Gantry Crane with An Aluminum I-Beam

I am kicking around the idea of building a gantry crane with an aluminum I-beam.

Last week I found a source of surplus aluminum 6061-T6 beams (10' long by 8" high by 5" wide).

With the plan of using one for a gantry crane, what would the lifting capacity be for a 10' beam with adequate safety factors included?

I am hoping to make this a one man assembly effort so I am looking for ideas for an adjustable height supports.

Here are some approaches...

http://www.wallacecranes.com/ratchet.htm

http://www.spanco.com/pages/gantryaalum.php

Any other suggestions for buiiding an aluminum gantry crane?

Thanks

TMT

What concerns me about aluminum for a beam on a portable gantry is lateral bracing. Aluminum has 1/3 the stiffness (Modulus of elaxticity) of steel. Resistance to bucking is inversely proportional to the modulus of the material under consideration other things being equal.

The failure mode of a gantry beam is usually buckling in the upper flange instantly followed by rotation and collapse. It doens't work like in the movies there there's an ominous creaking followed by a slow sag while the hero rescues the girl. There will be a big crash and that's it for whoever was under it.

If it was a steel beam, I can offer this: the load tables I refer to all the time are rated for steel structural members. For a 10 S 25.4# end supported steel American Standard I beam they give a distributed load cap of 39,000 lb load. The steel beam will sag about 3/8" under this load; expect the alumnum one to sag just over an inch. You cannot take these figures to the bank for lifting equipment. You have to consider the effect of a near point load at the trolley, attachments, bracing etc AND material characteristics.

That said, your beam may be short enough and have wide enough flanges to be OK for automotive engine lifting. The beauty is the aluminum beam will weigh about 84 lb instead of steel's 260 lb. It would be an excellent idea for a good ME analyist to look at the problem and your plan. He can give you figures and load test parameters.

My only input is remember that 6061-T6 does not have much elongegation- the yield point is pretty close to the ultimate tensil strength. That means not a lot of warning before it fails. You should be designing for minimal deflection anyway, I don't think I would excede 25%-30% of yield strength in the worst case load, and that is not conservative, keep out from under suspended loads at this level. It just means that instead of bending if the unexpected happens, it may break.

All good points so far.

Little more info is needed...to calculate the I-beam moment of inertia the web thickness and the flange thickness is going to be needed.

There isn't really an exact match from the books. A wide flange W8 x 18 (steel) has a flange base of 5.25" and has a web thickness of .230" and a flange thickness of .330"

I'd think that the aluminum would have greater thickness, at least in the web.

There's nothing inherently wrong with using aluminum. Wallace Cranes sells aluminum versions for portability.

I would be concerned about any bolted connections...make sure they don't tear out...any trolley loading, and any degredation of the material from load cycling...i.e. aluminum has no "ultimate" strength like steel...it will eventually fail due to cyclic loading. Granted this takes a long time, but it's firmly proven in from materials science theory.

If you could assume that the aluminum beam matches the steel in cross-section, I would rate that setup for 1200 lbs. Of course I am not qualified to make such decisions and have no idea what I am doing.

While I'd have no opinion half as good as those already given concerning your aluminum beam question, I would note one thing. All the aluminum flatbed trailers I've been around were designed with a rise in their length (bowed upward)to copensate for their sag that will come to them when loaded. Your beam won't likely have that feature pre-made into it!

Richard

How surplus is this surplus? Have these beams been salvaged or are the new surplus? A beam that was salvaged out of an outdoor structure or boat could have corrosion damage in micro cracks that is not easily seen and any conculsions should take this into consideration.

Richard, that's a good point about the "pre-cambering" of the beams.

It might not work out right for a gantry though...under certain light loads the trolley would be unstable in the middle.

I think it's better (safer) to have the depression in the center, and not be able to push the trolley from the center towards the supports if the deflection is extreme (and that means overloaded!...or out of level if it will push one way but not the other).

It would be a good idea to test it with a "proof load", I think hoists are typically tested to 125% of rated unless there's a purpose-built load limiter. Best way would be to have an electric chain or air hoist hoist and stand 50 feet back as you raise the proof load. After that you are not authorized by anyone to lift over the 100% rated.

When using aluminum alloy you need about twice the cross section compared to steel. I have a truck (Kenworth) with an aluminum frame. Usual steel frames are either 1/4" or 5/16" channel about 9 3/4" wide. The aluminum rails are 10 1/2" wide with a 1/2" thick web and 3/4" thick flanges. The aluminum rails weigh about 30% less than the steel rails.

