Linear Motor Selection Guide: How to Choose the Right Linear Motor

Learn how to select and size a linear motor based on force, speed, acceleration, precision, stroke, thermal performance, feedback, environment, and application requirements.

Choosing the right linear motor is not simply a matter of finding the motor with the highest thrust or the fastest rated speed. A properly selected linear motor must work as part of a complete motion system—including the motor, drive, encoder, linear guide, mechanical structure, cooling system, and motion-control software.

For high-speed automation, semiconductor equipment, precision inspection, packaging machinery, machine tools, robotics, and other demanding applications, the key question is:

Can the linear motor deliver the required force, speed, acceleration, precision, and thermal performance throughout the complete motion cycle?

This guide explains how to select and size a linear motor step by step, what specifications matter most, common selection mistakes, and how to validate a motor before mass production.

What Is a Linear Motor?

A linear motor is essentially a rotary motor that has been “unrolled” to produce linear force directly.

Unlike a conventional rotary motor combined with a ball screw, belt, rack-and-pinion, or gearbox, a direct-drive linear motor converts electrical energy directly into linear motion.

This eliminates many mechanical transmission components and can provide:

• High acceleration
• High maximum speed
• Zero mechanical backlash
• High positioning accuracy
• Excellent dynamic response
• Low mechanical wear in the drive mechanism
• Flexible long-stroke configurations

However, eliminating the mechanical transmission does not automatically guarantee higher system accuracy. The final performance depends on the complete motion system, including the mechanical structure, encoder, servo drive, control loop, thermal behavior, and force ripple.

For this reason, linear motor selection should always be treated as a system-level engineering problem rather than a motor-only decision

The Four Most Important Linear Motor Selection Criteria

At the beginning of a project, engineers should focus on four fundamental requirements:

1. Force
2. Speed and acceleration
3. Positioning accuracy and repeatability
4. Thermal performance and service life

These four parameters provide the foundation for motor sizing.

But in real applications, several additional factors can determine whether a selected motor actually performs as expected.

1. Linear Motor Force: Peak Force vs. Continuous Force

Force is usually the first parameter engineers look at—but it is also one of the most commonly misunderstood.

A linear motor datasheet normally provides at least two important force ratings:

• Peak force
• Continuous force

Peak Force

Peak force is the maximum force the motor can generate for a limited period.

It is mainly important during:

• Rapid acceleration
• Rapid deceleration
• High-load positioning
• Short-duration process forces
• Dynamic trajectory changes

For example, a high-speed pick-and-place machine may require very high peak thrust during acceleration but relatively little force while moving at constant velocity.

Continuous Force

Continuous force is the force the motor can sustain under the specified thermal conditions.

It is strongly related to:

• Winding temperature
• Copper losses
• Thermal resistance
• Cooling method
• Ambient temperature
• Duty cycle
• RMS force

Therefore, a motor with a very high peak thrust rating may still be unsuitable for a high-duty-cycle application.

This is one of the most important rules in linear motor sizing:

Peak force determines whether the motor can perform the dynamic move. Continuous/RMS force determines whether it can survive the complete duty cycle thermally.

Engineering sizing guides similarly recommend calculating the motion profile first and comparing both peak force and RMS force against the motor’s ratings

2. How to Calculate Linear Motor Force

or a simple horizontal axis, the required motor force can be approximated as:

F = ma + Ff + Fext

Where:

• F = required motor force
• m = total moving mass
• a = required acceleration
• Ff = friction and guide resistance
• Fext = external process force

For a vertical axis, gravity must also be considered:

F = ma + mg + Ff + Fext

The direction of each force must be considered carefully.

Example

Suppose:

• Moving mass = 10 kg
• Required acceleration = 5 m/s²
• Friction = 10 N
• External process force = 20 N

The acceleration force is:

Facc = 10 × 5 = 50 N

Therefore:

Frequired = 50 + 10 + 20 = 80 N

The selected motor must have sufficient peak force for this dynamic requirement, while its continuous capability must also satisfy the thermal requirements of the entire motion cycle.

Do not forget to include the mass of the moving motor component itself. In high-acceleration applications, the motor’s moving mass can become a significant part of the total inertia.

3. Why RMS Force Matters for Linear Motor Sizing

A machine rarely operates at maximum force continuously.

A typical motion cycle may contain:

1. Acceleration
2. Constant-speed travel
3. Deceleration
4. Dwell
5. Return motion

The motor force therefore changes continuously.

