The Axis That Couldn’t Accelerate Fast Enough
A customer needed a pick-and-place axis: 500 mm travel, 15 kg payload, move in 0.8 seconds. We specified a ball screw with a servo motor. The simulation said it should work — the screw was sized for the load. On the floor, the axis took 1.4 seconds, not 0.8. The servo could accelerate faster, but the ball screw had a critical speed limit (the screw resonated above 2,000 RPM). To move 500 mm in 0.8 seconds, the screw needed to spin at 3,500 RPM at peak — beyond the critical speed. We couldn’t accelerate the axis without the screw whipping. The fix was a linear motor. It did 0.8 seconds flat, with no mechanical transmission to limit the acceleration.
Linear motor vs. ball screw for high-speed axes is the classic trade-off: cost vs. performance. For standard travel and moderate speed, the ball screw wins on price. But once you need high acceleration, long travel, or high duty cycle, the ball screw hits limits that a linear motor doesn’t have. This article is how I decide which to use.
How Each System Works
Ball Screw Axis
A rotary servo motor spins a ball screw. The ball nut (attached to the carriage) converts the rotation to linear motion. The transmission (screw + nut) provides mechanical advantage and positions the load.
The ball screw is a mechanical part. It has inertia (the spinning mass), critical speed (resonance at high RPM), and wear (the balls recirculate over time).
Linear Motor Axis
A linear motor is a rotary motor unrolled flat. The coil (forcer) moves directly on the magnet track. There’s no screw, no nut, no gearbox. The carriage is the motor. Position feedback is a linear encoder (directly on the axis). The motion is direct drive.
The linear motor has no mechanical transmission. What you command is what you get — no backlash, no compliance, no screw whip. But it costs more and needs a linear encoder.
| Feature | Ball Screw | Linear Motor |
|---|---|---|
| Max acceleration | 1–2 g (limited by critical speed) | 5–10 g (direct drive) |
| Max speed | 0.5–1.5 m/s (RPM limited) | 3–5 m/s (electrical limited) |
| Position accuracy | ±0.01–0.05 mm (screw pitch error) | ±0.001–0.01 mm (encoder limited) |
| Backlash | Small (preloaded nut) or zero | Zero (direct drive) |
| Wear / maintenance | Screw life (billions of revs), lubrication | Minimal (no contact), air/cooling required |
| Cost | Low (screw + rotary motor) | High (forcer + track + linear encoder) |
| Heat | Minimal (screw efficiency ~90%) | Significant (forcer heat, needs cooling) |
When the Ball Screw Hits Its Limits
The ball screw works well until three things push it past its limits.
1. Critical Speed (Screw Whipping)
A ball screw is a long shaft supported at two ends. At a certain RPM, it resonates (whips) like a jumping rope. This is the critical speed. It depends on the screw diameter, length, and support bearings. A Ø20 mm screw 1,000 mm long has a critical speed around 2,000–2,500 RPM. Above that, the screw vibrates and can fail.
To move fast, the screw must spin fast. A Ø20 mm screw with 10 mm lead moves 10 mm per revolution. To go 1 m/s, it must spin at 6,000 RPM. That’s way above critical speed. To go faster, you either use a bigger screw (lower critical speed, heavier) or a shorter screw.
The linear motor doesn’t have this limit. Its speed is limited by the encoder and the drive’s electrical bandwidth, not by a mechanical resonance.
2. Acceleration (Inertia Matching)
The screw has inertia. To accelerate the load, the motor must accelerate both the load and the screw itself. For high acceleration (2 g+), the screw’s inertia dominates. The motor spends most of its torque spinning the screw, not moving the load.
The linear motor has no spinning mass. The forcer accelerates the load directly. All the motor’s force goes to the load, not to spinning a screw.
3. Duty Cycle (Heat and Wear)
A ball screw running continuously at high speed generates heat (friction in the nut). The heat expands the screw, causing thermal growth and positioning drift. The nut wears over millions of cycles. For a 24/7 high-duty machine, the screw needs periodic replacement and thermal compensation.
The linear motor has no contact (the forcer floats on a bearing rail). No wear. The heat is in the forcer (copper losses), which is cooled by air or water. No screw to wear or thermally expand.
