Hysteresis loss and gearbox precision

Hysteresis loss is unavoidable. It is present even in zero backlash gearboxes. That is due to internal friction and material deformations that occur when the rotary direction changes. But there are differences. The level of hysteresis loss depends on several factors. We explain what you should watch out for when selecting gearboxes and why cycloidal gears and strain wave gears are unrivalled in terms of precision.

What is meant by hysteresis loss in gearboxes?

Hysteresis loss describes the torsional strain on the gear output shaft when the input shaft is blocked with a defined torque (at Nabtesco up to the rated torque); this reading is taken at the 0 axis. It consists of the purely mechanical backlash and the inherent friction in the hysteresis measurement process.

How is it measured?

Hysteresis loss is measured by sweeping through the hysteresis cycle. This is a standardised testing process: With the gear output shaft fixed, it is subjected to a torque – first increasing, then decreasing, and then increasing again. The angular tolerance between the gear input and output shaft at a torque of 0 Nm is referred to as hysteresis loss. 

How important is the level of hysteresis loss for gearbox precision?

Hysteresis loss is an important indicator for gearbox precision. A lower value corresponds to higher gear precision. In cycloidal gears and strain wave gears it is between 0.2 and 1 arcmin, depending on the series. Values below 0.25 arcmin are also technically possible.

What is the difference between hysteresis loss, backlash, and lost motion?

There are various parameters that are relevant for gearbox precision. They are closely related and are crucial for the precision, repeatability, and performance of the drive system:

• Backlash refers to purely mechanical clearance.
• Hysteresis loss is the sum of backlash and friction.
• Lost motion describes the angle of torsion at low torques (3 % of the rated torque).

Strain wave gears have absolutely zero backlash. Cycloidal gears have reduced backlash or nearly zero backlash, with pure backlash of only 0.1–0.2 arcmin throughout the entire service life – there is no perceptible backlash at the gear output shaft. The average lost motion of our gearboxes is within a very low range of 0.3 to 0.6 arcmin. By way of comparison: Conventional reduced backlash planetary gears normally have backlash between 1 and 3 arcmin, standard planetary gears even as high as >7 arcmin.

Which factors affect hysteresis loss in gearboxes?

The level of hysteresis loss depends on numerous factors. These include the design principle, materials, temperature, lubricants, wear, and application parameters.

Import factors:

• Gear principle
• Materials
• Elasticity and rigidity
• Temperature in the gearbox
• Choice of lubricant
• Production tolerances
• Assembly precision
• Operating conditions

Which gearboxes are especially suitable for precision applications?

Our cycloidal gears and strain wave gears are unrivalled in terms of precision, torque capacity, and torque density – which makes them ideal for all applications that require precisely executed motions and pin-point positioning. We will be happy to advise you!

Would you like to learn more about hysteresis loss, or do you have questions relating to a specific project? Then don’t hesitate to contact our experts.