There are always numerous factors that go into choosing the right linear actuator for an application. One consideration that is often overlooked is moment loading. Moment load comes into play when the cylinder is located some distance away from the actual load that it's lifting. That distance may be very short, but you have to account for it along with the weight of the load in order to spec the proper cylinder.
Why? Because “moment” is the tendency of the driving force exerted to cause actuator rotation.
Calculating moment load is not simple, by any means. We asked Chad Randleman, our VP of Engineering and General Manager here at W.C. Branham, to explain.
Q: What is moment load?
Chad: A moment load is the result of a force (F) applied at a given moment arm distance (d) from an axis which otherwise would create rotation around the axis. Moment loads can be applied in any of three directions, or in multiple directions:
- Pitch (upward or downward movement)
- Yaw (movement to the left or fight)
- Roll (rotational movement)
During operation, gravity and the cylinder’s own acceleration or deceleration can cause moment loading. These dynamic moments can pull the cylinder out of alignment, degrading performance and ultimately damaging the cylinder. Linear actuators are designed to withstand certain levels of moment loading – called the permissible range – so it is critical to spec the right cylinder for the task.
There is also a static moment – loading when the cylinder is stopped. The permissible static moment depends on the physical strength of the table and linear guide.
Q: Why is moment load important (especially for linear actuators)?
Chad: Moment load calculations should be part of the analysis for any linear actuator application. The presence of moment loads creates additional forces on the actuator guide system and negatively influences performance and service life of the actuator. Properly analyzing moment loads will ensure that you choose the proper cylinder and guiding system.
Q: How do you calculate moment load for your application?
Chad: Moment loads are the product of the applied force (F) x the moment arm distance (d) as measured from the axis origin (F x d). Moment loads are typically specified in inch-pounds.
Although the formula seems simple, several factors influence moment loading.
- The size of the cylinder and the mass it is capable of transporting
- Positioning (vertical vs. horizontal stroke) – gravity has significantly greater effect on vertically-mounted cylinders, in effect increasing downward speed and slowing upward motion
- Positioning of the load – for example, if the load’s center of gravity overhangs the center of the table
- The actuator’s capabilities
To determine the overall moment load, you need to:
- Figure pitching direction moment, yawing direction moment, and rolling direction moment
- Divide those by the permissible dynamic moments for each direction as spec’d for the cylinder you’re considering
- Add up the resulting ratios
An acceptable result is less than 1.0.
Just as linear actuators vary in size and configuration, they also vary in their ability to support lifting or pulling loads as opposed to side loads.
Q: Are there any important factors to consider?
Chad: It is important to consider all applied moment loads for the application and verify that they are within the product’s specified limits. Excessive moment loads can result in poor performance and decreased service life. This is why we offer many linear actuators with various guiding options to handle a wide range of moment loads.
More questions about moment loading?
We aren’t surprised. The calculations can be complicated, and you want to choose an actuator that will perform well and reliably. We’re here to answer all your questions, about component spec’ing or other industrial design considerations, and we’re only a call or an email away.