Force in - force out 1

[Hoosier1]

I'm trying to develop a mechanical mechanism that will allow 3-6 lbs of force-in to get 18-21 lbs out. Size would be 5sq" X 2” I would like to construct it without using electricity, fluid, etc. Must be portable. Could flat spring material be designed to do this?

 

[IRstuff]

Conservation of energy still applies. You need a gear system. Consider the modern crank-loaded crossbow.

TTFN

FAQ731-376

 

[CorBlimeyLimey]

A little more information would be helpful.

In-line or offset forces? Rotary? Input/output stroke lengths involved? Materials? Usage? Cost?

 

[Hoosier1]

IRstuff - Can you show an example of a modern crank-loaded crossbow?


CorBlimeyLimey - In-line, Stroke=2", Materials: any, Cost:?

 

[TheTick]

A lever or a gear train. Perhaps a 4-bar mechanism.

 

[JStephen]

Wouldn't a little see-saw work? IE, the Lever, with off-center fulcrum? Or what am I missing here?

 

[NickE]

is 2" the input or output stroke?

If its the input you are going to have ~0.5" ouput stroke at 24lbs with 6lbs input.

If its the output stroke you are going to need ~8" of input stroke.

how about 2 facing racks, and two twin gear shafts?

Kinda like:

------------------                Input rack
   OoOo                           Gears (x4 force -- direction change)
   -----------------------------  Ouput rack

Nick
I love materials science!

 

[Hoosier1]

NickE - The overall depth is 2". I see your method as mechanical advantage. I need to keep the force on the same axis. The 6lbs in with an output of 24lbs is much what I'm trying reach.

 

[eromlignod]

Unless you set it up as an amplifier, a simple mechanical advantage will cost you in stroke. So if you design a mechanism with a 4:1 mechanical advantage and you push the input one inch, the output will push four times as hard as the input, but only 1/4 as far (1/4"). This is because of conservation of energy, though it can be done with simple mechanisms like levers, pulleys, or inclined planes, etc.

In the case of an amplifier, you can have more force at the same stroke, or even a larger stroke, but you have to have an external source of additional energy, like a hydraulic pump or motor. In this case, a small pressure on a spring-loaded valve can cause massive, though proportional, amounts of force at the output, but it is technically produced by the pump, not the input force itself.

Don
Kansas City

 

[sreid]

Even though you said no fluid, coaxial pistons with a fluid between them might be the simplest. A 2 to 1 diameter difference gives a 4 to 1 force increase.

 

[MintJulep]

And don't forget that the "whatever" will need to be anchored to something capable of resisting the difference between in and out, 18 pounds in the example case.

NickE's solution could be made co-axial with a few extra characters:

[------------------
-[ OoOo ]-
-----------------------------]

 

[IRstuff]

Wikipedia has a nice little drawing of a rachet mechanism on crossbow:

http://en.wikipedia.org/wiki/Image:Armborst_3,_Nordisk_familjebok.png

http://en.wikipedia.org/wiki/Image:Armborst_4,_Nordisk_familjebok.png

Full article:

http://en.wikipedia.org/wiki/Crossbow

Something that's unclear is whether you want 20 lb over the entire stroke, which is different than a spring compressed 1 in with a 20 lb/in spring constant. This would give you an inch to put in a crank and rachet.

TTFN

FAQ731-376