Industrial Valve Actuation: Rack and Pinion vs. Scotch-Yoke Actuators

Valve actuation in the industrial sector is a crucial component in regulating and controlling fluid flow. With a myriad of applications ranging from chemical processing to oil and gas extraction, the right choice of actuation can impact efficiency, safety, and system longevity. Among the popular options for pneumatic actuation are the rack and pinion actuators and scotch-yoke actuators. But how does one choose between them? Let's delve deeper into the mechanics, advantages, and ideal scenarios.

Basics of Valve Actuation:

Valve actuation is the mechanism that drives the valve to open, close, or modulate, controlling the flow of the medium (gas, liquid, or slurry). Pneumatic actuators utilize air pressure to provide the necessary motion, transforming the energy from the compressed air into mechanical movement.

Rack and Pinion Actuators:

Mechanism: Rack and pinion actuators comprise a cylindrical chamber housing a piston connected to a rack. When air pressure is applied, the piston moves, driving the rack to engage with a pinion, resulting in rotational movement.

Advantages:

1. Consistent Torque: They offer a uniform torque output throughout the rotation, aligning well with valves that require a nearly constant force, like ball or butterfly valves.
2. Compact Design: Typically more lightweight and compact, they're ideal for situations with limited space.
3. Durability: With fewer moving parts, they often exhibit longer lifespans in specific environments.
4. Cost-effective: Generally less expensive than their scotch-yoke counterparts for the same torque output.

Scotch-Yoke Actuators:

Mechanism: The scotch-yoke design converts linear motion into rotational motion using a yoke mechanism and a rotating pin. As the piston rod moves, the yoke slides along the rotating pin, producing rotational movement.

Advantages:

1. Variable Torque Profile: Their torque output isn't constant—it increases near the ends of rotation, which can be beneficial for valves like a gate or globe valves where torque needs to rise at the start and end of a stroke.
2. High Torque Capabilities: They can offer higher torque outputs for the same size as rack and pinion actuators in specific configurations.
3. Robustness: They can be more robust in heavy-duty applications requiring high torques.

Rack and Pinion vs. Scotch-Yoke: When to Specify Which?

• Torque Profile Needs:
• For ball or butterfly valves that demand consistent torque throughout the rotation, rack, and pinion actuators are preferable.
• The scotch-yoke design is advantageous for gate or globe valves that require high torque at the start and end of operation.
• Space & Weight Considerations:
• In constrained spaces or where weight is a concern, the compact nature of rack and pinion actuators may be beneficial.
• Durability & Maintenance:
• In clean environments, the more straightforward design of the rack and pinion might have an edge in terms of longevity.
• The wear and tear on the scotch-yoke's sliding parts may require more frequent maintenance in dirty or harsh conditions.
• Cost Sensitivity:
• Budget constraints might lean towards rack and pinion actuators, though weighing initial costs against potential maintenance and longevity is essential.
• Operational Speed:
• Depending on design specifics, speed requirements may favor one actuator type over the other.

Choosing between rack and pinion and scotch-yoke actuators is not a one-size-fits-all decision. Factors like torque requirements, environmental conditions, space constraints, and costs play vital roles. Engaging with manufacturers and understanding the specific needs of your application will ensure optimal system performance and longevity.

Mead O'Brien
https://meadobrien.com
(800) 874-9655

White Paper :Ten Reasons to Consider Brushless DCV Motors in Electric Valve Actuators

This paper, courtesy of Flowserve Limitorque, aims to investigate the most recent advancements in these motors, consider alternatives, and discuss how to make an informed decision about when and where to use BLDC motor technologies.

Brushless DC motors are synchronous motors powered by a direct current source via an electric controller rather than the brush/commutator mechanism used in brushed DC motors. The electric controller, an integrated inverter/switching power supply, generates an alternating current signal that drives the rotor. Electronically commutated motors, ECMs, or EC motors are other names for them.

Brushless DC electric (BLDC) motors have been around for nearly 50 years, but their use for intelligent actuation is relatively new. Their adaptation is critical to the improvement of process control and plant safety systems.

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DOWNLOAD THE WHITE PAPER HERE

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Leading Edge Electric Actuators - The Limitorque MXb

The Limitorque MXb electric actuator performs across a broad range of challenging applications where reliability is critical, including oil and gas; commercial power; chemical; fresh and wastewater; and general industries.

Improved reliability

The actuator's design isolates critical components, protecting them from electrical shock and interference, typical in extreme environments. High-quality materials extend actuator service life, operating ranges, and mean time between failure.

Enhanced user experience

An updated user interface coupled with a simplified, intuitive menu structure and larger, high-resolution LCD screen makes navigation easy and enables 50% faster commissioning, set-up, and operation. Users of any skill level can configure the actuator through various pre-configured or customization options for quick and error-free set-up and operation. A larger, higher-resolution LCD with a built-in ambient light sensor offers eight times the previous display's resolution to extend viewing distances up to 30 feet. Real-time torque graphs, alarm and event logs, and other data are accessible in higher-quality resolution.