All interesting points, but help me out on something. I thought that aluminum had much worse problems with fatigue and thus catastrophic failure more than steel. Is this true or not? Is aluminum therefore more prone to failure when put into situations that flex a lot? I assumed that even makking the aluminum part beefier wouldn't really compensate totally for that assuming it still flexes.

I know nothing about the mechanics of materials so I'm hoping you guys can educate me.

Thanks,
-Art K

Art K

For both steel and aluminum there is some load below which fatigue damage will not occur, or occur so slowly that it is immaterial. That load will vary with type of steel or aluminum.

If my memory serves me correctly, the fatigue life of steel is calculated for no damage after 10 million cycles. This is because steels show a fatigue life versus load curve which is essentially flat above 10 million cycles. That is, if you apply the limit fatigue load, it doesn't matter how many cycles you run it for. It will not fail. Aluminum, on the other hand shows no such flattening, so a fatigue life of 50 million cycles is assumed and a load that will not cause failure until that number of cycles is taken as the fatigue limit, the thinking being that a part is unlikely to see more than 50 million cycles in its life.

With either material, it is generally possible to design for "infinite" fatigue life. You just may not like the size of the parts and they may not fit within the bounds of your design constrants.

If you cannot stay within the fatigue limit, you may design a part for a specific number of load cycles at a load above the fatigue limit, then assure it is removed from service before that number of cycles is exceeded. This process is often followed in aircraft design.

For the subject at hand, I don't think fatigue life is a significant limit. I believe Forrest has nailed the important consideration.

Stu Miller

Yes. aluminum alloy has a shorter fatigue life than steel. In this application though,as long as it is of the proper cross section and alloy, the fatigue life would probably be measured in centuries. Just not that many fatigue cycles (load applied and removed). My dump truck with the aluminum frame is 35 years old this year, and has been tipped over once (about 12 years ago) where the frame was twisted about 45 degrees. It popped back when up-righted and has been fine ever since (probably just jinxed myself). Aluminum alloy is alot better of a material than most people give it credit for. It does have a finite fatigue cycle life though, as evidenced by the wings coming of of old bomber planes being used as fire tankers in recent years. Those planes were at least 50 to 60 years old, so it's all kinda relative. And of course, the beams in question, being "surplus", there is probably no way of knowing the service they were in and for how long.

Here's a useful link for calculating strength of various standard shapes - http://www.xcalcs.com
Not terribly user-friendly but still helpful. I've used it a couple of times, so if you need help, email me.

With the usual disclaimers (not a structural engineer, use at your own risk...). There are basically 2 types of fatigue behavior, low cycle, and high cycle. Low-cycle fatigue (the kind of interest for a home-shop crane) occurs in many materials when stressed near the fatigue limit for a relatively few cycles, e.g. 100. The fatigue limit is the stress level below which there will be no fatigue damage. With a conservative factor of safety (I'd use 4:1) for a shop-built crane), fatigue is not likely to be a design constraint.

High-cycle fatigue is similar, but occurs when the material is subjected to many cycles, like millions.

Keep in mind that its not just the I-beam, but also the columns, and the connections, that can fail.

Then there's the problem of headroom. A gantry needs the height of the beam, plus the trolley (about 4-6") plus the hook-to-hook distance of the chain hoist. For a 1-ton hoist that's about a foot, so you need ~2 feet of clearance above the hook, all assuming that the height is infinitely adjustable, which it probably won't be. Add to that a couple of feet for slings, and you need a pretty high ceiling to handle anything like a Bridgeport. A commercial 2-ton folding engine hoist, that you can get from HF or many others for $150 - $200, is probably worth it, and take up little space when not in use.

The fatigue strength is less than significant in a gantry application....but you have to think about all the possibilities. I have no idea what the material has already been subjected to...Could have already been in a semi truck trailer and cut out as scrap?

Steels have basically 1/2 of their publicized ultimate strength available for cyclic loading ad infinitum. The strength curve stabilizes...I was thinking 10^6 cycles where the "knee" is.

Aluminums have no fatigue strength...it will eventually break. Guaranteed. The curve is a slow decrease...seems like there's basically zero strength at 10^9 cycles. As I recall that's like a piston connecting rod driving a car 60 mph for 400,000 miles with an average gear ratio, something like 2000 engine rpms.

