Good points Kestas. Well, the overkill may be the wisest route, along with the inspections. Also the bolt holes preferably should be close to the other leg of the angle, with nuts supported by large steel washers.
Engineers: Strengh of Materials Question
- Thread starter TallPaul
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I agree with Stuart. Light weight + vibration + aluminum =fatigue
2*2*1/8 is strong enough if it is 6061T6 but not if it is 6063T4. So it sort of depends on the kind of aluminum you are using. Also note steel aluminum interfaces corrode with galvanic action.
However, I basically agree that any kind of steel is better for this application due to steel being a grey metal and aluminum being a white metal, and the long term strength issues with white metals.
However, I basically agree that any kind of steel is better for this application due to steel being a grey metal and aluminum being a white metal, and the long term strength issues with white metals.
30" span, .128" material (Al - what alloy?, what temper?) How will you attach it holes and bolts?
85 lbs static is not huge but, but dynamically due to the flex there will be some decent forces.....
Funny my first thought was the steel/Al interface and corrosion.
I would get some steel supports made up. A local shop shouldn't be TOO expensive (but not cheap)
85 lbs static is not huge but, but dynamically due to the flex there will be some decent forces.....
Funny my first thought was the steel/Al interface and corrosion.
I would get some steel supports made up. A local shop shouldn't be TOO expensive (but not cheap)
You need to be aware of random loads and vibrations that will cause even small cracks to propagate over time. I'd inspect any sharp corners of the structure on a yearly basis. Typically you'd "dye pen" or NDE these, but that may be beyond what you're prepared to do.
TS
TS
hmm... try this for a solution.
http://www.efunda.com/formulae/soli...study_display.cfm?case=simple_2symload#target
if we can assume the each beams holds half the weight of the generator, a single beam is 30" long, two point loads 12" from end ends @ 21.25lb/ft, the modulus for aluminum is 10.2e6 psi. for 2x2x.125 channel, the section modulus is .18 and the distance from the neutral to the extreme stress fibers is .58" (this is from a table).
this yields -0.0123" displacement and a maximum moment of 822 psi on the beam.
you need to find the properties for your aluminum bar, but typically the values are in the few tens of 1,000 psi for the yeild/tensile strength.
this is a static computation. however, you have a safety factor of a couple orders of magnitude.
btw, i am an ee not a me -- i may not remember enough about my statics course.
personally i would pick unistrut. it comes zinc plated and already has holes. this makes it easy to mount stuff later or shift the load around for balance.
http://www.efunda.com/formulae/soli...study_display.cfm?case=simple_2symload#target
if we can assume the each beams holds half the weight of the generator, a single beam is 30" long, two point loads 12" from end ends @ 21.25lb/ft, the modulus for aluminum is 10.2e6 psi. for 2x2x.125 channel, the section modulus is .18 and the distance from the neutral to the extreme stress fibers is .58" (this is from a table).
this yields -0.0123" displacement and a maximum moment of 822 psi on the beam.
you need to find the properties for your aluminum bar, but typically the values are in the few tens of 1,000 psi for the yeild/tensile strength.
this is a static computation. however, you have a safety factor of a couple orders of magnitude.
btw, i am an ee not a me -- i may not remember enough about my statics course.
personally i would pick unistrut. it comes zinc plated and already has holes. this makes it easy to mount stuff later or shift the load around for balance.
Just looked it up. The generator with roll cage is 88 pounds. I will be removing if from the roll cage (frame) so it will weigh less. The engine, a Honda GC160 5 hp, weighs 30 pounds. So the dynamo is less than 58 pounds by the roll cage weight. So the dynamo is probably 40 to 45 pounds. I think my 1/4 stock will handle it nicely (even though one brace is two nested 1/8" stock).
A top brace would be very difficult.
I'll just have to inspect it now and then.
A top brace would be very difficult.
I'll just have to inspect it now and then.
Hmmm, wood does seem like a good idea. Maybe I will get some treated 2x2's to run under the aluminum. BTW, LarryL, I bet your generator weighed more like 150+ pounds.
wood isnt a wrong choice.
while wood has a lower elastic modulus (about 1/6 of aluminum), it still has decent tensile strength. a 2x4 also has a section modulus of about 3. of course, a 2x4 is a bit bigger and heavier than a 2x2x.125 aluminum.
wood typically doesnt have a high shear strength.
while wood has a lower elastic modulus (about 1/6 of aluminum), it still has decent tensile strength. a 2x4 also has a section modulus of about 3. of course, a 2x4 is a bit bigger and heavier than a 2x2x.125 aluminum.
wood typically doesnt have a high shear strength.