For a simplified duty cycle, RMS force can be calculated as:

F_RMS = √[(F₁²t₁ + F₂²t₂ + F₃²t₃ + … + Fₙ²tₙ) / T]

Where:

• 
  F₁, F₂… are forces during different motion segments
• t₁, t₂… are the corresponding times
• T is the total cycle time

The selected motor should have a continuous force capability greater than the required RMS force under the actual thermal conditions.

This is why sizing a motor purely from the maximum or peak force can result in either:

• An undersized motor that overheats, or
• An unnecessarily oversized motor that increases cost, moving mass, and system size.

4. Speed, Acceleration and Stroke

Speed alone is not enough to determine whether a linear motor is suitable.

You should define the complete motion profile.

At minimum, specify:

• Stroke length
• Maximum velocity
• Acceleration
• Deceleration
• Cycle time
• Dwell time
• Number of cycles per minute
• Load mass
• Motion direction

Stroke

Stroke is the required travel distance.

Long-stroke applications may require modular magnetic tracks, longer guide systems, cable management, and additional structural considerations.

Maximum Speed

Maximum speed is normally specified in m/s.

However, the maximum achievable speed is influenced by more than the motor’s mechanical capability.

It can also be limited by:

• Back EMF
• DC bus voltage
• Drive voltage capability
• Motor winding
• Encoder bandwidth
• Cable management
• Mechanical resonance
• Bearing or guide limitations

5. Acceleration Is Not the Same as Acceleration Time

This is an important technical distinction.

Acceleration is measured in:

m/s²

Acceleration describes how quickly velocity changes.

Acceleration time is measured in:

seconds

For example, if a stage accelerates from 0 to 2 m/s in 0.2 seconds:

a = Δv / Δt

a = 2 / 0.2 = 10 m/s²

Therefore, when requesting a linear motor quotation or sizing calculation, provide both the required velocity and the required acceleration or motion time.

Linear Motor Sizing Workflow

Step 1: Define the Application

Determine:

• Payload
• Moving mass
• Stroke
• Orientation
• Process force
• Required lifetime
• Operating environment

Step 2: Define the Motion Profile

Determine:

• Maximum velocity
• Acceleration
• Deceleration
• Cycle time
• Dwell time
• Repetition frequency

Step 3: Calculate Required Force

Calculate acceleration, friction, gravity, and external process forces.

Step 4: Calculate Peak and RMS Force

Confirm:

Peak motor force > required peak force

and

Continuous motor force > required RMS force

with appropriate engineering margin.

Step 5: Check Speed and Voltage

Verify that the motor and drive can achieve the required maximum speed considering back EMF and available bus voltage.

Step 6: Check Thermal Performance

Evaluate:

• RMS current
• Winding temperature
• Thermal resistance
• Ambient temperature
• Cooling method
• Duty cycle

Step 7: Select Encoder and Guide System

Match:

• Encoder resolution
• Accuracy
• Feedback bandwidth
• Guide load capacity
• Magnetic attraction
• Mechanical stiffness

Step 8: Validate the Complete System

Common Linear Motor Selection Mistakes

Mistake #1: Selecting a Motor by Peak Thrust Alone

A datasheet may advertise an impressive peak thrust value.

But peak thrust may only be available for a limited period.

Mistake #2: Assuming a High-Resolution Encoder Guarantees High Accuracy

A high-resolution encoder is valuable, but it cannot compensate for:

• Mechanical resonance
• Structural deformation
• Thermal expansion
• Poor installation
• Force ripple
• Poor servo tuning

Mistake #3: Ignoring Magnetic Attraction Force

This is particularly important with iron-core flat linear motors.

The magnetic attraction force can significantly load the guide system.

Mistake #4: Ignoring Motor Moving Mass

In high-acceleration applications, the moving portion of the motor is part of the accelerated mass.

Ignoring it can lead to an underestimated force requirement.

Mistake #5: Treating Maximum Speed as the Only Dynamic Requirement

Two machines may have the same maximum speed but completely different performance requirements.

For example:

• Machine A reaches 2 m/s slowly.
• Machine B reaches 2 m/s in 50 ms.

Machine B requires significantly higher acceleration and peak force.

Mistake #6: Ignoring Thermal Conditions

A continuous-force rating is meaningful only under its specified thermal conditions.

If the machine operates at:

• 40°C ambient
• High duty cycle
• Poor heat dissipation
• Enclosed installation

the actual continuous capability may be lower than the catalog value.

