Linear Actuator

What is a Linear Actuator

A linear actuator is a device that creates motion in a straight line, in contrast to the rotary motion of a conventional electric motor. In the most common electromechanical configuration, a rotary motor turns a screw or pulley, and a nut or belt rider converts that rotation into linear travel of a rod or carriage. The actuator therefore packages three functions into one assembly: a prime mover (the motor), a motion-conversion element (the screw or belt), and a guidance and housing structure that carries the load and resists side forces. When end-of-stroke limit switches, position feedback, and a motor controller are integrated, the unit becomes a self-contained motion axis that a PLC or a vehicle controller can command directly.

The distinction between an actuator and a bare motion component matters for procurement. A ball screw, a lead screw, or a linear guide on its own is a mechanical element with no motor and, per IEC 60529, no ingress protection rating. A linear actuator integrates the motor and, usually, the electronics, so it carries a thrust rating, a duty cycle, and an IP rating as a complete product. This is why a purchase order that says linear actuator almost always means the motorized, ready-to-install assembly rather than the screw inside it.

Linear motion has industrial roots stretching back to the screw and the lever of antiquity, but the modern electric linear actuator emerged from the marriage of the precision lead screw and the compact electric motor in the twentieth century. The lead screw and its more efficient descendant the ball screw provided controllable, self-supporting linear force; the addition of brushed DC, brushless DC, stepper, and servo motors gave the actuator its speed and positioning behavior. More recently, integrated electronics and fieldbus interfaces such as SAE J1939 and CANopen have turned the actuator from a dumb push-rod into a networked, diagnosable smart device that reports its own position, force, and overload state.

The application range is wide because the same architecture scales across orders of magnitude. Compact 12 V and 24 V DC actuators move a few hundred newtons over strokes of tens to hundreds of millimeters in furniture, medical beds, and light machinery. Mid-range industrial actuators handle a few thousand newtons in assembly lines, conveyors, and valve operation. Heavy-duty smart actuators reach into the tens of thousands of newtons: the Thomson Electrak XD, for example, is rated to a dynamic load of 25,000 N and a static load of 32,000 N, explicitly positioned as an alternative to hydraulic cylinders in agricultural vehicles, automated guided vehicles, and factory automation.

Four engineering metrics determine whether a linear actuator is fit for purpose: thrust force, stroke, speed, and duty cycle. These are not independent, because they all draw on the same motor power and the same thermal budget. A higher speed at a given thrust demands more power and generates more heat, which lowers the achievable duty cycle. Sound selection is the act of finding a single product whose rated envelope contains every point of the application's force-speed-duty operating profile, with margin, rather than chasing one headline number in isolation.

Actuator Types and Classification

Linear actuators are classified first by energy source (electromechanical, pneumatic, hydraulic, piezoelectric) and then, within the dominant electromechanical family, by mechanical layout (rod style versus rodless) and by drive element (screw versus belt). Choosing the wrong family is the most expensive selection error, because it locks in the force ceiling, the achievable stroke, and the maintenance regime for the life of the machine. The table below summarizes the main families and their engineering envelopes.

Electromechanical actuators are the default for new design because they draw power only while moving, are clean and leak-free, and expose digital control and diagnostics. They subdivide by mechanical layout. A rod-style actuator extends and retracts a push tube or piston rod from one end, so its retracted body length is roughly the stroke plus the motor and gear pack, and the extended rod is exposed to bending and buckling. A rodless actuator carries the load on a carriage that rides along the body, so the installed footprint is only slightly longer than the stroke, there is no exposed rod, and buckling is eliminated. Rodless layouts dominate long horizontal travel and tight-footprint gantries.

