How to Size a Linear Module: Payload, Speed, Thrust and Lead

Choosing a linear module by stroke alone is the most common cause of axes that are too slow, overloaded or noisy. This guide walks through the sizing steps engineers use – payload, motion profile, thrust, moments and motor – and finishes with a worked example based on real module data.

1. Collect the application data

• Moving mass: part + gripper or tool + adapter plate.
• Mounting orientation: horizontal, vertical or wall-mounted.
• Stroke and the required cycle time (or travel time per move).
• Load position: how far the centre of mass sits from the carriage (overhang).
• Accuracy: required repeatability.
• Process forces: pressing, screwing or machining forces acting on the carriage.
• Environment: dust, chips, coolant, clean room.

2. Choose the drive type

Accuracy, stroke and speed usually decide the drive: ball screw for accurate, stiff and vertical axes; timing belt for long, fast strokes; rack and pinion for very long, heavy axes; linear motor for the highest dynamics. See Ball screw vs. belt drive linear modules.

3. Check the payload for your orientation and lead

The permitted payload depends strongly on mounting orientation and, for ball screw modules, on the lead. Example: BSS140 with a 400 W servo motor:

The pattern is typical: a larger lead gives more speed but less thrust and payload; vertical payload is much lower because the drive must carry the full weight at all times.

4. Define the motion profile

Most moves follow a trapezoidal profile: accelerate, travel at constant speed, decelerate. With stroke s, speed v and acceleration a (same value for acceleration and deceleration), the travel time is:

t = s / v + v / a

(valid when the stroke is long enough to reach full speed, i.e. s ≥ v² / a). The required speed then sets the minimum lead: lead = v / motor speed. With a 3000 rpm servo motor (50 rev/s), 500 mm/s needs a 10 mm lead and 1000 mm/s a 20 mm lead.

5. Check the thrust

The drive must provide the force to accelerate the load, overcome friction and, on vertical axes, lift the weight:

• Horizontal: F = m · a + μ · m · g + F_process
• Vertical (moving up): F = m · (g + a) + F_process

with μ ≈ 0.01 for linear ball guides and g = 9.81 m/s². For a ball screw, the available thrust follows from motor torque T, screw efficiency η (typically 0.85–0.9 for ball screws) and lead P: F = 2π · T · η / P. A 400 W servo motor delivers about 1.27 N·m rated torque; with η ≈ 0.87 this gives about 1390 N at a 5 mm lead – the value in the table above. Compare the required force with the rated thrust and keep a reserve.

6. Check the moments

When the centre of mass is offset from the carriage, the load creates pitch, yaw and roll moments on the guide. A long overhang can overload a module even when the mass is well within the payload rating. Check the allowable static and dynamic moments on the product page; if they are exceeded, reduce the overhang, choose a wider module or use a dual-rail or dual-drive module.

7. Motor, brake and controls

• Use the recommended motor size as a starting point and confirm torque, speed and inertia ratio with the motor supplier’s sizing tool.
• Vertical axes need a motor brake so that the load cannot drop when power is off.
• Decide the home and limit sensor arrangement and the motor position (in-line or parallel) to fit the available space.

Worked example

Task: move a 15 kg load (part + gripper) horizontally over 600 mm in 0.8 s, repeatability ±0.02 mm, no process forces.

1. Drive type: ±0.02 mm and 600 mm stroke → ball screw.
2. Motion profile: with v = 1.0 m/s and a = 5 m/s², t = 0.6 / 1.0 + 1.0 / 5 = 0.8 s. ✔
3. Lead: 1000 mm/s at 3000 rpm → 20 mm lead.
4. Payload: BSS140 with 20 mm lead allows 40 kg horizontal → 15 kg is fine.
5. Thrust: F = 15 · 5 + 0.01 · 15 · 9.81 ≈ 77 N. Rated thrust at 20 mm lead is 347 N, so there is ample reserve for the carriage mass and acceleration peaks.
6. Accuracy and stroke: ±0.01 mm repeatability and 100–1050 mm stroke range. ✔

Result: BSS140, lead 20 mm, 600 mm stroke, 400 W servo motor – then check the moments for the actual gripper offset.

Same task, but vertical: at a 20 mm lead the vertical payload is only 10 kg, so 15 kg is too much. A 10 mm lead allows 22 kg vertical, but limits speed to 500 mm/s, which makes the move take about 1.3 s. If 0.8 s is required vertically, the next larger size (for example BSS170) or a counterbalance should be considered – and a brake motor is mandatory.

Frequently asked questions

How much reserve should I plan?

Many engineers keep a factor of about 1.5 to 2 between the calculated load or thrust and the rated value, more for shock loads, high duty cycles or uncertain process forces.

Why is the vertical payload so much lower than the horizontal one?

On a vertical axis the drive carries the full weight continuously and must also accelerate it upwards, so the same motor and screw can move far less mass. A brake is required to hold the load when power is off.

What is the difference between repeatability and accuracy?

Repeatability is how precisely the axis returns to the same position. Absolute accuracy is how close it gets to a commanded position anywhere on the stroke. Most automation tasks depend on repeatability; absolute accuracy additionally depends on screw grade, mounting and calibration.

Let us check your sizing

Send us the moving mass, its offset, orientation, stroke, cycle time and required accuracy. We will check payload, thrust and moments and recommend a module with a drawing for review. Contact us or browse the single-axis linear modules.