• Injoon Hwang

CRUX-170HD REVIEW (Jiyong Yun)

Harmonic Drive

I’ve received a brand-new ‘Harmonic Drive’ equatorial mount CRUX-170HD about a month ago. CRUX-170HD uses a special driving mechanism different from other equatorial mounts. Baisically, almost all commercially-available equatorial mounts are worm-driven. They uses worm and worm gear system for final reduction. But there are few drawbacks here.

Worm & worm gear system

Worm screw and worm wheel are engaged together in worm drive system. However, to achieve high precision required for astro-imaging, tension of the worm must be precisely adjusted. If the tension is not enough, guide accuracy with high payload is lowered. It may also suffer from backlash. If the tension is too strong, guide accuracy is lowered, and the gear may be damaged. If ambient temperature is changed, the tension of the gear have to be adjusted. In addition to that, because only 2-3 gear teeth are engaged in worm drive systems, they cannot withstand high torque. Worm-driven equatorial mounts have to be precisely balanced for precise guide.

The Harmonic Drive is definitely better than worm drive to be used in equatorial mounts.

Basic principle of Harmonic Drive

Harmonic Drive baisically consists of only three components – Wave Generator(green), Flexipline(red), and Circular Spline(blue). If the elliptical wave generator rotates, it deforms a thin steel cylinder called flexipline, and the flexipline rotates slowly with respect to the cylinder called circular spline. Because about 1/3 of gears are always engaged, it can withstand very high torque, and there is no backlash – which means extremely high precision guiding.

Harmonic Drive is actually not new – neither unpopular. They are already widely used in industrial robots. Three mars rovers, Spirit, Opportunity, and Curiosity also utilized Harmonic Drive in their wheels. Mars rovers show us one thing – extreme lifespan and reliability of Harmonic Drive. In the case of Opportunity, it is wandering around the Mars for 13 years and 5 months! But there is just one problem in Harmonic Drive – the cost. Just single gear unit of Harmonic Drive costs more than $1000 – almost equivalent to chinese-made GOTO equatorial mounts. Because two Harmonic Drive units are needed for single mount for RA and DEC, it costs a LOT much than traditional worm driven mounts. Because of the cost, there were no consumer-targeted Harmonic Drive equatorial mounts before.


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