Doug72

[WIP] Intermittent Rotation Mechanism

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For some time I have been trying to develop a mechanism to allow intermittent rotation for various GBC modules I have built or plan to build.
A Geneva mechanism would be ideal but its not easy to build using Lego, and some solutions are quite large.

I have developed a fairly small version which uses a rotating arm to intermittently turn the black gear with 4 rollers.

Indexing is acheived by using a 24T gear with four blue pin/axles. As the wheel rotates and drive arm disengages the 24T gear & wheel is held stationary by an arm kept in contact with two of the blue pins by a shock absorber with a soft spring.

I tried using knob gears but due to them being slighty smaller diameter the indexing is no longer exactly 0, 90, 180, 270, degs.

The two black 36T gears are only used to hold the rollers and drive arm as along with the 24T gear as they are the only lego parts which 4 equally space holes in line with the axle hole.

At the moment the output axle stops every 90 degs but could be made to stop at 180 degs if use a 12T gear meshed the the 24T indexing gear to give a1:2 step up. (An alternative would be to use a 20T meshing with a 40T gear holding the drive rollers.)

Hope that makes sense.

Timings should be adjustable via varying the drive input / output gear ratios and choice of PF motors.

Video will make it clearer how it works.

Next step to try and make it more compact and see if I can use it for my build of DaFokka's Ballkirk Wheel GBC Lift.

 

Edited by Doug72

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Nice work, Doug. The spring-loaded locking is a very good feature, as Lego doesn't really have parts to make a Geneva mechanism without slack:

 

You can probably use the 90 degree limiter that @Didumos69 developed for gear shifting, it's a very solid solution.

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51 minutes ago, BusterHaus said:

Nice work, Doug. The spring-loaded locking is a very good feature, as Lego doesn't really have parts to make a Geneva mechanism without slack:
You can probably use the 90 degree limiter that @Didumos69 developed for gear shifting, it's a very solid solution.

Thanks'

I came across that video when looking for ideas on Google. Also found a three leg Geneva mech. but I am after 180 deg. indexing.

Only way at present is to use 1:2 step up gears so a 90 deg. step gives 180 deg out.

There is a little bit of slack causing a slight jolt as the locking comes into operation. Looking for away damping it..

The gearshift solution might possible work with a reciprocating arm to move the gear lever back & forth but not sure how much it power it would have to could rotate if used on the Ballkirk lift.

Found and downloaded the LXX file - so will give it a try.

Doug.

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I like your solution.  Looks really robust.

aeh uses a wonderful Geneva mechanism in his Pen-dragon drawing machine that you should check out.  Akiyuki also uses an incremental motion device in his Ball Factory GBC for 1/16 motion of the bucket wheel.

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13 hours ago, Blakbird said:

I like your solution.  Looks really robust.

aeh uses a wonderful Geneva mechanism in his Pen-dragon drawing machine that you should check out.  Akiyuki also uses an incremental motion device in his Ball Factory GBC for 1/16 motion of the bucket wheel.

Thanks for the comment, I hope to make it a bit more compact and try to get rid of the little bit of slack.

Can you provide the links to the two you mention. Can,t seem to locate them.         Now found.

Edited by Doug72

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I have built a test rig to demostrate the intermittent rotation when applied to one side of a Ballkirk wheel (note turntables are not interconnected).

Biggest challenge was getting the index wheel and compression of the shock absorber correct.

An XL motor gives enough power to rotate the wheel and overcome the index wheel giving 4 stops per revolution at  90, 180, 270 & 360 degs. To get 180 deg steps would require a 1:2 step up between star wheel and ball lift wheel.

Video showing the action:-

 

 

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Update:

Found using 1:2 step up gears to get 180 deg. steps has too much backlash in the gears and rotating arm fails to align correctly.

Changed the "Star" wheel to 4 arms and improved the indexing arrangement made.
The index mechanism aligns the arms rotating wheel  at 180 & 360 degs
So for each revoltion there are four stops - intention is to load / unload balls at 180 & 360 degs.

An alternative is to have only two arms at 90 degs. to each other to align for ball transfers but this gives a long dwell time for the other 270 degs of rotation. 

It took a lot of trial & error to get the "star" wheel, rollers and indexing system to work well.
The 16L tie rod is required to stop the shock absorber pusing everything apart !

Short video:

 

Edited by Doug72
Added video

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