aluminum vs steel - strength question (help please)

I need some materials advice. On my COE, I am building large hinges for the storage boxes (Git R' Done thread). I have to choose the materials to make the hinges. They are 12" long and I am worried that oif they are too weak, someone will lean on the door when it is open and bend them sideways. I know how strong 1/4" x 2" steel bar is, and also 3/8" x 2" steel. I have no idea how strong aluminum is in comparison.

Can anyone compare (and somehow explain) 1/4" and 3/8" aluminum 2" bar stock strength vs. steel to me?

I really need to know if aluminum be too weak and bend too easy for this project to move forward.

Thanks.

 

Some Aluminum plate matls range from soft: (T0) up to hard: (T6). Most will exhibit a greater flexibility (the E value), if I recall correctly...it is 3X greater in AL.

Perhaps there is a way that you could set up a 'model' of the design, and lay into it, to see how well it takes the load?

 

from what I understand. Alluminum as different grades. The best being T6066 aircraft grade. It is what better mountain bikes are made of. But, it has a life expectancy... It flexes slightly, but eventually cracks because it is brittle.... Kind of an oxymoron, but I know this from Mountain bike racing. Chromoly 4130 is a great grade, its heavy but lasts a lifetime.

There are so many grades of stock, I am thinkin alluminum will simply crack if it is over flexed since its pretty rigid and light.....

 

I repair trailer trucks for a customer that has alum. hinges on his trailer doors. They break so much that we order them by the case.

 

Is 4130 Chrome Moly a better choice than standard steel? I can buy it locally in various thicknesses.

 

It is a strong high quality alloy. Takes more knowledge to weld properly too.

 

Does it drill and tap OK?

 

Maybe try a search here, the race chassis are built with it for the most part, and roll cages. Every time you see the long range videos of the dragsters flexing underway, That's chromemoly put to good use. Nice stuff.
Specs: http://www.azom.com/article.aspx?ArticleID=5742
Looking over your box and hinges CAD drawings and the wood mock-up, I was not sure of the exposure of the hinges to bending, but thicker is always helpful. Or a section/shape form, check out any OEM auto bear-claw style door hinge, that would make it strong enough for daily use. Looks good by-the-way.

 

Design the doors and how they open so that there isn't any failure points - they can even open inside of themselves if you want.

 

Mild steel is not as strong as chrome moly. Race cars use it to shave a little weight. A thinner CM tube has the same strenght as a thicker mild steel tube. Most Salt racers use mild steel for ease of fabricating. CM cuts and threads harder than mild steel, unless you are machining with abrasives.

Aluminum, to have the same strenght as steel needs to be almost as heavy as steel. Most aluminum parts are lighter Than the steel parts they replace because they are not as strong. Steel has a predictable fatique point (if you are an engineer) and aluminum is unpredictable.

 

As a general rule steel is 3 times stronger than aluminum. So to get the same strength aluminum must be 3 times thicker. This is just a general statement not accounting for all the different alloys available. I've built custom carts for high end recording equipment out of 16 ga mild steel. One customer wanted the same design built out of aluminum. I explained that it would be heaver and not a rigid because to gain the same strength the material would have to be much thicker and then would weigh more than steel. No advantage structurally to steel. Go with 4130 chrome moly, lighter and stronger.

 

X2 w/Fogger. Steel or moly are more forgiving, they let you know by bending, (usually before) they break.

 

It sounds like the Chromoly is the better choice.

I'll have to talk to the waterjet guys to see how it cuts on their end.

I do have to drill and tap some holes in the hinges. Will using good sharp bits, drill oil and a slow drilling rate do the job?

 

If you don't have to do any welding on the hinge, I'd use 7075 T6 aluminum. It has tensile strength about equal to mild steel and it machines beautifully.

 

Yes. And Tap Magic for the tapping.

 

Just wondering, why don't you want to use steel? Make your cardboard shapes and have them water jet cut. Drill your holes and have everything powder coated.... Love your truck and good luck with your build....

 

I have to agree with some here as to use Chrome Molly . It's super strong and has been used in parts and roll cages due to the strength . Now it does cost more but then you will be buying the top of the line in alum. material . If you are going to weld it , make sure you are a good welder of alum. or have someone else that is qualified to do it for you .

Retro Jim

 

aluminum wears a lot faster because 'it isnt hard', strength and hardness are seperate concepts here. Also consider how brittle something is.

Aluminum say 6061 t511 or t6 are very commonly available alloys. Strongand easy to weld, relatively good for machine work. Not very hard so it wears fast and can be brittle at the weld site.

Mild steel; is relatively ductile, stronger than alum, and will last longer in a hinge full of grit. Less prone to cracking, really easy to weld, probly the best and easiest choice.

Chromoly; is much stronger, a little harder, and more difficult to weld than mild steels(tig or torch). You can use less material for equal strenth but chromoly doesnt bent very good, it usually just breaks. But if you dont use the minimum size material, it will be more than stron enough.

Try looking up piano wire, usually stainless and up to a half inch or so. Its got very tight tolerances and stainless is a good idea in hunges with no grease zerks.