Mistake #7: Treating Installation Tolerances as an Afterthought

Incorrect flatness, alignment, parallelism, or air gap can reduce motor performance and create mechanical problems.

Better approach: design adjustment mechanisms and measurement procedures into the machine from the beginning.

Linear Motor Maintenance and Operation

Although linear motors eliminate many mechanical transmission components, they are not completely maintenance-free.

Before operation, check:

• Motor mounting
• Air gap
• Encoder installation
• Electrical connections
• Grounding
• Cable routing
• Guide alignment
• Cooling system
• Environmental protection

During operation, monitor:

• Motor temperature
• Vibration
• Noise
• Position error
• Following error
• Current
• Abnormal force behavior

Periodic maintenance should include:

• Cleaning exposed surfaces
• Checking connectors
• Inspecting cables
• Checking guides
• Verifying encoder condition
• Checking cooling performance
• Lubricating guide components where required

Linear Motor Applications

Semiconductor Equipment

• Wafer inspection
• Wafer handling
• Die inspection
• Lithography-related positioning
• Precision stages

Factory Automation

• Pick-and-place systems
• High-speed sorting
• Packaging
• Assembly
• Material handling

Machine Tools

Linear motors can provide high dynamic response and eliminate mechanical transmission elements in suitable machine architectures.

Inspection and Metrology

Applications such as optical inspection and precision measurement can benefit from smooth motion and high-resolution feedback.

Robotics

Linear motors can also be used in high-speed Cartesian motion systems and other precision automation platforms.

The Future of Linear Motor Selection: Think in Systems, Not Components

The trend is moving toward:

• Higher speed
• Higher acceleration
• Higher positioning accuracy
• Lower vibration
• Higher throughput
• Lower energy consumption
• Longer service life
• Smarter servo control
• More compact machine architectures

As performance requirements increase, selecting a linear motor by one specification becomes increasingly unreliable.

The real optimization target is the complete system:

Motor + Drive + Encoder + Guide + Structure + Cooling + Control Algorithm + Environment

Final Thoughts: How to Select the Right Linear Motor

A good linear motor selection process should answer five questions:

1. Can the motor provide the required peak force?
2. Can it provide the required RMS/continuous force without overheating?
3. Can the motor and drive reach the required speed and acceleration?
4. Can the encoder, mechanics, and control system achieve the required precision?
5. Can the complete system maintain these specifications throughout its intended service life?

Don’t select the biggest linear motor. Select the right linear motor for the complete motion profile.

Frequently Asked Questions About Linear Motor Selection

What is the most important parameter when selecting a linear motor?

There is no single most important parameter. Peak force, continuous/RMS force, maximum speed, acceleration, precision, thermal performance, and feedback requirements should be evaluated together.

How do I calculate the required linear motor thrust?

Start with the motion profile and calculate acceleration force:

F = ma

Then add friction, gravity where applicable, and external process forces.

Is peak force or continuous force more important?

Both are important. Peak force determines short-duration dynamic capability, while continuous/RMS force determines long-term thermal capability.

How do I calculate RMS force for a linear motor?

Divide the motion cycle into force/time segments and calculate the square root of the time-weighted mean square force:

F_RMS = √[Σ(Fᵢ²tᵢ)/T]

Is an ironless linear motor better than an iron-core motor?

Not necessarily. Ironless motors are attractive for smooth, high-dynamic motion, while iron-core motors can provide higher force density. The correct choice depends on the application.

Does a higher-resolution encoder always provide better positioning accuracy?

No. Encoder resolution is only one part of system performance. Mechanical accuracy, thermal expansion, vibration, servo tuning, force ripple, and structural stiffness also affect actual positioning accuracy.

Why does a linear motor need a linear encoder?

A direct-drive linear motor does not inherently provide absolute position information. A linear encoder provides position feedback to the servo controller, enabling closed-loop positioning and motion control.

Why is magnetic attraction important?

Iron-core linear motors can generate significant normal magnetic force between the motor and magnetic track. This additional load must be considered when selecting the linear guide and mechanical structure.

Can a linear motor replace a ball screw?

In many applications, yes. Linear motors can be particularly advantageous when high speed, high acceleration, long stroke, or high dynamic response is required. However, ball screws may remain more economical or advantageous for some applications.

What should I provide to a linear motor manufacturer?

At minimum, provide payload, moving mass, stroke, maximum speed, acceleration, duty cycle, installation orientation, external force, required accuracy, ambient temperature, available drive voltage, and cooling conditions.