Every linear motion design starts with the same choice: How do you convert rotary motor motion into linear travel? The two most common answers are the Lead Screw (simple, cheap, friction-based) and the Ball Screw (complex, expensive, rolling-based). Making the wrong choice here is costly. Use a lead screw where you need precision, and you get backlash. Use a ball screw in a vertical lift without a brake, and your load crashes to the floor. In this guide, we compare them side-by-side. Table of Contents 1. The Physics: Sliding vs. Rolling 2. Efficiency & The "Back-Driving" Danger 3. Accuracy and Backlash 4. Selection Table Advertisement 1. The Physics: Sliding vs. Rolling The fundamental difference is friction. Lead Screws rely on Sliding Friction . The nut (often bronze or plastic) slides directly against the steel screw threads. This generates heat and wear. Ball Screws re...
Every linear motion design starts with the same choice: How do you convert rotary motor motion into linear travel?
The two most common answers are the Lead Screw (simple, cheap, friction-based) and the Ball Screw (complex, expensive, rolling-based).
Making the wrong choice here is costly. Use a lead screw where you need precision, and you get backlash. Use a ball screw in a vertical lift without a brake, and your load crashes to the floor. In this guide, we compare them side-by-side.
Table of Contents
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1. The Physics: Sliding vs. Rolling
The fundamental difference is friction.
- Lead Screws rely on Sliding Friction. The nut (often bronze or plastic) slides directly against the steel screw threads. This generates heat and wear.
- Ball Screws rely on Rolling Friction. The nut contains recirculating steel bearings that roll inside the grooves. This is the same principle as a ball bearing vs. a bushing.
Figure 1: Ball screws use recirculating balls to minimize friction, while lead screws rely on surface contact.
2. Efficiency & The "Back-Driving" Danger
This is the most critical design factor for vertical Z-axis applications (like 3D printers or CNC mills).
Lead Screw Efficiency: ~30% to 50%
Because of high friction, lead screws are inefficient. However, this inefficiency is a feature, not a bug. They are often Self-Locking, meaning gravity cannot push the nut down. If the motor loses power, the load stays in place.
Note: Self-locking depends on lead angle and friction coefficient; not all lead screws are inherently safe. Always verify your specific screw geometry.
Ball Screw Efficiency: ~90% to 95%
Ball screws are incredibly efficient. This means you can use a smaller motor to drive a heavier load.
The Trap: Because they are so efficient, they are Not Self-Locking.
Many vertical CNC Z-axes fail not during motion, but during power loss events when an unbraked ball screw back-drives under gravity.
If you use a ball screw on a vertical lift, you must use a motor with a brake to prevent this crash.
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3. Accuracy and Backlash
Backlash is the "play" or "slop" felt when you reverse direction. Ideally, when the motor reverses, the load should move instantly. In reality, there is a tiny gap between threads.
- Lead Screws: High backlash (unless using spring-loaded "anti-backlash nuts"). As the nut wears, backlash gets worse.
- Ball Screws: Near-zero backlash. The balls are pre-loaded to ensure constant contact. This makes them essential for CNC machines where 0.01mm accuracy is required.
4. Selection Table
When should you pay the premium for a ball screw?
| Feature | Lead Screw (Acme) | Ball Screw |
|---|---|---|
| Cost | $ (Low) | $$$ (High) |
| Efficiency | 30-50% (High Friction) | 90-95% (Low Friction) |
| Backlash | High (Needs compensation) | Zero / Very Low |
| Vertical Load | Self-Locking (Safe) | Back-Drives (Needs Brake) |
| Typical Use | 3D Printers, Vises, Lifts | CNC Mills, Industrial Automation |
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About the Author:
This article is written by a mechanical design engineer specializing in machine design, linear motion systems, and automation components.
This article is written by a mechanical design engineer specializing in machine design, linear motion systems, and automation components.
As an Amazon Associate, I earn from qualifying purchases.
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