Re: ZWO harmonic drive


Having been following the harmonic drive issue for almost twenty years partly for 4th/5th axis CNC applications and to a lesser extent AP applications I would like to make the following points:

You can find bargain priced harmonic drives second hand either as an actuator, actuator with motor (dc servo, ac servo, stepper) or just the cups to fit into your own housing.  But like most things the second hand market fluctuates and some items are priced not to sell but just because some one else is asking a high price.  Its annoying and impacts on other items of interest like encoders and motor drive electronics.  You can also pick up used robotic arms as a source, for example I picked up a SCARA for £150 which includes 3 harmonic actuators / motors.

A lot of the early drive/motor setups were axial and quite long (partly due to the harmonic cup and use of dc servo motors) but they ranged in size from 1cm by 10cm for a gearbox/dc motor and encoder to 20cm by 30cm.  Its now increasingly common to see much shorter versions with AC servos but of course the drive electronics get more expensive, and there are increasing numbers of bare actuators out there that can be adopted to AP use especially with steppers and hence my interest in onstep.   Common gear ratios on auction sites range from 30:1 to 160:1 but you can get much higher ratios if you buy through the manufacturers as the likes of ZWO do.  Whilst some tend to be long they are small compared to some worm drive setups, I have one from Beacon Hill which is 275mm dia!

Like I think Bryan used, I have some 17-80 actuators which measure 85x40mm (1:80) ratio and looked on one of the far east sites last night for suitable stepper based gearboxes to drive these, to be surprised by a few things.  Firstly a ZWO AM5 for £250.00 (obviously an error) but more importantly a range of harmonic drives ranging from short integrated NEMA 17-Harmonic drives 1:30 ratio for <<$100 new and a wide range of other HDs priced competitively .  This perhaps explains why ZWO and others are starting to use harmonic drives in consumer based mounts.  I have bought some of the NEMA 17 drives so will let you know what they are like when they turn up.  I also now have some boards for the Max onstep 3.6 so can move forward on the electronics.
Clear skies

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