Safety Couplings in the Rolling Mill
Longer Service Lives in the Case of Damage
The universal shafts are the weak point of this design. Fig. 1 shows the typical damage of a broken universal shaft. The causes of such damage are almost always too high torques occurring briefly during accidents (strip breakage) or overloads, if the mill stand is operated to the limit. Since the gears are designed with higher safety factors than the universal shafts, the universal shafts are the weak point in the drive train, for these are destroyed first of all in such cases.
This means that the universal shafts have to be stocked as spare parts to minimise the downtime caused by a universal shaft fracture, since such universal shafts have a delivery time of several weeks. But replacing the universal shafts is also relatively time-intensive and thus involves an expensive loss of production.
Integrating a safety or overload coupling is an expedient investment for such cases. This is frequently omitted in new plants for cost reasons. Opinion is frequently widespread in existing plants that retrofitting is not possible. However, if one compares the spare part inventory and production downtime costs with the purchasing price of a safety coupling, then everything speaks for such a solution. It is also usually possible in existing mill stands to simply retrofit the coupling between the motor and gearbox instead of the existing standard coupling. This is usually a torsionally stiff gear or disc pack coupling. As an alternative, the safety coupling can also be placed on the driven side between gearbox and universal shaft. The mode of operation of a safety coupling is simplicity itself: If a predefined torque is applied to the coupling, it disengages the connection between drive and driven side within a few milliseconds. This disengagement also has the effect that the existing kinetic energy can no longer have a destructive effect on the drive components to be protected. Once the damage has been rectified, only a mallet or a lever is required to re-engage the coupling. In contrast to the previously widely used shear pin couplings, special knowledge about additional consumable materials such as pins, lubricants and similar is not required. The coupling is completely maintenance-free.
The MK Metallfolien GmbH company based in Hagen supplies a practical example. Foils used in particulate filters for cars are produced here. The 20-high reversing stand of a North American manufacturer rolls the stainless steel between 178 mm and 380 mm wide from an incoming strip thickness of 1 mm down to less than 0.025 mm. The strip is rolled from 2,500 m to a length of 20 km at the most. To this end, the asynchronous motor delivers a nominal torque of just on 3.7 kNm at around 225 kW and 565 rpm in the lower speed range. There was a crash last year of the type described above. Apart from breakage of the joint of the universal shaft, the gear coupling between motor and gearbox was also damaged. Enquiries to several coupling manufacturers resulted in delivery times of more than 10 weeks. Standstill of the entire plant for such a long period would have been more than unsatisfactory. In addition, within the scope of the continuous improvement process, a solution whereby such cases could be avoided in the future was sought.
The HPL Neugnadenfelder Maschinenfabrik comes into play at his point. As external service provider, HPL has been looking after MK Metallfolien as a customer for a long time in the field of rolling mill technology and strip production plants as well as in all requirements for longitudinal and transverse shears. The suggestion to use a safety coupling came from HPL. Yet these types of coupling are not standard stock goods that can be delivered off the shelf. HPL could help here with support by the R+W Antriebselemente GmbH company, the coupling specialist based in Klingenberg. A solution available at relatively short notice was found jointly. In less than three weeks, R+W could deliver a solution that replaced the old gear coupling as shaft-to-shaft connection and at the same time fulfilled the function of an overload coupling.
However, the design of the coupling is unusual. The model of the TORQSET ST2/25 type is preset at 6,610 Nm and is combined with a torsionally elastic coupling. In contrast to a torsionally stiff coupling, this version requires a considerably shorter delivery time. A torsionally elastic coupling in the main drive of a mill stand is surely unusual, frequently because of the required stiffness, but the elasticity at this place is negligible. The elastic compensating element of the selected coupling has a dynamic torsional stiffness of 230x103 Nm/rad. This means an angle of torsion of less than one degree in normal operation at around 3,700 Nm. It was not possible to assess reliably in advance whether this angle of torsion would influence the result in production. All concerned were aware of the risk. However, the prospect of being able to bring the rolling mill back into production several weeks earlier was worth the attempt to use this coupling. After three months' running in three shift operation at around 40 passes per week, the coupling was inspected and appeared to be just as fresh as on the day of delivery. According to Jörg Giersbach, the Technical Manager of MK Metallfolien GmbH, the low angle of torsion generated by the elastic coupling had no negative effect on the result in production.
In another case R+W was able to deliver a safety coupling for the 10" 2-high reversing mill stand of the rolling mill plant in Bochum of the R. Mendritzki Kaltwalzwerk GmbH & Co. KG company with headquarters in Plettenberg. Here as well there had occasionally been failures of the universal shafts in the past due to production-induced overloads. Steel for widely differing industries is rolled in Bochum. The max. 300 mm wide material is rolled with an incoming thickness of 10 mm down to 0.5 mm at a speed of 150 m/min. According to Production Manager Waldemar Steier, the coupling has already disengaged a few times and has thus paid for itself several times over.
The coupling of the TORQSET ST4/10 type is a safety coupling integrated in a gear coupling with minimal backlash. In addition the customer received at its request an integrated brake disc for the existing parking brake.
Naturally the TORQSET ST couplings can be used not only in reversing stands. Their use is justified in all machines in which components in the drive train must be protected in the case of overloads.
The mode of operation of the TORQSET couplings is just as simple as it is ingenious: The engagement between drive and driven side of the coupling is by ST safety elements. Inside these ST safety elements, a ball, one half of which protrudes from the element and sits in a detent on the other side of the coupling, is prestressed axially by disc spring packs. Once the circumferential force acting on the ball reaches a certain torque, the ball slips into the safety element and the coupling is disengaged within a few milliseconds. To re-engage the coupling, it has only to be turned into the correct position and a slight axial force must be applied to the safety element, e.g. by striking with a mallet or by the leverage of a crowbar. The TORQSET couplings are available in different sizes and are of modular construction. If required, the preset disengaging torque can be adapted easily to the application at any time subsequently. The TORQSET ST couplings can be delivered for indirect drives with individual attachment flange and as ready-made shaft-to-shaft connection in torsionally stiff or elastic versions .