Torque can be defined as any force that tends to create mechanical rotation. Put simply, torque is a measure of the quantity of force required to rotate a given body around a specific point or a pivot point.
Mathematically speaking, torque is the vector cross product of the distance of separation between the pivot and the point of application of the force, and the value of the force itself. While this results in a unit of measure for torque that is dimensionally identical to that of energy, the two terms are conceptually different. Torque is a vector (having both magnitude and direction), while energy is a scalar value that has magnitude only. Therefore, torque is usually expressed in the unit of Newton-meters (N⋅m) while energy or work is represented in Joules (J).
It’s important to remember that, unlike force, torque is also a function of the distance between the point of application of the force and the pivot.
Actuator Torque and Torque Ratings
Actuator torque is the quantity of torque that an actuator is capable of generating — a critical aspect of the rating specifications for any type of actuator. Torque ratings will determine the type of applications in which a specific actuator can be used. And when sizing an actuator, it is important to give consideration to all of the factors that determine the torque requirement for that application. These may include:
- breaking torque, which is the amount of torque needed to unseat an initially closed valve
- running torque, which is used to overcome the frictional forces of the load
- closing torque, which is needed to properly seat a valve to a closed position
If the actuator torque is not sufficient for a given application, serious issues may result. For example, if actuator torque is insufficient to turn a valve stem, the valve may not open or can become struck mid-stroke. Alternatively, if the actuator torque is significantly higher than required, the valve may be damaged. When specifying the requirement for actuator torque, it is customary to build in margins of safety such as an additional 25% requirement on the actuator torque value to prevent such problems.
Actuators are essentially chosen based on their ability to respond to torque values that are available within the target system. Therefore, the actuator torque values should match the torque values of the system within which it operates. Valve actuators play a critical role in the performance of any control loop, which means proper actuator selection is crucial.
Torque Value Considerations
As mentioned above, accurate torque ratings are critical for determining the ideal applications for an actuator. Consider the example of an automobile being driven up a hill. When the torque is insufficient to facilitate the climb, the gear ratio can be altered to increase the available torque. Similarly, an actuator motor converts the latent force available to the actuator into actuator torque.
Before choosing a valve actuator, users must know the actuator torque for the options being considered. The manufacturer will usually provide a specification sheet outlining different types of valves and the various torque values for the valves, such as breaking torque, closing torque, and running torque. An ideal actuator can then be chosen based on specific system characteristics, such as minimum and maximum supply pressure. If the actuator is pneumatically operated, the actuator should be capable of supplying the needed torque to the valve under the worst case (minimum) supply pressure and not just with the nominal value. Also, the specific type of valve (e.g. ball vs. gate) plus the properties of the media contained in the system (e.g. viscosity), and the material used in the seal and seat of the valve all impact the actuator sizing requirement.
A very low torque rating means that the actuator in question can only function over a small range of loads; a higher torque rating indicates that an actuator can function over a wide range of load variations.
Choosing the Right Actuator
Fully grasping torque ratings is essential for choosing the right type of actuator for a specific job, ensuring smooth, efficient, and safe operations. So before embarking on the buying journey, take the time to develop an understanding of torque and actuator torque ratings, and seek out a knowledgeable partner who can assist throughout every step of the selection process.
References:
- http://www.indelac.com/products.aspx
- https://www.gedore-torque.com/service-centre/torque/
- https://www.physics.uoguelph.ca/tutorials/torque/Q.torque.intro.html
- https://www.mpofcinci.com/blog/how-to-select-a-valve-actuator/#Considerations
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