Metal VS Plastic

[Jasonfrank]

Hey guys,

I am working on an invention which involved a frame with a build in arm. The arm slides out of the frame and then is clamped forn at various lengths. I'm trying to figure out the most cost effective material to use for the frame and arm. They will be running against each other and will be clamped in place. It needs to be durable and cannot bend, if it does it must go back to shape, it must also be light.

I have the prototype being built right now our of aluminum but its too soft and will corrode. When it goes into mass production I was thinking about possibly using some sort of plastic, but the whole frame and arm must remain perfectly straight. I also need to keep costs down.

Does anyone have any ideas on what I should use?


Thanks!

 

[KENAT]

You'll probably need to nail down your requirements a bit more quantitatively and explain your device a little bit.

For instance "cannot bend" is impossible, for given load pretty much any material will deflect some amount (I can't think of an exception off the top of my head). However, through correct design and material choice the deflection can be kept to acceptable limits. " if it does it must go back to shape" means that at maximum load you mustn't exceed the yield strength of your chosen material (assuming fatigue etc. isn't a significant issue).

What do you mean by too soft? Do you mean it dents and dings too easily? Or do you actually mean it bends too far in which case it is too flexible/not stiff enough not too soft. Or do you mean it actually fails either by permanent deformation or actual breakage in which case it is not strong enough.

In bending situation the Youngs or elastic modulus is only one parameter that determines how much something deflects. The cross section (second moment of area) is potentially more significant - think I beam verses flat strip.

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

Back to aluminum, if you improve the cross section you may be able to give it adequate stiffness in your application. While different grades of aluminum vary quite a lot in strength, the youngs modulus doesn't vary much from memory. However, different grades can be more or less corrosion resistant. Additionally there are various finishes that can be applied to aluminum to improve it's corrosion resistance. Finally while aluminum is fundamentally very reactive (so prone to corrosion) the corrosion (aluminum oxide) adheres to the surface creating it's own protective layer (unlike typical red rust on iron which flakes off). So for some grades of aluminum alloy no finish may be required, as long as you are not scraping off the oxide layer during operation.

What environment is this item being used in as this may affect choice/definition of corrosion resistant?

What volumes of production as this will impact potential manufacturing methods.

From memory I want to say Youngs modulus of most simple plastics is somewhat lower than aluminum. Some fiber reinforced plastic composites may be options though.

As described, I'm tempted to suggest

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

What kind of engineering education do you have, no offense but much of this should be pretty elementary structures to any kind of mechanical, structural, aeronautical, civil or similar engineering degree.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[zdas04]

While I was reading the OP, I had several of the same thoughts that KENAT described. I also wondered about scale. Is this arm 3 inches unsupported or 30 m unsupported? Is the force on the extended arm a few grams or a couple of tons? Does it cycle daily or on a nano-second scale? How are you planning to transfer the forces to ground?

All that stuff matters. If this is a device to count grains of rice then you may be able to afford carbon fibers or titanium. If you are loading a ship then those materials get kind of pricey. I'm an inventor as well (3 patents and a dozen or so inventions that I've given to industry because they didn't have enough market potential to pay for the cost of getting a patent) and I applaud anyone able and willing start down that road, but if your design is lacking as much as your description of it, I don't hold out much hope for a successful result.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[tbuelna]

When it comes to structures, metals are usually a better choice. High strength structures made of composites or plastics can be quite expensive.

As the old joke goes: "Lightweight, strong, corrosion resistant, low cost. Pick any two."

 

[zdas04]

The OP hasn't been back since the day he made his first post. I Think we're talking to ourselves.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[MadMango]

As long as we don't start answering ourselves...

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?