Gears, most wanted

[efferman]

sorry guys, but the harmonic drive is a level to high for me. calculating the teeths and the diameter of the rotor needs a natural math prof and iam not one.

[Cumulonimbus]

No problem, as said: I don't have a practical use for it, I just wanted to set a challenge :).

I was looking around for harmoinic drive examples and I found a cycloidal drive, which is based on similar principles as the harmonic drive.

The animation on Wikipedia seems a lot more feasible in Technic, it might inspire you:

More info at: http://en.wikipedia.org/wiki/Cycloidal_drive

[efferman]

indeed, iam working currently on a cycloidal drive

[Cumulonimbus]

i think he want use it on grapples and similar things to syncronize them, without four bevel gears or let them work asymetric.

two gears with 17 teeths can do it

but because more narrow theeths and distances between them, it is not really compatible with standard gears

maybe on some uneven distances on bigger opponents.

uploading the 17Z?

Thanks for the work, I'll put it on my wish-list but I can't promise I'll buy soon.

A grapple was exactly the application I needed it for, a while back. It was a MOC meant as a modification of the grapple and the arm of the 8094 Control Centre but actuated as the pneumatic grapple from 8443 Pneumatic Log Loader. The misalignment resulted in a lack of accuracy. Other solutions were tricky as I needed the assembly at the end of the arm as simple and as light as possible. The MOC itself died a silent death.

The other occasion I needed it was in an airplane MOC were the gears linked the two ailerons, but again the misalignment lead to a poorly functioning control stick. I solved it by using a lever system, which was easily calibrated.

[Boxerlego]

A gear design that would be great to have is a Stationary gear that will allow an axle to rotate in the center. Works like the Sun and planet Gear in this video.

Edited April 27, 2014 by Boxerlego

[efferman]

is there an practical use in lego or is it only a wet dream of math nerds?

[Boxerlego]

Yes, the gear has practical use in lego.

added text:

The above gear video can be made in lego but the kinematic example is difficult to build in lego because the most gears do not have a circle hole o in the center for the shaft to rotate and provide a base to build on so the gear can remain fixed in position.

Edited April 27, 2014 by Boxerlego

[DrJB]

Effe, it's really not that complicated. Let us say that the stator (rigid internal gear) has 30 gears. The rotor (flexible cylinder with external teeth) needs to have few teeth (2-4) less (in this example 28). The mechanism that flexes the rotor, and make it mesh, can be made with 2 small rollers that push the cylindrical shell (and meshes it against the internal gear (stator). I'll try to make a sketch and post it.

[efferman]

Doc JB, the principle behind the harmonic drive is very simple to understand.

but calculating the unflexed inner gear ring is the problem. how have to look the teeth, how large has the diameter to be?

the Master akiyuki has done one, but we want a transmission which is much smaller and has a large reduction

Edited April 28, 2014 by efferman

[OzShan]

You may have sighted this already,

http://mechatronics.ece.usu.edu/ece5320/Schedule/hw01-2010/hw1_Zeke_Susman_a.ppt

The profile of the teeth is not described unfortunately, but the three key components are discussed in different configurations. A difference of two teeth will give the highest gear ratio.

Having played with the real things, I can assure you a technic version would be awesome! :)

[efferman]

thank you very much. one orf the pictures in this presentation helps a lot and my rotor should work.

i only hope the walls ar not to thin. the rotor has 116 teeths and the stator will have 120 teeth so we should have a reduction of 34 to 1 if my math is correct

[OzShan]

That looks awesome! Instantly recognisable as a flex spline :)

[Boxerlego]

It would be great to see you perform some test for these parts and show the limitation.

[efferman]

i have a bad news about the harmonic drive. the teeths are to small for save printing. Currently it has the diameter of the XL motor. make it bigger?

[OzShan]

Yeah, why not. I think diameter is not the main issue if you can keep it short/thin. i.e compact enough to mount under a turret or crane turntable etc :)

Maybe have the output the same as the large turntable and it would drop into a lot of projects more readily?

