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MODULE 6 · UNIT 4 OF 5

Linear Actuators: Design Types

approx. 7 min · Learning goals, example, knowledge check

Learning goals — after this unit, you will be able to …

  • distinguish between the four types of linear actuators (push rod, open spindle, telescopic, servo-electric cylinder);
  • Identify the force, stroke, and compactness characteristics for each design type and assign them to an application;
  • Distinguish between DC, AC, and servo drive types in terms of controllability and applications.

What is a linear actuator?

A linear actuator is an electromechanical drive that converts rotational motion into linear motion: the motor, gearbox, and spindle are integrated into a single housing. This makes it space-saving and ready for use without tools—typically up to about 100 kN of force, with good positioning accuracy and low maintenance due to its enclosed design.

A Comparison of Four Designs

Push rod: The spindle rotates axially within the housing, and the anti-rotation nut pushes out a push tube. High forces up to 100 kN, but limited stroke height (100–500 mm)—compact and robust.

Open spindle: The nut is movable and travels with the spindle; the housing remains stationary. Longer strokes (500–3,000 mm) at 50–100 kN (typical, depending on the manufacturer), but requires external guides because the nut cannot absorb lateral forces.

Telescopic spindle: Multiple tubes nested inside one another—extremely compact in the rest position; the stroke can be 3 to 4 times the rest length. Moderate forces (30–80 kN, typical and manufacturer-dependent), slightly lower stiffness at full extension.

Servo electric cylinders: Equipped with servo control—repeatability of ±0.1 mm or better, force control, digital communication (CANopen, EtherCAT, Profibus), and programmable end positions instead of purely mechanical limit switches.

Practical Tip

With push-rod actuators, the limiting factor under compression is often not the motor power but the buckling strength of the extended rod: The allowable compressive force decreases with the square of the free buckling length (Euler)—with double the stroke, it roughly drops to one-quarter.

Duty Cycle and Drive Type

In practice, electric linear actuators are often limited by thermal factors rather than mechanical ones. The duty cycle (ED) specifies the proportion of a working stroke during which the drive is permitted to operate under load; the duty type is classified according to IEC 60034-1 (e.g., S1 continuous duty, S3 intermittent duty). Simple DC push-rod actuators often achieve only a duty cycle (ED) of ≈ 10–25%, while servo-controlled electric cylinders can be designed for continuous operation (S1).

Regarding drive types: DC motors can be easily controlled via voltage and their direction of rotation can be reversed by changing polarity—cost-effective and ideal for portable or battery-powered devices. Induction motors (AC) operate on the mains at a fixed speed (≈ 1,400/2,900 min⁻¹) and can only be controlled using a frequency inverter. Servo motors offer the most precise digital position control but require a servo controller; the sequence control is usually handled by a PLC.

Knowledge check

Answer all three questions, then click "Check". From 2 of 3 correct answers, the unit counts as completed. You can retry at any time.

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Question 1 of 3: Which design is suitable for long strokes ranging from 500 mm to 3,000 mm?
Explanation

For strokes of 500 mm or more (up to 3,000 mm), the open spindle is the right choice—the push-rod design becomes uneconomical for strokes of this length, while the telescopic version is better suited for extreme compactness with medium strokes.

Source: Linear Actuators: An Overview of Types →
Question 2 of 3: What distinguishes a servo electric cylinder from a standard linear actuator?
Explanation

Servo-electric cylinders offer precise positioning (repeatability of ±0.1 mm or better), force control, digital communication (e.g., CANopen, EtherCAT, Profibus), and programmable end positions rather than purely mechanical ones.

Source: Linear Actuators: An Overview of Types →
Question 3 of 3: A push-rod actuator is designed with double the stroke while retaining the same overall design. To what fraction does the permissible thrust (buckling stability) approximately decrease?
Explanation

The permissible compressive force decreases with the square of the free buckling length (Euler)—so with double the stroke, it roughly drops to one-quarter. For push-rod actuators, therefore, the limiting factor in compression is often not the motor power but the buckling stability of the extended rod.

Source: Linear Actuators: An Overview of Types →

Please answer all three questions to activate "Check".

Further reading (optional)

Guide: Linear Actuators—An Overview of Types (opens in a new tab)

Learning purpose: calculation methods and figures are simplified teaching examples. For a real machine, the manufacturer’s specifications, the relevant standards and a check by a qualified person apply.

Curriculum v0.1 (Beta) · Status 17.09.2026 · content carefully prepared and reviewed – final sign-off to follow

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