Torsionally flexible couplings are among the proven machine components used in various applications for a long time. During the course of time, with increasing need of better performance, the quality demands on the couplings have continuously increased. As a result the standard spider materials often reach their limits and then necessitate the need of meeting the higher parameters by using special compounds. Following the reputation of upgrading products with the latest technology, KTR has developed the new spider material T-PUR? for the jaw couplings ROTEX? (illustration 1) which will become the future standard and would make KTR jaw couplings coping with the increasing expectations on quality, performance and service.

The new material T-PUR? offers high fatigue strength and temperature resistance compared to standard elastomers, while maintaining all other positive technical features such as damping and resistance to wear.

Right since 1967, KTR Kupplungstechnik GmbH has offered jaw couplings type ROTEX, which are used most frequently throughout the world today. The couplings are characterised by small dimensions, low weight and low flywheel effects with high torque transmission. The accurate, all-over machining has a positive influence on the running characteristics and increases the service life of the coupling considerably. ROTEX couplings ensure for a torsional vibration-damping power transmission in case of shocks in the drive train being produced by machines with non-uniform operation.

Why has a new material been used?

Often the environment in which gearboxes, pumps, etc., are used, are not known beforehand. With this, the existing and well-approved spider compounds meet their limits on several occasions. The spiders made of PEEK or other special compounds have to be used under rugged ambient conditions more and more frequently. These are not only more expensive than standard elements, but they are also disadvantageous with regard to damping, displacement, etc. This necessitated an all-round spider material for the "heart" of the jaw coupling. KTR now offers their customers such kind of solution by T-PUR - at the same price, which has been charged for standard materials up to now. The innovative spider material is particularly predestined for those drives being operated with variable speeds, reversing operation or high ambient temperatures.

Outstanding properties

The well-approved ROTEX? couplings offer considerable improvements with the new spider material T-PUR?.

With the new material significant improvements compared to standard elastomers in particular with fatigue strength and temperature resistance were realized, while maintaining all positive technical features such as damping and resistance to wear. Thus, the new material can be used with temperatures from -50?C to +120?C. In contrast to that other standard spider materials only offer a temperature resistance from -30?C to +90?C. The increased service life allows reducing the maintenance intervals and accordingly the expenses.

The new material T-PUR is used as a standard spider in ROTEX jaw couplings from size 14 to size 90 (illustration 2). From size 100 KTR supplies double tooth elements being made of the new material, too (illustration 3). That is why the double tooth elements can also be offered in 64 Sh now. Thus, the new material is available in different elasticities for all kinds of hardness from 92 Sh-A to 64 Sh-D.

New products by extensive know-how only

There is no light without shadow: That is why products being market leaders often run the risk of being copied. For KTR this is an additional incentive to be always one step ahead of their competitors. A permanent further development of the product as well as customised solutions call for a strong research and development department. Out of the 850 employees within the KTR group of companies 60 engineers and technicians are in charge of the development of new products and customized solutions.

The material was preceded by many years of cooperation between KTR's experts and the experts of the nylon engineering industry. The results of this development were optimised over several months until the requested elastomer material was found: T-PUR. Afterwards the spider material was tested under real operating conditions with such applications where standard elastomers did not achieve the necessary fatigue strength or temperature resistance, respectively. The new elastomer achieved the requested service life in the field without problems.

In addition to a close cooperation with the experts of the nylon industry one decisive factor for the success of this development were above all extensive tests on various test benches in the R + D test centre of KTR (illustration 4). Eleven different, mainly servo-hydraulic test benches with a testing torque of up to 100.000 Nm cover the entire torque range. This allows to perform tests of all relevant coupling data regarding operation, strength of components, wear and service life with many different temperatures up to 120?C.

ROTEX? coupling with double tooth elements made of T-PUR?.

Apart from these tests the new T-PUR spiders were used in field tests with selected customers under operating conditions mainly for such applications where standard spiders have met their power limits before which did not have the necessary fatigue strength or temperature strength, respectively, as proven by the following examples.

Applications of customers that have been realized

Nylon injection machines have immense demands on the couplings used. Thus, they have to resist high ambient temperatures and frequent overload, often with 24 hours of operation. As a result one of the first customers using the new spider material was a well-known manufacturer of injection-moulding machines. Subject to the excellent price-performance-ratio they meanwhile only use ROTEX? couplings with the new spider material T-Pur.

Couplings used on shredders and recycling machines are subject to extreme dynamics with heavy shock loads. In addition operating errors arise frequently. As an example, wood that was dropped may cause jams, the coupling having to protect the adjacent components. The shaft displacement, which is usual in such machines, does not allow using high-power, yet torsionally stiff elastomers, since these are disadvantageous with misalignment and damping. For these machines the ROTEX couplings with T-PUR are most suitable, since on the one hand they resist the loads that arise and dampen dynamic shock loads and on the other hand compensate for shaft misalignment without problems.

Summary and prospects

In KTR's test laboratories the new spider material T-PUR? was tested thoroughly.

For many drives expensive customized solutions instead of standard components have to be used. Here the ideal consequence is to make use of the experiences gained with the standard production. This was the case with the spider material T-PUR? by KTR. The material which was originally developed for a special drive could be realized as a standard solution which will be beneficial for all future users of ROTEX? couplings.

New developments are very complex - which is often reflected by the price. This is not the case with the new standard spider material T-PUR?. Maintaining the costs at the same level, it provides for significant improvements regarding fatigue strength and temperature resistance compared to previous solutions, while maintaining all other positive features of the previous elastomers. As an example, the temperature resistance of the new material could be improved by 35 per cent compared to the previous standard elastomers.

The new spider material was presented to the general public at the Hannover Fair in April 2011 and for the Indian customers, this innovation will be presented for the first time, during the GMC Fair, at Mumbai in Hall 5, from 6th to 8th May 2011. From that date ROTEX couplings with T-PUR would be available for customers in India as well.

Kumar Karandikar, Sales Manager, Email: k.karandikar@ktr.com