Pneumatic cylinders deliver fast, simple, and inexpensive linear motion and are ubiquitous in high-cycle clamping and sorting. Their dimensions and mounting are standardized under ISO 15552 (the former ISO 6431 and VDMA 24562 profile cylinders), which makes them interchangeable across suppliers. Their weakness is precise mid-stroke positioning and energy efficiency, since compressed air is costly to produce and air is compressible. Hydraulic cylinders provide the highest force density and can stall against load indefinitely, which is why they still own presses and heavy mobile equipment, but they bring a pump, reservoir, hoses, valves, and oil-leak and fire risk.

Piezoelectric actuators and linear motors occupy the high-dynamic, high-precision extreme. Piezo stacks give sub-micrometer resolution over very short travel for optics and semiconductor alignment. Linear motors are rotary motors unrolled flat, with the electromagnetic force acting directly on a forcer, giving zero backlash and very high acceleration over long travel, at a high cost per newton and with no self-locking. Because these serve fundamentally different duties from a general-purpose push-pull actuator, they are usually specified by their own performance class rather than cross-shopped against a screw actuator.

Mechanical Drive Technologies

Within electromechanical actuators, the motion-conversion element determines efficiency, load capacity, positioning accuracy, speed, and whether the actuator self-locks. Four drive technologies dominate: lead (Acme) screw, ball screw, planetary roller screw, and toothed belt. Each occupies a distinct point in the trade-off space, and no single element is best across all four metrics. The table below compares them on the parameters that drive selection.

Lead screws use a trapezoidal or Acme thread engaging a sliding nut. Friction between thread flanks limits efficiency to roughly 20 to 50 percent, which sounds like a drawback but delivers the key benefit of self-locking: when efficiency falls below about 50 percent, the screw will not back-drive, so a vertical load is held with no separate brake. Lead screws are quiet and low cost, which makes them the standard for low-duty, intermittent positioning and for vertical lift where holding without power is valued. Their wear depends on the nut material, and they should be avoided where high load and high duty cycle combine.

Ball screws replace sliding friction with recirculating ball bearings between the screw and nut, raising efficiency to about 90 percent. This higher efficiency lets a smaller motor produce more thrust and run continuously without overheating, so ball screws carry far higher dynamic loads and suit high duty cycle, high speed, and accurate positioning. The same low friction that makes them efficient also makes them back-drivable: under power loss a vertical ball-screw actuator falls unless an integral holding brake is fitted. Heavy-duty smart actuators such as the Thomson Electrak XD pair a screw drive with an internal load-holding brake for exactly this reason.

Planetary roller screws replace the nut's internal thread with multiple planetary rollers that engage the screw thread, distributing load across many contact points. The result is the highest force density and shock resistance of the screw family, with long service life under demanding loads. The penalty is high cost and manufacturing complexity, so roller screws are reserved for the most demanding duties such as high-force presses, aerospace and defense actuation, and high-shock robotics where ball screws would not survive.

Toothed-belt drives use a steel- or aramid-reinforced timing belt running over toothed pulleys to move a carriage. Belt drives reach about 90 percent efficiency, run at high linear speed (commonly up to several meters per second), and can be built in very long strokes (multiple meters) that no practical screw can match, because a long screw whirls at its critical speed. The trade-off is lower load capacity and positioning accuracy than a ball screw, and the belt is a wear item. Belt actuators are the standard for long, fast, lower-load horizontal travel such as gantry axes and material transfer.

Duty Cycle, Ingress, and Standards

Two ratings decide whether an actuator survives its working environment: duty cycle, which governs thermal load, and ingress protection, which governs dust and water exposure. Both are defined by international standards, and reading them correctly prevents the two most common field failures: a burned-out motor from over-running the duty cycle, and a corroded or shorted unit from an inadequate IP rating.

Duty cycle is the proportion of time an actuator may run before it must rest to shed motor and gear heat. It is expressed either as a percentage or as one of the duty types defined in IEC 60034-1 for rotating electrical machines. A 25 percent duty cycle means the actuator runs one quarter of each cycle: for a 4-minute cycle that is 1 minute of motion and 3 minutes of rest, or about 15 minutes of operation per hour. Exceeding the rating drives winding temperature past the insulation limit and shortens life. The three duty types most relevant to actuators are summarized below.

