How electric linear actuators work

In short:An electric linear actuator turns a motor's rotation into straight-line motion, usually through a screw and nut (lead, ball or roller screw) or a toothed belt; a linear motor skips the conversion and drives the load directly. The mechanism sets force, speed, stroke, efficiency and whether the load can back-drive the motor, while the motor and feedback set how precisely it is controlled.

What an electric linear actuator is

An electric linear actuator produces straight-line push, pull or positioning from an electric motor. Almost every industrial design has the same chain:

1. Motor: brushed DC, stepper, or brushless servo.
2. Reduction (optional): a gearbox, worm gear or belt-and-pulley stage between motor and screw.
3. Rotary-to-linear mechanism: a screw and nut, or a toothed belt and pulleys.
4. Output: a rod that extends from a tube, or a carriage that runs along the body.
5. Guidance and bearings: a thrust bearing to take the screw’s axial load, and guides that take side load.
6. Feedback: limit switches, a potentiometer, Hall sensors or an encoder.

A linear motor is the exception: it creates force directly on the moving part, with no screw or belt.

The choice of mechanism fixes most of the actuator’s behaviour: force, speed, stroke, efficiency, accuracy, life and whether it holds position without power. For hands-on sizing, use how to size a linear actuator; this guide explains the mechanisms.

Lead screws, ball screws and roller screws compared

A screw converts one motor revolution into one lead of travel. A 10 mm lead screw moves the nut 10 mm per turn. Force and torque are linked by the lead and efficiency: drive torque T = F × P / (2000 × π × η), with F in N, lead P in mm and T in Nm (Linear Motion Tips).

Sources: Thomson, Moog.

Lead screws suit low-cost DC actuators, medical and furniture products, and set-up axes. Friction heats the nut, which is why duty cycle is limited: Thomson notes bronze-nut assemblies may drop as low as 10% duty under heavy load.

Ball screws are the industrial default for servo and stepper actuators. Higher efficiency means a smaller motor and cooler running. Long screws are limited by critical speed (screw whip), which falls with unsupported length.

Roller screws carry more load in the same diameter because the rollers give many more contact points, which Moog links to higher load capacity, longer life and higher axial stiffness. They are the usual route for replacing hydraulic cylinders; see hydraulic to electric actuator and ball screw vs roller screw.

Belt-driven actuators

A timing belt runs between a drive pulley and an idler, with the carriage clamped to it. Travel per motor revolution equals the pulley’s pitch circumference, typically 60 to 170 mm, so belts are fast but need more torque (or a gearbox) for a given force.

Tolomatic’s own comparison of its screw and belt rodless actuators (Tolomatic):

Glikin at Tolomatic sums it up: belts for long stroke, high velocity and high acceleration; screws for moderate stroke and speed where accuracy matters (Tolomatic blog). More in rodless belt vs screw actuators.

Linear motors: direct drive

A linear motor is a servo motor unrolled flat (or into a tube). The coil assembly, or forcer, rides on the carriage, and the magnet track lies along the stroke. With no mechanical transmission there is no backlash, and Parker’s engineers say response can be more than 100 times that of a mechanical transmission, with typical speeds of 3 m/s at 1 micron resolution (Process Online).

The trade-offs:

• Force density is low. Parker gives the example of a linear motor producing about 65 N continuous in a 50 × 40 mm cross-section, while a 10 mm ball screw produces over 400 N.
• Heat goes into the load, because the forcer is attached to the carriage.
• Iron-core motors give the most force and cost least but have cogging and a magnetic attraction of up to about ten times continuous thrust, which the bearings must carry. Ironless (air-core) motors have no cogging or attraction but less force and poorer thermal performance.
• No holding force when de-energised, so vertical axes need a brake or counterbalance (Machine Design).

Linear motors pay off on fast, light, precise axes such as pick-and-place, inspection and semiconductor stages. See how to select a linear motor.

Rod vs rodless actuators

• Rod actuators push and pull through a rod extending from a sealed tube, like a pneumatic or hydraulic cylinder. They suit pressing, lifting, clamping and anything reaching into a machine. The rod should be guided externally if side loads are present.
• Rodless actuators drive a carriage along the body. Tolomatic notes that the stroke lies entirely within the body length, giving a smaller working footprint, and that rodless actuators carry the load rather than push it, which can remove separate guides (Tolomatic). Check pitch, roll and yaw moments on the carriage bearings: Tolomatic’s example is a 30 lb load at 3 in offset giving a 90 in-lb pitch moment.

Motors and feedback

For DC actuators, Progressive Automations explains the options: potentiometers give absolute position at start-up but have sliding contacts that wear; Hall-effect sensors count pulses from a magnet on a gear, giving finer resolution but needing a homing run; limit switches stop the motor at the ends but give no position in between (Progressive Automations).

