Quarter-turn valve actuator

A power-driven actuator for a valve having a valve stem rotatable approximately 90 degrees between a valve shut position and a valve open position, the actuator including a support structure adapted to be mounted on a valve adjacent to the valve stem, a drive member, means for mounting the drive member on the support structure for axial reciprocation in a plane substantially perpendicular to the axis of the valve stem such that the axis of the drive member will be spaced from the axis of the valve stem by a preselected fixed distance, and power driven means for reciprocating the drive member along a fixed axis between a first position corresponding to the valve shut position and a second position corresponding to the valve open position, wherein the improvement comprises:

• a crank arm having one end adapted to be coupled to the valve stem for rotation thereof such that the axis of the crank arm makes an angle of between approximately 60.degree. and 100.degree. with a line intersecting the stem axis and perpendicular to the drive axis of the actuator when the valve is in the shut position and is within approximately minus 30.degree. to plus 10.degree. of perpendicularity to the drive axis of the actuator when the valve is in the open position;
• and
• a link member having a first end pivotally attached to the other end of the crank arm and a second end pivotally attached to the drive member such that force is transmitted from the drive member to the crank arm only along the axis of said link member, the length of the link member being greater than the preselected distance between the drive axis of the actuator and the axis of the valve stem, the length of the crank arm is greater than the length of the link member, and the axis of the link member makes an angle of between about 60.degree. and about 70.degree. with the crank member when the drive member is in the first position, the lengths of the crank arm and the link in relation to said preselected distance are predetermined to produce a torque at one end of the crank arm having a first maximum torque value when the crank is in the valve shut position and a second maximum torque value when the crank arm is at a position intermediate the valve shut and valve open positions, a first minimum output torque value is produced between the first and second maximum output torques values and a second minimum output torque value being produced at the full valve open position.