Selecting a motor for a linear module is not simply a matter of choosing a higher wattage. The motor, drive, feedback system and mechanical transmission must work together as one motion system.

A motor that is too small may fail to reach the required acceleration or overheat during repeated cycles. A motor that is unnecessarily large can increase rotor inertia, cost and tuning difficulty without improving the machine. The correctlinear module motor selectiontherefore begins with the motion requirement, not with a preferred motor size.

This guide explains how to choose between servo and stepper motors, how to match motor torque and speed to the module, when encoder feedback matters, and which control details should be checked before finalizing the drive system.

How to Select a Motor for Your Linear Module

Start With the Motion Requirement, Not the Motor Catalog

Before comparing motor types, define what the axis must actually do.

The minimum information should include:

  • Linear module drive type
  • Stroke
  • Moving mass
  • Horizontal, vertical or other mounting orientation
  • Maximum speed
  • Acceleration and deceleration
  • Move time and complete cycle time
  • Required positioning accuracy and repeatability
  • External process force
  • Required holding behavior
  • Available controller and communication interface

Selection rule:the motor should be selected from the complete motion profile. Payload alone does not determine motor size.

Servo or Stepper: The First Decision

Both servo motors and stepper motors can drive industriallinear modules, but their operating characteristics are different.

Selection Factor Servo Motor Stepper Motor
Feedback Normally closed-loop with encoder feedback Can be open-loop or closed-loop depending on system
High-speed performance Generally strong across a wide speed range Available torque usually decreases more rapidly as speed rises
Acceleration Well suited to high dynamic response Suitable for moderate dynamics when properly sized
Position confirmation Encoder continuously reports motor position Open-loop systems infer position from commanded steps; closed-loop versions add feedback
Control complexity Requires servo drive and tuning Often simpler, especially in open-loop systems
Cost Usually higher Usually lower for comparable basic applications
Typical fit High speed, high acceleration, dynamic positioning, coordinated axes Moderate-speed positioning, indexing, cost-sensitive automation

When a Servo Motor Is Usually the Better Choice

Alinear module servo motoris often preferred when the application requires:

  • High acceleration and deceleration
  • Wide operating speed range
  • Frequent direction reversals
  • Closed-loop position monitoring
  • High cycle rate
  • Coordinated multi-axis motion
  • Torque or force monitoring
  • More advanced motion profiles

Servo systems are especially useful when the axis must operate near its dynamic limits while still maintaining stable control.

Why servo systems handle dynamic motion well

The encoder continuously reports motor position and speed to the drive. The servo controller compares actual motion with the command and adjusts motor current accordingly.

This closed-loop behavior allows the drive to correct following error and adapt motor torque throughout acceleration, constant-speed motion and deceleration.

When a Stepper Motor Can Be the Better Choice

Alinear module stepper motorcan be a practical solution when the application has predictable loads, moderate speed and relatively simple positioning requirements.

Typical applications include:

  • Indexing
  • Fixture adjustment
  • Light pick-and-place motion
  • Dispensing equipment
  • Inspection positioning
  • Low- to medium-speed automation

Stepper systems are attractive because the control structure can be simple and holding torque at standstill can be useful in some applications.

The main limitation of open-loop stepper systems

An open-loop stepper controller assumes that the motor follows every commanded step. If the load exceeds available torque or acceleration is too aggressive, the motor can lose synchronization without the controller directly knowing the true shaft position.

This is why stepper motors should not be selected only from their low-speed holding torque.

What Is a Closed-Loop Stepper Motor?

A closed-loop stepper system adds encoder feedback to the stepper motor and drive. The controller can compare commanded motion with measured motor position and detect or correct position error depending on the system design.

This can reduce some of the uncertainty associated with open-loop operation and improve usable performance under changing loads.

However, a closed-loop stepper is not automatically equivalent to a servo motor. The motor construction, torque-speed behavior, control algorithm and dynamic capability remain different.

Useful way to decide:choose the motor from the required motion performance, not from the label “servo,” “stepper” or “closed loop.”

Motor Power Alone Is Not a Sizing Method

Motor power is related to torque and speed:

P = T × ω

where:

  • P= mechanical power in watts
  • T= torque in N·m
  • oh= angular speed in rad/s

Two motors with similar rated power can have very different torque-speed curves, rotor inertia and peak torque capability. For this reason, selecting “a 400 W servo” without checking the actual operating point is incomplete.

Match Motor Speed to the Linear Module Transmission

The mechanical transmission determines how motor rotation becomes linear speed.

Ball screw module

For a screw leadpand desired linear speedv:

n = 60 × v ÷ p

wherevis in m/s,pis in m/rev andnis in rpm.

A 10 mm lead screw moving at 0.5 m/s requires 3000 rpm. The motor and screw must both be suitable for that speed.

