Selecting shaft couplings to suit application requirements

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This article from Huco Dynatork explains what types of shaft couplings are suitable for different applications depending on the requirements - such as angular misalignment, axial compliance and torsional stiffness.

>True angular misalignment

The common causes of true angular misalignment are when one of the connected shafts is compliantly mounted; for example, when it is located by a self-aligning bearing. Alternatively, it could be that an unsupported intermediate shaft is placed between the driver and the load. Because the shafts are not mounted conventionally, they will self-align to intersect at the centre of the coupling, which acts as a hinge and, to a degree, a radial bearing. As the coupling is locating the shafts on a stable axis of rotation, it should be of the single-stage type due to the fact that any radial compliance in the coupling is counterproductive.

While couplings based on the flexible shaft can be used in these circumstances, there is a possibility that the coupled system may go into lateral oscillation. This is best described by visualising the effect of a belt and pulley drive mounted on the compliant shaft. Having a lateral compliance capacity, the coupling responds to fluctuating tension in the belt by allowing lateral oscillation of the shaft.

Under no circumstances should a coupling with lateral displacement be used with floating shafts. The reason is that this type of coupling has no self-centring action and its use would allow the shafts to orbit in an uncontrolled way.

Couplings capable of overcoming true angular misalignment include the single universal joint with its capacity to handle large offsets, torsional damping, water resistance and lubrication-free operation.

Single-stage disc couplings are also suitable thanks to their near-infinite life and built-in dynamically balanced properties. Similarly, single-stage bellows couplings with their high torsional stiffness are a good choice in this application.

Zero misalignment

By assembling both shafts in self-aligning bearings, zero misalignment can be achieved. In this way both shafts can float into concentric relationships, allowing the use of a solid coupling that simply supports the shaft in perfect alignment.

Difficulties arise when attempting to connect fixed-axis shafts in this way, as the level of alignment is difficult to both achieve and maintain due to settlement, creep, thermal expansion and contraction. The influence of these factors results in relative movement between the shafts and the alignment achieved in the factory machine may not be achievable 'in the field'. Therefore a flexible coupling is always the preferred option.

Before installing a solid coupling, an interesting test is to try a flexible coupling first. With the machine at normal operating temperature, measure the speed and/or the current drawn by the motor. The difference between these readings and those with the solid coupling indicate the losses generated by the additional friction at the bearings.

Axial compliance

Longitudinal shaft displacement (axial shaft displacement) can either be intentional or unintentional. The movements due to tolerances, settlement and thermal expansion or contraction cause the latter. While these movements may be small, they can contribute to substantial thrust loads and result in bearing damage.

In these cases a coupling with axial compliance capacity should be selected, predominantly bellows, sliding disc or even helical beam couplings. Multi-stage bellows create the greatest amount of axial compliance, while the single-stage disc or bellows provides the smallest.

For intentional shaft displacement, such as on push/pull systems or those with extensible drives where the distance between actuator and load is variable, a teleshaft coupling should be used. The Huco Dynatork HUCO-POL is a typical example; it comprises precision drawn nesting tubes manufactured from square-section brass that can be cut to the appropriate length to provide a wide range of axial movement. This ability to customise the length of the coupling means it can be tailored to the required stroke length.

Another option is the Oldham demountable, three part coupling. By mounting the hubs slightly out of full engagement, a limited amount of axial compliance is created.

Torque capacity

Rotating loads have frictional and inertial components and are classified according to whichever dominates; for example, the resistance encountered by a pump delivering fluid is a frictional load as the inertial part is secondary, assuming that the pump runs continuously at a steady speed. If the pump runs at a constant speed, it produces a uniform load and the required power would be given in kW or HP. The kW rating is related to torque by the following formula: torque Nm = kW x 9550 divided by revolutions per minute.

Conversely, a ball-mounted slide table, typified by short cycles of rapid acceleration and deceleration in both directions of rotation, will have inertial loads as the predominant factor. These will determine the reversing torque factor of the coupling. To be more precise, the maximum torque experienced by the coupling may be dictated by whether braking is applied by the load or the motor.

Once the maximum torque in the system is known, the correct coupling can be selected by relating it to the peak torque rating. The coupling should be selected using the following formula: peak torque > application torque x service factor.

Torsional stiffness

Torsional stiffness may be expressed in different units but the most common and easiest to work with is Nm/rad. Often described as torque per unit deflection, torsional stiffness is significant in positional system and describes a coupling's resistance to torsional deflection. The inverse of torsional stiffness, torsional deflection, is defined by deflection per unit torque. This also has many denominations but is best expressed in degrees/Nm.

When used in a closed-loop or velocity control system, a coupling's torsional stiffness becomes more critical and forms a contributory factor in calculating the upper limit of dynamic performance and stability. Therefore, the stiffness of a coupling should be such that its torsional resonance frequency exceeds 300–600Hz, depending on dynamics. Stiffness is at its most critical when load inertia is dominant and becomes less so when the dominance swings in the motor's favour.

Follow the link for more technical guidance and to see Huco's range of shaft couplings.

16 July 2013

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