Sunday, November 14, 2010

Week TechVideo, 2010 #45 - Applying the Geneva mechanism

Some days ago Thiago Bouzan wrote me about his improvements on a Geneva mechanism design, presented here at TBs long ago.

However, what I found most genius in his model, was not the Geneva mechanism implementation itself, but the application he found to command a functions switch box sequentially, with such indexing mechanism.
Who says it can command a switch box, may also say a sequential gearbox for a car transmission.

Here the Geneva mechanism is used to switch among functions the easy way, using a secondary selection motor. Ideal for remote operation of any 4-stage switch box or gearbox.
The mechanism allows for a larger 'window' to tune the secondary motor, as by conception it locks always in one of four positions.
Now it would be great if someone would manage to build a 6 or 8-station Geneva mechanism, out of LEGO parts, to command 5+R and 7+R car sequential gearboxes...
Probably an alternative way to build a remote controlled sequential like gearbox. A bit like doing the same as the one at Sheepo's Buggati Veyron.

Still regarding the Geneva mechanism, Thiago explains:

In this mechanism we have a driving wheel and a driven wheel (or star wheel). This document explains that in order to work properly, a Geneva mechanism should fulfill three conditions:
  1. r is the distance between the center of the driving wheel to the center of its outer pin.
  2. The center distance of the two wheels should be the square root of 2 times r.
  3. r should also be the distance of  the outer radius of the star wheel.
Consider the space between the centers of the holes in a Technic beam as a unit of measurement. In my design I have r equal to 4 in both wheels and a distance between then of 6.

This fulfill conditions 1) and 3). Condition 2) could be match with r = 5 and distance between the wheels = 7., because 7 is a good approximation to square root 2 times 5. It looks like that the example from the original post meet these criteria.
The reason why I didn't choose this particular combination was to make full use of two (32249) 'Red Technic Beam Quarter Circle' and achieve a more compact size. But I intend to make a new model that will meet all these conditions.

Simply a brilliant idea on how to apply the Geneva mechanism into a LEGO model.


ChinShanky said...

Looks like someone else thought of using those quarter circle parts as well. This is my iteration of the Geneva mechanism. Note that the arm by itself doesn't rotate the mechanism a full quarter revolution, but it is aided by the quarter circle triangles I mentioned earlier. It doesn't work as well as I'd like, but it works.

TechnicBRICKs said...

Not so easy to understand without a video, but good to know there are some always working trying to find different solutions. :)

It looks it works only one way.
Don't know however, how it behaves the Thiago implementation in what regards this point.

Torsten Mumme said...

I found this solution last year.

Thiago T5 said...

Wow, this previous solution with two 'star wheels' is great!

Erwin said...

Just take a look at page 145 of the Tora no Maki ...

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