Rack and pinion actuators are indispensable mechanical devices renowned for their ability to use fluid power to transform linear motion of a moving piston into a precise rotary motion of a drive shaft. This makes them a prime operator for automating the open and closing of valves and dampers. There are many variants within the over banner of rack and pinion actuator actuators so does need very careful and meticulous consideration when selecting a model to ensure it is tailored to the applications unique requirements so it will give a seamless integration within the system it is to be used.
The following guide will give some of the essential and key factors to consider during the selection process of a rack and pinion actuator:
1. Function – Double Acting or Spring Return
Rack and pinion actuators come in two main formats, double acting and spring return. Double acting (DA) units will use the operating media to push the pistons in both directions to either open or close the actuator. The spring return (SR) actuator only requires the operating media to power open the actuator, during this power stroke springs in the end of the actuator are compressed which can then close the actuator when operating media pressure is removed and the springs can push the pistons back to the closed position. The smaller cheaper option would be to use a double acting actuator but if the application requires a ‘Fail Safe’ position for the valve/actuator to return to in the event of an emergency then the spring return actuator will be the one to use.
One other function to consider in actuator style is the direction of operation, in the majority of cases valves need to conform to the convention of clockwise rotation to close the valve with the anti-clockwise operation opening the valve. During assembly configuration the actuator needs matching to the valve operating condition, i.e. the need for the valve to fail open or fail close and the spring powering the actuator in the correct direction.
2. Operating Media
Actuators are fluid power devices, meaning they can be operated by either a compressed gas or liquid with the most common media being compressed air as it is a relatively cheap and a widely available power source. It is commonplace for sites, normally those operating with hazardous working environments to use other inert gasses such as nitrogen to operate the actuator. Just because it is preferred to operate these units on inert gasses, providing a suitable actuator is chosen more explosive gas can used, this is a common practice on gas pipeline stations in remote areas where they use the natural gas in the actuator.
The majority of rack and pinion actuators in the valve industry are pneumatic but as expected with fluid power devices, they can also by operated hydraulically. With oil based systems operating at higher pressures this gives the opportunity to achieve greater torque outputs from the actuator. It is not unknown for actuators to work on water, these tend to be special applications often in the hydro power generation industry.
3. Degrees of Rotation
Due to the nature of the rack and pinion design these actuators cannot be multi-turn so are restricted to being ‘part-turn’ which suits a vast array of valves such as ball and butterfly as they only need a 90deg rotation. It is possible to have slight variants to this for other applications such as ventilation dampers in buildings and tunnels or Y-pattern valves for the pigging industry, alternative rotations can 120°, 135° and 180°.
Many rack and pinion actuators are fitted with external travel stop adjustments, this will allow the end of stroke position to be finely tuned ensuring the valve it is being fitted to will be fully and precisely ported.
4. Sizing – Torque Output
Understanding the minimum and maximum outputs of a rack and pinion actuator in comparison to the valve it is to use on is fundamental to its successful integration into any application. Having an actuator that has too low an output will not fully or reliably operate the valve whereas having too high an output initially sounds ok as it will certainly mean the valve will turn but the consequence could be if the valve locked or seized the actuator would continue to drive and potentially damage any component within the drive train including the valve stem.
The output of a rack and pinion actuator in its double acting form will give a constant torque whereas the spring return equivalent gives a linear output, decreasing at a constant rate during the media power stroke as it puts energy into the springs and in a similar way on the return stroke torque decreases in a linear pattern as the energy held in the springs diminishes.
5. Mount & Drive Connection
Most modern rack and pinion actuators now comply with the international standard ISO5211 ‘Industrial Valves – Part-Turn Actuator Attachment’ that covers the bolting arrangement and drive connection for mounting the actuator to valve. Even with the international standard not all valves will direct mount onto the valve so a bespoke mount kit is often required to marry the two pieces together.
6. Speed Requirements
The speed at which a rack and pinion actuator moves can be a crucial factor that directly impacts the efficiency and effectiveness of your application, having a water valve close to fast may cause ‘water hammer’ down a pipeline but not closing an emergency gas valve quick enough could lead to an explosion both of which can result in catastrophic and expensive damage.
7. Environmental Considerations
The environmental conditions in which your rack and pinion actuator operates can significantly impact its performance, reliability, and lifespan. Factors such as temperature, humidity, dust, and exposure to corrosive substances can all affect the actuator’s functionality. To ensure optimal performance, it’s crucial to assess and address these environmental considerations.
8. Control & Automation
The control and automation of a rack and pinion actuator are pivotal aspects of optimising its functionality within your application. There is a vast range of accessories specifically designed for fitting to rack and pinion actuators, providing they comply to modern international design standards, these include limit switchboxes and positioners for position feedback & modulating control and directional control solenoid valves that fix directly to the actuator body using the VDI/VDE 3845 NAMUR mount arrangement. The choice of control method can significantly impact the precision, speed, and adaptability of your system.
9. Maintenance Requirements
Long term performance is a critical aspect of the modern engineering world so it becomes important to ensure an actuator is chosen that can give required reliability and durability with low levels of failure and maintenance. Actuator internal construction can vary significantly among different manufacturers with some having additional bearings amongst other features to guide the pistons and lubrication that will last its lifetime.
9. Safety Considerations
With an increasing consideration to health and safety in the modern workplace and environmental concerns during emergency situations it has highlighted the importance of making sure safety critical equipment will work as intended when required. To this has been the introduction of the IEC 61508 Safety Integratory Level (SIL) standard which calls for all devices used within these critical systems, including rack and pinion actuators, to demonstrate suitability with proven reliability data.
- Customisation Options
Depending on your application’s unique requirements, you might need a customised rack and pinion actuator. As well as the fairly standard spring return and double acting features below is some of the special features that can be requested from some manufacturers:
- Varying external & Internal levels of environmental protection
- Alternative sealing options for differing medias and temperatures
- Varying degrees of rotation
- Fast Acting
- 3 Position operation
- 180° rotation with a fail position to the mid-point
- Hydraulically dampened for more precise speed control
Air Torque Rack & Pinion
By carefully considering these factors, you can make an informed decision about selecting a rack and pinion actuator for your application. Our strong collaboration with Air Torque Italy at AC Valve Alliance positions us as a prominent player in the pneumatic actuation market. We maintain extensive inventory levels of both aluminium and stainless-steel rack and pinion actuators, along with a comprehensive assortment of heavy-duty carbon steel scotch yoke actuators. In addition to our diverse actuator stock, we offer a wide variety of local control options and accessories in aluminium and stainless steel to enhance your actuation systems. Contact us today to ensure you choose the most suitable solution that aligns with your specific needs and objectives.
