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.
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.