Knowing the load does not always tell you how much force a linear actuator needs to produce.
If an actuator pushes or pulls a load directly along a straight path, the relationship between the load and actuator force can be relatively straightforward. Once the actuator drives a lever, hinged arm, or another pivoting mechanism, the geometry between the actuator and the load affects how much actuator force is required.
The position of the pivot, the actuator attachment point, and the angle between the actuator and the lever all affect how effectively the actuator force moves the lever. This relationship is described by mechanical advantage.
For engineers working with compact mechanisms, this matters because the same load can require very different actuator forces depending on the actuator attachment point and the angle at which it acts on the lever.
What Is Mechanical Advantage?
Mechanical advantage describes how the geometry of a lever changes the amount of actuator force needed to move or hold a load.
A lever is one of the simplest examples. It consists of a rigid member rotating around a pivot, or fulcrum. Actuator force applied at one point on the lever creates a turning effect around that pivot.
For an ideal lever where the input force and load force act perpendicular to their respective lever arms:
Ideal Mechanical Advantage = Effort Arm ÷ Load Arm
The effort arm is the distance from the pivot to the point where the input force is applied. In the lever-driven actuator mechanism shown here, this is the actuator attachment point.
The load arm is the distance from the pivot to the point where the resisting load acts.
This relationship shows that the force required from an actuator depends not only on the load, but also on where the actuator and load act relative to the pivot.
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[DIAGRAM A: Show a pivoting lever with the pivot at one end, a load acting farther along the lever, and a linear actuator connected at an intermediate point. Label actuator force Fa, attachment distance ra, load force Fl, load distance rl, and actuator angle θa.]
Why the Actuator Attachment Point Matters
The actuator attachment point is the point where the actuator connects to the lever. Its distance from the pivot affects how much actuator force is needed to rotate the lever.
If the actuator attaches farther from the pivot, and the actuator angle remains comparable, the same actuator force creates a greater turning effect around the pivot. Less actuator force may therefore be needed to move the same load.
If the actuator attaches closer to the pivot, the same actuator force creates a smaller turning effect. More actuator force may therefore be required.
The attachment point also affects how far the actuator must extend or retract as the lever moves. For this reason, the attachment position should be considered together with the required actuator stroke and available installation space. The actuator mounting arrangement must also allow the actuator to pivot through the required movement without introducing side loading or binding .
Why Actuator Angle Matters
The actuator attachment point is not the only factor that affects how effectively the actuator rotates the lever. The angle between the actuator and the lever also matters.
A linear actuator produces force along its own axis. Only part of that actuator force contributes to rotation around the pivot, depending on the angle at which the actuator acts on the lever.
For the actuator side of the mechanism, the basic torque relationship is:
τ = rF sin θ
Where:
- τ is the turning effect, or torque, around the pivot
- r is the distance from the pivot to the actuator attachment point
- F is the actuator force
- θ is the angle between the lever arm and the direction of the actuator force
When the actuator acts closer to perpendicular to the lever, more of the actuator force contributes to rotating the lever. As the actuator becomes more closely aligned with the lever, its turning effect decreases, so more actuator force may be required to move the same load.
This is why both the actuator attachment point and the actuator angle need to be considered when evaluating a lever-driven mechanism.
How Geometry Changes Force and Stroke During Movement
As the actuator extends or retracts, the lever rotates and the angle between the actuator and lever changes. The actuator attachment point stays at the same distance from the pivot, but the changing angle affects how effectively the actuator force produces rotation.
At some positions, the actuator force creates a greater turning effect around the pivot. At other positions, particularly when the actuator becomes more closely aligned with the lever, its turning effect decreases and more actuator force may be required. The resisting torque created by the load can also change as the lever moves, depending on the direction of the load and the mechanism geometry. This is why checking the mechanism at only one position can give an incomplete picture of the actuator force required.
The same geometry also affects actuator stroke. If the actuator attachment point is moved farther from the pivot, that point travels through a larger arc as the lever rotates. Depending on the location of the actuator's fixed mount, the actuator may need to change length by a greater amount to move the lever through the required angle.
Moving the attachment point closer to the pivot may reduce the required actuator travel, but it can also increase the actuator force required.
This creates an important design trade-off: geometry that reduces the required actuator force can also require more actuator travel.
The mechanism should therefore be evaluated throughout its intended range of motion. Engineers should check both the highest actuator force requirement and the total actuator stroke needed between the required end positions. The goal is not simply to maximize mechanical advantage, but to find a geometry that provides the required force and movement within the available actuator stroke and installation space.
[DIAGRAM B: Show the lever in retracted and extended positions. Keep the actuator base fixed and the actuator attachment point fixed on the lever. Show that the angle between the actuator and lever changes while the distance from the pivot to the attachment point remains constant. Label the actuator lengths in both positions to show how required stroke comes from the change in actuator length.]
Related Application: See how actuator stroke and linkage geometry were considered in a robotic harvesting end effector using a P16 linear actuator .
When Does Mechanical Advantage Matter?
Mechanical advantage becomes important when a linear actuator transfers force through a pivot, lever, or linkage before that force reaches the load. The geometry of the mechanism can either reduce or increase the actuator force required. In some arrangements, the actuator may need to produce more force than the applied load force because of the actuator attachment point and the angle at which the actuator acts on the lever.
When an actuator moves a load directly in a straight line, there is no lever or pivot changing the force required. The actuator force is therefore more directly related to the load being moved.
A Practical Way to Evaluate a Lever-Driven Mechanism
When evaluating a lever-driven actuator mechanism:
- Define the required movement. Identify the pivot, load location, and required lever rotation.
- Determine the load's turning effect around the pivot. Consider where the load acts relative to the pivot.
- Choose an actuator attachment point. Consider both its distance from the pivot and the available installation space.
- Check the actuator angle. Evaluate the direction of the actuator force relative to the lever arm.
- Review the full range of motion. Check how the actuator angle and actuator force requirement changes as the mechanism moves.
- Determine the required actuator stroke. Measure the actuator length at the mechanism's two required positions. The difference between them is the required stroke.
- Identify the highest actuator force requirement. Do not base actuator selection on only one position of the mechanism.
- Compare the required force and stroke with the actuator specifications.
Real assemblies can also introduce friction, component flex, tolerances, and additional loads. Physical testing is therefore important before finalizing the design.
Mechanical advantage helps determine how much force the actuator must provide to move the mechanism. The actuator specifications then determine whether a particular model can provide the required force, stroke, speed , and other operating requirements.
For a lever-driven mechanism, actuator selection should therefore follow the geometry evaluation rather than begin with the applied load alone. Once the required actuator force and stroke are known, those values can be compared with the available actuator options .
Find an Actuator for Your Force and Stroke Requirements
Once the required actuator force and stroke have been established, use the Actuonix Actuator Selector Tool to compare actuator options based on the requirements of the mechanism.
