Linear Module Basics: Stroke, Load, Speed, and Accuracy Explained

Why engineers ask for these four values first

A linear module is a system: ball screw, guide rail, carriage, support units, and motor connection. When an engineer asks for stroke, load, speed, and accuracy, they are not following a routine. These four values decide the screw diameter and lead, the rail width and quantity, the accuracy grade, and the end machining. If any of them is unclear, the quotation comes back vague and the specification changes later.

Stroke: how far the slide can actually travel

Stroke is the usable travel distance of the carriage on the rail, normally stated in mm. A module marked 300 mm stroke gives about 300 mm of movement from one end to the other.

Two points are worth separating.

Mechanical stroke: the theoretical travel between the end limits.
Usable stroke: the practical range after allowing for deceleration zones, buffers, and mounting tolerance.

If the machine needs 250 mm of movement, buyers usually keep a safety margin and choose roughly 300 mm so limit switches and adjustment are easier. Another common mistake is confusing stroke with the total module length. Total length equals stroke plus the two support units, motor housing, and screw ends — often 100–200 mm longer. State both the stroke and the available mounting space in the inquiry.

Load: not only how heavy, but where the weight sits

Load on the carriage is more than one total weight number.

Vertical load: the total mass of the workpiece, fixture, and gripper mounted on the slide.
Inertia load: the load created by mass and acceleration during start and stop.
Moment load: tilting moment (Mx, My, Mz) when the center of gravity is off the carriage center.
Combined load: the total effect in multi-axis setups.

Two loads with the same weight behave differently when the center of gravity moves. Centered weight loads the rails evenly; an offset weight forces the rails and drive parts to carry more tilting moment and may require a wider rail or an extra rail. In practice, send the total mass, the approximate distance from the center of gravity to the carriage center, the expected acceleration, and whether the axis is horizontal or vertical. For vertical axes, state whether the load must stay held or braked after power loss.

Speed: tied to screw lead and cycle time

A single top-speed number is not enough. Linear speed of a screw module is roughly motor speed times screw lead: a larger lead moves faster at the same RPM but produces less thrust, so speed and load are a trade-off to balance.

More important than top speed is the cycle: how far each move is, how many seconds it takes, and how many cycles run per minute. Work backward from travel and time to the required speed and acceleration, then compare with the manufacturer's recommended range. Drive types differ.

Screw-driven module: medium speed with tight positioning control, good for dispensing and inspection stations.
Belt-driven module: long travel and a wide speed range, common for transfer and handling.
Linear motor module: fast response for tight cycle times, at a higher cost.

Very high speed increases inertia, extends braking distance, and adds structural vibration, so the practical choice is a balance between process cycle time and structural limits.

Ball-screw-driven linear module covered in this stroke, load, speed, and accuracy guide

Accuracy: positioning, repeatability, and system error

The "accuracy" on a module datasheet is a set of indicators, not one number.

Positioning accuracy: the error between the commanded position and the actual position reached.
Repeatability: how tightly the stop positions cluster over many back-and-forth moves, for example ±0.01 mm.
Straightness and parallelism: deviation of the carriage from the reference surface along the stroke.
Backlash-related values: screw backlash and rail clearance, which affect direction-change vibration.

Many stations combine repeatability with process compensation, so the module only has to stop consistently and the vision system or fixture corrects the rest. That is why repeatability is often the key value for automation buyers. Accuracy also maps to the ball screw grade: C3, C5, and C7 — the smaller the number, the tighter and more expensive. C7 suits general handling, C5 suits precision positioning, and C3 is for high-accuracy work.

Keep system error in mind as well: an uneven mounting surface, weak base, offset load, large temperature swings, or poor control tuning can multiply the module's own error at the workpiece. When you state an accuracy target, add the mounting method, base condition, temperature range, and whether vision compensation is used.

Turn the four values into a selection table

Before contacting a supplier, prepare a short table with these items.

Axes and direction: single or multi-axis, XYZ layout, horizontal or vertical mounting.
Stroke: usable travel and mounting space per axis.
Load: total mass, approximate center of gravity, obvious moments, and acceleration.
Speed and cycle: travel per move, time per cycle, cycles per minute.
Accuracy: allowable process tolerance and whether vision or compensation is used.

How to write the RFQ

Avoid sending only "a module with 600 mm stroke." A request like the example below lets the supplier confirm the design in one round.

Screw-driven linear module, horizontal, single axis.
600 mm stroke with no more than 800 mm of mounting space.
About 15 kg load (10 kg workpiece plus 5 kg fixture), centered, acceleration 0.5 g.
Average speed 200 mm/s, one cycle every 3 seconds, continuous operation.
±0.02 mm repeatability.
Quantity, target delivery, and packing requirements.

These points translate directly into screw diameter, lead, thread length, accuracy grade, and rail size. For the full list of what to include in a module inquiry, see the linear module RFQ parameters guide.

FAQ

Is stroke the same as the total module length? No. Total length also includes the support units, motor housing, and screw ends on both sides. Send the available mounting space as well.

Is it enough to give the total weight? No. Center of gravity, moments, acceleration, and mounting direction all change the rail and screw sizing.

What is the difference between positioning accuracy and repeatability? Positioning accuracy is the absolute error from the target position. Repeatability is how consistent the stopping position is over many cycles — usually the critical value for automation.

Next step

Turn this guide into an RFQ

When the specification direction is clear, send the details below together with quantity, lead time, and packing requirements.

Include these details

Model, diameter, lead, accuracy grade, or target application.
Load, speed, travel, mounting method, and matching rail or support-unit needs.
Quantity, lead time, packing, and whether inspection records or shipment photos are needed.

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