Features of the ROTEX DRS Scotch Yoke Actuator

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Actuator Modules 

• ROTEX actuator spring modules and pneumatic modules can be removed, swapped, rebuilt, or reinstalled on the frame module without disassembling the frame module. No need even to remove top cover of frame module, because all bolting for removal of the modules is external. 
• Actuators can easily be changed in the field from fail close to fail open by just swapping the positions of the spring and pneumatic modules. No need to dismantle the frame cover or cylinder assembly. 
• Actuators can be modified from double acting to spring return easily in the field by adding a spring module. 

Springs are lifetime lubricated and epoxy coated.

• Springs are pre-compressed inside their welded spring modules for safety and ease of use.
• Pneumatic cylinders are lined with hard chrome plating. Smooth to eliminate wear of piston seals, the surface is very hard, for long life. 
• Inside the pneumatic module, dual oversized tie rods are protected from external corrosion. Where the tie rods pass through the piston head, ROTEX has improved the design by providing a slide bearing, where most designs don’t. This bearing is important to help maintain alignment and assure smooth operation. 

Frame Module

• Inside the frame module, the yoke is supported by life time lubricated bearings. Needle roller bearings ensure the most efficient torque transmission. 
• Extra heavy duty travel adjustment bolts allow for +/- 10% adjustment of the 90 degree stroke. The area of contact is large, to withstand the impact forces. 
• The yoke thrust pin is made of case hardened steel to resist wear. 
• The piston shaft is supported on each side of the frame module by solid bearings that ensure proper alignment and smooth operation. 

Flexijoint

• Each piston shaft of the ROTEX DRS is equipped with two Flexijoints. These specially designed joints compensate for movement and changes in alignment to reduce internal stresses and increase output torque. 
• The thrust transmitted through the Flexijoints ensures no side thrust on the cylinder walls and bearings. 

Unique Sealing Features

• Piston seal is ROTEX X design. Flexibility of the sealing material reduces friction, prevents leakage, and extends actuator life. The X shaped seal is wear compensating, and the unique shape of the seal retains the grease for longer periods. The piston is guided by a PTFE thrust bearing, which helps keep the piston centered, and seal wear to a minimum. 
• The tie rod seal is also of the ROTEX X design. Old fashioned O-rings are replaced by these flexible, low friction, resilient seals to reduce the failure rate of tie rod seals. Also reduces the wear and tear to the tie rods themselves. 
• ROTEX has also replaced the standard O-ring of the shaft seal. In this position, we provide a proven V-seal design. This self-energized, wear compensating seal guarantees low hysteresis and minimum friction. Like the X seal of the piston and tie rod seals, the inherent shape of the V-seal helps keep the grease where it needs to be. 

Optional Hydraulic Override

• ROTEX DRS actuators may be ordered with an optional hydraulic override, or the override may be retrofitted to an existing actuator. 
• The override is fitted into the spring cartridge, to keep the overall dimensions of the actuator and override to a minimum. Can withstand up to 2,000 psi hydraulic pressure.
• Easy to operate.
• Special Z seal ensures zero leakage and extended life.

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ROTEX Pneumatic Valve Actuator Introduction

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Here is a movie/slideshow introducing the ROTEX family of pneumatic valve actuators. The movie includes the ECV "Infinity line of rack and pinion actuators, the DRS scotch yoke series and the DRC and DRV large scale rack and pinion series.

Unique design and high performance features are explained and pictures of automated valve packages are included.

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Interesting Video of ROTEX Scotch Yoke Actuator and Manual Override

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An actuation package supplied with ROTEX DRS actuator and manual override.

For any new engineer involved in process control, beginner valve technician, sales or service professional or newbie to the valve industry, here is a helpful 7 minute video on how scotch yoke valve actuators are designed and constructed.

While this video outlines how ROTEX Controls builds their actuators, the video gives you a good general idea of the construction and major components.

ROTEX is a a recognized worldwide leader in manufacturer of valves actuators and valve accessories.

