How to Choose a Linear Actuator | XW Machinery
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How to Choose a Linear Actuator
How to choose a linear actuator is a decision problem, not a catalog lookup. The right unit is the one whose motion profile, force, duty cycle, and feedback match the control loop your machine actually runs—not the one with the biggest number on the page. Whether you are specifying a servo linear actuator for EPC web guiding or a simple lift, the method below turns an overwhelming spec sheet into a short list of well-justified choices. For the underlying principle, see our companion posts What Is a Linear Actuator? Complete Guide and How Does a Linear Actuator Work?.
Start With the Motion Profile, Not the Spec Sheet
Before you read a single datasheet, write down what the actuator must do. Four numbers define the motion profile: stroke (travel distance), peak speed, acceleration, and the positioning accuracy the process demands. A web-guide correction is a short, frequent, high-acceleration move; a guard lift is a long, infrequent, gentle travel. Those two applications want completely different actuators even at the same force.
The shape of the move matters as much as the extremes. A trapezoidal move (accelerate, cruise, decelerate) drives peak current and heat; a near-constant-velocity travel does not. Capture the motion profile first and you eliminate half the catalog before comparing part numbers.
Step 1: Define the Load — Peak, Holding, and Side-Load
"Force" is not one number. The peak force is what the actuator must deliver during the hardest move: accelerating the load mass plus the reaction from web tension plus friction. The holding force is what it needs when stationary. Size for peak, never for average—an actuator that holds but cannot accelerate will lag the loop and cause oscillation.
For web-handling lines, the silent killer is side-load: the lateral force the running web exerts on the correction roller. A web guide actuator that only sees axial thrust on paper will wear its guides prematurely when real web tension pushes sideways. We cover the exact force and load calculation in our upcoming Linear Actuator Force and Load Selection Guide; here, the job is to capture the inputs the calculation needs.
Step 2: Classify the Duty Cycle
The duty cycle—what fraction of time the actuator is actually moving or energized—decides whether a thermally fine unit stays fine. An actuator sized for peak force but run continuously can overheat and trip, just as a magnetic powder brake does under sustained slip. Classify your application before you trust a force rating.
| Duty class | Typical on-time | What to specify |
|---|---|---|
| Intermittent | < 20% | Standard unit, no extra cooling |
| Regular | 20–50% | Verify thermal rating, consider brake |
| Continuous | > 50% | Forced cooling or oversized frame |
| 24/7 correction | Near-constant | Servo with feedback, thermal margin |
If your application sits in the top two rows, the next steps still apply—but plan for heat from the start rather than discovering it on the line.
Step 3: Pick the Drive Family With a Decision Tree
Once the load and duty are known, resolve the drive family with a short decision tree. This step sets the type; the later steps refine it. (For a full comparison of every type, see Linear Actuator Types and Applications.)
- Must it self-correct at line speed with closed-loop accuracy? → Choose a servo (ball screw + encoder). Pair it with an EPC-600 servo web guide controller so actuator and controller share the same position language.
- Only occasional positioning (a setup stop, a lift, a clamp)? → A lead-screw electric actuator is enough and cheaper.
- Required thrust exceeds what a screw can deliver in the space, or the cycle is very slow and very heavy? → Move to hydraulic.
- Simple, low-cost push/pull at end stops, accuracy not critical? → Pneumatic is the cheapest entry.
The tree resolves the family; if you cannot have everything, relax cost before you relax accuracy on a guiding application—an overspecified-but-stable loop beats a cheap one that drifts.
Step 4: Match Feedback and Repeatability to the Loop
Closed-loop web guiding needs encoder feedback, and the actuator must speak the same bus as the controller. An all-in-one web guide control system bundles sensor, controller, and actuator so the loop is tuned as one unit. Decide between an incremental encoder (counts from a reference, loses position on power loss) and an absolute encoder (knows position immediately after restart) based on whether the machine must recover its preset on a reboot.
Bandwidth matters more than the headline repeatability number. The actuator's response must exceed the disturbance frequency—if the web can drift faster than the actuator can correct, no accuracy rating saves you.
Step 5: Account for the Real Environment
Temperature, dust, and washdown change the seal and material choice. For web lines, two environment factors are easy to miss: continuous side-load (covered in Step 1) and the washdown or corrosive exposure of coating and packaging rooms. Specify rigid guidance and the right seal rating, or expect premature wear regardless of how perfect the force calculation was.
A Worked Selection Example
Consider an EPC edge guide on a 600 m/min slitter. The correction roller needs 50 mm of stroke, corrects roughly twice per second, peaks at about 800 N including web reaction, must hold ±0.02 mm repeatability, and runs 24/7. Walking the method: the motion profile is short and high-frequency; peak force is 800 N; duty is continuous; it must self-correct at line speed; the environment is a dusty converting floor. The decision tree points straight to a servo ball-screw actuator with encoder feedback, paired with the EPC-600 controller and a steering frame for web guiding to convert linear travel into web angle. A lead screw or pneumatic option fails the closed-loop and continuous-duty tests outright.
Common Selection Mistakes
- Over-specifying. Buying a servo where a lead-screw electric would do wastes budget on accuracy the process never uses.
- Under-specifying duty cycle. A unit fine for intermittent use overheats and trips under continuous correction.
- Ignoring side-load. Axial-only ratings hide the lateral wear that web tension actually causes.
- Mismatching the bus. A great servo actuator behind a controller it cannot talk to wastes its feedback.
- Forgetting fail-safe. A jammed or powered-down carriage should fault or retract safely, not bend the frame.
- Treating catalog max stroke as the spec. Size the real travel and add 10–20% margin.
Selection Worksheet
| Decision factor | What to capture | Where it points |
|---|---|---|
| Motion profile | Stroke, peak speed, acceleration, accuracy | Rules out families immediately |
| Load | Peak vs holding force, side-load | Sizes the actuator |
| Duty cycle | On-time percentage | Sets cooling / frame size |
| Drive family | Tree result (servo / lead screw / hydraulic / pneumatic) | Picks the type |
| Feedback | Encoder type, bus, bandwidth | Matches the controller |
| Environment | Temp, dust, washdown, side-load | Drives seals and materials |
Fill this worksheet before requesting a quote and the supplier can confirm the spec in minutes instead of guessing.
FAQ
Q: How do I choose between a servo and a lead-screw linear actuator? A: If the load must self-correct at line speed, choose servo with encoder feedback. If you only position occasionally at a setup stop or lift, a lead-screw electric actuator is usually enough and cheaper.
Q: What duty cycle can a servo linear actuator handle? A: A servo with encoder feedback handles continuous and 24/7 correction duty when sized with thermal margin. Intermittent and regular duty classes need no special cooling. Confirm the thermal rating against your on-time percentage.
Q: Do I need an absolute or incremental encoder? A: Choose absolute if the machine must recover its preset position immediately after a power loss; choose incremental where a homing move at startup is acceptable and cost matters.
Q: How much stroke margin should I add? A: Add 10–20% to the real correction travel so the actuator is never driven to its mechanical limit during setup or web variation.
Conclusion & Next Step
How to choose a linear actuator comes down to a method: define the motion profile, size the load, classify the duty cycle, resolve the drive family with a decision tree, then match feedback and environment to the loop. Do that and the catalog stops being a maze.
Need help specifying the right servo linear actuator for EPC web guide for your slitter, rewinder, or printing press? Contact XW Machinery for a free solution and factory-direct quote.