Mini gantry crane

[LD1]

The center of the tee dropped 1/32" and the legs, on the casters, spread 1/8". seems ok.
Material used:
The legs are 3"x6"x1/4". 45s are 3x3x1/4". The H beam base of the tee is 10"x 6 and the cross is 8"x5?

Okay until moved under a load. Then the game changes ALOT. THAT is my concern.

Dynamic loading is a WHOLE lot different animal than a static test. And where on the beam was the load placed that spread the legs 1/8"? was it in the middle? What happens if the load is closer to the door? What happens to your connection to the main beam if the load is all the way that way?

 

[Jim Timber]

Not to mention welding aluminum anneals it and it needs to be re-heat treated to achieve it's tempered strength.

I can certainly appreciate the intentions of this project, but I would've bolted the T connection and used steel brackets on the corners for the legs and uprights. Aluminum is much easier to bend and set, where steel will flex and more readily return to it's original shape.

 

[robstaples]

LD1, In the simple load test, the winch was about 2ft from the door end. There may have been some compression in the rubber covered casters. Cast iron centers. What I don't know is how much force to rip the flange off of the top of the web. That would be a probable failure mode for the main beam. The grade 8 bolts won't be the weak point. There will be a interesting dynamic of the gantry on rollers and a heavy load that I push when suspended. The load will pull the gantry. But if I lift something heavy only fractions off the floor, it should minimize risk.

 

[Jim Timber]

What I don't know is how much force to rip the flange off of the top of the web. That would be a probable failure mode for the main beam.

You think the red end is the weak point??

If you're only lifting things inches off the floor, then failure isn't a big deal. Sure, a lathe or mill won't like being dropped, a surface grinder could be ruined, and toes do crush pretty easily - but no one's likely going to die.

What if you're trying to put a mower in a pickup? You didn't get the mower high enough (because they don't balance for crap), and you smack it with the tailgate causing it to flip over in an unsupported direction - then that starts the whole thing swinging. Now what?

What if that swinging mower makes the legs spread out and the garage door side of the main beam goes down, then the load goes flying at the truck, but you're standing between them? Then what? Can you handle 500# coming at you with the force of gravity. I know I can't.

Static loads are the lowest importance when figuring what's safe. Dynamic loads are why cranes have safety factors of 5:1. Gravity can be a cruel master. I have two crushed toes because an 840# beam decided it didn't want to play nice while hovering 2" above the floor. I should've been wearing steel toes too. My cross to bear. My second toe next to my big toe doesn't bend in the middle anymore because of that mistake.

 

[robstaples]

Jim, Thanks for your feedback. No, the white end.- Aluminum is the weak link. Your comments lead me to think about adding a 1/2" steel plate under the tee joint bolted thru both members on 3 sides. That would leave the top of the legs as the weakest joints. I'm seriously considering thsee additions. Thanks. Rob

 

[Jim Timber]

Having built my own crane, and having a 5,000# load shift when a strap slipped - I take these projects very seriously. The engineer I was bouncing ideas off of when I built it was giving me a 2K safe limit before finding out I was tying all my columns into the building's walls at 2 points. His concern was always "what if it shifts?" Now that I have 6 years use of that crane, and countless wiggling loads (usually just weldments that aren't well hooked), I've come to realize what he was trying to beat so deeply into my head. We can't plan for best case scenarios. We have to look at what's the worst thing that might happen - will the crane survive it, and will we?

700# isn't a lot of weight in the grand scheme, but it's enough to change your life. I just want what Dave wanted for me: that you stay safe. Crane's are handier than sliced bread. When you get used to having it, you'll use it all the time.

You might be able to sneak some L brackets in on the web of your T and not lose much travel of the dolly. If that joint holds, then the next concern is the short side of the T twisting and spitting the feet fore or aft along the same line as the long leg. Side to side bowing that you're measuring isn't the failure mode I'd be worried about (it might not even be what you're actually measuring either). I'd look at how the short side of the T is reacting to the load and adjust the strategy accordingly. If it's twisting in the horizontal plane, then boxing it or adding another stiffener along it's length would be needed.

I was looking for pics of how my runway trollies are bolted to the web of my bridge to give you an idea about bracing, but I don't have it on the web. I did find the x-ray and pin shots (I'll spare ya the icky feet).

As for your 30' push-pull gun - you need to unfurl it before use. I have a 35' Miller and it's a pain to use because of how much extra leads I have. I tig all my aluminum instead. I've probably run a pound of wire through the PP rig in 5 years of owning it. When I move and start doing dock repairs, I expect it'll be a lot more useful.

 

[Walker1]

As a pro welder I wouldn't work under it should have used bolted connections. You might consider retro fitting some bolted bracket on each of your aluminum joints.

 

[Coyote machine]

I'm not clear on why you only have the two legs by the garage door? Is there no way to have 2 more where your current T joint is located? Even if they were roll into place for support just when using the crane, then roll away after use?

 

[Jim Timber]

Can't load in/out of a trailer or truck bed if the middle is blocked.

I think the concept is good, but the materials/construction/assembly needs a little tweaking.

 

[Sodo]

That should be handy. I like your scale too.

I think you should proof it to a good amount beyond 1000 lbs. can do it in increments, & don't lift the test weight high, lift it so it can only fall 1/2 inch. Watch the paint on all the welds. Nows a good time because the paint is easy to monitor.

Agree the casters look undersized. What are they rated at? Maybe double them on a walking beam?