A linear actuator is a device that converts energy into controlled straight-line motion and force, as opposed to the rotary motion produced by an ordinary motor. In robotics, linear actuators push, pull, lift, clamp and position loads along an axis, for example in Cartesian gantries, robot grippers, legged robots, exoskeletons and adjustable mechanisms.
Linear actuators are classified by their energy source and transmission: electromechanical designs with screws, belts or direct-drive linear motors, fluid-powered pneumatic and hydraulic cylinders, and piezoelectric or other smart-material actuators for very small, precise strokes. Each family offers a different balance of force, speed, precision, stiffness and controllability.
How linear actuators create straight-line motion
Most electric linear actuators start with a rotary motor and a mechanical conversion stage. A servo motor or stepper motor turns a lead screw or ball screw, and a nut riding on the screw moves linearly. The screw lead, the distance traveled per revolution, sets the ratio between motor speed and linear speed and between motor torque and output force.
For an ideal screw drive, force equals 2π × torque × efficiency ÷ lead. A smaller lead therefore yields more force and finer resolution but lower speed, which is the central trade-off when selecting an electromechanical actuator.
Other designs avoid the conversion stage entirely. Direct-drive linear motors generate force electromagnetically along a magnet track, while pneumatic and hydraulic cylinders use pressurized fluid acting on a piston, where force equals pressure multiplied by piston area. Piezoelectric actuators change their dimensions when voltage is applied and produce movements in the micrometer range.
Types of linear actuators
The table compares the main actuator families used in robotics and automation. Values are typical orders of magnitude; specific products may exceed them at the cost of size, price or energy use.
| Type | Force range | Speed range | Positioning precision | Typical robotic use |
|---|---|---|---|---|
| Lead screw actuator | Up to several kN | Low to moderate | Moderate | Adjustments, light-duty axes |
| Ball screw actuator | Up to tens of kN | Moderate to high | High, micrometer level with good components | Cartesian robots, precision stages |
| Belt-driven actuator | Low to moderate | High, several m/s | Moderate | Long gantry axes, pick-and-place |
| Linear motor | Low to moderate continuous force | Very high | Very high | High-speed positioning stages |
| Pneumatic cylinder | Up to several kN | High | Low, mostly end-to-end | Grippers, simple push and pull |
| Hydraulic cylinder | Up to hundreds of kN | Moderate | Moderate to high with servo valves | Heavy legged robots, construction machines |
| Piezoelectric actuator | Small to moderate | Very fast response | Nanometer to micrometer | Micro-positioning and fine alignment |
Key selection parameters
Selecting a linear actuator means matching its specification to the load profile, motion profile and environment. The parameters below appear on almost every datasheet and decide whether a design works reliably over its service life.
| Parameter | Typical unit | Why it matters |
|---|---|---|
| Stroke | mm | Usable travel of the axis |
| Peak and continuous force | N | Acceleration and holding capacity |
| Maximum speed | mm/s or m/s | Cycle time |
| Repeatability | mm or µm | Ability to return to the same position |
| Duty cycle | Percent of time in motion | Heating and service life |
| Back-drivability | Yes or no | Holding position without power, safety behavior |
| Side load capacity | N or N·m | Need for external guides |
| Ingress protection | IP rating | Resistance to dust and liquids |
Two parameters are often misjudged. Repeatability is not accuracy: an axis can return to the same point within a few micrometers yet be offset from the commanded coordinate. And screw actuators tolerate little side load, so cantilevered loads require linear guides that carry the moment while the actuator provides only axial force.
Electric, pneumatic and hydraulic actuation compared
The choice between energy sources is one of the first decisions in actuator design. Electric actuators dominate where programmable positioning and clean operation matter, while fluid power remains strong where force density or simplicity is decisive.
| Criterion | Electric | Pneumatic | Hydraulic |
|---|---|---|---|
| Position control | Precise, any intermediate position | Mostly end positions | Precise with servo valves |
| Force density | Moderate | Low to moderate | Very high |
| Energy efficiency | High | Low, compressed air losses | Moderate |
| Infrastructure | Power and drive electronics | Compressor, valves, tubing | Pump, reservoir, hoses |
| Compliance | Stiff unless controlled | Naturally compliant | Very stiff |
| Maintenance concern | Screw wear, lubrication | Leaks, air quality | Leaks, fluid contamination |
Inside robots, the decision interacts with the rest of the drivetrain. A robot gripper may use a small pneumatic cylinder for speed and simplicity, while a precision end effector for assembly typically uses an electric actuator with force feedback.
Common mistakes and failure modes
Most actuator failures trace back to underestimated loads or overlooked operating conditions, rather than to defective components.
- Ignoring acceleration forces: the force needed to accelerate a mass can exceed the static load several times.
- Exceeding critical screw speed: long, thin screws whip and vibrate at high rotational speeds.
- Oversized duty cycle: continuous operation beyond the rating overheats motors and wears screws.
- Missing end-of-travel protection: crashes into hard stops damage nuts, couplings and gearboxes.
- Unexpected back-driving: ball screws can move under gravity when power is lost, so vertical axes need a brake.
Good practice includes calculating rated life from load and travel distance, adding a safety margin to force estimates and confirming critical speed and buckling limits for long strokes.
The idea in brief
Most motors spin. Many robot tasks, however, need a straight push or pull, such as raising a platform, closing a gripper or sliding a tool along a rail. A linear actuator is the part that turns power into straight-line movement.
It works much like a car jack. You turn a screw, and the lifting arm moves up in a straight line. Electric, air and oil-powered versions exist, and each suits a different mix of strength, speed and precision.
A practical scenario
A laboratory automation company builds a Cartesian robot that moves sample tubes between racks, a centrifuge and an analyzer. The horizontal axes are 1.2 meters long and must move quickly, so the engineers choose belt-driven actuators reaching about 1.5 m/s. The vertical axis carries the gripper and must place tubes within a fraction of a millimeter, so it uses a ball screw actuator with a 5 mm lead, a servo motor and a holding brake.
The gripper itself is driven by a compact electric actuator with force control, which prevents cracking glass tubes. After testing, the system achieves the required throughput, and the brake keeps the vertical axis safely in place during power interruptions.