Simulating cable over pulley?

[sacherjj]

I'm working on a system that takes a pivot bracket to cable, then through pulleys and back to a pivot bracket.

This is a single cable and pulley system, so no mechanical advantage and displacement ratios. Just a distance from the first pivot to tangent of first pulley and the tangent of last pulley and last pivot.

As long as the pivot is in plane with the pulley, there is no cosine error, but with 90 degrees offset that is a factor. I'm looking at taking total length of pivot to pulley, subtracting the first pivot to pulley position for the last length.

Anyone have any pointers on how to approach this? I'm trying to simulate all the linked pieces around this and make sure my ratios are working as expected and allow changing of pivot pieces to tune in.

 

[HaroldL]

Can you post a screen shot of the assembly to better visualize it?

 

[sacherjj]

The idea is that we assume the levers at the left make tension on two cables (that are not there now). This runs from the fork connector under the pulleys oriented xy to pivot on the z axis and run up to the outside of the pulleys at a 40 degree and onto the final pivot piece. The levers should work opposite each other based on the equal pivot at the piece on the right top.

I'm not tying to solve this specific problem, but trying to see of there is a methodology that would allow making constraints to get motion, so I can assure range of motion works out with a mix of pushrods and cable actuation that is more complex.

I attached a package of this setup.

 

[sacherjj]

So I can link the pulleys with realistic motion using 1 to 1 plain gear constraints. Trying to see if I can use a rack gear setup to handle the cable as if it were rods to first pulleys. Regardless of the complexity, it should work to motion control at each end. Not sure how the angle changes will work. Thinking about a helper piece that lives near the pulley as a surface constraint.

 

[sacherjj]

This basically works using 1:1 gear ratios on the pulleys and 35mm pitch on rack (radius of pulley). Having issues with tangents or other methods of getting the "cable" to stay in the pulley.

 

[HaroldL]

Having issues with tangents or other methods of getting the "cable" to stay in the pulley.

Maybe RMB>Show Reference Geometry on the cable and pulleys. then use the parts default planes for alignment to center of pulley and offset from its axle.

 

[sacherjj]

That is a good idea. I was watching something about plane center references for parts and will look at that to make this cleaner. Actually building the reference geometry that the pulleys mount to will also help.