect shaft coupling may not be the first thing that comes to mind when developing a new technical solution. The efficient and reliable connection of two shafts is essential for the reliability of the overall system and to avoid costly reworking during the design. Huco, a leading brand of Altra Industrial Motion Corporation, offers a comprehensive selection of standard and custom couplings for almost all applications.
David Lockett, Managing Director at Huco, explains the criteria to be considered when selecting shaft couplings.
Think about the shaft coupling as early as possible
Although easily overlooked alongside electric motors, gearboxes, clutches, and other major components, the shaft coupling can have a major impact on the overall performance of the powertrain. Once the central elements of the drive train have been determined, the options for the shaft couplings must be checked to see how the respective designs can optimize the performance of the overall design.
With development schedules tight, the temptation is to simply choose the most readily available coupling type. Under certain circumstances, savings are made at the wrong end, because an unsuitable shaft coupling can lead to premature wear of components, poor efficiency, vibrations or insufficient torque in the coupling. In these cases, an expensive and time-consuming conversion is necessary.
Compensation for misalignment
Offset is the deviation between the ideal and the actual position and orientation of the two shafts. Depending on the situation, shaft couplings have to compensate for angular, axial and radial misalignments. Offsets often result from manufacturing tolerances, but can also arise during operation due to different temperatures of the components, wear, loss of preload and deformations under load.
The best way to determine the misalignment of two shafts is to measure the system at operating temperature and when it is cold. An analysis of the tolerances of other individual elements of the drive train is also informative. Failure to take the offset into account can increase the wear and tear on the components involved, for example the motor bearings, and shorten the service life of the shaft coupling.
Typically, the larger the offset, the lower the torque capacity and the life of the coupling. In addition, the compensation of larger misalignments usually requires a longer shaft coupling. Bellows couplings with a larger number of bellows can accommodate larger angular misalignments, for example. Radial misalignment has a major impact on the length of the shaft coupling, as two angles have to be compensated for in order to connect the shafts. However, there are construction methods for which these basic rules do not apply. With the Oldham and UniLat couplings from Huco, even larger displacements can be compensated for in the smallest of spaces.
In general, with increasing flexibility of the shaft coupling to compensate for misalignments, the positioning accuracy decreases. For successful product development, it is important to find the optimal compromise between these two factors based on the requirements of the application.
A higher nominal torque usually means a larger coupling, whereby the materials and the construction naturally also play a role. A stainless steel bellows coupling offers a higher torque capacity than an Oldham coupling for the same size. This is because in the Oldham design, a plastic disc inside limits the torque capacity.
Another aspect is the way in which the torque is applied in the drive train. The torque of a shaft coupling specified in the catalog is based on its power at constant speed in one direction. However, if acceleration, deceleration or a change in the direction of rotation are added, the torque capacity of the clutch is reduced. A shaft coupling with a nominal torque of 10 Nm only achieves 3 to 4 Nm when accelerating and decelerating with alternating direction of rotation. In such applications – for example in robotics or in automatic placement machines – the coupling must be designed larger.
This is the resistance that the coupling opposes to a rotation around the longitudinal axis (torsion). High torsional rigidity is important for applications that require high signal integrity or positioning accuracy. Conversely, a lower torsional stiffness is important if larger misalignments are to be compensated for or if torsional damping is sought in the event of shock loads.
Again, it is a matter of checking the requirements of the application and finding the optimal compromise between precision and compensation for misalignments. Bellow couplings are characterized by their high torsional rigidity and are therefore ideal for precision applications. To do this, they can only compensate for offsets to a limited extent. Double loop couplings with their plastic elements pressed between two hubs, on the other hand, can also compensate for larger misalignments, but are less suitable for precision applications due to their low torsional rigidity.
In couplings subject to frictional loads (pumps, roller doors and machines are good examples), torsional stiffness plays a subordinate role because the shafts do not have to be synchronized. In the case of inertial loads, on the other hand, torsional stiffness is important because it prevents the driven component from “lagging”. If it is not torsionally rigid under load, the coupling can spring, which means that the transmission of movement is slightly delayed. This is undesirable for high signal integrity or position accuracy.
For example, the moment of inertia of the motor and driven load is a potential source of instability that can be affected by the torsional stiffness of the coupling. The coupling between the rotor of a motor and the rotor of a pump, each representing an inertial load, acts like a spring. When the motor starts, it twists the clutch before the motion is transmitted. However, if this effect is too great, it will cause a delay that will lead to instability. The greater the inertia loads, the more pronounced the effect.
These instabilities can «build up» and lead to resonances, which can considerably shorten the service life of the entire drive train due to excessive vibration loads on elementary components. This phenomenon cannot be completely eliminated, but it can be reduced to a minimum by appropriately adapting the torsional rigidity or load-bearing capacity of the coupling. Various designs, materials and sizes are available for this. It is important to take the risk of vibration into account during the specification so that a coupling is installed with the correct tolerance.
Take space requirements into account
It is always important that sufficient space is provided for the coupling. While each design is different, the coupling outside diameter is typically at least twice the shaft diameter. The length, on the other hand, depends on the type and extent of the displacement that has to be compensated for. With our Oldham coupling, the length is usually three times the shaft diameter. In addition, of course, the size of the clutch also depends on the peak torque capacity required.
For example, to connect shafts with a diameter of 3 mm and to compensate for an angular misalignment of 0.5 °, a coupling about 12.7 mm in length is required.