# spring design help

I have a part that needs about five lbs. of spring force over a
movement of 0.140"
The only design that will work in this space is a flat stock spring
3/8" wide. Think of a diving board sticking out. The force on the end
of the board needs to be 5 lbs over this 0.140" movement. Not exactly
constant but close.
I can select several material thickness from .035 to .062, could
double or triple layers.

I can easily change the length of the spring from near 0 to over two
inches with solid mounting behind.
I can change the amount of preload by adjusting the height of the
mount.
OK, what amount of length and deflection gives about 5lb. force with
these material choices? Must be some kind of calculator someplace.
Karl

Machinery's Handbook has spring formulas, including flat springs. If the spring section doesn't have what you need, use the section on beams. You may have to rearrange the equations to solve for the spring dimensions with a known force and deflection.
Don't forget to check max stress once you have dimensions.
Beamboy will work, but you'd need to use an iterative process to zero in on the dimensions. But it'll work well to check your calculations.
Do you have space outside that immediate area? Could you use some form of constant force spring coil (like a watch spring) outside the immediate work area to act on a lever that intrudes into the work area?

Karl - the figures below came from a beam calculator using, mostly, the equations found in "Machinery Handbook". They indicate you might think of a diving board with a "z" shape.
assuming 2.00 inches by .375 by thick, thick lbs deflection max stress .062 5 .057 41623 .062 10 .119 83246 .050 5 .113 64000 .050 10 .227 128000
Hul
Karl Townsend wrote:
Man your facts are messing up my great theory :) I assume yield strength is around 80,000. Mcmaster doesn't list it.
I don't get what you mean by Z shape.
I have an area 0.45" by 0.45" by 3" deep that lays horizontal. I need to give a vertical force right at the end of this area. Maybe i can dream up a way to pivot the force from a compression spring 90 degrees.
Got any other ideas?
Yes -- an 'L' shaped lever could do that. Either from a compression or an extension spring, depending on how you orient the lever.
But -- have you looked at Belville washers for the task? (Cup-shaped washers of spring material -- which can be stacked like nested spoons for more force, or alternating --()()()- for greater stroke.) These are used -- among other things -- to maintain constant compression when holding high current diodes or SCRs between two heat sinks.
The required diameter would be a function of the diameter of the rod or screw which is carrying the force.
Enjoy, DoN.
Could you make a small piston and cylinder fit where you need the force and another piston ond cylinder with a weight or spring to pressurize the first cylinder? That could give you a very constant force.
Dan
Well, Hul's calcs show that a simple "diving board" shape pushing up probably won't do it given the lack of travel for the force needed within the yield limit. One extension of that idea may work. Make a short, stiff diving board shape that gives more force, downwards, pushing down on the end of a "seesaw" transfer link to amplify the distance and push upwards. This will divide the force by the same factor as the distance is increased, so you will need 20-40 lbs now depending on the preload. Or, wind a mainspring type spring with od 3/8" or whatever your available vertical space is, anchor the inner end to a pin, and either straighten the end of the spring material to form the arm that pushes up on your target, or straighten out a short piece and attach to a longer, stiffer arm. Wind it up to get your five lbs force preload, and the force should be pretty constant over your short travel. Anyway, just some Sat afternoon musings :-).
----- Regards, Carl Ijames
Karl - the figures below came from a beam calculator using, mostly, the equations found in "Machinery Handbook". They indicate you might think of a diving board with a "z" shape.
assuming 2.00 inches by .375 by thick, thick lbs deflection max stress .062 5 .057 41623 .062 10 .119 83246 .050 5 .113 64000 .050 10 .227 128000
Hul
Karl Townsend wrote:
Karl - the idea behind the 'z' being to increase the length greater than 2 inches and use a heaver thickness. Then .14 inches of travel would not make such a difference in applied force (the 5 lbs you mentioned) or the max stress on the material. Carl mentioned the same approach using a circular spring, a-la screen door spring, made of heavier material. Check the local hardware store?
Hul
Karl Townsend wrote:
yup... don't forget the "slipper spring", conventional on a lot of autos of the recent past. It's a cantelever spring, anchored on one end, supported on a "sliding" support on the other, so when it bends, it "slips" on the loose support. Double the support for a given length.
'doubles the force for a given spring.
Lloyd

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