Bridge Crane build

[rabler]

Randal, I think you would be chasing your tail trying to level it with a machinists level. That's a lot of accuracy for something that will probably start deflecting and lose much of that instantly. Just an opinion. I do agree about keeping it loose until you get it all together, but you can probably start tight , then loosen it so that it's self constrained and as true as it will get without your further intervention.

I think 5 gal concrete pails a bunch of them with eyebolts in the concrete will be your friend in trying to setup and secure the iron. That, or some holes in that nice new floor.

Jeff,
Buckets would scare me, once things got to running away. There is a bit of significant pendulum mass so a little oscillation in the structure (imaging it bouncing around 1/2") could overcome static friction. Of course, I could guy wire it to one of my bigger machines. Rather than that, the current plan, subject to more pondering...

I ordered 4 eyebolts. I'm going to strap the rail to my tractor loader bucket using multiple tractor trailer binder straps. I have bunches of those from hauling hay. I'll use the bucket because that is a 6' width rather than the not quite 5 that the forklift attachment will span, and the forklift has a back guard that will interfere. I'll set an eyebolt into the ceiling into a 4x6 that spans 2-3 ceiling trusses. Not a rigorous analysis but each truss will hold my weight plus my wife's, which is one wife-unit heavier than the beam half (i.e., I weight about the same as one end of the beam). The beam will go up the loader and be bolted in place, and safetied to those ceiling eyebolts and bolted loosely to the top of the posts before the loader is removed. That gives me enough redundancy that I can then use the loader to hoist the other rail and start tying them together.

As far as the level, I'm not feeling like crunching the numbers to convert precision right now, but I don't think trying for level to within 1/4" across the entire structure (roughly 40' x 24') is that unreasonable. I'm not leveling it to within a few thou, but *guessing* the span and angle is getting within the same ballpark. I have a 6' carpenters level I'll start with of course. I'm not THAT stupid even fresh out of the hospital.

edit: .250" over 40' is .0062/ft. Not lathe leveling accuracy but tough with a carpenters level. I do have a carpentry laser level that could also used.

 

[dkemppai]

Sounds like you do the same thing I do when analyzing a problem like this, that is be overly cautious. (Not a bad thing...)

From what I gather, you've got a lot of stout j bolts in the concrete. If you're uprights are snugly in place, setting a 500lb beam on top of them would probably be fine without cross bracing for initial assembly. Once you get a couple of bolts between the rails and the uprights, they're not going anywhere.

For a point of reference, I have a two post portable lift installed. That uses 5 epoxy in anchors with 5/8" bolts in a 4000 psi slab for each post. They are in a approximately 10" by 12" plate. And that's designed to safely lift a 3ton vehicle, including live load pushing/pulling/wrenching and possible imbalance, etc.

I don't recall what your truss spacing is. Even on 4ft centers a 4x6 spanning them should be plenty for a few hundred pounds on each end of the rail for safety lines. Compare that to the possible live load for your trusses (snow load, etc) for your part of the country.

Final cross bracing might warrant more thought. But, I wonder if you could have too much bracing??? Having some spring/flex to take any impact loads without imparting them directly into the concrete might be a good thing.

 

[rabler]

Sounds like you do the same thing I do when analyzing a problem like this, that is be overly cautious. (Not a bad thing...)

From what I gather, you've got a lot of stout j bolts in the concrete. If you're uprights are snugly in place, setting a 500lb beam on top of them would probably be fine without cross bracing for initial assembly. Once you get a couple of bolts between the rails and the uprights, they're not going anywhere.

For a point of reference, I have a two post portable lift installed. That uses 5 epoxy in anchors with 5/8" bolts in a 4000 psi slab for each post. They are in a approximately 10" by 12" plate. And that's designed to safely lift a 3ton vehicle, including live load pushing/pulling/wrenching and possible imbalance, etc.

I've... umh ... dropped big things before. I try not to repeat stupid mistakes. Especially when they get hazardous or expensive. I knew a civil engineer that would look at things dropped and say “Yep, gravity still works.”

My thought on the bracing was that if I put in the attachment points on the posts and beams, I have the option to add bracing. Once the beams are up in the air, I won't want to take it down again. So I'd rather have unused than insufficient brace points. You're certainly right that some flex is desireable. Just want to make sure it is stiff enough to not start moving/swinging/oscillating perceptably under load. That's disconcerting, like wings bouncing on an airliner. And since I'm running close to, but independent from the walls (2"), more deflection than that could damage the walls.

