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.

Note rack and pinion gears
and how they are prone to
wear and slop.

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:

Scotch yoke operation. Easy to see its highly
susceptible to wear and resulting slop.

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.

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

Operation of the Kinetrol Model 20 Vane Actuator with Limit Switch

The following video demonstrates the Kinetrol Model 20, double acting, pneumatic vane actuator as it closes a large butterfly valve.

The Model 20 actuator is capable of delivering 89,000 inch pounds of torque at 80 PSI. The unit you see below is equipped with a dual coil, 4-way solenoid valve to control air flow to either side of the actuator, as well as control the opening / closing time. The actuator below also is equipped with a Kinetrol Universal Limit Switch with visual indication and internal electrical contacts.

EL Electropneumatic Positioner

The EL positioner controls airflow to an actuator and moves it to a position determined by a 4-20mA signal and using a unique low power proportional servo valve.

The microprocessor in the loop-powered 4- 20mA position circuit reads the signal via one channel of a 12-bit A-D converter, reads the position voltage from the feedback potentiometer via the second channel of the

A-D converter, and compares the two. If it detects a position which is different from that required by the signal, it changes the output to the servo valve, in order to drive the actuator in the direction required to reach the correct position. As the actuator moves, the feedback potentiometer voltage changes and the microprocessor continually calculates the adjustments required for the servo valve in order to guide the actuator accurately into position. The microprocessor is programmed with a sophisticated but compact algorithm which allows this critical dynamic valve adjustment to be made correctly. This in turn gives optimal results with any actuator/load combination - slow or fast, low or high friction, low or high inertia. All can be optimized by tuning the PGAIN and DAMP push buttons via the positioner circuit push buttons.

Actuators for Sludge Blowdown Valves in Municipal and Industrial Water and Wastewater Treatment Plants

Large, bulky and unreliable old sludge
blow-down valve actuator packages.

Sludge blowdown valves are used in water and wastewater treatment systems that separate solids prior to filtration. They are typically plug valves with double acting pneumatic actuators that operate on a controlled time frequency. The frequency depends on the amount of solids in the media and cycle times of every 20 minutes are common.

The pneumatic actuators that are supplied with the original equipment have been known, in some cases, to have a short performance life due to the cycle frequency and environment. Some end users have experienced the original actuators failing in 3 - 4 months. Customers who have upgraded to Kinetrol actuators have experienced performance lives of over 5 times this and still counting.

New Compact and highly reliable Kinetrol install.

Getting ready to remove old actuators.

Welcome to the Kinetrol USA Blog

We'll be using this site as a place to educate people about the advantages of rotary vane actuators and showcase interesting and unique applications.

We'll try to post as often as possible and keep the site fresh. Please check back frequently.