In my motorhome, an old MCI bus, the generator kept cracking it's mounts. I rebuild the support framework out of wood, fastened with bolts and epoxy and those metal brackets, and it not only never cracked again, it was very quiet. It lasted 10 years until I sold the bus. Was it wrong to use wood?
I'd be using the 2x4 laying flat, so the section modulus (SM) would be smaller. I believe it is
SM = width * height^2 / 6
Of course there is a lot more to it than just section modulus, but coupled with the aluminum pieces it should be pretty solid.
I guess I could use the 1/8" aluminum stock with the 2x2's and take back the $34 piece of 1/4" stock. Remember I only have to support a total of about 75 pounds, or about 30 to 40 pounds on each of the two supports.
I am inclined to install the wood with the grain flat so the bolt holes go through the successive growth layers, but it may be stronger with the grain vertical?
Also, I would want the wood to extend well past the bolts (I guess not a bad idea for the aluminum too), several inches anyway.
SM = width * height^2 / 6
Of course there is a lot more to it than just section modulus, but coupled with the aluminum pieces it should be pretty solid.
I guess I could use the 1/8" aluminum stock with the 2x2's and take back the $34 piece of 1/4" stock. Remember I only have to support a total of about 75 pounds, or about 30 to 40 pounds on each of the two supports.
I am inclined to install the wood with the grain flat so the bolt holes go through the successive growth layers, but it may be stronger with the grain vertical?
Also, I would want the wood to extend well past the bolts (I guess not a bad idea for the aluminum too), several inches anyway.
S would be smaller and the distance to the neutral fibers is less, so its not as strong in that orientation. but, its still far more than you need. wood is pretty amazing. its not by accident that its a popular building material. perpendicular to the grain its hard to shear. parallel to the grain it has high tensile strength. and the low elastic modulus means that additional vibration damping is likely unneeded. just paint it for weather resistance (i would use an oil base/alkyld) and use some fender washers where you bolt through to spread the load across.
you wont need the aluminum.
you wont need the aluminum.
Thanks chas3. Well in a belt and suspenders sort of way I plan to do the wood and aluminum. I'll get some treated lumber or maybe cedar (natural rot resistance), and use the 1/8 aluminum stock (since I can't return it as I lost the receipt and bought it last year). Hopefully the 1/4 stock can be returned as I just bought it the other day.
I see on Google that fender washers are exactly what I planned on, just very big washers. I assume we don't want the wood to crush when I tighten things down. What would be a good torque, 15 foot-pounds maybe?
I see on Google that fender washers are exactly what I planned on, just very big washers. I assume we don't want the wood to crush when I tighten things down. What would be a good torque, 15 foot-pounds maybe?
Kestas
Staff member
Whatever the torque, they will need to be checked frequently. The wood can easily shrink and leave a loose fastener. I'm always retightening the fasteners on my wood chairs.
use either lock-tite (blue so you can get it apart later), split-ring washers or nuts w/ nylon inserts (dont know the name for this). any of these are resistant to vibration.
no idea for a torque. google should have some suggestions i guess.
no idea for a torque. google should have some suggestions i guess.
TallPaul, you're right, then generator was a lot heaver than 75 pounds. The engine was actually a 1966 Renault R8, 1108cc, wet sleeve engine. It was the best little engine I've ever had. Started easly, ran smooth, great on gas. The engine also ran an air compressor through a pto gearbox, on side for the generator, the other for the compressor. The compressor was also mounted on a wooden framework. It was amazing how much quieter the whole setup was vs the original aluminum setup. Wood is a great material to work with, and I had no problems with bolts comming loose, or joints comming apart. The epoxy was some stuff from a boat store. It swallowed one of my screwdrivers.
Well, the cedar 2x2s are at a relatively distant store and then I have to return the 1/4 thick aluminum stock. Also all my bolts have to be longer with the wood. By the time I am done and get my $34 refund on the 1/4" stock, I will have wasted as much or more running around and so I guess I will stick with all aluminum, 1/4 stock on one brace and two nested 1/8 stock on the other. I found out the rubber vibration mounts have a short threaded part to mount to the frame and will not work with 1/4 stock, but I will run two 16" cross braces of 1/8" steel and mount the gen to those. Anyway, no wood, nyloc nuts, grade 5 bolts.
Oh and I figure a short steel piece on top of the notorhome frame flange, extending a few inches out the brace and bolted once through the brace and once through the brace and frame should provide a lot of added security.
Oh and I figure a short steel piece on top of the notorhome frame flange, extending a few inches out the brace and bolted once through the brace and once through the brace and frame should provide a lot of added security.
i wouldnt bother with cedar (or pressure treated). a plain 2x4 (look for a straight piece) would be fine painted. if you wanted something exotic for paint, a one-part marine epoxy is fine. you only need to protect a little road spray right?
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