Duty cycle matters most on DC actuators. Thomson’s Electrak HD, for example, is rated at 25% full-load duty at 25 °C on most models (Thomson). Servo actuators are usually rated for continuous duty within their thermal limits. See servo vs stepper for the motor choice.

Efficiency and back-driving

Efficiency decides two things: how much motor torque you need, and whether the load can drive the motor backwards when power is off.

• Thomson: a lead screw becomes back-drivable once its efficiency exceeds 50%; ball screws are back-drivable because of their high efficiency (Thomson).
• Tolomatic: belt drives always back-drive, and vertical belt drives need brakes (Tolomatic).

Rule of thumb: never rely on screw friction to hold a suspended load where a drop would hurt someone or damage the machine. Vibration can creep a nominally self-locking screw, and lubricant changes friction. Specify a spring-applied brake, sized on the back-driving torque, and treat the screw’s resistance as a bonus.

Worked example: a vertical lift axis

Duty: lift a 50 kg tool vertically at 200 mm/s, accelerating at 2 m/s². Stroke 400 mm.

Force: gravity 50 × 9.81 = 490.5 N; acceleration 50 × 2 = 100 N; total 590.5 N during the upward acceleration.

Ball screw, 10 mm lead, η = 0.9:

• Torque T = 590.5 × 10 / (2000 × π × 0.9) = 1.04 Nm
• Screw speed = 200 / 10 = 20 rev/s = 1,200 rpm
• Mechanical output 590.5 × 0.2 = 118 W; motor output about 131 W
• At rest, gravity alone back-drives the screw with roughly 490.5 × 10 × 0.9 / (2000 × π) ≈ 0.7 Nm at the motor. A brake is mandatory.

Lead screw, 10 mm lead, η = 0.35:

• Torque = 590.5 × 10 / (2000 × π × 0.35) = 2.69 Nm, about 2.6 times the ball screw
• Motor output about 337 W, of which some 219 W becomes heat in the nut. At that loss, duty cycle limits bite hard.
• At 35% efficiency it should resist back-driving, but add a brake anyway for a suspended tool.

Belt drive: fast enough, but it always back-drives and has limited thrust. Brake essential, and check belt stretch under a hanging load.

Linear motor: it must produce 490.5 N continuously just to hover, all of it as heat in the carriage. That exceeds the continuous rating of many compact iron-core motors, so a pneumatic counterbalance and a brake would be needed.

Verdict: a ball screw rod or rodless actuator with a servo motor, a holding brake sized above 0.7 Nm with margin, and a check of the screw’s critical speed at 1,200 rpm over a 400 mm stroke.

Products in our range to consider

These are available through our supply network:

• Tolomatic RSA-ST: general-purpose rod actuator with lead, bronze or ball screws, up to about 17.9 kN and 1,524 mm stroke.
• Tolomatic RSA-HT: high-thrust version with planetary roller screw options, up to 58 kN.
• SRB Linear Servo Actuator: ball-screw servo electric cylinder in seven frame sizes, up to about 49 kN.
• Tolomatic B3W: belt-drive rodless actuator for strokes to 14.6 m and speeds to 5 m/s.
• LB Belt-Drive Linear Actuator: steel-belt module to 3,050 mm stroke and 2,000 mm/s.
• LA In-line Linear Actuators: 12/24 V DC lead-screw actuators up to 6,000 N, IP66 standard.
• DINGS captive linear stepper: hybrid stepper with integral anti-rotation lead screw, NEMA 11 to 34, up to 2,400 N.
• Akribis AJM iron-core linear motor: compact iron-core motor, up to 446.8 N continuous and 5 m/s.

Things that catch engineers out

• Sizing on average force and missing acceleration or process peaks.
• Forgetting the brake on a vertical ball screw or belt axis.
• Exceeding a lead screw’s duty cycle and cooking the nut.
• Side-loading a rod actuator that has no external guide.
• Long screws at high speed: critical speed falls with length.
• Linear motors on vertical axes without a counterbalance.

Checklist

Send these to our engineers, or enter them in the spec builder:

• Force: static, dynamic and peak, and direction (push, pull or both)
• Moving mass, orientation (horizontal, vertical, inclined) and any side or moment loads
• Stroke, speed, acceleration and move profile
• Duty cycle: moves per minute and hours per day
• Accuracy and repeatability required
• Whether the load must hold without power, and the consequences if it does not
• Supply: 12/24/48 V DC or mains AC; control interface
• Feedback needed: end of stroke, absolute position, encoder
• Environment: IP rating, washdown, temperature, dust
• Mounting: rod or rodless, envelope and fixing

For the full family, see the linear actuators hub and how to select a linear actuator.