High Speed or Heavy Load? 7 Trade-Off Rules for Sizing Linear Modules

1. Start by Choosing Your “Primary Mission”

Before you open a catalog or call a supplier, answer two brutally honest questions:

1. What really sets the cycle time on this machine?
   • Is it the longest X axis shuttling parts between stations?
   • A short Z axis that must move in and out dozens of times per cycle?
2. What happens if this axis misbehaves?
   • Does the product go out of tolerance?
   • Does the line simply slow down?
   • Or does something expensive get smashed?

Put differently, every axis is usually one of these:

• A speed axis – throughput is king.
• A precision axis – accuracy and stiffness win.
• A reliability axis – it just has to run 24/7 for years.

You rarely get all three at once from the same linear module without paying a lot. Decide early which role each axis plays. That will drive your later choices of ball screw drive vs timing belt drive, motor size, and structure.

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2. The Physics: Why Speed and Load Fight Each Other

2.1 One equation you can’t escape

Regardless of how fancy the catalog looks, every linear actuator is bound by:

F ≈ m × a + F_friction + F_process

• m – total moving mass (carriage + tooling + workpiece + cable carrier)
• a – acceleration
• F_friction – friction in bearings, seals, etc.
• F_process – external forces: pressing, cutting, clamping, etc.

If you want higher speed in less time, you increase a. For a given mass, that means more force. More force means:

• Higher motor torque and current
• Higher stress on linear guide rails and the ball screw drive or belt
• More vibration and potential overshoot

At the same time, when you ask the axis to carry more load, the moving mass m grows. For the same acceleration, required force goes up again. That’s why “heavy and fast and long stroke” is a very different beast from “light and fast”.

2.2 A quick example

Say you want:

• Payload: 20 kg (tooling + part)
• Stroke: 800 mm
• Target: accelerate from 0 to 1 m/s in 0.2 s

Average acceleration:
a = 1 / 0.2 = 5 m/s²

Just to accelerate the mass:
F = 20 × 5 = 100 N

Add friction, safety factor, and process forces and you’re easily sizing for 200+ N thrust.

Now check your design:

• Can your motor and drive deliver that continuously?
• Is the ball screw drive diameter large enough to avoid buckling or whipping?
• Are the linear guide rails and housing stiff enough to handle these forces without deflection?

If any answer is “not sure”, that’s your hint you must compromise: either slow down or reduce the load on that axis.

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3. Drive Choice: Screw vs Belt vs Linear Motor

Drive technology is your biggest lever when balancing speed and load.

3.1 Ball screw drive – precision and stiffness first

A ball screw drive converts motor torque into linear force via a screw and ball nut.

Best for:

• High positioning accuracy and repeatability
• Good thrust and stiffness (pressing, precise Z motion)
• Short to medium strokes (say 100–800 mm)

Typical uses:

• Dispensing, screwing, welding heads
• High-precision pick-and-place
• Z axes in vision or assembly equipment

Pros:

• Excellent low-speed smoothness
• Predictable backlash when properly preloaded
• Easy to build into a fully enclosed linear module for dust or liquid protection

Cons:

• Limited by critical screw speed on longer strokes
• High loads and aggressive speeds reduce bearing life

If your axis must carry heavy loads at very high speed over long stroke, a screw may not be your friend—even if a catalog shows a tempting “max speed” number.

3.2 Timing belt drive – long stroke and high speed

A timing belt drive uses a toothed belt and pulley.

Best for:

• Long travel (1–3 m or more)
• High speed with moderate precision (±0.05–0.1 mm)
• Conveyor-like transfers and gantries

Pros:

• Not limited by screw critical speed
• Lower moving mass than a long screw
• Great for “get it from A to B fast” tasks

Cons:

• Lower stiffness and repeatability than a screw
• Belt elasticity and wear under high load
• Sensitive to tensioning and temperature

Rule of thumb: if your problem is “fast and far” more than “fast and heavy”, a timing belt drive is usually the pragmatic choice.

3.3 Linear motor – when dynamics are everything

A linear motor is a direct-drive system: no screw, no belt, just a forcer and magnets.

Use it when:

• You need very high acceleration and jerk
• You want ultra-smooth velocity (e.g., for scanning)
• You’re already in premium territory (semicon, metrology, high-end linear module for electronics inspection)

Pros:

• No mechanical transmission backlash
• Excellent dynamic response
• Quiet and clean when combined with good guides

Cons:

• Higher cost
• Demands a very stiff base and high-quality linear guide rails
• More challenging to tune and integrate

For most general machines, ball screw drive and timing belt drive modules will cover 90% of use cases. Keep linear motors for those last 10% where physics simply won’t cooperate.

