British couplings protect Swiss slab mill

British firm Renold Hi-Tec Couplings was chosen by the owners of a Swiss slab mill to provide a solution to problems with shock loading and torque overload before the installation of a new reduction gearbox. The Halifax based firm recommended the installation of rubber-in-compression couplings after considering the cost advantages of a maintenance free coupling together with the protection offered to the motor from torque amplification of shock loads through the gearbox.

The 880kW motor driving the mill was operating at speeds between 600 and 1,200rpm, and producing torques of up to14kNm on the gearbox's input shaft. To protect the gearbox from torque overload under near stall conditions, Renold Hi-Tec's engineers fitted a torque limiting device to a PM60 rubber-in-compression coupling on the new gearbox's input shaft. The output side of the gearbox operated at 27rpm and generated torque of 31kNm. Here a PM850 coupling was used with a spacer to bridge the gap between the gearbox and the slab mill.

The rubber-in-compression couplings significantly reduced shock loads on the gearbox by reducing torque amplification and absorbing vibratory energy, the frequency of which could be determined by careful selection of the rubber compounds for the rubber blocks used in the couplings' construction. Since installing the couplings the plant is now more reliable and even runs more quietly due to the fact that there is no metal-to-metal contact in rubber-in-compression coupling design.