4-bar linkage question

[sigurd]

Hoekens linkage can drive with a constant enough x-axis velocity for about 110 degrees crank rotation.
I need a more or less constant velocity for >180 degrees. Is there a different eccentric linkage which can do that, preferably as simple as a 4-bar? y-axis movement is not any bother.

http://www.mekanizmalar.com/hoekens_linkage.html

[SamSam]

Just a guess, but it would seem if you had constant velocity for 180 degrees x-axis, the y-axis would be exactly the same in the opposite direction. Anything more than 180 would require a lengthening linkage, somehow. Remember, I'm just guessing.

Nice site, btw.

[sigurd]

Not sure about that. Actually I don't know that the Hoeken linkage has CV for about 110' either, because my simulation looks like this!!
What I ment is that as long as there is constant velocity along one axis for 180'+, I don't care what the other axis does.
I intend to try out different parameters on the excel sheet if I can find out what it is suffering from.

[SamSam]

What are you making? Does the 180' CV have to be in a straight line?

[sigurd]

drivetrain/transmission. If I were to use the hoekens linkage, the output link would be clamped at one end around an eccentric beaing (A), where the inner surface of that bearing is attached to a freewheel on the crankshaft (O). In the middle of the output link would the rocker be pivoting (B). the other end of the rocker would be pivoting on the frame (D). A rope is attached to the end of the output link (C). The other end of the rope is wrapped a few turns around a freewheel, mounted on the driven wheel/hub. This whole assembly is doubled except for the crank freewheel, and the eccentric bearings are mounted opposite so that while one is pulling, the other is giving slack.

It is easy to change the leverage when the rotation is converted to linear motion and then back.

If the x values on my spreadsheet are correct, then there is about 20% output speed reduction at say 0 and 180 degrees crank rotation. This would be like changing gear in the middle of a pedal stroke and then back. It is a bit much, since one doesn't know where in the crank stroke it is going to appear because of the crank freewheel. 10% might be good enough.

If the eccentric linkage can't be improved a little bit, then the same thing could be done with cams on the crankshaft freewheel. But I am not sure that it is as compact/insensitive to dirt/durable. There are fewer necessary pivots with cams, that is neat.

Since there is a decent amount of distance between output point (C) and the driven wheel, the ratio is mostly unaffected by whether the line is straight or not.

[SamSam]

I can't follow what you're saying, but I got to thinking of steam locomotive drive wheel and steam paddle wheeler drive mechanisms and it seems a 'slider-crank' sort of thing will get you close to what you might want. Here's a Cornell Kinetic Energy site that might help if you're able to download the running programs.
http://ecommons2.library.cornell.edu...20Gallery%2002 Sometimes a problem with 180' rotation/movement is the mechanism getting locked up at the ends of the movements. Inertia can carry it through, but sometimes things can go into reverse.

[sigurd]

Imagine that you are a big fish, and up on the surface in a boat are two sisters, each with their fishing pole. Their hooks are tangled, but you eat it anyway. When the two sisters notice it, they start to reel in. Since you are so big, they use a technique to ease the cranking force: They alternately lift the rod, then lower it while reeling. The thing is, they do this in opposite phase - one is reeling while the other is lowering her rod. The result is that you feel a continuous pull, even though each of them is not pulling continuously.