The ball screw is the high-performance standard for precision linear drives. With efficiencies of 90–98% and accuracies in the hundredths-of-a-millimeter range, it is a key component of modern machinery. This guide explains sizing per ISO 3408/DIN 69051.
Understanding Ball Screws
A ball screw is a highly efficient mechanical conversion of rotary motion into linear motion. Unlike a trapezoidal thread (friction via sliding friction), a ball screw uses rolling ball elements in helical grooves.
Key Advantages at a Glance
- High efficiency: 90–98%, significantly better than trapezoidal threads (25–50%)
- Zero-backlash: With preload, play can be completely eliminated
- Precision positioning: Repeatability of ±0.05–0.1 mm achievable
- Long service life: Hundreds of thousands of operating hours possible
- Self-locking excluded: Not suitable for loads that can "back-drive"
Design and Components
A ball screw consists of:
1. The Screw (Ball Screw Shaft)
A steel cylinder with a precision-ground helical groove in which balls roll. The groove geometry is precisely defined per DIN 69051. Standard diameters: 8–80 mm, lead: 1–20 mm/revolution.
2. The Nut (Ball Screw Nut)
It sits on the screw and carries the load. Inside, it also has grooves that form a ball circuit with the screw groove. Variants exist with return tube (ball return channel) and without (open nut).
3. The Balls
Hardened steel balls in precision diameters (typically 4–10 mm). They roll between the screw and nut and are held at even spacing by retainer cages.
4. The Retainer Cage
Made of plastic or sheet metal, it holds the balls at constant spacing and prevents them from rubbing against each other.
Practical Tip from TEA:
Store ball screws dry and protected from contamination. A single particle of dust can significantly impair running accuracy.
Key Sizing Parameters
Lead (P)
The lead defines the linear travel per screw revolution. Smaller leads produce higher torque; larger leads produce higher speed.
- P = 1–3 mm: High-torque design, e.g. for precise positioning
- P = 5–10 mm: Standard design, balanced between torque and speed
- P > 10 mm: High-speed design, requires higher motor power
Screw Diameter (d)
Determines load capacity and stiffness. Larger diameters handle higher loads but also require higher torques for rotation.
Rule of thumb for load rating: The dynamic load rating Ca approximately doubles when the diameter grows by about 25%.
Accuracy Classes per ISO 3408
| Class | Lead deviation | Typical application |
|---|---|---|
| C1 (highest) | ±0.006 mm/300 mm | Metrology, optics, robotics |
| C5 | ±0.023 mm/300 mm | Standard industrial equipment |
| C7 | ±0.050 mm/300 mm | Robust machinery and equipment |
| C10 (basic) | ±0.210 mm/300 mm | Cost-effective mass production |
Service Life Calculation per ISO 3408
The nominal service life L10 is a statistical measure indicating how long 90% of all identical screws can be operated before fatigue occurs.
Formula
Ca = dynamic load rating [N] (from catalog)
F = operating load [N]
To obtain the service life in hours:
n = speed [rpm]
Sample Calculation
Task:
Ball screw 16 mm × 5 mm, Ca = 4,200 N (from catalog)
Operating load F = 800 N, speed n = 600 rpm
Calculation:
L10 = (4200 / 800)³ × 10⁶ = 5.25³ × 10⁶ = 144.7 × 10⁶ revolutions
T10 = 144.7 × 10⁶ / (600 × 60) ≈ 4,020 hours ≈ 1.9 years (8 h/day, 5 days/week)
Preload and Zero-Backlash
Manufacturing tolerances always result in a small amount of play between ball and grooves. Preload is an additional tightening torque that eliminates this play.
Preload Levels
- C0 (none): For simple positioners, accepts play
- C1, C2, C3: Increasing preload force, typically 3–8% of the dynamic load rating
Effect of preload:
- Higher stiffness (less deflection under load)
- Eliminates play (backlash-free movement)
- Higher wear rate (friction forces increase)
- Shorter service life (L10 decreases with high preload)
Practical Tip from TEA:
Choose preload C2 or C3 only when zero-backlash is truly critical. For simple positioners, C0 or C1 is sufficient and more economical.
Lubrication and Maintenance
Ball screws are relatively low-maintenance. Nevertheless, proper lubrication is essential:
Lubrication Schedule
- Lubricant: Lithium complex grease (DIN 51825 K2K-30), or specialty greases for high speed
- Frequency: Every 100–200 operating hours
- Quantity: Small amounts; over-greasing worsens efficiency and thermal behavior
- Cleanliness: Use only clean tools, prevent contamination
Inspection Intervals
- Monthly: Visual inspection for contamination or damage
- Semi-annually: Functional check, verify positioning accuracy
- Annually: Cleaning, lubrication, wear check
TEA Selection Recommendations
Ball screws are indispensable for highly accurate and efficient linear drives. Sizing per ISO 3408 requires careful consideration of lead, diameter, preload, and expected service life. For critical applications (robotics, metrology), we recommend a detailed sizing review by experts. Use our online sizing tools or contact our application engineering team for your project.
Select the Right Ball Screw
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