TR
Why the Rotary Vane Actuator Design is Superior to Rack and Pinion & Scotch Yoke Designs
When it comes to pneumatically actuating an industrial quarter-turn valve, you basically have only three types of mechanical technologies to choose from: rack and pinion, scotch yoke and the rotary vane design. This post describes why a rotary vane design is the clear winner when it comes to efficiency and reliability.
First, let’s describe how rack and pinion and scotch yoke actuators work.
A rack and pinion actuator is comprised of two opposing pistons, each with its own gear (referred to as the “rack”). The two piston racks are set against a round pinion gear. As pressure increases against one side of each piston, each rack moves linearly against the opposite sides of the pinion gear causing rotational movement. This rotational movement is used to open and close a valve. Pretty basic stuff. See the animation (provided by Wikipedia) below for a visual understanding.
A scotch yoke actuator relies on the scotch yoke mechanism to convert linear movement into rotary motion. In this case, a piston is coupled to the sliding yoke, which in turn moves a fixed pin on the shaft of the actuator to provide rotation. As one side of the piston is pressurized, the piston forces the yoke to move linearly, which allows a slot in the yoke to drive the pin on the actuator shaft. See the animation (from Wikipedia) below for clarification:
Both of these valve actuator mechanisms use several interconnected, mechanical moving parts. As a result, they are very susceptible to wear.
It All Comes Down to “A Single Moving Part”
The vane actuator has only one moving part and there is no linear-to-rotary conversion. An internal vane moves uniformly in response to inlet air pressure, without gears, slots, or levers. This is a clear advantage when you consider wear and tear, and also machine efficiency. See the video below for a visual explanation.
Prepared through Kinetrol USA.
| Vane = Simple, reliable design | Piston = Complicated, less reliable |
| One moving part | Many moving parts |
| No O-rings | Several sets of O-rings |
| Dynamic Memory Seals | Static seals |
| No linear to rotary motion | Linear to rotary = friction/wear |
| Spring isolated | Spring exposed to atmosphere |
| Very accurate control | Hysteresis = poor control |
| Non-pressurized shafts | Pressurized shafts |
| 4 million operations | 500,000 to 1 million operations |
