Geometry and Crank Mechanisms

Khoala

I'm trying to design a device that will have a motor drive a circular motion and turn it into a reciprocating pivot motion. My goal is to rotate an axle +/- 5, 10,15, or 20 degrees, located at the top of the green frame. I have stared at this problem all day and thought it solved for +/- 10 degrees pivot, but when I move the arm connected to the motor out to a larger radius to have a +/- 20 degree pivot, it doesn't look like it will do it because it displaces the lever arm downward. It's like I broke something before it was even built.

Can anyone give tips or suggestions? I know it's some variation of a crank mechanism, but I'm not an engineer in the least. Help

Here is a diagram:

Aioua

Does it need to have different radiuses or can it just be one? So you'd turn the crank -/+ 90º for the 20º offset, turn it some lesser amount for 5º, 10º, or 15º.

Donovan Puppyfucker

Can you do it with a worm gear set instead of the crank and conrod setup?

Aioua

chrishallett83 wrote:

Can you do it with a worm gear set instead of the crank and conrod setup?

Haha yeah I just did up a rack/pinion setup.

Ignore my lazy depiction of gears.
This has some issues, the stops between A and E don't exactly correspond to the positions on the arc (they're all a little bit higher), but I'm thinking it'd run a bit smoother as you don't have two different arc of movement fighting each other.
(Also this is 'close enough' style engineering. I can't do trig for shit so i'd just make this IRL, measure out the positions I want and program in the stops on the motor :P)

Dunadan019

your design will move it in the desired arc but the center point of those arcs will move down as your r increases.

if you want to look at it mathmatically, the vertical distance h changes (not s which is fixed) so your h should be equal to sqrt(s^2 - r^2) which means that as r increases more, h decreases and your hinge point moves down.

Dunadan019

the easiest fix would be to redisgn your wheel (link 2) into a bar with slots and redesign your arm (link 3) to have variable pegs. each peg fits into its own paired hole and h is therefore made constant.

Dunadan019

you could also do an extension in the middle of link 3 if you didn't want to deal with possible collisions or change the wheel design.

Aioua

Ooo I though up convoluted solution!

Okay I'm going to stop now. I am the worst engineer.

Dunadan019

chrishallett83 wrote:

Can you do it with a worm gear set instead of the crank and conrod setup?

if you use a worm gear you need a reversible motor.

Khoala

Dunadan019 wrote:

the easiest fix would be to redisgn your wheel (link 2) into a bar with slots and redesign your arm (link 3) to have variable pegs. each peg fits into its own paired hole and h is therefore made constant.

Wow, it's so simple! (I'm an idiot)
Thank you! Please lock