Timing belt module

Motor speed depends on pulley pitch circumference and any reduction ratio between the motor and pulley. Larger pulleys move more belt per revolution but require more motor torque for the same linear force.

Rack-and-pinion module

Pinion pitch diameter and gearbox ratio determine the relationship between motor speed and linear travel. Long-stroke rack systems often use gear reduction to balance motor speed, output torque and reflected inertia.

Match Motor Torque to the Complete Motion Cycle

Motor torque must cover more than the static load.

Important torque components include:

  • Torque needed to overcome linear resistance
  • Torque required for acceleration
  • Rotational inertia of the screw, coupling, pulley or gearbox
  • Gravity torque on vertical axes
  • External process forces
  • Preload and seal resistance

After the torque profile is calculated, check both:

  • Peak torquefor acceleration and short-duration demand
  • RMS or continuous torquefor thermal suitability over the complete cycle

A motor that can complete one move successfully may still overheat during continuous production if RMS torque is too high.

Do Not Ignore the Torque-Speed Curve

Available motor torque changes with speed.

This is particularly important for stepper motors, whose usable torque generally falls significantly as speed rises. Servo motors also have defined continuous and peak operating regions.

For every candidate motor, verify that:

  • Required maximum speed is within the motor's operating range.
  • Required peak torque is available at that speed.
  • Continuous or RMS torque remains within the continuous operating region.
  • The selected drive can supply the required current and voltage.

Inertia Matching: Why the Mechanical Load Matters to Control

The motor must accelerate both its own rotor and the inertia reflected from the linear module.

For a ball screw, translating mass can be represented as equivalent rotational inertia at the screw shaft:

Jtrans= m × (p ÷ 2π)2

Rotating inertia from the screw, coupling and other components is then added.

If a gearbox is used, load inertia reflected to the motor decreases with the square of the reduction ratio.

Why an oversized motor can be counterproductive

A larger motor normally has a larger rotor inertia. If the motor is selected far above the required size, this extra inertia may reduce acceleration efficiency and make the system more difficult to tune.

This is one reason motor sizing should aim for a suitable operating margin rather than simply choosing the largest frame that fits.

There is no universal inertia ratio

Acceptable load-to-motor inertia ratio depends on the motor, drive, control method and motion profile. Follow the actual motor manufacturer's sizing criteria rather than applying one fixed rule to every servo system.

Encoder Choice: What Does the Feedback Need to Tell You?

An encoder does more than provide a “resolution” number. It determines what information the controller can observe about the motor or axis.

Incremental encoder

An incremental encoder reports position changes through pulses or digital counts. After power-up, many systems require a homing routine to establish the machine reference.

Absolute encoder

An absolute encoder retains or reconstructs a unique position reference so the control system can know the motor position after power cycling, depending on encoder and drive architecture.

This can reduce or eliminate certain homing requirements and is useful in machines where restart time or axis position retention matters.

Single-turn and multi-turn feedback

Single-turn absolute feedback identifies the shaft position within one revolution. Multi-turn systems also track the number of revolutions, which can be useful when motor position must be preserved across many turns.

Encoder Resolution Is Not the Same as Linear Accuracy

A high-resolution encoder allows the controller to represent very small motor-angle increments, but it does not automatically remove mechanical errors in the module.

Ball screw lead error, belt elasticity, backlash, guide straightness, structural deflection and thermal expansion can all exist between the motor shaft and the actual tool position.

For very demanding applications, direct linear feedback may be used to measure carriage position more directly.

Important distinction:encoder resolution describes feedback granularity. System positioning accuracy describes the complete mechanical and control result.

Motor Encoder or Linear Scale?

With motor-side feedback, the controller calculates linear position from motor rotation and transmission geometry. This is common and effective for many automation applications.

A linear scale mounted along the axis can directly measure carriage position. It can therefore detect position errors that occur after the motor, including some screw, belt or transmission effects.

Direct linear feedback is valuable in high-precision systems, but it adds cost, installation requirements and control complexity. It is not necessary for every linear module.

Vertical Linear Modules Need a Different Motor Check

On a vertical axis, gravity continuously affects the drive.

The motor must be checked for:

  • Upward acceleration torque
  • Continuous gravity load
  • Downward regenerative behavior
  • Holding requirements at standstill
  • Brake capability
  • Safe behavior during power loss

When is a motor brake needed?

A brake is commonly considered when a vertical load must remain in position after motor power is removed. However, motor holding brakes have specific ratings and intended functions.

If unintended descent could create a safety hazard, the machine may require additional safety-rated mechanical measures rather than relying only on a standard motor brake.

How Gearboxes Affect Motor Selection

A gearbox can increase output torque and reduce the load inertia reflected to the motor, but it also changes speed and adds mechanical characteristics of its own.