 

My concern is that someone will lean on my cargo box door when it is open and bend the hinges (around the easy axis). If you've seen my COE build thread you see that the hinges really stick out as the door goes up. It sounds like using chromoly would help prevent bending, since it kind of bounces back.

I also have a limited amount of room for material width to make the hinges fit between my jambs. (1/2" max)

I can get chromoly in a .040 thickness. But the real question is: how much will the .040 resist bending?



One more additional question - in order not to waste material, I could have brackets cut from the leftover sheet of chromoly, but can they be welded to DOM tubing?

 

http://www.engineeringcalculator.net/beam_calculator.html
http://www.matweb.com/

do some math. Saying AL and steel really is misleading there are hundreds of alloys of both. The all have different stiffness and yield strengths. Properly designed a hinge could be made out of straw and mud and work fine. MOst aircraft use 6601 or 7075 Al instead of steel and they will see much more force than your door. Maybe post some sketches of your hinge you need the length of the door attached to is and the estimated force applied by a person leaning on it.

 

Chrome moly steel hinges is probably overkill. Why not just buy some premade steel hinge stock and cut it to length?

Bob

 

This is Not your normal hinge design. The hinges lift the storage box doors up like those on a bus cargo compartment. All the pieces will be cut with a water jet.

 

1018 cold rolled would be the best for a novice, 4130 needs to be welded with some care. Alum in 6061 t6 will crack easy after it in welded 7075 is even worse if you are not welding it either of these can be very strong with a properly designed part.
good luck with whatever you choose

 

Use good old 1018, All Alum. Will work harden, 4130 is not worth it and if you have to weld it it will get too hard unless you stress relieve it. I spent over 20 years in a r&d stress lab.

 

I was distracted by a pretty girl on the day they taught how to calculate elastic (plastic?) bending of materials.

This is the basic setup. What I need to know is how much force does it take to bend steel (permanently) if it is 1-3/4 wide but only 1/4" or 3/8" thick. i.e., will someone bend it if they lean on the hinge with a force of say 50 pounds, 10 pounds or ?

Anyone know how to calculate the force required to bend the steel if it sticks out 8 inches?

 

Carefully measure your distance from the fulcrum point, to the hinge pin center.
Then the force at the fulcrum point should reveal as: 50 lb X (8 + dist) in / (dist) in = Force
Force, in lbs, will translate into surface stress values. Tension on outside, and compression on the inside (of the hinge's bend axis). It is not a brittle matl, so the bend is likely to happen by a compression (think column-in-collapse) form of "plastic" (or permanent) yielding.
Ya got a little leverage working against the hinge.

 

Your question needs a lot more than a simple answer. I am a metallurgical engr.

Material stiffness is defined by modulus of elasticity, E, and for steel it is 30 x 10E6 (30,000,000) psi and alum is 10 x 10E6. This means steel is three times stiffer than alum. Heat treating and tensile strength DO NOT change E. Chrome-moly and mild steel are both the same E value.

Tensile strength has two values: yield and ultimate. Yield is what you want as once yielded it will have permanent deformation. Higher yield strength gives more eleastic deformation before it becomes permanent.

Fatigue strength is another issue, it is not the materiall losing tensile strength. It is cyclic loading under yield that can lead to a crack forming and propagating. Steel has fatigue limit as long as less than half of yield it will never crack. Alum does not have fatigue limit and it will crack eventually, maybe after 10E9 cycles, but it will crack.

Chrome-moly is a higher strength material, but must be TIG welded, can't MIG or it will crack. A bit harder to machine and tap, etc due to being higher strength.

Stiffness of your doors is really a function of the internal bracing and the structural design. Not so much the materials. You can get a lot stiffer part by adding section thickness, than by going stiffer materials. In other words, some tubing is better than a thick plate.

You can get the same performance of mild steel for the hinge by using thicker material vs thinner chrome-moly. Making the hinge more of a box shape is even better. Also location of the hinge, farther apart is better than close together. There are just a lot of variables you can juggle.

 

To give you own real world experience. My brother raced GT Zaskar L.E. mountain bikes, placed 7th over all, ate frames 4 or 5 a year... T6066. GT when owned by GT and sponsored him. ( Not pacific).... I being a power lifter didnt care about a few pounds, since I am 232 lbs, I went chromoly 4130.. He weight 150.... I raced a clydesdale class for fun...

I am still riding my 1997 timberline chromoly 4130 frame. He has been through so many mountain bike frames we cant count, and looking at pics, we dont even know which ones which....

I hope this gives you an idea of the strength of chromoly. For strength and durability, chromoly is the best choice

 

Even if the strength of an aluminum and steel part are the same, aluminum flexes more without yielding, and the fatigue resistance of aluminum is lower than steel. Those are two major factors in why a well engineered aluminum part isn't always as much lighter than steel as you would expect.

 

for a storage box in your project, I'd go with aluminum. It's not like your making a work truck. The weight savings will pay for itself.

You can add stiffeners on the large span areas (frames & stringers)