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4. Stiffness: The Hidden Limiter You Forget to Ask About

Most “why can’t we run faster?” complaints are not about torque. They’re about stiffness.

You can size the motor correctly and still end up with:

• Blurry images in a vision station
• Parts slamming into nests
• Oscillation when reversing direction

because your structure behaves like a diving board.

4.1 Load is not just mass — it’s also moment

Real machines rarely put the mass neatly on top of the carriage:

• A gripper hangs 150–200 mm in front of the axis
• A camera bar sticks out from a crossbeam
• A vertical Z axis drags a cable chain

All of this creates bending moments (Mx, My, Mz) on the linear guide rail and housing.

If you only size the axis using a “max 50 kg” line in the catalog, but ignore the moment limits, you risk:

• Micro-tilt when accelerating, hurting precision
• Uneven rail wear and shortened life
• Strange resonances that show up only at high speed

4.2 What to do about it

When you know you’ll have heavy, offset tooling:

• Choose wider profiles and rails
• Use twin carriages or even twin rails for large overhangs
• Reinforce gantry beams with higher sections or steel structure

It’s not glamorous, but often the correct answer to “we want high speed and heavy load” is simply “use a bigger, stiffer linear module”.

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5. Patterns That Let You Have “Fast” and “Heavy”

Advanced machines that seem to do everything well usually don’t achieve it with a single heroic axis. They split the problem.

5.1 Combine a fast coarse axis with a precise fine axis

Classic pattern:

• A long-stroke X axis with timing belt drive carries a lighter payload at high speed.
• On top of it sits a short-stroke, high-stiffness ball screw drive module for fine positioning.

The belt axis delivers throughput. The screw axis delivers microns. Together they:

• Keep the heavy, stiff “precision stuff” short and manageable
• Let the fast axis be optimized for speed, not ultimate rigidity

You see this approach in linear modules for semiconductor equipment, high-end AOI machines, and some battery manufacturing lines.

5.2 Split big payloads into multiple moves

If moving 40 kg in one shot is killing your dynamics, ask:

• Could we move 20 kg twice instead?
• Could we split the station into two axes sharing the work?

From a cost perspective, two medium-sized actuators often outperform one “monster” axis:

• Lower inertia per axis
• Easier handling and service
• Less brutal requirements on supports and frames

When you ask how to choose linear module in a heavy application, sometimes the best answer is, “Choose two, and let each do less.”

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6. A 7-Point Checklist for Speed–Load Trade-Offs

When you’re down to a few candidate modules, run them through this checklist:

1. Stroke & Cycle Time
   • What speed and acceleration are truly required?
   • Is there enough time for accelerate + run + decelerate + settle?
2. Total Moving Mass & Process Forces
   • Did you include tooling, workpiece, fasteners, and cable carriers?
   • What is the worst-case pressing or cutting force?
3. Drive Type Fit
   • Short/medium stroke + high precision → screw-driven linear module.
   • Long stroke + high speed + moderate load → belt-driven module.
   • Extreme dynamics or scanning → consider linear motor.
4. Guide and Structure Stiffness
   • Check rail load AND moment ratings, not just drive thrust.
   • Are width, profile, and support structure appropriate for your overhang?
5. Environment & Protection
   • Dust, chips, spray? Need semi-closed or fully enclosed linear module?
   • Any cleaning chemicals that might attack seals or grease?
6. Motor & Drive Sizing
   • Stepper vs servo? Open vs closed loop?
   • Do continuous and peak torque cover the real force profile with margin?
7. Lifetime & Maintenance
   • Expected life in cycles at your load and speed?
   • Lubrication interval and ease of access?

If a candidate fails on more than one of these, it’s probably the wrong tool, no matter how attractive the price looks.

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7. Wrap-Up: Don’t Let the Spec Sheet Daydream Design Your Machine

Many painful projects have the same origin story:

• Someone read the linear module catalog and assumed the “max speed” and “max load” lines applied simultaneously.
• No one checked what happens to life and stiffness when you run near both limits.
• When problems appeared, they were “fixed” with lower speeds, extra supports, bigger motors—anything but rethinking the original assumptions.

If you remember only three things from this article, let them be:

1. Speed and load are physically linked through F = m × a. You can’t change one without affecting the other.
2. Your choice of ball screw drive vs timing belt drive vs linear motor should follow the mission of the axis: precision, stroke, and cycle time.
3. Stiffness—of the linear guide rails, profile, and frame—is often the real limiter at high speed and high load.