The break-even point: Below 1 m/s and 1 g acceleration, the ball screw is cheaper and simpler. Above 1.5 m/s or 2 g, or for 24/7 high-duty, the linear motor wins on performance and lifetime. For most general automation (pick and place at moderate speed), the ball screw is the default. Reserve the linear motor for high-performance axes.
Cost Comparison: Why the Linear Motor Is More Expensive
The linear motor costs 2–3× more than a comparable ball screw axis. Why?
- Magnet track: The permanent magnets along the rail are expensive (rare earth magnets). The longer the travel, the more track you buy.
- Linear encoder: A high-resolution linear encoder is required (no rotary encoder on the motor). This adds cost.
- Forcer and drive: The linear motor drive is specialized, not a standard servo drive.
- Cooling: High-duty linear motors need water cooling (another system).
- Mechanical guidance: The linear motor provides force but not guidance. You still need a linear guide (rail + carriage) to carry the load. The ball screw self-supports somewhat (the nut rides on the screw); the linear motor doesn’t.
The total cost of ownership changes the picture. A ball screw replaced every 2–3 years in a high-duty application costs labor + parts + downtime. The linear motor runs for 10+ years with minimal maintenance. Over a machine’s life, the linear motor can be cheaper.
When to Choose Which
Choose a Ball Screw When
- Speed is below 1 m/s (or below the critical speed limit).
- Acceleration is below 1–2 g.
- Travel is under 1,500 mm (longer = lower critical speed).
- Budget is constrained.
- Duty cycle is moderate (not 24/7 high-speed).
- Position accuracy of ±0.02 mm is sufficient.
Choose a Linear Motor When
- Speed needs to be 2 m/s or higher.
- Acceleration needs to be 2 g+.
- Long travel (over 2 m) where the ball screw critical speed is limiting.
- 24/7 high-duty operation (screw wear is a problem).
- Highest accuracy and minimal settling time needed.
- Silent operation or no lubrication is required (clean room, food).
Designing a Linear Motor Axis
If you go linear, the design considerations are different from a ball screw.
Guidance Is Critical
The linear motor doesn’t guide the load. The linear guide (profile rail) must carry all the moments (pitch, yaw, roll). A ball screw axis has some inherent stiffness (the nut rides on the screw). A linear motor axis is entirely dependent on the guide rail. Use a preloaded double carriage or two rails for moment loads.
Encoder Mounting
The linear encoder must be mounted to the machine frame (not to the moving carriage). The encoder measures the actual carriage position directly. If the encoder is on the motor (as in a ball screw), it measures the screw rotation, not the actual load position. Direct encoder mounting gives true position feedback.
Cable Management
The forcer has a cable (power + encoder) that moves with the carriage. A ball screw axis also has a cable (the motor is stationary), but the linear motor’s cable moves more. Use a proper cable carrier (energy chain) rated for the flex cycles. The cable is a wear item in a high-duty linear axis.
Cooling
For continuous high-force operation, the forcer needs cooling. Air cooling works for intermittent duty. Water cooling (a chilled plate on the forcer) is for continuous high force. Without cooling, the forcer overheats and the drive derates.
A Linear Axis Selection Checklist
- What is the required max speed? (Ball screw critical speed check.)
- What is the required acceleration? (Inertia ratio check for ball screw.)
- What is the travel length? (Longer = lower ball screw critical speed.)
- What is the duty cycle? (24/7 favors linear motor.)
- What position accuracy is needed? (±0.02 mm = ball screw; ±0.005 mm = linear.)
- What is the budget? (Linear motor costs 2–3× upfront.)
- For linear: is the guidance (rail) sized for the moment loads?
- For linear: is there a cooling plan for the forcer?
- For linear: is the linear encoder mounted to the frame (not the carriage)?
- What is the total cost of ownership? (Screw replacement vs. linear motor maintenance?)
The Bottom Line
High-speed linear axis design isn’t picking between two products. It’s matching the transmission to the required speed, acceleration, and duty cycle. For most automation, the ball screw is the right answer — cheap, proven, sufficient. But when the axis needs to move 500 mm in under a second, or run 24/7 at 2 m/s, the ball screw’s critical speed and wear make it the wrong tool. The linear motor costs more upfront, but it doesn’t have a screw to whip, wear, or thermally expand. Pick the transmission that matches the performance requirement, not the one with the lower purchase price.
