Sizing a linear actuator means figuring out the force, speed, and stroke you need, then matching those numbers to what actuators can actually deliver. You’re also looking at duty cycle, where it’ll operate, and how you’ll mount it. Don’t forget safety margins because you want 1.5 to 2 times what you calculated as your minimum requirements.
Step 1: Determine Force Requirements and Calculate Motor Input
First thing you need to nail down is how much pushing or pulling power your actuator needs. There are different forces at play here, and they all add up.
Static Force
This is the straightforward part. Take the weight of what you’re moving, multiply mass by gravity, and add in whatever friction you’re dealing with. If you’re lifting something straight up, gravity fights you the whole way. Moving things horizontally? You’re mainly battling friction instead.
Dynamic Force from Inertia
Here’s where it gets more interesting. When you accelerate or slow down your load, inertia comes into play, and that takes extra force. The math is pretty simple:
- Force equals mass times acceleration
- To find acceleration, divide how fast you want to go by how long it takes to get there
- Triangular movement profiles (go from zero to peak speed, then immediately back to zero) need the highest acceleration forces
- Trapezoidal profiles (ramp up, cruise at steady speed, then ramp down) spread the forces out and reduce your peak requirements
Angled Loads and Safety Factors
If your actuator pushes or pulls at an angle instead of straight, you need to work out the force component along the actuator’s centerline. The toughest case usually happens when it’s fully retracted because that’s where the angle is steepest.
After you add up everything, multiply by a safety factor between 1.5 and 2. This cushion handles unexpected situations and keeps things running reliably over the long haul.
Calculate Motor Input Requirements
Once you know the forces and speeds for each step of your cycle, you can calculate what the motor needs to provide. For each cycle step, you’ll need:
Input torque: Min = (F · p · 100) / (2π · ηscrew) · 100 / (i · ηcoupler)
Where:
- F = Required force at this step (N)
- p = Screw lead (mm)
- ηscrew = Screw efficiency (%)
- i = Reduction ratio
- ηcoupler = Coupler efficiency (%)
Input speed: nin = (60 · i · v) / p
Where:
- v = Required screw output speed at this step (mm/s)
- i = Reduction ratio
- p = Screw lead (mm)
These calculations tell you what torque and speed the motor needs to deliver at each point in the cycle, which helps you select the right motor to pair with your actuator.
Step 2: Define Speed and Stroke Requirements
Once you know the force you need, it’s time to map out how your load actually moves.
Stroke Length
Measure how far your load travels from one end to the other. That distance is your required stroke, and whatever actuator you pick needs to handle at least that much travel.
Speed and Movement Profile
How quickly does your load need to move? You’ll usually see speed measured in millimeters per second or inches per second. Here’s something to remember: speed and force work against each other in most actuators. More pushing power typically means slower top speed because of how the gears are set up.
Figure out how fast you need to get up to speed and how quickly you need to stop. This ties right back into your force calculations:
- Triangular profiles move fastest but demand higher forces
- Trapezoidal profiles take more time but ease up on peak force needs
- Quicker acceleration always equals higher dynamic forces
Step 3: Verify Speed Requirements Against Actuator Limits
Your actuator doesn’t work in some perfect lab setting, so real world conditions and mechanical limits really matter here. You need to verify three different speed checks to make sure the actuator can handle what you’re asking.
Check 1: Critical Speed vs Maximum Speed
Long lead screws can resonate at certain speeds, kind of like a guitar string vibrating. This critical speed depends on stroke length and how the screw is supported. You’ll find the standard critical speed (Vcr_std) in the actuator datasheet based on the bearing configuration (fixed free or fixed single support).
If your stroke differs from standard, calculate the actual critical speed:
Vcr_l = Vcr_std · (lstd² / ls²)
Where:
- Vcr_std = Standard critical speed from datasheet (mm/s)
- lstd = Standard stroke length (mm)
- ls = Your actual stroke length (mm)
Make sure your maximum cycle speed stays below this critical speed, or you’ll get vibration and accelerated wear.
Check 2: Peak Output Speed vs Required Maximum Speed
Look at the datasheet for each available gear ratio and verify that the peak output speed (Vp max) exceeds your required maximum speed (Vmax). Not all gear ratios will work because higher force ratios trade off against maximum speed capability.
Check 3: Continuous Output Speed vs Average Speed
Your actuator might handle peak speeds fine but overheat if the average speed over the whole cycle exceeds its continuous rating. Calculate your average output speed across the cycle:
Vm = Σ(vi · ti / ttot)
Where:
- vi = Speed at each step of the cycle (mm/s)
- ti = Time spent at that speed (s)
- ttot = Total cycle time (s)
This weighted average tells you the continuous speed capability you actually need. Check that the continuous output speed rating (Vc max) for your chosen gear ratio exceeds this average.
Duty Cycle Considerations
Duty cycle tells you what percentage of time the actuator works versus rests. A 25% duty cycle means it runs a quarter of the time and sits idle the other three quarters. Light duty work can get by with lower ratings, but if you’re running frequently or nonstop, you need something rated for higher duty cycles or continuous operation.
