Zero backlash means drive components with no measurable reversal play — typically <1 arcmin. Systems with backlash have 5–50 arcmin of torsional play. This choice directly determines positioning accuracy, cost, and maintenance effort.
Zero-backlash components often cost 2–5 times as much as standard components. But for some applications this investment is indispensable; for others it is completely unnecessary. This guide shows when zero backlash is critical and when backlash is sufficient.
Takeaway: Zero backlash (<1 arcmin) is essential for positioning drives, robotics, and measurement technology. For conveyor technology and force-controlled drives, 5–10 arcmin is sufficient. The choice is based on control accuracy, not cost alone.
What Is Backlash?
Torsional backlash (also called lost motion or simply backlash) is the angular deviation between the input axis and the output axis when the direction of rotation reverses. It is measured by applying torque to the drive side and recording the angle by which the output side shifts before it responds.
Measurement and units:
- Unit: arcminutes (arcmin) or degrees (°). 1° = 60 arcmin.
- Practical threshold values:
- <0.5 arcmin = High precision (robotics, medical technology)
- <1 arcmin = Zero backlash (positioning drives)
- 1–3 arcmin = Reduced backlash
- 3–10 arcmin = Moderate
- >10 arcmin = With backlash (conveyor technology)
Torsional backlash is caused by several factors: tooth flank clearance (backlash) in gearboxes, bearing play, elastic deformation under load, and manufacturing tolerances.
From Backlash to Positioning Error
Backlash expressed in arcminutes only becomes tangible once you convert it into the linear positioning error at the effective radius. For small angles the following applies:
s ≈ R · φ (φ in radians)
s = positioning error · R = effective radius / lever · 1 arcmin = 2.909 · 10⁻⁴ rad
Worked example: R = 200 mm, backlash φ = 5 arcmin
- φ = 5 · 2.909 · 10⁻⁴ = 1.45 · 10⁻³ rad
- s ≈ 200 mm · 1.45 · 10⁻³ ≈ 0.29 mm
- A zero-backlash axis (< 1 arcmin) reduces the error to < 0.06 mm.
This illustrates the lever effect: the larger the effective radius, the more strongly the same backlash impacts positioning accuracy. For gears, the gear quality grade (quality grades per DIN 3961/3962) additionally determines the achievable flank clearance — the values are reference figures.
Practical Tip from TEA:
Always convert the backlash to the actual effective radius in millimetres before you specify a backlash class — the arcmin figure on its own invites mis-sizing, because the same backlash is harmless on a 50 mm lever yet unacceptable on a 1 m cantilever. In our consulting work we also often see that only the catalogue new-condition backlash is used as the basis: jaw couplings and especially worm gears develop additional backlash over their service life, so an allowance for the end-of-life backlash belongs in the specification. When submitting an enquiry, always state the lever arm and the required linear positioning accuracy alongside the permissible arcmin value — this lets us derive the most economical backlash class cleanly.
Zero-Backlash Components
Zero-Backlash Planetary Gearbox
High-quality planetary gearboxes can be designed with torsional backlash <0.5 arcmin. This is achieved through: (1) Increased manufacturing accuracy (quality grade 5–6 per DIN 3961–3967), (2) Preloaded bearing packs, (3) Elastic or spring-loaded planet carriers, (4) Profile correction to optimize tooth flank contacts. Such gearboxes are standard in robotics and medical technology. They cost approximately 3–4 times more than standard planetary gearboxes.
Bellows Coupling
Bellows couplings guarantee torsional backlash <1 arcmin and are the standard interface for zero-backlash positioning drives. The corrugated bellows element (elastomer or metal) transmits torque without mechanical play. Bellows couplings cost 200–500 € for small sizes, but offer an extremely reliable zero-backlash connection.
Preloaded Guides and Bearings
Angular contact bearings or needle bearing packs can be mounted under preload to eliminate radial and axial play. This is often more economical than a new gearbox generation. Typically, two bearings with opposing contact angles are arranged and preloaded by locking nuts. This reduces bearing service life by approx. 20–30 %, but provides zero-backlash function. Zero-backlash roller guides such as TEA LinRol/LinTrek systems achieve <1 arcmin thanks to preloaded guide rails. The choice between a press fit (e.g., H7/p6 for permanent mounting) and a transition fit (e.g., H7/k6 for serviceable connections) is governed by ISO 286 — see our reference for ISO fits H7/H6 with µm-values for the most common shaft diameters.
