615,469. Fluid-pressure servomotor-control systems. AKTIEBOLAGET A. EKSTROMS MASKINAFFAR, and SALL, E. May 3, 1946, No. 13518. [Class 135] A servomotor device for regulating, e.g. a valve for controlling the temperature of the liquid in a tank or a rheostat in an electrical circuit, comprises e.g. a thermostat 31 in a tank 32 for adjusting a valve 30 at the mouth of a nozzle 30a on a pipe connected to an air blower 27 and having a constriction 29 and an outlet branch to a bell 35 disposed in a liquid tank 37. The bell contains a float 36 supported by a rod 39 connected by a counterweighted crank 40 to the piston 7 of a control valve provided with a rotary sleeve 6 and controlling a servomotor 14, the piston of which is connected by a cable 17 to the valve 19 in a hot liquid supply pipe connected to a cold liquid pipe 33. The servomotor is operated by liquid, e.g. oil, drawn from a tank 1 by a gear pump 2 which, in the central position of the piston, discharges through a relief valve to a constant level tank 26 connected through a port controlled by a valve 18 with the tank 37. When the temperature in the tank 32 exceeds the desired value, the thermostat adjusts the valve 30 so as to restrict the outlet from the nozzle and increase the air pressure in the bell, thus depressing the float 36 and moving the piston 7 to the right to admit oil to the top of the servomotor cylinder and depress the piston and close the valve 19. A follow-up action is effected by the oil displaced from the cylinder through the pipe 44 to the tank which results in the raising of the float 36 and re-centring of the control valve piston 7.