This article walks through the decision framework that Stard’s application engineers apply daily, helping procurement and project teams match the right actuator to the real field condition.<\/p>\n
When the Valve Demands a Scotch Yoke Actuator<\/strong><\/p>\n A Scotch Yoke Actuator<\/strong>\u200b generates rotary motion via a piston-driven yoke mechanism. Its defining characteristic is the torque curve: maximum torque occurs at the beginning and end of the 90-degree stroke, which aligns precisely with the breakaway and reseating requirements of ball and butterfly valves.<\/p>\n Stard’s YP Series Scotch Yoke Actuator is typically specified when:<\/p>\n Valve size exceeds DN300.<\/strong>\u200b Larger ball and butterfly valves have proportionally higher breakaway torque relative to running torque. A Scotch yoke matches that profile efficiently.<\/p>\n Spring-return is required for emergency shutdown (ESD).<\/strong>\u200b The compact yoke design accommodates powerful spring cartridges within a shorter overall length compared to rack-and-pinion alternatives.<\/p>\n Space is constrained on skids or pipe racks.<\/strong>\u200b The yoke mechanism achieves higher torque density per unit volume.<\/p>\n Operating pressure fluctuates below 5 bar.<\/strong>\u200b The Scotch yoke geometry compensates for reduced supply pressure better than some other rotary designs.<\/p>\n When the Application Calls for a Linear Pneumatic Actuator<\/strong><\/p>\n A Linear Pneumatic Actuator<\/strong>\u200b converts compressed air directly into straight-line thrust. There is no rotary intermediate; the piston rod couples directly to the valve stem. This eliminates mechanical losses inherent in converting rotation back to linear motion.<\/p>\n Stard’s LSP Series Linear Pneumatic Actuator is the preferred choice when:<\/p>\n The valve is a gate, globe, pinch, or knife-gate type.<\/strong>\u200b These valves require axial thrust, not rotation.<\/p>\n Throttling or modulating service is needed.<\/strong>\u200b Linear actuators pair naturally with positioners for continuous control. The LSP Series accepts 4–20 mA input and can achieve positioning accuracy within ±1% of span.<\/p>\n Stroke length must be customized.<\/strong>\u200b Unlike rotary actuators with fixed angular travel, linear actuators can be ordered with specific stroke lengths to match valve stem travel.<\/p>\n High thrust at mid-stroke is critical.<\/strong>\u200b Some control valves require maximum force not at the seat but at the plug’s regulating position. A linear pneumatic actuator delivers consistent thrust throughout the stroke.<\/p>\n Torque vs Thrust: Understanding the Numbers That Matter<\/strong><\/p>\n One common mistake in actuator selection is treating torque and thrust as interchangeable. They are not.<\/p>\n