There are two different fundamental types of pneumatic actuators, and we’d like to explain the differences between them. Each has advantages and our guide will help you decide which is best for your application. Don’t hesitate to contact us if you require more information.
Aspect |
Scotch Yoke |
Rack & Pinion |
| Operational Principle | Upon air supply, pistons move horizontally, turning the shaft by the arm connected to the pistons. | Upon air supply, pistons move horizontally, turning the shaft that is linked to the piston. |
| Operation | Quarter turn (90°) | Quarter turn (90°) |
| Working Principle | Piston driving crankhaft | Piston driving pinion shaft |
| Body Enclosure | Anodized Aluminium | Anodized Aluminium |
| Shaft | Nickel Plated Steel Alloy | Nickel Plated Steel Alloy |
| Seals | Viton (high temp allowed) | Viton (high temp allowed) |
| Seals | Viton (high temp allowed) | Viton (high temp allowed) |
| Operational Principle (diagram) | 
|  |
| Major parts | Piston / crank / shaft | Piston / shaft |
| Cost | Lower | Higher |
| Torque | Higher | Lower |
| Lifecycle | 1 million + | 1 million + |
| Scaleability | Highly scalable | Limited |
| Spring Adjustment | External | Internal |
| Frame size | slightly longer | slightly shorter |
| Benefits | - Smooth on/off with the torque curve synchronised with the valve | - Short body length makes it easier to fit in small spaces |
| - Relatively higher torque than the comparable rack & pinion type | - The multi-spring structure enables adjustment of number of springs to suit the site requirements | |
| Torque Illustration | 
|
 |
| Torque as function of stroke | non linear (ideal fit for butterfly valves) | constant |
| Torque Curve | Highest at beginning / end of stroke | essentially constant throughout stroke |
| Positioning (throttling) applications | No | Yes (better suited for positioning applications) |
| Available in Spring Return (SR) or double acting (DA) | Yes | Yes |
| Retrofit between SR and DA | No | Yes |
| Weakness | - Long length due to the piston stroke | - Rigid torque |
| - Multi-spring disabled and therefore a jig is required for spring adjustment | - It requires a bigger cylinder inner-diameter to achieve the same torque as a scotch-yoke type | |
| Additional Information | - Due to high torque in larger models scotch-yoke types are commonly used | - Due to short piston stroke rack & pinions type commonly used where compact design is required |
| - In order to ensure that each part withstands high torque, design works and durability tests must be carefully conducted | - Gears being the media connecting pistons to shafts, their quality affects the actuator’s performance directly. High precision gear design technique is therefore essential | |
| ISO 5211 Mounting Flange | Standard | Standard |
| IP Rating | IP67 | IP67 |
| Temperature Range | -4 to 167° F (-20 to 75° C) | -4 to 180° F (-20 to 82° C) |