In practice, compact 12 V and 24 V DC actuators are commonly rated for 10 to 25 percent duty cycle, reflecting their small motors and limited heat-dissipation mass. Industrial and heavy-duty units extend further: the Thomson Electrak XD is rated for duty cycles up to 100 percent depending on the loading condition, meaning it can run continuously at reduced load. Always read the duty cycle together with the ambient temperature derating, because a rating valid at 20 degrees C shrinks at 40 or 60 degrees C.

Ingress protection is defined by IEC 60529 and written as IP followed by two digits. The first digit (0 to 6) rates protection against solid objects and dust; the second digit (0 to 9K) rates protection against water. A crucial subtlety from IEC 60529 is that the rating applies only to enclosures of electrical equipment, so a bare ball screw, lead screw, linear guide, or coupling cannot carry an IP rating; only an actuator with integrated, enclosed electrics is tested and rated. Selecting an IP class is a matter of matching both digits to the real exposure, summarized below.

Note that a higher second digit does not guarantee a higher first digit, and the two water tests are not strictly cumulative: an IP66 unit withstands powerful jets but is not certified for immersion, while an IP67 unit survives temporary immersion but is not tested against pressure jets. Where both jet and immersion exposure occur, look for a dual rating such as IP66/IP69K. Beyond duty cycle and IP, machinery actuators are also governed by safety frameworks: in the EU, the Machinery Directive (and its successor Machinery Regulation) requires technical documentation and conformity, with functional safety addressed through ISO 13849 and IEC 62061 for the control system around the actuator.

Key Specification Parameters

A linear actuator datasheet lists many figures, but eight parameters drive nearly every selection decision: dynamic thrust, static (holding) load, stroke length, travel speed, duty cycle, positioning accuracy or repeatability, voltage and control interface, and ingress and temperature limits. Each is explained below, with the traps that catch first-time buyers.

Dynamic thrust versus static load. Dynamic thrust is the force the actuator can exert while moving; static or holding load is the force it can hold when stopped, usually a higher number because no acceleration or back-EMF limit applies. The two must be read separately. The Thomson Electrak XD, for instance, lists dynamic load up to 25,000 N but static load up to 32,000 N. Sizing only to the dynamic figure can under-rate the holding case, and sizing only to the static figure can over-promise the moving force.

Stroke length. Stroke is the linear travel between full retraction and full extension. For rod-style actuators the installed length is roughly stroke plus the motor and gear pack, so a long stroke needs physical room. Long strokes also reintroduce the buckling problem: a rod cylinder becomes risky once the stroke exceeds roughly ten times the rod or bore diameter without external guiding, which is the central reason rodless and belt actuators dominate long horizontal travel.

Travel speed. Speed is set by the motor speed, the gear ratio, and the screw lead or belt pitch, and it trades against thrust: a higher lead gives more speed but less mechanical advantage and thus less force. Belt drives reach the highest speeds (commonly several meters per second), while heavy screw actuators are far slower under load: the Thomson Electrak XD, for example, runs at speeds up to 75 mm/s. Always check speed at the rated load, not the no-load headline.

Positioning accuracy and repeatability. Accuracy is how close the carriage stops to the commanded position; repeatability is how consistently it returns to the same point. These depend on the feedback element and the drive: a ball screw with an absolute encoder positions far more tightly than an open-loop lead screw with only limit switches. Belt systems typically reach repeatability on the order of plus-or-minus 0.1 mm or coarser because belts stretch, while preloaded ball-screw axes reach roughly plus-or-minus 0.005 to 0.05 mm; lead-screw systems fall between the two and improve markedly with an anti-backlash nut.

