Flexible drive couplings

Flexible drive couplings are non-engageable elastic couplings that are backlash-free. Due to their design, they possess elastic properties and are usually used for the transmission of torque.

The typical design of a flexible drive coupling is a combination of wound fiber bundles and elastic material, usually rubber. The material used and the design of the coupling result in characteristics that differ significantly from those of the popular Hardy disk couplings and full elastomer couplings.

Due to their structure, flexible drive couplings combine the advantages of a number of other coupling systems.

HARDY DISK COUPLINGS

Hardy disk couplings are usually simply called flexible couplings in everyday speech. Unlike flexible drive couplings, Hardy disk couplings are often made entirely out of rubber or other elastic materials combined with simple and non-directional fibers for reinforcement, such as textile fabric.

Hardy disks are used in drive shafts, where they are connected to the shaft via bolt holes in plates that are riveted to the disk. In this position, they ensure that alignment errors and slight offsets are compensated for. Thanks to their design, they also intercept and absorb shocks that may occur during changes in rotational speed or when the shaft starts up.

In order to ensure that Hardy disks are installed effectively and optimally, they are often stabilized radially with a centering mechanism. For shafts that rotate at high speed or particularly long shafts, this is crucial in order to minimize any imbalance.

Hardy disks are always screwed to the shafts via the bolt holes. In alternation, one bolt hole is connected to the drive shaft, and the next one to the output shaft.

The name "Hardy disk" is used primarily in the German-speaking world, and can be traced back to the Englishman John Leslie Hardy, the inventor of the design, who registered it with the Royal Patent Office in the UK in 1938 under the number GB497903 via his company HARDY SPICER & CO LTD. In English, this disk is often simply called a "flexible joint disk" or "flex disk", without mentioning Hardy's name.

Figure 1: Hardy disk coupling with textile fabric.

FULL ELASTOMER COUPLINGS

Full elastomer couplings are used as elements for dampening vibrations and shocks on particularly susceptible components and applications. The connecting element, via which the torque is also transmitted, is made entirely of elastomer, which gives this coupling its name. This results in good damping properties.

Depending on the vibrational requirements, these couplings are offered with various Shore hardnesses and in various shapes, whereby the suitability of the coupling for a particular application is determined exclusively via the elastomer element used.

Depending on the coupling system, pressure buffers, elastomer rings , and elastomer springs for shear oads may be used. Here too, the coupling design will need to be customized for each particular application scenario. In cases where a high degree of torsional stiffness is desired, elastomers with a high Shore hardness are used. For low torsional stiffnesses, elastomers with a low shore hardness are used. Full elastomer couplings have a material-specific load limit.

If there is insufficient space to install a full elastomer coupling of the corresponding size, or if the rotational speed is simply too high, fiber- or metal-reinforced elastomer couplings are used, such as flexible drive couplings or link couplings. In these couplings, the torque is transmitted via internal fiber bundles and the elastomer only serves to provide damping.

DESIGN AND FUNCTIONING OF FLEXIBLE DRIVE COUPLINGS

Flexible drive couplings have multiple bolt holes that are surrounded by wound fiber bundles and rubber. The bolt holes are alternately connected to the drive flange and the output flange via a bolt connection, a threaded bush, or a socket pin. The torque is transmitted via the fiber bundles in the flexible drive coupling.

During this process, the fiber bundles are subject to stress along the direction of pull, and exhibit elastic behavior under strain. Apart from compensating for axial, radial, and angular misalignment, flexible drive couplings can therefore also be used to dampen torsional vibrations.  Drive shaft couplings have a particularly high degree of efficiency due to the fiber technology used. This fiber technology allows the strength of fiber bundles to be combined with the damping properties of rubber (Figure 2). It allows extremely slim space- and material-saving designs to be integrated into all types of couplings. This not only has the advantage of reducing the weight of the coupling, but also provides good accessibility and makes maintenance extremely easy.

In flexible drive couplings, the elastomer serves to support and protect the fiber bundles, and also insulates against noise, as the solid-borne noise is disrupted via the structure of the material.

Figure 2: Cross-section of flexible drive coupling

In flexible drive couplings, torque is transmitted by a composite of fiber bundles and rubber inside the couplings. Due to the combination of materials, the permissible tension in the rubber-fiber composite is about 15 times that what elements made purely of rubber would be able to withstand. This allows extremely high torques to be transmitted without any problems in very tight spaces.

Particularly in automobile construction, these properties have led to the widespread use of flexible drive couplings to connect drive shafts.

