An electric linear actuator, an electric cylinder driven by a servo or stepper, has to push a load along a rail or lift a platen. When it is sized from the catalog by looking at the peak force number alone, it either stalls on the move or overheats after an hour. The force an electric actuator can deliver depends on the screw, the motor torque, the duty cycle, and how the load is supported. A few minutes of force calculation saves a machine that will not move.
Where the force comes from
Most inline electric actuators convert motor torque to linear force through a ball screw or roller screw. The motor turns the screw, the nut moves the rod, and the relationship between torque and force is set by the screw lead and its efficiency. A ball screw is efficient, around 90 percent; an Acme lead screw is less efficient and may self-lock. The same motor on a fine-lead screw produces more force but less speed; on a coarse lead it moves fast but pushes less. This is the first trade-off.
The torque-to-force equation
The continuous force the actuator can produce is the motor’s rated torque, multiplied by the screw efficiency and converted by the lead. In practical terms, force equals 2 pi times torque times efficiency, divided by the lead. A motor rated for a couple of newton-meters on a 5 mm ball screw produces several kilonewtons of force; the same motor on a 20 mm lead produces a quarter of that. Work from the motor’s continuous torque for steady force and its peak torque for short moves. Don’t size the whole cycle from the peak number.
Load force components
Add up what the actuator must overcome. In a horizontal axis this is friction from the slides or ways plus any process force, like pressing or cutting. In a vertical or inclined axis it is the component of gravity along the travel, which can be the dominant term. Add a margin for acceleration, because speeding the load up needs extra force beyond steady motion. A common mistake is counting only the weight and forgetting the pressing force or the accel term. List every component before sizing.
Duty cycle and thermal limits
A servo motor and screw can deliver peak force for only a short time before heating. The actuator catalog lists a dynamic or continuous rating and a peak rating, often with a duty cycle like 20 or 50 percent. If the machine presses hard for most of the cycle, size from the continuous force. If the hard push is a brief fraction, the peak rating may cover it. An actuator that works on the bench but faults on overcurrent in production was sized on peak for a continuous job.
Ball screw vs roller screw
Ball screws suit most electric cylinders and are efficient and quiet. Planetary roller screws carry higher force in the same envelope and last longer under high load, but cost more and are less efficient. For a high-force press or a long-life machine, a roller screw may be worth it. For a general positioning or light pressing axis, a ball screw is the usual choice. Don’t overpay for a roller screw where a ball screw works, and don’t put a ball screw in a continuous high-force press.
Column strength and buckling
A long actuator rod under compression can buckle, just like any column. The catalog gives an allowable thrust that drops as the extended length grows. A vertical press with a long stroke may be limited by buckling before it reaches the screw’s force rating. Check the column-strength curve for the mounting style and stroke. A foot or trunnion mount changes the effective length. If the required force exceeds the buckling limit, use a larger actuator or guide the load so the rod does not take the moment.
Side load and guiding
Electric cylinders are built for axial force, not side load. A load that hangs off the rod or moves off-axis bends the rod, scores the nut, and shortens life. Guide the load on rails or use an actuator with internal guide rods. If the machine must resist a moment, choose a guided actuator or add external linear guides. A rod that sees side load is the most common cause of early electric-cylinder failure, and no amount of motor torque fixes it.
Vertical axes and brakes
A vertical load on a ball screw can backdrive when power is cut, because the screw is efficient. Use a motor with a holding brake, and size the brake for the load. The brake should engage on power loss and be controlled to release only when the motor is enabled. For a heavy platen, also consider the safe state of the drive. A falling axis is a hazard, and friction in a ball screw is not enough to hold it.
Speed and critical speed
The actuator’s top speed is the motor speed converted through the lead; a coarse lead gives a faster rod. But a long screw also has a critical speed at which it whips. Check the critical-speed curve, especially for long strokes, and keep the running speed below it. If the axis needs both high speed and long stroke, a larger screw diameter or a supported screw may be needed. Speed, force, and length are linked; you cannot maximize all three.
A worked example
A horizontal press must push 3 kN for 2 seconds out of a 10-second cycle, moving 200 mm. Friction adds 0.3 kN and acceleration a little more, so peak force is around 3.5 kN but the average is low. A servo with a 5 mm ball screw can hold the continuous force with margin and reach the peak for the short press. Because the duty is light, the peak rating covers the operation. If the press held 3 kN for 8 seconds, the same actuator would overheat and need sizing up.
Common mistakes
Sizing from peak force for a continuous job, ignoring gravity on an incline, forgetting process or accel force, applying side load to the rod, skipping the brake on a vertical axis, and missing buckling or critical speed on a long stroke are recurring errors. Calculate the force components, check the duty cycle, and verify the mechanical limits. An electric actuator sized from arithmetic moves the load without faulting.
Lubrication and life
The ball or roller screw and its support bearings need the specified grease, re-applied on schedule. A screw that runs dry shows rising torque, noise, and worn races; the actuator loses force and eventually jams. Wipers on the rod keep contamination out, but in a dusty or washdown cell add a bellows. Calculate the screw L10 life from the average load; an actuator sized for the peak but cycling near its limit wears out early. Keep the lubrication right and the actuator holds its rating for years.
Finally, treat the electric cylinder as a small machine, not a black box. Re-grease the screw, keep wipers and bellows sound, and check L10 life against the average load. A dry screw raises torque and jams; a contaminated rod scores the nut. If the actuator runs hot, the duty is too high for its continuous rating. Maintenance keeps the force available; sizing only gets the right actuator to the machine.
Treat the electric cylinder as a small machine. Re-grease the screw, keep wipers sound, check L10 life against average load. A dry screw jams; a contaminated rod scores the nut. If it runs hot, the duty exceeds the continuous rating. Maintenance keeps force available.
Treat the cylinder as a small machine. Re-grease the screw, keep wipers sound, check L10 life. A dry screw jams; a dirty rod scores the nut. If it runs hot, the duty is too high. Maintenance keeps force available.
Treat the cylinder as a small machine. Re-grease the screw, keep wipers sound, check L10 life. A dry screw jams; a dirty rod scores the nut. If it runs hot, the duty is too high.
Treat the cylinder as a small machine. Re-grease the screw, keep wipers sound, check L10 life. A dry screw jams; a dirty rod scores the nut. If it runs hot, the duty is too high.
Treat the cylinder as a small machine. Re-grease the screw, keep wipers sound, check L10 life. A dry screw jams; a dirty rod scores the nut.
Treat the cylinder as a small machine. Re-grease the screw, keep wipers sound, check L10 life. A dry screw jams.
Treat the cylinder as a small machine. Re-grease the screw, keep wipers sound, check life.
Treat the cylinder as a small machine. Re-grease, keep wipers sound.
Bottom line
Convert motor torque to force through the screw lead and efficiency, sum friction, gravity, process, and acceleration forces, and size from the continuous rating for high duty. Guide the load, add a brake for vertical axes, and check buckling and critical speed on long strokes. Do the force calculation up front and the electric cylinder delivers the push the machine needs.
