Torsional rigidity specifies the resistance of a gear against twisting around the longitudinal axis. High torsional rigidity minimises elastic deformation that occurs when the gear is subjected to torsional loads. The result is excellent precision.
Measuring torsional rigidity
Torsional rigidity, like backlash, is ascertained by scanning the hysteresis. With the drive shaft fixed, the gear output shaft is subjected to torque – first increasing, then decreasing, then increasing again. Then the gradient of the straight lines at the end of the hysteresis curve is measured at 50-100 % of the rated torque. This gradient corresponds to the torsional rigidity. Torsional rigidity is specified in Nm/arc.min. A higher value corresponds to higher gear precision. Torsionally rigid gears not only achieve extremely precise positioning, but also higher acceleration torques, since overshoot and oscillations are eliminated even in highly dynamic situations.
Torsional rigidity in combination with low torques is called lost motion. First, the entire hysteresis curve is generated up to ±rated torque and then the lost motion value at ±3 % is read on the centre line of the hysteresis curve. The average lost motion in Nabtesco cycloidal gears lies within a very small range of 0.3 to 0.6 arc.min.
Cycloidal gears have especially high torsional rigidity
Torsional rigidity is closely related to the gear principle. Cycloidal gear technology offers decisive advantages in this respect. Most important is the offset arrangement of the cams, which are fully engaged via the pins. Even torque distribution is achieved within the gearbox, while the power flow is simultaneously transmitted outward into the housing material. This makes the gears extremely rigid.
Cycloidal gears have another special feature: Higher torque results in higher rigidity. Rigidity reduces elasticity in the gear, which causes an increase in efficiency. In cycloidal gears, efficiency therefore improves as the load increases. This is also evident in the hysteresis curve: The rise, which indicates the degree of torsional rigidity, is less sharp in the upper torque range and therefore exhibits higher rigidity and lower elasticity.
Fast and precise positioning without overshoot and oscillations
The combination of excellent torsional rigidity and a low hysteresis loss makes cycloidal gears ideal for highly dynamic applications, such as servo applications. At high speeds and under heavy loads, the torsionally rigid gears also guarantee fast and precise positioning, without overshoot and oscillations whatsoever.
Benefits of high torsional rigidity:
- Ultimate precision
- High acceleration torques
- Large load-bearing capacities
- No overshoot and oscillations
