A chain has proven to be the most economical solution for synchronous switching of the two change-over valves. However, more robust solutions are required for certain environmental conditions. Arctic climates are such a special case. To withstand snow and ice, angular gearboxes with a fixed shaft are used here for power transmission. Compensation for possible mounting tolerances between the change-over valves is achieved by using a metal bellows coupling. Especially in the Russian market, there is from time to time a demand for such a solution, which is also resistant to permafrost.