Showing posts with label rack and pinion. Show all posts
Showing posts with label rack and pinion. Show all posts

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.


Intro to ROTEX Solenoids, Valve Actuators and Limit Switches

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Introduction of ROTEX Controls product line of solenoid valves, Infinity ECV line of rack and pinion actuators, DRS and DRV scotch yoke actuators, and HYV hydraulic actuators and limits switches.

ROTEX offers a range of Solenoid Valves and Air Operated Valves used for handling almost any fluid with various of temperature, pressure or vacuum for use in Control and Automation applications.

ROTEX limit switches are designed to last and meet your required service conditions. Both the weather proof and explosion proof switches feature an aluminum die cast enclosure that is powder coated for superior corrosion resistance resulting in a weather proof enclosure to IP67. Stainless steel enclosures are available, also confirming to IP67.

Valve Actuation 101: Comparative Review of Pneumatic Actuators and Advantages of Rack and Pinion

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Here's is a good presentation on the various types of industrial pneumatic valve actuators, such as scotch yoke, vane, diaphragm and rack and pinion, and their respective advantages and disadvantages.



Assembly Animation of ROTEX Rack & Pinion Actuators

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Here is a good animation detailing the primary components and construction of the ROTEX ECV pneumatic valve actuator:



ROTEX ECV actuators distinguish themselves from other rack and pinion manufacturers by offering the following advanced features:
  • Self energizing V-Seal piston rings
  • Double rack & pinion
  • High corrosion resistance
  • Pinion supported on polyacetal bearings
  • Maximum working pressure 125 psig, optional 150 psig
  • No lubrication required
  • Temperature range -20 °F to +230 °F
  • Anodized body inside and out
  • Hard anodized pistons
  • ISO mounting for interchangeability
  • Anti-blowout pinion
  • Can be converted to spring return by just adding springs
  • Smooth burnished cylinder wall

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 and pinion actuator
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 actuator
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.

Proper Actuator to Valve Torque Sizing

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pneumatic actuator torque chart
A quarter turn pneumatic actuator, wether direct acting or spring return, has a given output torque based upon the air supply pressure provided. This output torque can be fairly linear, or very non-linear depending on the style of the actuator (rack and pinion vs. scotch yoke) or if the application calls for springs to be used for opening or closing.

It is critically important to properly select and match the design and torque requirements of the pneumatic actuator and the valve in any given process. Improper selection will result in failure, much higher installed cost, not to mention lost production.


There are two main ways an improperly sized actuator costs money. If the actuator is too small (underpowered at a given line pressure to provide ample seating and unseating torque), it will fail and need to be replaced. If an actuator is severely oversized, the cost of the actuation package is higher, and space is wasted accommodating the larger physical size.

Many actuator failures result before the valve and actuator are even installed because of improper sizing. Proper application engineering calls for proper use of safety factors.

A safety factor is applied when sizing a pneumatic actuator. These recommended safety factors vary slightly from manufacturer to manufacturer. However, applying a safety factor is not enough. Special consideration for the valve torque curve from fully closed to fully open must be evaluated. After the valve full-stroke torque is known, then the actuator torque curve should be evaluated side by side to assure that the safety factor exceeds all point of the valve torque curve. Otherwise, an accidental "dead-zone" where the actuator torque curve is inadequate to move the valve at a certain part of the valve stroke can happen.

Obviously, its not a good idea to apply an actuator with too small a safety factor. These actuators can add maintenance costs due to erratic operation or a higher likelihood of failing all together.

Quarter turn valves and actuators are often used in demanding application requiring high cycle rate, exposure to harsh environments and / or control critical processes such as hazardous chemicals. For these reasons a great deal of attention and care must be given to proper sizing. If you're unsure about any given application, make sure you contact an expert who can help you.

Industrial Valve Limit Switches

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ROTEX Limit Switches
In industrial valve terms, a limit switch is a device containing one or more magnetic or electrical switches, operated by the rotational or linear movement of the valve actuator. They are used to show the open or closed position of an industrial valve, or as safety interlocks to protect man or machine. The enclosure is usually classified in NEMA (National Electrical Manufacturers Association) Standards, such as NEMA 4, NEMA 4X or NEMA 7 (explosion resistant).

ROTEX Limit Switch
Internal Mechanical Limit Switch
The internal switches can be electromechanical or magnetic. Electromechanical switches respond to a mechanical cam on a shaft to change state (make or break electrical contacts). Magnetic switches use a magnetic field to change state in a reed switch, or  to sense a disruption in the magnetic field, or through capacitance.

ROTEX Limit Switch
Internal Magnetic Limit Switch
Along with an electrical signal, visual indication is an important part of a limit switch. Most switch come standard with a "dome" or "beacon" displaying "open" or "closed" or positions anywhere in between.

When applying limit switches on industrial valve application one should consider the following:


  1. Environment - is the limit switch in an area where there may be dust, spray-down, rain, corrosive gasses, combustible gases, or require sanitary conditions. Choosing the correct NEMA classification is critical.
  2. The electrical rating of the signal being transmitted. AC or DC? High voltage or low?
  3. Operation Frequency and Cycle life - will the switch only open/close occasionally, or will it cycle every few seconds? Mechanical switches have inherently shorter cycle lives than do proximity or magnetic switches. Choose the best switch considering load and cycle life.
  4. Visual indication - do you need to see the valve status from a distance? 
  5. Auxiliary outputs - besides opening of closing the primary signal, are their other signals you must send by using additional switches in the housing. 


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.
ROTEX Rack and pinion actuator
Transparent actuator showing opposing pistons (ROTEX)

rack and pinion bearing
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.


Defining a "Quick Acting" Pneumatic Actuator

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ROTEX Fast acting actuator
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.