What is the difference between torsional backlash and radial play?

Torsional backlash (or reversal backlash) is the angular deviation between the input and output axis when the direction is reversed – measured in arcminutes or degrees. Radial play is a lateral displacement between bearing and shaft. For positioning drives, torsional backlash is critical because it affects control accuracy. Radial play is more of a bearing phenomenon. Both should be minimized, but their effects differ.

How is torsional backlash measured and in what unit?

Torsional backlash is measured by applying a known torque to one axis and measuring the angle by which the other axis shifts before it responds. The unit is arcminutes (arcmin) or degrees (°). 1° = 60 arcmin. For high-precision robots, the requirement is <0.5 arcmin. For standard industrial gearboxes, 3–10 arcmin is acceptable. Measuring instruments: indicators, angle encoders, or specialized test benches.

Can a preloaded bearing reduce torsional backlash?

Yes, definitely. Preloading via angular contact bearings, needle bearing packs, or special bearing arrangements can reduce radial and axial play. This is often cheaper than purchasing a new gearbox generation. However, preloading increases the bearing load and can shorten service life. For high-precision applications, a combination of low-backlash gearbox plus preloaded bearings is typical.

Why do zero-backlash components cost significantly more?

Zero-backlash components require higher manufacturing accuracy (quality grades 5–7 per DIN 3961–3967 instead of 9–10). This means: specialized manufacturing tools, longer cycle times, more quality control, and higher scrap rates. Bellows couplings and metal bellows couplings have complex elastic structures that are expensive. The higher costs are often justified by improved control accuracy and service life.

Is zero backlash always better, or are there disadvantages?

Zero backlash is not always better. Disadvantages of zero-backlash systems: (1) Higher cost, (2) Less tolerance for wear – after a few years, a zero-backlash coupling can develop backlash again, (3) Less shock absorption – shock loads are transmitted without reduction, (4) Higher demands on assembly and maintenance. For standard conveyor technology, jaw couplings with 5–10 arcmin are perfectly adequate and more economical.

Zero Backlash vs. Backlash: What Does Your Application Need?

Q: Can a preloaded bearing reduce torsional backlash?

Yes, definitely. Preloading via angular contact bearings, needle bearing packs, or special bearing arrangements can reduce radial and axial play. This is often cheaper than purchasing a new gearbox generation. However, preloading increases the bearing load and can shorten service life. For high-precision applications, a combination of low-backlash gearbox plus preloaded bearings is typical.

Q: Why do zero-backlash components cost significantly more?

Zero-backlash components require higher manufacturing accuracy (quality grades 5–7 per DIN 3961–3967 instead of 9–10). This means: specialized manufacturing tools, longer cycle times, more quality control, and higher scrap rates. Bellows couplings and metal bellows couplings have complex elastic structures that are expensive. The higher costs are often justified by improved control accuracy and service life.

Q: Is zero backlash always better, or are there disadvantages?

Zero backlash is not always better. Disadvantages of zero-backlash systems: (1) Higher cost, (2) Less tolerance for wear – after a few years, a zero-backlash coupling can develop backlash again, (3) Less shock absorption – shock loads are transmitted without reduction, (4) Higher demands on assembly and maintenance. For standard conveyor technology, jaw couplings with 5–10 arcmin are perfectly adequate and more economical.

One of the most important decisions when selecting drive components is the question: zero backlash or with backlash? This decision has a major impact on cost, service life, control accuracy, and maintenance.

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 reversal backlash or backlash) is the angular deviation between the drive axis and the output axis when the direction of rotation changes. It is measured by applying torque to the drive side and measuring 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 in gearboxes, bearing play, elastic deformations under load, manufacturing tolerances.

Zero-Backlash Components

Planetary Gearbox with Zero Backlash

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 folded bellows (usually elastomer or metal) transmits torque without mechanical play. Bellows couplings cost 200–500 dollars 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.

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.

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. Reversal-Backlash-Sensitive Applications

Applications where torque frequently changes direction (e.g., oscillators, valve drives with rapid closing) are particularly sensitive to reversal backlash. Control 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 play is "taken up" by the force and plays no role. 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

Do You Need Zero-Backlash or Backlash Components?

Our engineers help you choose the right technology for your application. We analyze your requirements and recommend the most cost-effective solution with the appropriate backlash level.

Contact Our Experts →