Advanced diagnostics and analytics

The MXb electric actuator's next-generation diagnostics and analytics capabilities help operators monitor and track its performance and quickly respond to upset conditions. The MXb actuator has 500 times the previous MX model's memory capacity, allowing increased data capture and storage for higher degrees of process monitoring, data logging, and information feedback. Additionally, a real-time clock enables data log time stamping to support asset management functions and lifecycle analysis.

Simplified maintenance

A new electric connector design removes the need for brackets and hold-downs, making maintenance more straightforward and faster. And the enhanced connector design ensures robust connectivity throughout the rated seismic and vibration envelope.

For more information about the Limitorque MXb contact Mead O'Brien by calling (800) 892-2769 or visiting their website at https://meadobrien.com. 

Industrial Valve Actuators

Valve actuators are selected based upon a number of factors including torque necessary to operate the valve and the need for automatic actuation. Types of actuators include manual handwheel, manual lever, electrical motor, pneumatic, and solenoid. All actuators except manual handwheel and lever are adaptable to automatic actuation.

Handwheel (Metso)

Manual Actuators

Manual actuators are capable of placing the valve in any position but do not permit automatic operation. The most common type mechanical actuator is the handwheel. This type includes handwheels fixed to the stem and handwheels connected to the stem through gears.

Electric Motor Actuators

Electric Actuator (Limitorque)

Electric motors permit manual, semi-automatic, and automatic operation of the valve. Motors are used mostly for open-close functions, although they are adaptable to positioning the valve to any point opening. The motor is usually a, reversible, high speed type connected through a gear train to reduce the motor speed and thereby increase the torque at the stem. Direction of motor rotation determines direction of disk motion. The electrical actuation can be semi-automatic, as when the motor is started by a control system. A handwheel, which can be engaged to the gear train, provides for manual operating of the valve. Limit switches are normally provided to stop the motor automatically at full open and full closed valve positions. Limit switches are operated either physically by position of the valve or torsionally by torque of the motor.

Pneumatic Actuators

Pneumatic Actuator
(Metso Neles)

Pneumatic actuators provide for automatic or semi-automatic valve operation. These actuators translate an air signal into valve stem motion by air pressure acting on a vane, diaphragm, or piston connected to the stem. Pneumatic actuators are used in throttle valves for open-close positioning where fast action is required. When air pressure closes the valve and spring action opens the valve, the actuator is termed direct-acting. When air pressure opens the valve and spring action closes the valve, the actuator is termed reverse-acting. Double acting actuators have air supplied to both sides of the vane, diaphragm, or piston. The differential pressure across the diaphragm positions the valve stem. Automatic operation is provided when the air signals are automatically  controlled by circuitry. Semi-automatic operation is provided by manual switches in the circuitry to the air control valves.


Hydraulic Actuators

Hydraulic actuators provide for semi-automatic or automatic positioning of the valve, similar to the pneumatic actuators. These actuators use a piston to convert a signal pressure into valve stem motion. Hydraulic fluid is fed to either side of the piston while the other side is drained or bled. Water or oil is used as the hydraulic fluid. Solenoid valves are typically used for automatic control of the hydraulic fluid to direct either opening or closing of the valve. Manual valves can also be used for controlling the hydraulic fluid; thus providing semi-automatic operation.

Solenoid Actuated Valves

Solenoid Valve (ASCO)

Solenoid actuated valves provide for automatic open-close valve positioning. Most solenoid actuated valves also have a manual override that permits manual positioning of the valve for as long as the override is manually positioned. Solenoids position the valve by attracting a magnetic slug attached to the valve stem. In single solenoid valves, spring pressure acts against the motion of the slug when power is applied to the solenoid. These valves can be arranged such that power to the solenoid either opens or closes the valve. When power to the solenoid is removed, the spring returns the valve to the opposite position. Two solenoids can be used to provide for both opening and closing by applying power to the appropriate solenoid.

Single solenoid valves are termed fail open or fail closed depending on the position of the valve with the solenoid de-energized. Fail open solenoid valves are opened by spring pressure and closed by energizing the solenoid. Fail closed solenoid valves are closed by spring pressure and opened by energizing the solenoid. Double solenoid valves typically fail "as is." That is, the valve position does not change when both solenoids are de-energized.

One application of solenoid valves is in air systems such as those used to supply air to pneumatic valve actuators. The solenoid valves are used to control the air supply to the pneumatic actuator and thus the position of the pneumatic actuated valve.

Mead O'Brien can handle any valve actuation requirement you have. Contact them by calling (800) 892-2769 or by visiting https://meadobrien.com.