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Adjustable spring tensioner for 90deg rotating arm

  1. Mar 17, 2017 #1
    Hi all,

    I'm an electrical engineer and I'm part way through the design of a little project and I'm struggling with a mechanical element which I'm hoping someone may be able to assist with. I'm going to keep the finer details out of this to keep things simple and just focus on the issue.

    This is the mechanism in 'state 1':

    - There's an arm (A) with a weight (B) attached to it. Both the weight of B and it's position along the arm can vary.
    - The arm can rotate 90 degrees about point C (CCW from this 'state 1'). This is carried out using a servo D (this servo is not permanently coupled to the arm, it's just used to raise/lower the arm, it can then 'release' itself)
    - There is an extension spring (E), which is in under very low tension when the mechanism is in this state (T1) so the servo will require minimal torque to lower the arm from it's vertical position.

    This is the mechanism in 'state 2':

    - The servo has now lowered the arm, the spring is under increased tension (T2). T2 is enough to assist the servo in lifting the arm back up to state 1.

    So, on to the problem! Rotating the adjustment knob (G) causes the tensioning assembly to pull back and pivot around point F which lengthens the spring. You can probably see where this is going, increasing the spring length gives increased tension when the mechanism is in state 2 (T2) to ease the load on the servo in raising the arm, but it also increases the tension when it's in state 1 (T1). So with heavier arms, adding enough spring tension to assist in lifting the arm from the horizontal position will result in too much tension in state 1 and the servo will be unable to lower the arm.

    So what I think I need to do here is re-design the tensioning mechanism so that when we're in state 1 (vertical), the springs are always at the same length and as such in the same state of low tension, regardless of the adjustment that's been made to the tensioner by the user. It's only when the arm is lowered that the adjustment comes into play and gives you a final spring length that varies depending on how you've set the adjustment.

    I hope this makes sense! There are just a couple of 'conditions' to bear in mind:
    - The adjustment method for the user to adjust has to penetrate the enclosure at the rear (side 1), I can't make adjustments from other sides.
    - Nothing's built yet but the general method of operation (servo actuated/spring assist) is set and can't be changed.
    - The tensioner mechanism can be redesigned completely. As many pivot points/arms etc as required.

    So there it is! I'd really appreciate any pointers you guys/gals could offer.

    Thanks, Jonathan.
  2. jcsd
  3. Mar 17, 2017 #2


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    Without any numbers it is difficult to determine if the result would work but what your diagram shows is that the springs connection point on the rotating disc is located such that the spring cannot provide it maximum leverage at when the arm & weight are at their horizontal position where they have the most leverage to resist being raised. A positioning of the spring connection point such that when the arm is vertical the spring connection is horizontal to the right from the disc center and the spring connection point is vertical above the disc center when the arm is horizontal will both maximize the pulling strength of the spring when the arm is down and minimize the pulling strength of the spring when the arm is vertical.
    Additionally, have you investigated whether you have the best spring rate on you spring selection for the combination of spring loads you require at your arm up and arm down positions.
  4. Mar 18, 2017 #3
    Hi, thanks for the response. I understand your point about the spring attachment position on the disc not giving maximum leverage when the target is down as it is shown, this is just an inaccuracy on the diagram, this is not the case with the actual mechanism.

    I have mocked up some prototypes with a selection of spring rates but in all cases, a spring that counter-weights the lightest loads and after adjusting the tensioning mechanism also provides enough counter-weight for the heaviest loads simply has too much tension when the arm is at rest in its vertical position for the servo to work effectively.

    I still believe that the solution here is to come up with a mechanism that can vary the amount of spring extension I get when the arms rotates through it's 90 degrees but with minimal change to the spring extension when the arm is up at its vertical position ('at rest').

    This popped into my head today (not even remotely to scale, it's just indicative of an idea!):

    So in this case, if I could change the axis of rotation for LINK 2 (i.e. to P1, P2, P3) then I can vary the amount of spring extension I get when the arm moves through its 90 degree rotation. But when the arm is vertical (as shown) the spring is always under the same minimal tension. But this is just an idea and I don't know how to implement it or if it's even the right route to go down. My mind is electrical and gives me headaches when mechanics come into it!

    Any further input much appreciated.
  5. Mar 18, 2017 #4


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    For the second configuration the spring force would remain the same in the up position but the force the spring exerts on the arm in that position would not be the same for the three different pivot positions. Using your diagram and some rough distance ratios:
    In the arm up position:
    If the spring force is 5 lbs and the pivot is in the P1 position and the distance from P1 to the top connection is 1/4 of that from P3 to the spring connection then the spring force on the lever will be 5 x 4/1 = 20 lbs; but,
    If the this is reversed with the pivot in the P3 position such that the distance from P3 to the top connection is 4 times of that from P3 to the spring connection then the spring force on the lever will be 5 x 1/4 = 1.25 lbs.
    So the amount of force required for servo to start pulling the arm down will vary proportionally.
  6. Mar 18, 2017 #5


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    Reduce the rate of the spring and add some preload . That makes the change in spring force less with extension but maintains the required minimum force .

    Probably easiest to do this in practice by using a much longer spring than the one shown .
  7. Mar 19, 2017 #6
    Thanks both for your input. JBL, I hadn't considered how the position of the pivot would change the force felt by the arm even though the spring was under the same tension, this is clear now.

    Nidum, I think I may follow your suggestion on this one. I think that if I use lower rate springs with some preload and ensure that the spring is applying maximum force to the arm when the arm is horizontal (as JBL detailed) then I may be able to find a happy medium for all of my arm weights.

    I think I'm going to do a bit of rough prototyping of the mechanism this week and see what the results are. I'll post some results when I have them, as I may need to call on you guys again!

    Thanks, Jonathan.
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