harmonic drives really have 'zero' backlash? really better than planetary reduction?

Or just marketing?

I'm not seeing why they are inherently 'zero backlash' designed into the concept. I think without a doubt the claim of having a high reduction ratio in a small volume is true, but the 'zero backlash' seems more marketing than anything.

Backlash in mechanical devices are normally eliminated or reduced by having constant engagement of mating or mechanically interacting components. For example, ball screws ensure the recirculating balls in the long axial race maintain maximum engagement between the ballscrew and the ball nut, thereby eliminating backlash because when the screw is rotated, there will be immediate reaction forces acting on the nut. The same idea behind using delrin nuts (delrin beyond low friction coefficient is a very resiliant material) or preloadinging of normal nuts in normal screws - i.e. to ensure constant mechanical engagement between mechanically interacting surfaces.

When it comes to harmonic drives, are they really 'zero' or low backlash? Are they actually any better than backlash involved in normal gears reductions? They too facilitate power transfer via meshing gear teeth. As far as that is concerned and backlash is concerned, harmonic drives are not inherently any less likely to experience backlash than a planetary gearbox. The only explaination is perhaps the less stages involved, thereby eliminating the additive effect of having multiple meshing gears and multiple stages allowing greater chance of backlash to add up, as is the case for multi-stage gearboxes.

In a similar fashion, you can simply decrease the module significantly to increase teeth count and hence any backlash is minimal (at the cost of lower torque rating); or have an entirely surface friction based power transfer, for which too normal gears can achieve zero backlash also.

IIRC The like for ex 500 teeth mating with the 501 teeth other side do make them indeed 0 backlash. but if that was all and they are 'better' than planetary boxes as your title asks, why have they not replaced plannetaries? there are reasons that go beyond pure backlash. one that prevents me (and I am sure others) from considering them is their inherent ripple: velocity output = velocity input * ratio * cyclic ratio that causes the output to no longer be constant speed. true it is small ripple but apparently large enough to show up in parts machines - unless you go with external scale feedback, in which case you MAY get rid of it if your velocity loop is high enough bandwidth, which it may not be at high speeds..... most of the time the 3 arc min backlash of a planetary will do the job, or the extra cost down to 1 arc min. we even use dual motors/planetaries/pinions on racks to eliminate backlash electrically rather than use one of these....

Thats what I mean. It seems they get their 'zero' backlash capability based on having a high teeth count and a small gear module. This is not specific to harmonic drives. This is related to gear meshing principles in general. You can have a planetary gearbox reducer with tiny teeths and you can get 'zero' backlash effect also. The only advantage backlash wise, may be that harmonic drives involves use of less gear mesh stages, and so by means of additive backlash, it may have less since planetary reductions involve more stages of gear meshing so backlash can add up.

I always see the 'zero backlash' advertisement and sales pitch and I just dont see how 'harmonic drive' = zero backlash theoretically. I think its 'small gear teeth and low gear module' so close to zero backlash. Then again, any gearing system can do this. Use high teeth count and small teeth = low backlash at the expense of lower torque rating.

The only true zero backlash is a direct drive like a stepper motor.

[QUOTE=The only true zero backlash is a direct drive like a stepper motor.[/QUOTE]

So if this is true then can you mount 70 kg wait on 8.0 Nm stepper motor and rotate it with out any backlash.
And if motor is closed loop then it will be harder for weight to stall.
thanks

kindly reply

I will find that out this Winter ,.. I already gut the Drive now Building around it a trunnion setup ...


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Originally Posted by bebob1

Thats what I mean. It seems they get their 'zero' backlash capability based on having a high teeth count and a small gear module. This is not specific to harmonic drives. This is related to gear meshing principles in general. You can have a planetary gearbox reducer with tiny teeths and you can get 'zero' backlash effect also. The only advantage backlash wise, may be that harmonic drives involves use of less gear mesh stages, and so by means of additive backlash, it may have less since planetary reductions involve more stages of gear meshing so backlash can add up.

I always see the 'zero backlash' advertisement and sales pitch and I just dont see how 'harmonic drive' = zero backlash theoretically. I think its 'small gear teeth and low gear module' so close to zero backlash. Then again, any gearing system can do this. Use high teeth count and small teeth = low backlash at the expense of lower torque rating.

The only true zero backlash is a direct drive like a stepper motor.

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The Kinematics of a Harmonic Drive permit a mesh to utilize both sides of the tooth's flank without the adverse effects an involute gear would experience under the same condition.

while backlash sounds scary, on a cnc router 0.01- 0.03 mm backlash for practical life is neglectible..
If your concern to keeping minimal the backlash, then go with ballscrew.. they keep relatively long time their backlash..

on other side if your ballscrew, or rack n pinion not covered fully with bellow, then don't expect backlash free mechanism..

One question for you guys.
I am collecting parts for my cnc I am planning to build 5axis Cnc machine.

Doughty drive is expensive for me.
The price for a B/C drive is US$2950.00 + freight. The DD100 is US$1425.00.
I got the quote recently from them.

I was thinking for my B/C axis with harmonic drive and they are also very expensive and very hard to find online.

What about planetary gears with very low back lash 1arc or 3 arc.
will they work B/C Axis rather then using the harmonic drive gears.

I am talking about these ones.

Pl 90 Stage 1 Precision Planetary Gearbox for NEMA34 Stepper Motor and 750W Servomotor (PL90) - China Planetary, Gearbox

Kindly visit the link above and read the details of the gear box and their coupler which provide black lash free solution and then post the comments.


Thanks
Kindly reply

The problem with a planetary gearbox for a 5axis machine is that they take up much more space than a harmonic drive. If the size isn't an issue, then it should work fine.

Thanks ger21

harmonic drives achieve zero backlash because the tooth form can be preloaded and the oval shaped bearing preloads the system, without preload you would have backlash.

you could preload a planetary gearset if the ring gear is flexible and undersized a tiny bit, but then you have to engineer flexibility into the ring gear.. esentially turning the ring gear into a flexible splined cup, just like it is in a harmonic gear set. I wonder if you could preload a planetary gearset like this http://machinedesign.com/site-files/...ther-angle.jpg