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Torsionally flexible couplings

A torsionally flexible coupling requires a high torsional elasticity in order to keep the main resonance speeds below the idling speed (hypercritical operation). Furthermore the coupling should provide a large damping capacity in order to keep vibrations at a minimum.

For all torsionally flexible couplings in which the torsional elasticity and damping capacity are dependent only upon an elastic medium which cannot be influenced by external measures, there must be some compromise, since both requirements cannot always be met satisfactorily.

The demand for a high torsional elasticity requires the use of a soft elastomer which, naturally, has a low damping capacity. In order to achieve high damping, however, a harder elastomer must be used which, although offering higher damping values, increases the dynamic torsional stiffness.

Universally flexible 2-element Küsel coupling

- connects flywheel to stud The design developed by Voith Küsel allows these demands to be combined advantageously. While a soft rubber mixture can be selected in order to achieve a high torsional flexibility, an effect resembling that of a harder elastomer is produced in this soft elastomer by axial pre-loading of the coupling elements for couplings without frictional damping.
The advantage of all highly flexible couplings is that resonance speeds can be placed in a low speed range, with the disadvantage, however, that resonance amplitudes are increased. This amplification ratio is determined by the damping capacity of a coupling.
 

The described design minimizes undesirable and originally non-existent additional loads, so that burdens on the attached units are relieved by decreasing the alternating moments on passing through the resonance as well as in the speed range, and this has a positive effect on the life expectancy of the entire drive system.

The influence of the amplification ratios on the amplitudes are shown in the diagram on page 10 of the coupling systems catalog. Please contact Application Engineering Inc. for a Voith Küsel catalog.

The highly flexible couplings designed for connection to universal joint shafts are characterized by their extremely short and simple construction as well as by their insensitivity to larger bending angles of the universal joint shaft, while at the same time having high torsional flexibility and damping.
Since the dimensions of the internal fastenings of the rubber elements are identical to the corresponding types of universal joint shafts, the flanges for the joint shafts can, for some assemblies, be placed inside the coupling.

Due to the short and compact design, the shaft joints which produce the lateral and bending moments are brought so close to the coupling bearing that the bending moments are reduced to a minimum. As a result the universal joint shaft becomes longer compared to other designs, and the bending angles and resultant bearing loads are reduced.
 

The insensitivity of the couplings to larger bending angles of the attached universal joint shafts is a result of the axial arrangement of the friction disc, which additionally supports the bending and tilting moments caused by cross loads, and which reduces stress on the center bearing. The center bearing is subjected to load stress due only to the weight of the universal joint shaft and the remaining unbalanced centrifugal forces.

Since the rubber elements are axially pre-loaded, the friction discs cannot lift off. The relatively large diameter of the friction disc acts as a lever arm and is the determining factor. The friction disc rotates during the operation of the coupling, so that wear is minimized.

A low damping rubber mixture is used in the couplings with high torsional elasticity in order to prevent heating of the elements under high continuously alternating moments. The good damping characteristics come primarily from that part of the friction damping that increases the natural damping capacity of the elastomer.

The advantage of the separation of torsional flexibility and a large part of the damping capacity is that the heat which is created by the conversion of energy in the rubber, and which is normally not easily dissipated, can now be diverted outward through the friction disc over the adjacent steel parts.

Due to the large friction surface available, the required damping is achieved with very low contact pressures and a resulting reduction of wear on the friction discs.

Several rubber hardnesses from 45°-70° Shore are available for each size. Torsional flexibility and friction damping can be adapted to meet requirements.

The final determination of the suitable coupling demands the carrying out of a torsional vibration calculation which can be undertaken by us, if wished. The formula shown on page 12 of the coupling systems catalog, makes it possible to determine provisionally the appropriate coupling.  Please contact Application Engineering Inc. for a Voith Küsel catalog.

The load torque to be transmitted must be smaller than or equal to the nominal torque of the coupling. Under the influence of heat, the strength of the rubber reduces. An associated reduction in the permissible nominal torque of the coupling must be taken into consideration through the introduction of the temperature factor (see Fig. 1, page 12 of coupling systems catalog. Please contact Application Engineering Inc. for a Voith Küsel catalog.).
The highest periodic vibratory torque should not, taking the frequency and the surrounding temperature into consideration, exceed the allowable continuous vibratory torque of the coupling. At frequencies above 10 Hz and surrounding temperatures over 20°C, vibratory torque of the coupling must be reduced as shown in Fig 2. (see Fig. 2, page 12 of coupling systems catalog. Please contact Application Engineering Inc. for a Voith Küsel catalog.).
Due to torsional vibrations, energy is converted into heat by the damping effect of the coupling and this leads to an increase in the temperature in the rubber core. The permissible power loss of the coupling indicates how much damping energy the coupling can absorb and give up to its surroundings in the form of heat.
The Voith Kusel coupling exhibits in the working range a linear characteristic of the dynamic torsional stiffness. It is to be noted that the values stated in the list of the technical data are valid at a frequency of 10 Hz, a pre-load of 0 Nm and at a surrounding temperature of 20°C.