 

[Dabbler]

If I were 1/4 the distance I'd offer to drive over and help! I'm sure your bracing will be fine. The weakest direction of a free standing bridge crane is along the bridge direction anyway.

 

[rabler]

If I were 1/4 the distance I'd offer to drive over and help!

It reached a pleasant high of 52F today. Nevertheless I had fired up the wood burner this morning, so the shop was probably 60F.

 

[rabler]

A bit of late night rambling. Just how I work through things. A big part of the reason I enjoy hobby machining is the ability to dabble in the plan-design-build-revise cycle without having a lot of outside drivers, just my satisfaction.

My thought on trucks is still in flux a bit. Here is the initial CAD sketch of my end truck. The raised wheel wells and lowered saddle are to make the most of ceiling clearance, getting the bridge and rails as close to the ceiling as possible while still using larger wheels/rollers. As was discussed earlier, larger rollers handle imperfections/debris on the top rail a bit better. Frankly I don't see anything more than dust collecting on the top of the rails (at 10' up in the air) so that may not be much of a factor.


Length: One reference that I can't find at the moment suggested that the end-trucks length should 1/6th of the bridge length. That would be 4 ft from axle to axle for me. One trade off is longer trucks track better when the bridge is pulled from an off-center position, but shorter trucks give crane access to closer to the end walls. The later is important for me as that gets things up on my primary workbench, along the 24' wall. Side running bearings to keep the trucks centered on shorter trucks will probably be my solution. The end angle brackets on the sketch provide a place to mount track rollers for this purpose.

Another design factor in trucks is that while I don't necessarily plan to use a motor driven bridge, I'm exploring how allowing that down the road changes the design. That requires the truck wheels to be rigidly attached to the axle, with the axle coming through the side of the truck. That effects how the bearing layout, as the bearings need to be between the axle and the frame, not between the axle and the rollers/wheels. Allowing that means I can have an axle stub that could potentially be interfaced to a motor. I'm leaning toward 1641-2RS bearings, that gives an easy 1" axle size. Uniformly distributed load across 4 bearings means about 1000 lbs per bearing, those bearings have a static load rating over 1200 lbs each, so a bit of a margin. Not the same margin as the beam oversize, but bearing wear on a hobby-use crane isn't going to be significant, and a bearing failure doesn't cause the structure to collapse.

A third design feature I want but haven't designed, is brackets going under the lip of the rail flange so that the truck is captive to the rail, no way for it to bounce off the rail. Can't see that being an issue in normal use, but if the bridge ever got hit by a forklift mast it could be a significant safety factor in keeping the bridge beam from coming down.

 

[extropic]

What is your opinion of the axle shafts being connected between both end trucks?

 

[rabler]

What is your opinion of the axle shafts being connected between both end trucks?

Implied if it gets motorized, but adds significant complexity so won’t unless I have wracking/tracking problems. Would just motorize and cross-connect one of the two axles per truck.

 

[rabler]

Randal, could you assemble is sections? By that I mean install the temporary Turbuckle X between to of the posts on one side. Lift the top rail and secure in place then replace the turnbuckles with Y Bracing and then move to the next section. Would this be too ridged to still be able to adjust the height after it was all in place? Maybe level each section as you go. Does it really need to be as level as a lathe bed? I have no experience with but enjoy the thought experiment

Sorry I missed this. Something along these lines is what I'm thinking but I think it can be a little simpler. The tricky part is combining the final structure with the issues associated with rigging, lifting, and assembly. What I have learned there is that we humans do a lot "intuitively". But our intuition fails when dealing with 40' long steel beam sections, or things outside of the weights we normally can lift. So I tend to look at everything with a "Ok, I think it'll work this way. Now, what happens if something goes wrong in the middle of that process."

 

[dkemppai]

Implied if it gets motorized, but adds significant complexity so won’t unless I have wracking/tracking problems. Would just motorize and cross-connect one of the two axles per truck.

I decided to go down the route of running a shaft from one truck to the other on the one getting built up here. That shaft needs to be supported at several points, or be a pretty stout shaft. However, rather than drive the wheel directly the shaft drives a sprocket on each end, with roller chain run down each rail. It was the cheapest way I could come up with to drive/clock the trucks. $80 for chain, CNC milled sprockets, etc. Adding a motor later would be easy.

The big advantage was being able to use off the shelf rollers. As you're finding out bearings for that sort of load aren't easy/cheap to find. What ended up in the one here are 12,000 lb rated, but are smaller in diameter than what would have preferred. But at $80 ea, they didn't break the bank.

More complex, but the chain clocking should allow the lift hook to get within about 16"-18" of the wall.