In this application, the component functions much like a joint, but unlike universal and homokinetic joints, it also dampens torsional vibrations.

This means that it also insulates a wide range of vibrations and structure-borne noise. Noise paths are disrupted within the drivetrain, promoting smooth running.

Furthermore, flexible drive couplings are highly resistant to contamination and impact stresses. These properties make them ideal for use in mechanical engineering. Unlike other coupling systems, flexible drive couplings exhibit a long service life in these applications and need to be replaced much less frequently.

In addition, flexible drive couplings are maintenance-free and have damping properties. This has a positive effect on the service life of machine parts in the direct surroundings, as it greatly reduces wear and tear by damping vibrations and absorbing torque spikes.

Outdoor applications in harvesting machines or in the drived rains of vibrating screens are often outfitted with flexible drive couplings. In these cases, the advantages of flexible drive couplings can be fully exploited in a number of different ways, as these applications involve both extreme vibrations and high levels of dirt and contamination.

In pump and generator drives, both single and double versions of flexible drive couplings are used in the form of double cardan systems in order to compensate for axial, radial, and angular misalignment as well as assembly tolerances in individual machine units.

Flexible drive couplings are often used as insertable versions to couple combustion engines and hydraulic pumps. Due to the encapsulation of the drive, it is not possible to use bolts to screw components in place after combining the combustion engine and hydraulic pump.

Already during the manufacture of flexible drive couplings, it is a simple affair to take into consideration special requirements the elements may need to fulfill because of the environment or area where the coupling will later be used. Because it is primarily the fiber bundles which are responsible for the transmission of torque, many properties can be customized to the client's requirements in this respect, without having to modify the dimensions of the components.

Changes to the torsional stiffness, resistance to media, thermal resistance and suitability for high rotational speeds are properties that typically need to be customized.

FLEXIBLE DRIVE COUPLING DESIGNS

Flexible drive couplings are manufactured in a variety of different designs. One major method of differentiating between the various designs is according to their internal structure. For one, a distinction is made between flexible drive couplings with a preferred direction and flexible drive couplings for reversible operation.

For example, if the same torque is to be transmitted in both the forward and reverse operation of a machine (reversible operation), flexible couplings with equally strong fiber bundles for both directions are used.

If the load predominates in one direction (e.g. driving forward in automobiles), a flexible coupling with a preferred direction is used. In this case, if the coupling is to remain the same size, the fiber bundles for the preferred direction are designed to be much stronger — at the expense of the fiber bundles for driving in reverse — such that the flexible coupling is able to operate at a higher load in this direction and requires less space.

Generally, flexible drive couplings with 4-hole (Figure 3), 6-hole (Figure 4, Figure 5) or 8-hole designs are used, whereby the 4-hole design is able to work under particularly high bending angles due to its geometric structure, and exhibits the same kinematics as a universal joint.

The maximum torque that can be transmitted is limited by the size of the component.

If extremely high torques are to be transmitted, link assembly couplings or link couplings are used.

Figure 3: 6-hole flexible coupling in shim bushing design

Figure 4: 6-hole flexible coupling  in collar bushing design

Figure 5: Special design as a 4-hole flexible coupling

ADVANTAGES OF FLEXIBLE DRIVE COUPLINGS

Due to its design as a component made of rubber and fiber composite, a flexible drive coupling exhibits a number of advantageous properties. But above all, it makes the coupling maintenance-free, gives it a long service life, and makes it highly resistant to external influences.

Thanks to the tension-based system in flexible drive couplings, it requires less space than other coupling systems, such as the Hardy disk couplings mentioned at the beginning, and has a longer service life.

Through the combination of fiber bundles and rubber, as well as their special design, flexible drive couplings are able to serve multiple functions in the drive system.

Usually, these include torque transmission, compensating for axial, radial, and angular misalignment, as well as vibration damping.

One additional advantage of flexible drive couplings is the effective protection of all downstream and neighboring components through the damping of torsional spikes.

In addition, it compensates for misalignment due to component and assembly tolerances. For example, this makes it possible to avoid "audible and perceptible droning" from low-frequency vibrations due to the buckling of drive shafts when starting up or a sudden load change.

Due to its design, the technologies used, and the principles applied, such as the tension principle — there are significant advantages for maintenance. Flexible drive couplings are easily accessible, can be integrated into existing systems in a space-saving manner, and have longer service intervals than other solutions.

Author: Johann Löw, Roland Liessel, October 2015