What a linear actuator does
A linear actuator produces movement in a straight line: pushing a clamp closed, lifting a gate, shifting a part from one conveyor to another, or positioning a slide. It takes energy, whether electrical, compressed air or pressurized oil, and converts it into force and travel. The distance it can move is called the stroke, and the force it can deliver is its thrust.
Three technologies dominate in industrial settings: electric, pneumatic and hydraulic. Each makes sense in different circumstances, and many plants use all three.
Electric linear actuators
Most electric linear actuators combine a motor with a mechanism that converts rotation to linear motion. The usual mechanism is a lead screw or a ball screw, with a nut that travels along the screw and drives a rod or a carriage. Belt-driven slides are also used for long, fast travel, and linear motors, which produce straight-line force directly with no screw or belt, appear in high-end positioning.
Strengths:
- Accurate, programmable positioning. With a servo or stepper motor and a controller, an electric actuator can stop at many positions, not just the two ends.
- Controllable speed and force. Acceleration and speed profiles can be changed through software.
- Clean and quiet. There is no oil or air supply to leak, which suits food, medical and electronics work.
- Energy use follows demand. An electric axis uses power mainly while it moves or holds a load.
Limits:
- Cost per unit of force. For very high forces, an electric actuator can become large and costly compared with a hydraulic one.
- Mechanical wear. Screws and nuts wear over time, and lubrication needs attention.
- Holding a load. A screw that can be back-driven may need a brake to hold a load when power is off, while some lead screws with low efficiency hold by themselves through friction. Which applies depends on the design.
Pneumatic linear actuators
Pneumatic actuators, usually called air cylinders, use compressed air pushing on a piston. A control valve directs air to one side of the piston or the other, so the rod extends or retracts. Rodless cylinders and guided slides are variants for compact or long-stroke jobs.
Strengths:
- Simple and fast. An air cylinder is among the quickest and cheapest ways to produce back-and-forth motion.
- Easy to use for two-position movement. With sensors at the ends of stroke, a basic pick, clamp or push needs little control logic.
- Tolerant of harsh settings. Air systems cope well with dust, moisture and hot environments, and they are not an electrical ignition source at the actuator.
Limits:
- Air is compressible. It makes precise stopping at mid-stroke, smooth low-speed motion and stiff holding difficult, though special positioning kits exist.
- Efficiency. Compressed air is generally an expensive form of energy, and leaks waste a lot of it.
- Air quality needs managing. Moisture, particles and pressure variation affect cylinder life, which is why compressed air dryers and air receiver tanks matter for stable operation.
How cylinders fit into a complete system is discussed under pneumatic systems.
Hydraulic linear actuators
Hydraulic cylinders work on the same push-on-a-piston principle, but with oil at pressures far higher than normal shop air. Because oil is almost incompressible, hydraulic cylinders can deliver very large forces from a compact body and hold a position stiffly.
Strengths:
- High force in a small package. This is why they appear in presses, lifting equipment and heavy machinery.
- Stiff, controllable motion. With servo or proportional valves, hydraulic axes can be positioned and controlled well.
- Overload tolerance. Relief valves limit the maximum force.
Limits:
- Leaks and cleanliness. Oil leaks, contamination and fluid maintenance are ongoing concerns.
- Support equipment. A pump, reservoir, filters, cooling and piping are all needed.
- Heat and noise. Pumps and valves generate both, and energy is lost as heat if the system is not designed well.
- Stored energy. Pressurized fluid can cause serious injury. Procedures for depressurizing and isolating must be followed.
Some background on the sources of that force is found under hydraulic pump types.
Comparing the three
| Question | Electric | Pneumatic | Hydraulic |
|---|---|---|---|
| Positioning flexibility | High | Low to moderate | Moderate to high with the right valves |
| Force range | Low to moderate | Low to moderate | Moderate to very high |
| Speed | Moderate to high | High for simple strokes | Moderate |
| Cleanliness | Very good | Good | Needs attention |
| Setup | Programming and tuning | Valves, sensors, tubing | Power unit, valves, piping |
These are general tendencies, not guarantees; individual products vary widely.
Questions that help choose
- How much force and how long a stroke? Check the peak load as well as the average, including friction and any acceleration forces.
- How many positions? Two positions point toward air; many positions point toward electric or servo hydraulics.
- How precise and how smooth? Precision and low-speed smoothness favor electric.
- What happens when power or supply is lost? Decide whether the load should hold, fall or retract, and design for it.
- What is available on site? Plants with a good compressed air system or hydraulic power unit may already have the supply.
- What is the duty cycle? Continuous high-speed motion produces heat in electric and hydraulic systems and wears seals and screws.
- What environment? Washdown, dust, heat, hazardous areas and cleanrooms each limit the options.
Safety and upkeep
Actuators store energy, as compressed air, pressurized oil or a lifted load, and can move unexpectedly when valves are operated or power returns. Guarding, controlled energy isolation and lockout procedures are essential, and the machine maker’s manual and local regulations govern the details. Maintenance should be done by trained personnel. This article is general information, not engineering advice.