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Actuators 101: Quarter-turn Pneumatic Actuator Types

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Pneumatic valve actuators come in two basic types: rack & pinion and scotch-yoke. Both style actuators provide 90 degree rotational torque to drive a quarter-turn valve (ball, plug, butterfly) open or closed.

The scotch-yoke actuator uses a piston, which moves a linear push rod that in turn engages a pivoting lever arm to provide rotation. A rack & pinion actuator uses opposing pistons with gears to engage a pinion gear shaft. The practical difference however is in size, power, torque curve and ease of adding peripherals.

Rack & Pinion

Rack & pinion actuators tend to be more compact, have standardized mounting patterns and have an output torque range suitable for small to medium sized valves. They include standard bolting and coupling patterns to directly attach to a valve, solenoid, limit switch or positioner.

Scotch-yoke

Scotch-yoke actuators, for the most part, are much larger than rack & pinion actuators and consequently provide much higher torque output. These actuators are most often used on large quarter turn valves and require careful attention to the mounting requirements because of their power.

Both styles come in either "direct acting" or "spring return" versions. Direct acting actuators use the air supply to move the actuator in both directs (open and close). Spring return actuators, as the name implies, uses springs to move the actuator back to its "resting" state.

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Fire-Safe Scotch-Yoke Actuators with 3 Second Operation

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Critical Service Valves with
ROTEX DRS Scotch Yoke

A Central American power generation company needed 10" and 16", Class 900, triple-offset, critical service butterfly valves to operate flawlessly and quickly in the event of fire or loss of air supply.  They chose ROTEX Controls double acting DRS Scotch-Yoke actuators with hydraulic overrides.

The actuator package also specified a (5) second or less close time, a fusable link for emergency close in the event of fire, a declutchable manual override, and ASME air receiver tanks for "fail mode" that would allow for (2) full operational cycles.

ROTEX Controls engineers also included a 3-way fusable link that supplied air to a pilot operated shuttle valve. If the system was exposed to fire, the fusible link will fail, venting the air feed to the pilot causing the shuttle valve to shift and energize the actuator.

The result was an actuator that closed the valve in under (3) seconds. Additionally, the automated packages were delivered on time and under budget!

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What are NAMUR Mount and Pipe Mounted Solenoid Valve Mounting Styles?

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NAMUR Mount Solenoid

When selecting a solenoid valve for valve automation, the preferred mounting method needs to be considered. The two most common choices available are "NAMUR" mount and "PIPE" mount.

NAMUR on Actuator

NAMUR mount solenoids use a standardized "sub-plate" which months directly to the actuator on one side and to the solenoid valve on the other. This adaptor allows for quick change-out and minimizes the amount of plumbing required and is therefore less expensive to install.

Pipe Mount Solenoid

Pipe mount solenoids use the supply piping for support and mounting. Pipe mount solenoids are less compact and require more effort to service, but can be installed on any system as you are not limited to a pre-existing pattern and have more selection in valve size.

Pipe Mount on Actuator

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Advanced Scotch Yoke Design Increases Service Life

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ROTEX recently replaced some old "orange" scotch yoke actuators with the DRS series. The customer is a major international producer and marketer of food and agricultural products.  The "orange" actuators were experiencing high wear and had to be rotated out and serviced every few months. After the ROTEX DRS actors were installed, at last count, they had 125,000 cycles without any internal corrosion or degradation.

Let's examine the design improvements that allow this superior performance:

