A question for Mitch and all you engineer types ...

[ChrisNYC]

Howdy all!

 

I'm designing a gadget (Oh no! Not another!) and I'm trying to determine some leverage/fulcrum/range-of-motion numbers in a "virtual" way. I know there is sophisticated CAD-CAM software out there that lets you design and test out the ROM's and clearances etc. before you spend money on building an actual prototype, but that software is mucho $$$$.

 

I was hoping to find some simple website (or even a spreadsheet would be ok) that lets you play "virtually" and figure out "If I make a lever x inches long, and if the fulcrum is at point x, what is the ROM at each end" etc.

 

I suppose I could figure this out on my kitchen counter with popsicle sticks and pins and a ruler, but I'm lazy ....

 

 

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Chris (aka Tender Vittles )

Little '77 KZ400 in the Big Apple

Black '99 RT for Everywhere Else, such as ...

 

 

[Lone_RT_rider]

There might be something here that could help.

 

Shawn

[Gregori]

For a simple lever, it's a direct ratio.

 

(a)___________(b)

. . . . . . . ^

(fulcrum)

 

Divide (a) by (b) to determine movement, or mechanical advantage.

 

If side (a) is 2 inches long, and side (b) is 1 inch long, your ratio is 2:1.

 

Simply put (assuming friction and other parasitic loads are close to zero) that means:

10 lbs of force applied to (a) will be 20 lbs of force at (b)

However - (b) will only move 1 inch for every two inches you apply to (a)

 

 

[Mister Tee]

Well, you could pick up a textbook on machine dynamics, and write a set of equations that describes spatial references to line endpoints of a fixed length, and solve them using Excel, but what a pain in the ass. You can probably get a free (or very cheap) outdated version of Microstation or Autocad, and just construct your model graphically and accurately. I'm pretty sure Microstation has the functionality you need built in, but you could probably make Autocad work too.

[Joe Frickin' Friday]

Well, you could pick up a textbook on machine dynamics, and write a set of equations that describes spatial references to line endpoints of a fixed length, and solve them using Excel, but what a pain in the ass. You can probably get a free (or very cheap) outdated version of Microstation or Autocad, and just construct your model graphically and accurately. I'm pretty sure Microstation has the functionality you need built in, but you could probably make Autocad work too.

 

Right on both counts. For arbitrarily complex machinery, you can do a sophisticated motion analysis - including collision detection between 3-dimensional components - with a program like Solidworks. But that ain't cheap; my annual site license is ~$6000, and that's cheap for solid-modelling software.

 

For a basic kinematic analysis (if I move part A by this much, then part B moves by that much), you can write a system of equations. If you're lucky/smart you can solve explicitly for the variable you're interested; if not, you can plug them into Excel and chug through it. I spent a whole semester of undergrad doing this by hand; if you're doing anything more than a simple lever-and-fulcrum, the math gets ugly pretty quickly this way.

 

A lot of times I've used plain old 2D CAD software to check for range-of-motion and interference/clearance. CAD software can be pretty cheap (TurboCAD Designer, which is what I use at home, is only $40; more advanced versions available there for more $), and it's easy to quickly draw up outlines of parts, add some pivot points, and see how your mechanism will fit together and move. Take your virtual components, assemble them on-screen at one extreme of their range of motion, and note their positions; then rearrange the parts at the opposite extreme position.

 

If I was in your shoes, I'd go with TurboCAD. It'll be useful for other projects in the future besides this one.

[ChrisNYC]

Thanks everybody, I've made use of all your suggestions. I've plugged Gregori's numbers into a spreadsheet to get me started (my gadget has a counter-lever with a variable fulcrum, so things can get funny quickly).

 

Thanks Mitch, I'll go for the TurboCAD!

 

 

 

 

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Chris (aka Tender Vittles )

Little '77 KZ400 in the Big Apple

Black '99 RT for Everywhere Else, such as ...

 

[Pilgrim]

Chris, if I feel the earth start to move, I'll know that you and Archimedes have been working together on a solution for global warming . . .

Pilgrim

[ShovelStrokeEd]

For simple lever stuff, even complex stuff, some graph paper, a couple of map pins, a ruler, some twine and a pencil will get you most of what you need, or close enough unless you are designing some form of complex balance.

 

Strike the arcs of motion, measure the displacements and from there it is simple to calculate the forces.

 

I have even approximated reciprocal motion using this method. Think piston movement in a bore with piston pin offset, rod length and crank swing all variable.

[ChrisNYC]

Pilgrim said I'll know that you and Archimedes have been working together on a solution for global warming . . .

 

... Yeah, now if I could just figure out where to drill that first hole ...

 

 

ShovelstrokeEd said For simple lever stuff, even complex stuff, some graph paper, a couple of map pins, a ruler, some twine and a pencil will get you most of what you need ....

 

I often do it that way Ed! But for this new project, my time is limited and it would benefit from a little more math up front .... if this gadget works, y'all be the first to know! (It's a gadget for drummers and percussionists...)

 

 

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Chris (aka Tender Vittles )

Little '77 KZ400 in the Big Apple

Black '99 RT for Everywhere Else, such as ...