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What is Rack and Pinion Valve Actuator?

Rack and pinion actuators are mechanical devices used to automatically open and close valves or dampers, usually for industrial applications.

“Rack and pinion” is a generic term for a pair of gears that convert linear motion into rotational motion. A linear gear bar called “the rack” engages teeth on a circular gear called “the pinion”. The linear force exerted on the rack will cause a rotational motion of the pinion.

For valve automation, a common actuator design uses two piston-type racks moving in opposite directions to ensure balanced forces on the pinion. Typically, pneumatic air pressure is used to power the actuator. By applying pressure to the piston racks, the pinion can be turned to the desired position. The pinion bottom connects to the valve stem to open and close the valve as the pinion turns.

Components of Rack and Pinion Valve Actuator

Features of Rack and Pinion Valve Actuator

Pneumatic rack and pinion actuators offer many features and benefits for the operational actuation of the ball, butterfly, and plug valves.

• • The compact and lightweight design provides an easy installation with a virtual maintenance free fit and forget unit. Whatever your location this is the perfect choice for hassle free installation.

• • Varied operational usage provides industries and applications with fast acting, fail-safe solutions for cycles requiring single to hundreds of operations per day.

• • Low initial cost outlays and extended life cycles offer cost-effective solutions to that expensive electric equivalents. When serviced correctly airlines provide little downside.

• • Direct mount facilities and international standard iso platforms allow quick assembly, testing, and despatch for critical applications. Solenoid Valves & Valve Limit Switches offer bolt on additions.

• • Clockwise or counterclockwise setup is available based on end users’ requirements. In-House or In-Field adaption is offered as part of after sales or pre-installation supply service.

• • Multiple protection coatings are available from Manufacturers as standard or as an OEM addition. This includes PTFE, Epoxy, Enamel & Marine specific RAL coatings.

Single Rack vs Dual Rack Design

Rack-and-pinion actuators offer the widest ranges of torque and rotation compared to other conversion mechanisms for converting linear force to rotational torque. It has high mechanical efficiency and the torques that they are able to produce range from a couple of Nm to multiple thousands of Nm.

However, one potential drawback of the rack-and-pinion design is backlash. Backlash occurs when rack and pinion gears are not completely aligned and there is a small gap between each geared connection. This misalignment can cause wear on the gears during the life cycle of the actuator, which in turn increases backlash.

A double rack unit uses a pair of racks on opposite sides of the pinion. This helps eliminate backlash due to a counter force and also doubles the output torque of the unit and increases the mechanical efficiency of the system.

Single Acting vs Double Acting

In a single-acting actuator, the air is only supplied to one side of the piston and is responsible for the movement of the piston in only one direction. The movement of the piston in the opposite direction is performed by a mechanical spring. Single-acting actuators conserve compressed air but perform work in only one direction. A downside of single-acting cylinders is the inconsistent output force through a full stroke due to the opposing spring force.

In a double-acting actuator, the air is supplied to chambers on both sides of the piston(s). Higher air pressure on one side can drive the piston(s) to the other side. Double-acting actuators are used when work needs to be performed in both directions.

One of the advantages of double-acting cylinders is the constant output force through a full rotation range. The drawbacks of double-acting cylinders are their need for compressed air for movement in both directions and a lack of a defined position in case of a power or pressure failure.

General Troubleshooting Procedure

Problems	Inspection	Solutions
Pneumatic valve isn’t working	The solenoid valve or its coil is damaged./The solenoid ports are plugged.	Replace the solenoid valve or its coil. Remove contaminants attached to the solenoid ports.
Pneumatic valve isn’t working	Test with the actuator under air supply pressure, to check the piston seals for leaking and the actuator for damage.	Replace the worn down piston seals or damaged actuator.
Pneumatic valve isn’t working	The valve is plugged with contaminants.	Remove contaminants or replace the damaged parts.
Pneumatic valve isn’t working	The valve is switched to manual mode.	Turn the handle of gear operator back to pneumatic mode.
Pneumatic valve is acting slowly, sluggishly	The motive pressure is incorrect.	Increase the air supply pressure to 0.4-0.7Mpa.
Pneumatic valve is acting slowly, sluggishly	The actuator is undersized with insufficient torque output.	Select the correct actuator model.
Pneumatic valve is acting slowly, sluggishly	There are problems with the valve binding or the other parts.	Re-adjust the valve binding.
Pneumatic valve is acting slowly, sluggishly	The power gas pressure is too low./The tubing connected to the gas is plugged.	Replace the accessories with larger ports./Remove blockage or filter
There is no indication from the limited switch.	There is a power board failure.	Check and repair the power supply.
There is no indication from the limited switch.	The position of the limited switch cam is not in accurate position.	Adjust the limited switch cam to the correct position.
There is no indication from the limited switch.	The microswitch has failed.	Replace the microswitch.