Voltage, control, and feedback. Common supplies are 12 V and 24 V DC for mobile and compact units and 24 V DC, 48 V DC, or AC mains for industrial units. Control ranges from simple limit-switch on/off, through potentiometer or Hall-effect position feedback, to full closed-loop servo control with a fieldbus. Smart heavy-duty actuators such as the Thomson Electrak family expose position, current, and overload diagnostics over SAE J1939 or CANopen, allowing several actuators to be synchronized on one bus and commanded by a vehicle or machine controller.

• On/off with limit switches: simplest, lowest cost, no mid-stroke position knowledge.
• Potentiometer / Hall-effect feedback: approximate analog or pulse position for basic closed-loop control.
• Encoder (incremental or absolute): precise position and repeatability for servo-grade positioning.
• Fieldbus (SAE J1939 / CANopen): networked control, diagnostics, force and overload feedback, multi-actuator synchronization.

Duty cycle, IP, and temperature close the spec sheet, and they are covered in Chapter 4. The single most important reading habit is to treat thrust, speed, and duty cycle as a coupled triple, derated for ambient temperature, rather than as three independent maxima that can all be reached at once.

Selection Decision Factors

To turn the preceding chapters into a specific model, follow the ordered decision sequence below. Most selection mistakes come not from a single wrong number but from deciding a downstream parameter before the upstream constraint that bounds it. These eight steps can serve as a fixed RFQ template.

1. Motion and mounting layout: First decide rod-style versus rodless, and horizontal versus vertical. Long horizontal travel above roughly 1,000 mm points to rodless or belt to avoid buckling; vertical holding without power points to a self-locking lead screw or a ball screw with an internal brake.
2. Thrust force, dynamic and static: Size dynamic thrust to the worst-case moving load (weight, friction, acceleration inertia, process force) with a 1.5 to 2 times margin, and separately verify the static holding load. Do not assume one figure covers both cases.
3. Stroke and speed: Specify required stroke plus a small reserve, then the speed at full load (not no-load). Confirm the chosen stroke does not push a rod-style unit into the buckling regime, and that the screw is below its critical-speed length.
4. Drive technology: Match lead screw, ball screw, roller screw, or belt to the duty profile per Chapter 3. Low-duty quiet holding favors lead screw; high-duty high-thrust accurate motion favors ball screw; extreme force or shock favors roller screw; long fast light travel favors belt.
5. Duty cycle and thermal environment: Confirm the rated duty cycle (S1/S2/S3 or percentage per IEC 60034-1) covers the worst-case run-to-rest ratio at the actual ambient temperature, applying the manufacturer's temperature derating.
6. Ingress and protection: Pick an IP class per IEC 60529 to match real exposure, reading both digits: IP54 indoor, IP66 outdoor and washdown, IP67 temporary immersion, IP69K high-pressure hot-water washdown. Add corrosion and seal-material considerations for chemical or marine service.
7. Control, feedback, and interface: Choose limit-switch on/off, analog feedback, encoder servo, or a fieldbus (SAE J1939 / CANopen) to match the positioning tolerance, the need for force and overload monitoring, and the host controller (relay, PLC, or vehicle bus).
8. Total cost of ownership and safety: Sum purchase, installation, energy (electric actuators draw power only while moving), maintenance of wear items (belts, nuts), and downtime risk. For machinery, confirm the surrounding safety function (ISO 13849 / IEC 62061) and Machinery Directive conformity where applicable.

One last dimension that buyers often overlook is serviceability: availability of spare belts, nuts, and seals, manual-override capability in a power failure, firmware and configuration tooling for smart units, and local support. A self-contained smart actuator with onboard diagnostics shortens fault-finding, while a manual override (offered on units such as the Thomson Electrak XD) lets an operator recover a stalled axis without power. Major suppliers including Thomson, LINAK, Ewellix, Tolomatic, igus, SKF, and Festo maintain catalog parts and regional support that reduce repair response time over a 5 to 10 year service life, which matters as much to total cost as the headline thrust figure.