[DrJB]

Effe,

Based on the image below, I think a harmonic drive is feasible with the number of teeth below:

- Stator: Internal gear, with 36 teeth

- Rotor: External gear, with 34 teeth (mounted on flexible cylindrical shell.

For the rotor, the thickness under the shell should be as small as possible (1 mm?)

The wave-generator is made as shown in the attachment, with two rubber wheels.

Also, the teeth shall be the same size as Lego Teeth, for compatibility of course.

One can make a larger drive, but the one below is the minimum 'possible' based on the size of the wave generator.

[efferman]

ok, i will look after it, but first i have to get my hands on real parts

[Rockbrick]

Surely this could be done with a combination of a turntable and the 16tooth lego clutch gear with the teeth so it can link up to and old lift arm (or put a pin through a 24 tooth on any beam)

Edited April 29, 2014 by Rockbrick

[OzShan]

...

- Stator: Internal gear, with 36 teeth

- Rotor: External gear, with 34 teeth (mounted on flexible cylindrical shell.

...

There is also nothing stopping you from using the outside gear as the stator, and the flex spline as the output. It would give higher compactness in this application as the outer gear could be built to extend back and hold the motor directly and be used to mount the whole assembly.

While compatibility with other gears is great, bear in mind that the percentage deflection relative to the radius of the flex spline is very small on full size drives. Using deep full size teeth on such a small diameter flex spline will introduce a high wall angle as the spline deflects and flares out.

[DrJB]

There is also nothing stopping you from using the outside gear as the stator, and the flex spline as the output. It would give higher compactness in this application as the outer gear could be built to extend back and hold the motor directly and be used to mount the whole assembly.

While compatibility with other gears is great, bear in mind that the percentage deflection relative to the radius of the flex spline is very small on full size drives. Using deep full size teeth on such a small diameter flex spline will introduce a high wall angle as the spline deflects and flares out.

Correct, my attempt here was meant to build it and see how it behaves. Ultimately, what is important is the difference in number of teeth between stator and rotor. When the flex rotor is stretched by the wave-generator, the two anti-lobes on the rotor must retract enough to yield clearance between the teeth there and those on the stator (inner gear). I guess one can run a simple calculation, start with the circumferences of the two gears (primitive diameters) .. then push the lobes out, and calculate how much retraction in the anti-lobes. To simplify the calculation, one can assume that the deformed rotor has the shape of an ellipse with major axis aligned with the lobes, and minor axis perpendicular (aligned with the antilobes) ... the circumference of the ellipse is the easily calculated. The sketch below explains the approach.

[SNIPE]

Some crown gears that can mesh with regular lego spur gears would be good,crowns can act as spur gears too if the teeth are cut right. however unlike the current crown gear we don't need the bit sticking out in the middle for the axle hole.

Edited April 29, 2014 by SNIPE

[BondemandClausen]

I just wish for a 8 tooth gear that can run free, and likewise a 24 tooth. Would expand the possibilities to make compact gearboxes.

[DrJB]

I just wish for a 8 tooth gear that can run free, and likewise a 24 tooth. Would expand the possibilities to make compact gearboxes.

Can't you just take current gears and drill them? For sure will be much cheaper than a 3D printed version ...

Edited April 29, 2014 by DrJB

[BondemandClausen]

If you aim for participating in Lego World etc. then i prefer not to modify existing parts, not that i haven't done it, but still i rather not. I look with interest at the post with ideas here, and hope TLG sees them, allthough i know 3D printing is an opportunity you might want to take advantage of. Somehow i am just a bit concerned if 3D printing doesn't devaluates the LEGO Brand. Just a consideration.

[Boxerlego]

Surely this could be done with a combination of a turntable and the 16tooth lego clutch gear with the teeth so it can link up to and old lift arm (or put a pin through a 24 tooth on any beam)

The turntable is design to be the base for rotating cranes. Sure the turntable has it potential use here. But the kinematic geometry of gear elements behind the stationary sun gear and the planet gear must be be precise on many factors