Push an actuator past its duty cycle rating and it overheats. That heat breaks down lubricants and wears out components faster, especially in lead screw and ball screw designs where lubrication keeps everything working smoothly.
Step 4: Verify Force Requirements and Environmental Conditions
Now you need to check that the actuator can handle the forces without buckling, overloading, or failing over time.
Check 1: Buckling Force vs Maximum Force
Long strokes under compression can buckle like a column supporting too much weight. The datasheet gives you the standard buckling force (Fb_std) for the bearing configuration. If your stroke differs from standard, calculate actual buckling force:
Fb_l = Fb_std · (lstd² / ls²)
Where:
- Fb_std = Standard buckling force from datasheet (N)
- lstd = Standard stroke length (mm)
- ls = Your actual stroke length (mm)
Make sure this buckling force exceeds your maximum required force (Fmax) with a comfortable margin. Notice how longer strokes dramatically reduce buckling strength because stroke length is squared in the denominator.
Check 2: Peak Axial Force vs Required Maximum Force
For each available gear ratio, verify that the peak axial force rating (Fp max) exceeds your maximum required force (Fmax). The datasheet shows these limits for each gear ratio and drive stage.
Check 3: Continuous Axial Force vs Average Force
Just like with speed, you need to calculate the average force across your cycle to verify it doesn’t exceed continuous ratings. This uses a root mean cube calculation because force affects wear more dramatically than speed:
Fm = ³√[Σ(Fj³ · nj · tj / ttot) / nm]
Where:
- Fj = Force at each step of the cycle (N)
- nj = Number of direction changes at that force level
- tj = Time spent at that force (s)
- ttot = Total cycle time (s)
- nm = Total number of direction changes in cycle
Check that the continuous axial force rating (Fc max) for your chosen gear ratio exceeds this calculated average force.
Environmental Conditions
Think about where this thing will actually operate. What temperatures will it see? Is there dust floating around, moisture in the air, or chemicals that could cause problems? All this determines what protection rating you need. IP67 handles dust and brief water exposure pretty well, while IP68 steps up protection for tougher conditions.
If you’re dealing with corrosive environments or contamination issues, look for protective coatings and sealed construction. Modern linear electric actuators usually come with IP67 protection as standard, with IP68 available if you need it, plus corrosion resistant finishes that hold up in harsh industrial settings.
Life Expectancy
How many times does this actuator need to cycle over its life? Ball screw designs typically last longer and position more precisely than lead screw models, but they cost more upfront. If you’re looking at millions of cycles, this becomes pretty important.
Step 5: Calculate Power Requirements and Verify Selection
The final check is making sure your actuator can deliver the mechanical power your application needs.
Calculate Mechanical Power
For each step in your cycle, calculate the mechanical power required:
Pj = (vj · Fj) / 1000
Where:
- vj = Speed at this step (mm/s)
- Fj = Force at this step (N)
- Result is in Watts (W)
Find the maximum power across all cycle steps. This peak power (Pmax) must stay below the actuator’s power output limit (Pout_max) for the gear ratio you selected. The datasheet shows these power limits for each drive stage.
Match Specifications to Available Models
Now you’re comparing what you calculated against what actuators can actually do. When you’re looking at datasheets, consider these key specifications:
| Specification | What to Look For |
| Force Range | 2000N to 40000N depending on what you need |
| Control Modes | On off, modulating, or continuous operation |
| System Integration | Intelligent control or fieldbus options if needed |
| Environmental Protection | Explosion proof housing for hazardous locations |
Power and Physical Requirements
Check that voltage and current requirements line up with your power supply. A few things you can’t skip:
- Peak current draw during acceleration, when it pulls maximum power
- Physical fit in both retracted and fully extended positions
- Space for mounting brackets and any pivoting hardware
- Room for cable routing and maintenance access
Mounting Style and Safety Features
Verify the mounting configuration works for your application. Common options include clevis mounts when you need pivoting, flange mounts for fixed installations, and trunnion mounts for applications needing rotation around the actuator’s centerline.
Look for safety features built in, like electrical limit switches that stop travel automatically and prevent overtravel damage. Position feedback becomes important if you need precise control or you’re integrating into an automated system. Modern compact designs make installation easier, but you still need to confirm compatibility with your specific setup.
Iterate if Necessary
If nothing meets all your requirements after running through these calculations, you’ve got options. Dial back speed or acceleration to cut force demands. Adjust mounting geometry for better mechanical advantage. Or switch actuator technologies, like going from lead screw to ball screw construction. Sometimes you find that a different gear ratio solves multiple problems at once.
Bringing It All Together
Sizing correctly means working through these five steps in order. Calculate forces with proper safety buffers, map out your motion profile with input torque and speed requirements, verify critical speed and buckling limits against your stroke length, check continuous ratings against calculated averages using the proper formulas, then confirm power output and mounting works.
You’ll probably iterate a few times before finding something that hits all your requirements, and that’s normal. Adjust where you can until you land on a solution that meets what you actually need while delivering reliable performance over time.