Components with Backlash
Worm Gear
Worm gears have high susceptibility to wear and backlash development due to their design. Typical values: 5–20 arcmin when new, 20–50 arcmin after a few years of operation. This is due to the sliding contact type (worm on gear) and high friction. However, worm gears are very compact, enable high gear ratios, and are cost-effective. Not suitable for positioning drives, but acceptable for conveyor technology.
Jaw Coupling
Jaw couplings with elastomeric inserts have torsional backlash of typically 3–8 arcmin, depending on the elastomer material and wear. This is acceptable for general mechanical engineering drives but not for positioning. They are robust against contamination and shaft deflection.
Standard Rack and Pinion
Racks per DIN 8 with standard module have flank clearance of 1.0–2.0 mm (depending on module). This leads to torsional backlash of 10–30 arcmin for typical pinion sizes. They are cost-effective and ideal for conveyor technology, but unsuitable for precise positioning. Where reduced flank clearance is required, reduced-backlash racks made to drawing are the answer.
When Is Zero Backlash Critical?
1. Positioning Drives
Axes that must repeatedly travel to exact positions (machine tools, robots, handling systems) require zero backlash. Even a small backlash of 5 arcmin translates to a linear positioning error of approximately 1.45 mm at a 1-meter lever arm (tan(5/60°) × 1000 mm). In multi-axis systems, these errors compound. Typical requirement: <1 arcmin over the entire machine lifetime.
2. Highly Dynamic Applications with Rapid Direction Changes
Robot wrist axes that perform many motion reversals per second require zero backlash to minimize control deviations. Torsional backlash directly affects trajectory errors.
3. Applications Sensitive to Reversal Backlash
Applications where torque frequently reverses direction (e.g., oscillators, valve drives with rapid closure) are particularly sensitive to backlash on reversal. The control loop becomes unstable when play is large. Requirement: <3 arcmin for stable control.
4. High-Precision Measurement Technology and Medical Applications
Surgical robots, precision measuring instruments, and laboratory automation require <0.5 arcmin, since errors here can endanger human lives. Here, zero backlash is not optional but a safety requirement.
When Is Backlash Acceptable?
1. Conveyor Technology and Drives without Positioning Requirements
Conveyor belts, elevators, pump drives – anything where only uniform speed and torque matter, not exact positions. Here jaw couplings or worm gears are perfectly adequate with 5–20 arcmin.
2. Force-Controlled Applications
Applications that maintain a constant torque (not an exact position) – e.g., presses, roller drives, torque limiters. The backlash is absorbed by the applied force and has no adverse effect. Requirement: <10 arcmin is perfectly sufficient.
3. Non-Critical Drives with Long Cycle Times
Drives that rarely start/stop or run continuously at speed – e.g., fans, compressors, standard electric motors. Play only matters at start-up. Requirement: <15 arcmin is sufficient.
Comparison Table: Backlash of Components
| Component | Torsional Backlash | Cost Index | Application |
|---|---|---|---|
| High-Precision Planetary Gearbox | <0.5 arcmin | ★★★★★ | Medical technology, robotics |
| Metal Bellows Coupling | <0.5 arcmin | ★★★★ | High-precision wrist axes |
| Bellows Coupling (Elastomer) | <1 arcmin | ★★★ | Positioning drives, robotics |
| Precision Planetary Gearbox | 1–3 arcmin | ★★★ | Positioning, automation |
| Jaw Coupling (high quality) | 3–8 arcmin | ★★ | General mechanical engineering |
| Worm Gear (new) | 5–20 arcmin | ★★ | Conveyor technology, lifting drives |
| Standard Rack and Pinion | 10–30 arcmin | ★ | Conveyor technology, simple drives |
| Worm Gear (older) | 20–50 arcmin | ★ | Systems requiring maintenance |
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