Rubber-in-compression couplings, installed by Renold Hi-Tec Couplings at a district heating utility in Sweden, have proved their worth over rubber-in-shear couplings that were installed at the same time in 1986. The couplings link heat pumps to synchronous motors transmitting 9,400kW at 1,500rpm. Over the past 20 years, the Renold units have only required maintenance twice in order to replace the rubber elements - a simple job that costs less than 10% of a replacement coupling. Conversely, when the rubber elements in the rubber-in-shear couplings needed replacing it took two days and, significantly, the cost was higher than fitting brand new rubber-in-compression couplings from Renold!
Rubber-in-compression couplings are made up of two round, metal sections fitting one inside the other with what looks like the paddles of a paddle steamer projecting inwards from the outer section and outwards from the inner. Rubber blocks are placed in the spaces in-between the paddles, and, as the motor turns the outer section, it drives the inner section through the rubber blocks. As this happens, the rubber is compressed, hence the term 'rubber-in-compression.' Rubber-in-shear couplings are made up of two metal discs with a rubber block fused in between. The motor turns one disc, which, in turn, drives the other through the rubber element.