Annirak Wrote:
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> The problem with the spring is just the range of
> force required to move it. When the arm is at
> either extreme, the line is short and the spring
> is compressed. When the arm is in the middle, the
> line is long and the spring is extended.
>
> Because F=kx, if you want a reasonable force at
> the extremes, the force in the middle is quite
> large.
>
> Constant force springs are just coiled up leaf
> springs with a very small k and a prebiased x.
> You could probably pull one that would do the job
> out of a tape measure.
>
> The spring goes through an expansion of
> 2*(sqrt(2)-1)*F over the full range of motion.
>
> Here's how it works out on the meccano arm. You
> can see that the spring overcomes the motor quite
> easily.
Okay, I see what you are talking about. (Thanks so much for making a video. I am jealous of your meccano's.) I think I have a solution. I am going to use this spring. Currently, I am going with F=2" and E=8". That gives me a max working range of 4 inches (less if you give 0 degrees and 180 degrees a little breathing room which I will) on the spring. The spring I picked has a usable range of 6 inches. (4.5"-10.4") With a k of 4lbs/in, the springs force will vary by 16lbs from extreme to extreme. If I pick my mount point right, I can make that range 8-24lbs. Now the question is, can the string pull against such a large force? If I use a block and tackle setup along B then I get a mechanical advantage of 4 and the stepper only has to generate a tension in the wire between 2-6lbs. A 60oz-in stepper with a .5" diameter drive pulley can generate up to 15lbs. That is good to know since I can now run my steppers at half power to keep them cool and still get the job done. (It is right on the edge of Nema14. I am still hoping to do this but I am going to table that until I get a testing platform.)
I am ordering the springs and am going to have to mill or laser cut my arms. I am so close to being able to print them but oh well. I should have the parts ready for when the springs arrive so I can do a quick test to see if I am full of it. I will post a video when I do. I am getting closer on the CAD. I should be able to post a a decent mock up soon.
-------------------------------------------------------
> The problem with the spring is just the range of
> force required to move it. When the arm is at
> either extreme, the line is short and the spring
> is compressed. When the arm is in the middle, the
> line is long and the spring is extended.
>
> Because F=kx, if you want a reasonable force at
> the extremes, the force in the middle is quite
> large.
>
> Constant force springs are just coiled up leaf
> springs with a very small k and a prebiased x.
> You could probably pull one that would do the job
> out of a tape measure.
>
> The spring goes through an expansion of
> 2*(sqrt(2)-1)*F over the full range of motion.
>
> Here's how it works out on the meccano arm. You
> can see that the spring overcomes the motor quite
> easily.
Okay, I see what you are talking about. (Thanks so much for making a video. I am jealous of your meccano's.) I think I have a solution. I am going to use this spring. Currently, I am going with F=2" and E=8". That gives me a max working range of 4 inches (less if you give 0 degrees and 180 degrees a little breathing room which I will) on the spring. The spring I picked has a usable range of 6 inches. (4.5"-10.4") With a k of 4lbs/in, the springs force will vary by 16lbs from extreme to extreme. If I pick my mount point right, I can make that range 8-24lbs. Now the question is, can the string pull against such a large force? If I use a block and tackle setup along B then I get a mechanical advantage of 4 and the stepper only has to generate a tension in the wire between 2-6lbs. A 60oz-in stepper with a .5" diameter drive pulley can generate up to 15lbs. That is good to know since I can now run my steppers at half power to keep them cool and still get the job done. (It is right on the edge of Nema14. I am still hoping to do this but I am going to table that until I get a testing platform.)
I am ordering the springs and am going to have to mill or laser cut my arms. I am so close to being able to print them but oh well. I should have the parts ready for when the springs arrive so I can do a quick test to see if I am full of it. I will post a video when I do. I am getting closer on the CAD. I should be able to post a a decent mock up soon.
