steel vs al cantilever

[vbk]

I'm comparing the merits of using steel versus aluminum for an L-bracket. It is to be mounted on a motorized turntable oriented perpendicular to the ground. It'll be about 5' x 6' & have about 300 lbs at the end. Although I can use cntr-weight, I prefer to minimize it. Deflection is my main concern. I'm looking for rules of thumb based on trends I see. Can anyone confirm?
1) For a given cross-section, AL & steel cantilevered beams deflect about the same under their own weight, so AL is more attractive due to its light weight (ignoring thermal props, etc.)
2) Steel begins to look more attractive when there is a load at the end. Looking at square tubing; for a given load, in order to get the same deflection from the AL, the "I" must be increased so as to make the weight of the AL approach that of the steel (this assumes wall thickness can be increased but tube O.D. can't).

In my case since I have an L-bracket, I'm thinking that the part that is perpendicular to ground should be steel & the other leg I'm still debating.

Thanks, vbk

 

[brad]

1) Correct, for the same shape. The modulus and weight of aluminum are both @ 1/3 of steel. These cancel each other out for the case which you have described.

2) Steel looks more attractive for the same shape. For very thin hollow tubes (t<<d), you are correct that aluminum and steel would give eqivalent stiffness/mass performance for the same OD. However, if the OD can be increased, aluminum can be significantly superior to steel (as Ixx will increase much faster than mass).

Brad

 

[strokersix]

Perhaps considering available fabrication methods and equipment along with material availability will help you to decide steel vs. aluminum. Another consideration may be cost, particularly if this is a production application. Is the 300 lb load static? Fatigue performance of steel versus aluminum may be an issue with dynamic loading. Environmental considerations such as galvanic action may also be a deciding factor.

Above are just a few thoughts on issues to consider. Sorry no definitive answers!

Mike

 

[vbk]

Thanks,

Since this is an antenna measurement application & geometric bulk is undesirable, I'm leaning toward steel, at least for the vertical section. We've made variations on this in the past with heavier & lighter loads. Typically we use sandwich type steel construction with channels or I-beams welded inside of plates. It isn't a static load. It travels in Elevation (as opposed to Azimuth). As for fab processes, I guess for large welded structures steel is preferrable.

vbk

 

[trainguy]

Even people who have been involved with welding aluminum all their life get the jitters when specifying a welded aluminum joint. Some of these are my colleagues...

Although aluminum looks nice,
Steel rules.

 

[chicopee]

depending how the load is applied on the &quot;L&quot; shape structural bar, you may have to consider the effect of torsion since the shear center is outside one of the legs parallel to the load.

 

[jbknudsen]

I agree with trainguy. If some of the components or fixtures must be welded, I would try to stay away from alum welding. If the welds are critical to the structural integrity of the assembly, I would lean toward steel. Dont get me wrong, I have seen some great alum welding however, a good looking weld with aluminum does not insure that the weld had full penetration. A good looking uniformed weld on steel is pretty tought to beat. This ofcourse is totally negating if there are any plans for radiograph or ut of the welds.

It appears that you have quite a few good leads and comments with the previous threads.

Have a great weekend,

Jay

 

[vbk]

Thanks for all the input. I think I have a pretty clear picture. I'll try to weigh these factors each time.