torque established by the transmitter 12 is again reached the heater excitation is again connected and ring 3 is again unclamped this process is repeated periodically the duration of the periods being determined by the thermal time constant of the system until at a later time t 3 the end of the fully adjusted thread displacement has been reached motor 5 thereupon disconnects itself the voltage drop over the resistor 11 becomes zero and due to the cooling of ring 3 clamping torque m clamp within seconds again attains its original relatively high level it follows from the foregoing that thread play present during the rotary adjustment or the operative torque threshold m threshold can be selectably set at transmitter 12 and that it can be adapted to the output torque characteristic of the motor 5 of course the thread adjustment can also be effected manually fig2 shows a lever with gear toothed hub which can be used instead of gear 7 to rotate sleeve 1 for small corrective increments in whatever rotational direction is desired lever 17 is rotatable about a shaft 20 and it has two piezo elements 18 and 19 for finger applied pressure finger pressure on either or both of elements 18 19 produces an output signal to the comparator 10 for controlled excitation of element 4 for heating ring 3 and when ring 3 becomes sufficiently heated to reduce clamping torque below threshold torque continued finger torque applied to lever.