Compression Spring Buckling Design

[sumwonlost]

Hi

I require some assistance on a product I am working on. I need a design for a compession spring to buckle. The spring is attached to a 32mm OD x 3mm WT aluminium pole on each end. The first pole is 2.4m long and is fixed to ground vertically, the other is 1.5m long and has a 15kg weight attached to the end of it. Naturally the spring will buckle. I need the spring to buckle so the weighted pole is between 45 - 60 deg from horizontal at equilibrium. I need to be able to pull down the weighted end to 15 - 20 deg from horizontal and when released to spring back to equilibrium exerting 40-50kg force.

Material - any suitable but must be rust proof as will be used near sea water.
OD - Ideal 32mm larger if required
Wire Thickness - not important and therefore whatever size will do the job
Length - semi important and therefore the smallest length that will do the job

If I now use the recommended spring above and the weighted pole is reduced from 1.5m to 0.9m, what is the increase in upward force and the angle of equilibrium?

Any recommendations on spring design and calculations would be appreciated.

Thanks

 

[desertfox]

hi sumwonlost
Can I ask what is the application for the this device?

 

[sumwonlost]

Hi desertfox,

Without going into the workings in detail, I am looking at a "dancing light" concept using springs and wires as a stage back drop. The 15kg I required sprung suspended is the approximate weight of suspended pole and light fittings. I have attached a sketch. The light fitting will be pulled down to the required distance 15-20 deg by nylon wire with a safe limit of 50kg (viewers will not be able to see nylon wire) before being released.

The fitting must not spring back past the vertical. Rotation and horizontal movement of poles and spring will be restricted by nylon guide wires. It will only spring vertically.

Next Question, why 3mm thick aluminium instead of a lightweight plastic, let's say i need aluminium for a future application I will be testing on a boat as the lights are removable. But for now I am more interested in the initial reason that they are being created for.

I need spring design without going through the trial and error process so that I can order one to build prototype to test the concept as a starting point.

 

[desertfox]

Hi sumwonlost

Thanks for the sketch its much clearer now what you are trying to do.
Unfortunately I have no formula's in my spring design books that would help, I would recommend that you contact a local springmaker and explain what you are trying to do,he will probably help you a lot more than we can here.

desertfox

 

[TheTick]

I used to design similar spring hinge systems on a much smaller scale. It's a matter of balancing the torque caused by the weight of your system with the torque provided by the spring. Where torques are equal, that is where the system will rest.

I don't like the buckling spring idea. Too chaotic. Would you consider using a hinge wit a spring?

This is a kinetic system. Without damping, the system will overshoot the equilibrium point and probably oscillate at its natural frequency.

 

[handleman]

I doubt what you are asking for is possible. If you are going to be pulling it down that far and the spring has the force to return it to the "equilibrium" position, and you want it to "dance", you will not be able to prevent it from going vertical without adding some additional tether.

-handleman, CSWP (The new, easy test)

 

[zekeman]

I would recommend a torsion spring for this.
Should be straightforward, but the return to < 90 degrees would be limited by the fact that for a nondissipative system,you have to solve a nonlinear ODE for @ and make sure that @<pi/2
or by energy
To= torque after deflection
Kt=To/(@0-@1)
equate the increase in PE to the spring energy
MgLsin@f-MgLsin@0=.5*Kt(@0-@1)^2
@f<90 deg

 

[sumwonlost]

I could not find any formulas either on buckling of compression springs and have already contacted a local (smallish) springmaker who advised me that trial and error would be the way to go.

Looking at the alternatives suggested, I did look at a extension spring hinged system and discounted it preferring to use the properties of the spring alone to do the job. I did realise the possible choatic nature and was prepared to compensate for it by having guide wires attached. I had not considered a torsion spring, however now you mention it, you are right it is straightforward.

I'll do the calculations later tonight when I get some time. Thanks