As I understand it, that's the reason for the scheduled teardowns of aircraft for inspection and major airframe overhauls. The intervals are conservative considering worst-case fatigue. I think it is also part of the reason why pilots take great pains to get out of turbulence. Besides the additional expense of distributing barf bags, the chickens come home to roost when the airframe is inspected.

As Forrest mentions, stability of beams supported on the ends only is a real problem. A beam can have a load less than the load that would cause the fibers at the extreme points of the beam to yield, and can still fail due to some instability.

In many construction applictions, cross bracing is an option. Consider house floor joists, which are braced every 8 feet more or less.

With a crane beam, you do not have that option. For steel only, the Handbook of Steel Construction gives what they call the maximum unbraced length for all the sections used as beams. Some sections can handle a longer unbraced length than others.

Aluminum behaves differently, because, as Forrest pointed out, the modulus of elasticity is lower than for steel. Aluminum will deflect more, and will give more trouble with stability problems that are caused by deflection.

What to do? I suggest that either you dig into a good strength of materials text, if you have the background to understand it, or find somebody who knows something about aluminum beams to give you an answer. With overhead lifting equipment, the penalty for getting it wrong can be pretty serious, so there is a bit of liability for anybody that establishes the load on a beam like that.

Thermo1

This is a possibility if you are looking to reduce the span slightly as well as beef up the connection to the vertical members. I strongly recommend using bevel washers so you don't create a stress-point under the head of the bolt.

One concern in the forefront of my mind regarding a gantry is taking pains to keep the legs perpendicular (and thus centrically loaded). If the legs are allowed to deflect too far out of perpendicular, there will be instant problems as the legs will start to impart a bending moment into the end of the load beam instead of acting as a point of vertical shear. Not good.

Someone mentioned that Wallace makes an aluminum beam gantry. They have done the work, just coppy carefully and load conservatively. The length of the beam enters in to the equasion BIG TIME. A little shorter is quite a bit stronger, not directionaly porportional.

Saw some service guys changeing out a large motor for a screw air compressor. They had a aluminum gantry that they brought in in pieces and assymbled on site. Dont remember seeing any name on it and dont have any pictures but it did have a lot of braceing and doublers in it as I remember. Perhaps it would be worth your time to check with some of the service folks in your area and see what they use.

Charles

We have a couple of those all aluminum A frame/gantry cranes. One is rated at 2000Lb. and the other at 4000Lb. Couldn't find the 4K unit but the I Beam on the 2K unit measures like so:
16 Ft. long.
10 1/4" x 4 5/8" with a 3/8" thick web.
These measurments don't corrospond to any standard beam in any of my materials handbooks so would assume that the beam was engineered for this specific purpose. The beam is supported on the ends with uprights and angled supports that run from the base of the uprights to the top of the beam at about 30 Deg. Also, this thing is designed so that the beam floats and equalizes the weight strain thru the whole structure. I had never paid much attention to these things before, but on close examination it is evident that a lot of engineering went into the design. Not just as simple as slapping a beam on a couple of posts.

Great topic. I've been debating on building one myself. I already have a home-built gantry hoist that I bought when I was 17 and combined with a 1-ton come-along I've pulled a dozen engines with it, including big-blocks.

I have now debated on building a combination gantry hoist/auto-lift for dual purpose. I've seen auto-lifts come down to $1,500 for 7,000-lbs lifts but these are are permanent installation types bolted to concrete floors. My garage doesn't have 12 feet of interior clearance so I've thought about building a portable lift that could be assembled on-site. I'm currently pulling an engine/trans from a Chevy Corsica that should have come out the bottom and had the car lifted up. With no way of lifting a car 40" up, the engine now has to come out the top with the added work of removing hood, master cylinder, air conditioner pump, etc...

The car was purchased not running by a friend for his daughter. They are very tight on funds and the Dad has been re-activated (Army reserve) and will be sent to Iraq for a year leaving his family with no money and a 16 year old daughter with no car. A friend and I have made a pledge to the Dad to get the car running while he is off in the Tsunni triangle.

I've looked at hoists and concluded that I-beam posts will do the job and I can weld up and machine trolleys with the arms to ride up and down on the flanges of the beams. I've thought about using 2-ton chain hoists to lift and lower the trollies where commercial hoists use a single hydraulic cylinder in one beam that pulls a cable linking both beams and lifting both trollies simultaneously. I can't think of a better, cheaper alternative. I can buy a 2-ton chain hoist for $60.

Commercial portable lifts are $2,000 and are directly under the car, making a lot of under-car work impossible because the lift is in the way.

Anybody thought of a better way to do this?

Steve

Dig a pit.