# Pattern for oscillation with LEGO Technic

I would like to simulate an oscillation movement of 120/100 degrees around a fixed point.

Something like this one:

Basically, I want to rotate a gear around its axis, but only have a partial rotation and then have it return back.

• Easy. Hard to describe in words, but your illustration above offers the solution. I'll build something in LDD for you later today. Oct 1 '18 at 16:54
• While that picture shows what you need to do (you can replace the cam with either a large gear/wheel or even just a plate), it might be clearer to take a look at the driving rods on a steam engine that convert linear motion into rotating motion - for another example see here: Pattern for vertical motion. Note that the gear completes a full 360° rotation, but the cam is only rotating as far as the rod moves. Oct 2 '18 at 9:29
• umm, but the second link goes "up" "down" i see it before. i am intrested on something that rotate partially. i can connect a 3743 to the "up" "down" part and then a gear, but that's the only way to do that? it will take quite a few space :( Oct 2 '18 at 11:44
• It does, but fundamentally "up and down" is the same as "forwards and backwards" if you lay the mechanism down flat - by converting the circular motion of the first gear into linear motion on the arm, the cam no-longer completes a full circle, but rotates forwards a bit and then rotates backwards. Oct 2 '18 at 12:41

What you're looking for is reciprocal motion, or to convert rotation motion into linear motion (and then back to a limited rotation).

Building out the key parts of the fan head you have the following setup (using fairly standard technic pieces:

1. Drive the smaller cog normally, this will rotate 360° as normal.
2. The driving rod will only move as far as the limits of the smaller cog - from the side you can see that this appears to be only going forwards and backwards.
3. The larger cog will be driven forwards a few degrees and then be pulled backwards as the driving rod is pulled back in return.

Adjust the distance between the gears and the size of the gears as you require. If you need a larger gear in (3) you could replace the gear with one of the liftarms with an axle hole rather than circles, etc.