A question about constant force springs and telescoping tube arms

Our team is considering using constant force springs to power the extension of a telescoping tube arm, but we do not know how much force will be required to retract the tubes. The telescoping arm would be retracted with a rope that runs through the center of all of the tubes and is attached to Stage 2.


  • 1 individual spring is rated for 20 lbs of force, so each spring set will provide 40 lbs of force.
  • Stage 1 and Stage 2 move, the Frame is fixed in place
  • Spring Set 1 is attached to the frame and is pulling Stage 1 upward with 40 lbs of force.
  • Spring Set 2 is attached to Stage 1 and is pulling Stage 2 upward with 40 lbs of force.

Question: How much constant downward force (the green arrow) is required to fully compress the telescoping arm?

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40 lbs.

I would caution you, however. I believe you are using the thrifty version. The force required is going to vary dependent on the orientation of the mechanism because (as far as i know) thrifty uses delrin as the rear slide rather than bearings.

As for the safety, ask Nick jr if his broken nose has healed yet. The safety pin got pulled from one he was looking at and required some needlework to repair his face…

Not quite, I would say 40 lbs minus the weight of the upper telescoping arm section

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This just has me thinking a little how much more careful I need to be around robots this year with horizontal extensions with stronger springs.


How did you get to this conclusion? A think a lot of teams this year are gonna be using solutions like thrifty telescopes and a knowing the cf strength they may need could be a great help at building better robots and saving broken noses

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May I caution you against the use of constant force springs for this specific application? I assume you will be relying on the strength of the springs to lift the weight of the aluminum tubes themselves as well as the game piece and whatever mechanism holds it. These designs have numerous failure points when used to lift loads other than just the arm itself, including but not limited to:

-Constant up and down movement of the arm could cause tangles or improper spooling of the winch cord, leading to imprecise extension lengths, jamming, and more

-Assuming you will be mounting the arm at an angle, the increased load on the end could cause misalignment of the tubes, leading to increased friction that the CFS might not be able to overcome, so the arm gets stuck. My experience with the Thrifty telescoping kit included enough jamming, and that had next to no additional load and was mounted vertically.

Don’t get me wrong, I’m all for alternatives to motor and pneumatic power, but pick and place calls for a mechanism that can be powered by a motor up and down. If you’re looking to keep the telescoping design (our team is using it too to make it easy to put on a pivot!), I would look into internal chain drives with anchors to the tubes (preferred if you have room for wider tube) or cascade lift-esque rigging with a cord.

I did this on paper because I’m old fashioned:

Also, we have built several elevators that use constant force springs to lift. They do work very well, but we also found that by replacing a stage with a short section of tube created a PVC launcher. There is also a video somewhere of our 2020 sprung elevator launching out of our bot in a match when a rope came loose. Be careful.

To compress the arm, you need the force from the springs minus the weight of the arm (assuming you are vertical? You’ll need more if you aren’t vertical.) Of course, you should add a bit more to this when specing out motor/gearbox to account for friction. Then you should of course do torque calculations based on your pulley size (assuming you’re using a string).

We’ve used constant force springs this way in the past with mixed results. Our biggest problems have arisen from strings, not the springs themselves.

Just to describe the free body diagram here:

Think of each spring as a separate entity and work your way from higher spring to lower.

Individually, because it’s a constant force, the spring will not extend until the normal force exceeds the spring force. In a vertical orientation, the total force is your motor power + weight of structure countered by the compacted spring.
Same deal for the lower spring, it doesn’t know about the upper spring, just that it’s countering structural weight and the motor’s force.
If the structure had the same weight as the spring force, the entire system would move and stop indeterminently and if the structure was 1 LB over the force of the springs, both would extend fully (minimal acceleration).

In this setup, the lower spring would start extending at a lower applied force than the upper spring, since the lower is countering more arm weight than the upper. Hence the lower arm would contract slightly before the upper part.

To better visualize, put a 10 LB spring on top and a 100 lb spring on the bottom. When force is applied, the 10lb spring would extend (pulling the upper arm down) first, and the second half won’t start moving until nearly 100lb force is applied

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