Potential reasons to use a gearbox include:

  • Increasing output torque
  • Improving load-to-motor inertia ratio
  • Allowing a smaller motor to operate at a more suitable speed
  • Adapting the installation geometry

Trade-offs include:

  • Efficiency loss
  • Backlash
  • Additional inertia
  • Noise
  • Cost
  • Maintenance

Control Interface Matters Before the Machine Is Built

A mechanically suitable motor can still be the wrong choice if it does not integrate with the machine controller.

Confirm how the axis will receive commands:

  • Pulse and direction
  • Analog velocity or torque command
  • Industrial Ethernet or fieldbus
  • Built-in indexing commands
  • Motion controllernetwork

Also check whether the system needs synchronized motion, electronic gearing, camming, registration, torque control or coordinated interpolation.

Do You Need Position Control, Speed Control or Torque Control?

Position control

Most linear positioning applications use position control. The controller commands a target position and motion profile.

Speed control

Speed control can be useful when the axis must maintain a defined velocity while another mechanism controls position or process timing.

Torque control

Torque control may be used for pressing, tensioning, clamping or contact processes. However, motor torque is not always identical to actual linear process force because transmission efficiency, friction and mechanical compliance affect the relationship.

For critical force measurement, a dedicated load cell or force sensor may still be required.

Servo Tuning Is Part of Motor Selection

A motor that is technically large enough can still perform poorly if the system is difficult to tune.

Servo tuning is influenced by:

  • Load inertia
  • Structural stiffness
  • Coupling stiffness
  • Belt elasticity
  • Backlash
  • Resonance
  • Acceleration profile

A flexible mechanical system cannot be transformed into a rigid precision axis only by increasing servo gain. Mechanical and control design must support each other.

Motor Selection by Linear Module Type

Ball screw linear module

Check motor speed against screw lead and required linear speed. Then verify acceleration torque, screw inertia, critical screw speed and any vertical holding requirement.

Servo motors are common for high-speed precision positioning, while stepper motors can be suitable for moderate-speed applications with predictable loads.

Timing belt linear module

Timing belt modulesoften operate at high linear speed and acceleration. Motor selection should consider pulley diameter, belt tension, moving mass and frequent direction reversals.

Servo motors are often advantageous in high-cycle gantry or pick-and-place systems, while stepper motors can work well in lower-dynamic transfer axes.

Rack-and-pinion module

Long travel and higher loads often make gearbox selection important. Motor torque, gear ratio, pinion diameter and desired speed should be evaluated together.

Linear motor module

A linear motor produces force directly and does not require a rotary motor-to-linear transmission. In this case, sizing focuses on continuous force, peak force, velocity, acceleration, thermal load and feedback rather than rotary motor torque.

A Practical Motor-Selection Sequence

  1. Define the complete motion profile.Include stroke, speed, acceleration, deceleration and dwell.
  2. Calculate the real moving mass.Include tooling, upper axes and cable carriers where relevant.
  3. Determine required linear force.Include gravity, friction and process force.
  4. Convert the load to motor torque and speed.
  5. Calculate reflected inertia.
  6. Check peak and RMS torque.
  7. Compare servo, stepper and closed-loop stepper options.
  8. Select the feedback type.
  9. Check brake and vertical-axis requirements.
  10. Check the motor torque-speed curve.
  11. Confirm drive current, voltage and regenerative capability.
  12. Confirm controller communication and motion functions.
  13. Verify mechanical and electrical interfaces before release.

Common Motor-Selection Mistakes

Choosing by wattage only

Motor power does not show whether the required torque is available at the actual operating speed.

Choosing a stepper from holding torque

Holding torque is measured at standstill and does not represent high-speed running torque.

Oversizing “for safety”

A much larger motor can increase inertia and cost and may complicate tuning. Engineering margin is useful; uncontrolled oversizing is not.

Ignoring the vertical load

Vertical axes require gravity, braking, holding and regeneration to be considered.

Assuming encoder resolution equals positioning accuracy

The complete linear axis still contains transmission, structural and thermal error.

Ignoring drive and controller compatibility

The motor is only one part of the system. Drive voltage, communication protocol, I/O, braking resistor and control mode can determine whether the solution integrates successfully.

What Information Should You Provide for Motor Matching?

For QRXQ to recommend a motor for a linear module, provide:

  • Module type and transmission
  • Stroke
  • Total moving mass
  • Mounting orientation
  • Maximum speed
  • Acceleration or move time
  • Complete cycle time
  • External force
  • Accuracy and repeatability requirement
  • Preferred servo or stepper brand
  • Controller and communication protocol
  • Brake requirement
  • Daily operating time and duty cycle

The final motor should satisfy four things at the same time:torque, speed, inertia and control requirements. If one of these is ignored, the axis may work during a simple test but fail to deliver stable production performance.

A well-matched motor does not need to be the largest motor that fits the module. It needs to deliver the required motion with sufficient margin, stable control and acceptable thermal behavior over the actual machine cycle.