• Pneumatic cylinders are lined with hard chrome plating. Smooth to eliminate wear of piston seals, the surface is very hard, for long life.
• Springs are lifetime lubricated and epoxy coated.
• Inside the pneumatic module, dual oversized tie rods are protected from external corrosion. Where the tie rods pass through the piston head, ROTEX has improved the design by providing a slide bearing, where most designs don’t. This bearing is important to help maintain alignment and assure smooth operation.
• Inside the frame module, the yoke is supported by life time lubricated bearings. Needle roller bearings ensure the most efficient torque transmission. The piston shaft is supported on each side of the frame module by solid bearings that ensure proper alignment and smooth operation.
• Each piston shaft of the ROTEX DRS is equipped with two Flexijoints. These specially designed joints compensate for movement and changes in alignment to reduce internal stresses and increase output torque. The thrust transmitted through the Flexijoints ensures no side thrust on the cylinder walls and bearings.
• Piston seal is ROTEX 3X design. Flexibility of the sealing material reduces friction, prevents leakage, and extends actuator life. The 3X shaped seal is wear compensating, and the unique shape of the seal retains the grease for longer periods. The piston is guided by a PTFE thrust bearing, which helps keep the piston centered, and seal wear to a minimum.
• The tie rod seal is also of the ROTEX 3X design. Old fashioned O-rings are replaced by these flexible, low friction, resilient seals to reduce the failure rate of tie rod seals. Also reduces the wear and tear to the tie rods themselves.
• ROTEX has also replaced the standard O-ring of the shaft seal. In this position, we provide a proven V-seal design. This self-energized, wear compensating seal guarantees low hysteresis and minimum friction. Like the X seal of the piston and tie rod seals, the inherent shape of the V-seal helps keep the grease where it needs to be.

In 13 months, the ROTEX DRS went 125000 cycles without internal corrosion nor significant wear.

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What is a Rack & Pinion Actuator?

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According to Wikipedia, "A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby translating the rotational motion of the pinion into the linear motion of the rack."

Image courtesy of Wikipedia

When applied as a pneumatic valve actuator, opposing racks are attached to pistons in a cylinder. When one side is pressurized, the pinion bearing turns in one direction. When air or spring tension is applied to the opposite side, the pinion bearing turns in the other direction, thus allowing rotational motion in both directions.

Transparent actuator showing opposing pistons (ROTEX)

Pinion bearing and rack with piston and seal (ROTEX)

Housing which forms outer pneumatic cylinder walls (ROTEX)

Advantages of pneumatic rack and pinion actuators are compactness and space savings, reliability, durability and life cycle. Disadvantages are gear and bearing wear, piston seal fatigue and stem seal leakage.

Here is a great article on rack and pinion actuators from Valve Magazine.

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What is a Scotch Yoke Pneumatic Valve Actuator?

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According to Wikipedia, "The Scotch yoke is a mechanism for converting the linear motion of a slider into rotational motion or vice-versa. The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part. The shape of the motion of the piston is a pure sine wave over time given a constant rotational speed."

Animation of Scotch Yoke (courtesy of Wikipedia)

In a pneumatic actuator, the scotch yoke mechanism converts linear motion (provided by an air supply and pistons) to rotary motion. A return spring sets the rotation in the opposite direction.

Diagram of Scotch Yoke Internals (courtesy of ROTEX Controls)

Scotch yoke mechanism provides a great torque curve for many valve types and are most often used on larger valves. Additionally, as valve torque requirements increase, a scotch yoke actuator becomes a more economical choice considering production cost.

The disadvantages of scotch yoke actuators are typically the wear and tear on the pin and bearing mechanism. Manufacturers like ROTEX Controls have reduced this issue with innovative design improvements that reduce friction and wear.

ROTEX Controls DRS Scotch Yoke Actuator

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Defining a "Quick Acting" Pneumatic Actuator

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The desired speed of the pneumatic actuator opening or closing is a consideration of the application itself. For applications where an open valve may feed a fire, or overfill a tank, quick closing is required. Conversely, there are times when lines or tanks need to vented very quickly, or processes must be diverted quickly, and a fast opening actuator is applied.


Pneumatic Actuators can vary in opening and closing speed, depending on air supply, venting orifice, and/or size spring involved. A very general statement of valve actuator opening/closing times is normally in the "tens of seconds" whereas fast acting actuators open or close typically in less than 5 seconds.

The engineering of quick acting actuators must be done by seasoned professionals who fully understand the process. Other considerations when applying fast acting actuators are redundancy, failure mode (loss of energy supply), media, criticality and size.

Picture courtesy of Rencor Controls.

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