Abstract: The servomotor is constructed with a cylinder and a surrounding pressure tank with a construction formed of a plurality of radial grooves through which air can be throttled during an emergency closing of a valve. The constriction is formed with a flow resistance equivalent to a free cross-sectional area of not more than 0.02D.sup.2 ; D being the inside diameter of the cylinder. The spherical pressure tank may be secured about a cup-shaped cylinder to form the pressurized chamber while radial grooves in a flange of the cylinder form a constriction with a cover secured to the cylinder and spherical tank.

Abstract: The servomotor is constructed in one embodiment with a partition which forms a constriction through which air is throttled during an emergency closing of the valve. The constriction is formed with a flow resistance equivalent to a free cross-sectional area of not more than 0.02D.sup.2 ; D being the inside diameter of the cylinder. In addition, the chamber defined between the piston and the partition is not more than 10% of the combined volume of this chamber and a pressurized chamber below the partition.In another embodiment, the pressurized chamber may be formed by a pressure tank which is connected via a line which forms a constriction to the chamber below the piston. In still another embodiment, a spherical pressure tank may be secured about a cup-shaped cylinder to form the pressurized chamber while radial grooves in a flange of the cylinder form a constriction with a cover secured to the cylinder and spherical tank.