There are always numerous factors that go into choosing the right linear actuator or gearbox for an application. One consideration that is often overlooked is moment loading.
Moment load is when a force is applied that would cause an arm to rotate or bend in a way it was not necessarily designed to move. 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. It’s important to consider because if handled incorrectly even a minor force weighing on something like an actuator will warp it over time and make it not last as long. 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 right)
- 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 the 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 a 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. For example, your application may include a tension belt that holds the load in place. The force from that belt contributes to the moment load and needs to be calculated for. Excessive moment loads can result in poor performance and decreased service life, especially on machines running at higher RPMs.
This is why we offer many linear actuators with various guiding options to handle a wide range of moment loads. For instance, some customers add guides to our Duratrk rodless band cylinders to help them handle a greater moment load. You just have to make sure the load is secured to the guides and not just the carriage.
Proper installation and setup is key to getting the longest life out of a cylinder, gearbox, or really any of the products we sell and we’d be happy to answer any questions you have.
Q: What about moment load and right angle gearboxes?
Chad: W.C. Branham right angle gearboxes are not designed for taking on external radial or thrust load—the force will wear on the gears or put them out of alignment causing shortened service life or failure. We recommend using external bearing blocks to support the shafts on all sides of the gearbox.
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.
