Gantry design ?

In physics, there's something called a free-body diagram where you draw the weights and supporting members and calculate the forces to determine the basic geometry. Then you need to calculate the forces on the welds and bolts. It's about ideal to have base legs that are about 2/3 (or more) of the maximum height. But when it comes right to it, the leg length really depends on how it's used.

That's good intel!

Thank you Ray C,
 
I don't want to move any thing suspended in mid air , I just need the casters to move the crane from place to place in my shop. I got the I-beam from a jib crane at work for free but I have a pole building and need it in more than one spot. I agree with all that has been said SAFETY FIRST swinging loads are a recipe for disaster. I was thinking about the size of the feet because when the casters swivel I don't want it to tip.
 
Let's talk about torque for a minute... Look at the gantry in this picture. There are two critical parts. 1) the 4 bolts for the two cross struts at the top. Why? Torque! The side beams are approximately 8' long and the struts are bolted about 1' from the top. Torque is Distance x Force. And lets say you're pushing the gantry sideways (toward the wall in this picture) with most of your body weight. So... (and this is an approximate idea of how to solve the problem) the distance is 7 and the force is equal to your body weight, which is a rough guess. Torqe is 7 x 185 = 1295 ftlbs. Now go look up the shear strength of bolts. Guess what? You're at the borderline of a lot of bolts.
2) If the strut and bolts are strong enough and don't break, there is now a tremendous force on the pivot bolt in the top corner because the strut is acting like the pivot point in a teeter-totter. Because the distance from the strut connection to the top corner bolt is 1 foot (in this case), it turns-out there's also 1295 ftlbs of torqe on the corner bolt. If the strut were pinned 2 feet down, there would be less stress but there are other tradeoffs. You get the picture.

Calculating the weight capacity of the side beams and top beam is easy. Look it up in a book and there's the answer. Hard part is understanding where these forces are.

Same thing goes for the 4 struts at the bottom if you were pushing on it the other direction... Since there's 4 struts, you're less likely to have a failure there but, the calculation still need to be made so you know where the limits are.

And finally, having very long base legs has it's issues too. What did we just say about torque? (Force x Distance). Long base legs = a long distance that produces a torque somewhere. Guess what? Base legs too long put too much stress on the bottom struts.

Long story short (and I'm sorry for rambling on) this is a classic problem where basic mechanics meets-up with material strength... and if there's any time I don't feel bad about over-building, this is it.

Ray

Here's a gantry that was designed for dead lifts of my old diesel engine/generators that weighed up to 2 tons. I know the gantry can do more than that. It's 7.5' tall, 4 feet wide and has base legs 4'. I have used it for rolling things by putting wheels under it but, use very cautiously with the weight very low. The biggest problems you face are swaying of the weight and twist/stress on the struts. If something fails, it crumbles.

In physics, there's something called a free-body diagram where you draw the weights and supporting members and calculate the forces to determine the basic geometry. Then you need to calculate the forces on the welds and bolts. It's about ideal to have base legs that are about 2/3 (or more) of the maximum height. But when it comes right to it, the leg length really depends on how it's used.

Take a look at the gantry that Harbor Freight sells. The geometry is pretty good but due to component quality I don't think it's good for everyday, industrial use. Also keep in mind, it's almost certainly made of alloy steel and thus the seemingly thinner gauge metals.

View attachment 48672
 
A lot of great info Ray, more food for thought. I never thought about the changes in forces by having to long of legs. I'm going to have to sleep on this. Thanks again
 
Let's talk about torque for a minute... Look at the gantry in this picture. There are two critical parts. 1) the 4 bolts for the two cross struts at the top. Why? Torque! The side beams are approximately 8' long and the struts are bolted about 1' from the top. Torque is Distance x Force. And lets say you're pushing the gantry sideways (toward the wall in this picture) with most of your body weight. So... (and this is an approximate idea of how to solve the problem) the distance is 7 and the force is equal to your body weight, which is a rough guess. Torqe is 7 x 185 = 1295 ftlbs. Now go look up the shear strength of bolts. Guess what? You're at the borderline of a lot of bolts.
2) If the strut and bolts are strong enough and don't break, there is now a tremendous force on the pivot bolt in the top corner because the strut is acting like the pivot point in a teeter-totter. Because the distance from the strut connection to the top corner bolt is 1 foot (in this case), it turns-out there's also 1295 ftlbs of torqe on the corner bolt. If the strut were pinned 2 feet down, there would be less stress but there are other tradeoffs. You get the picture.

Calculating the weight capacity of the side beams and top beam is easy. Look it up in a book and there's the answer. Hard part is understanding where these forces are.

Same thing goes for the 4 struts at the bottom if you were pushing on it the other direction... Since there's 4 struts, you're less likely to have a failure there but, the calculation still need to be made so you know where the limits are.

And finally, having very long base legs has it's issues too. What did we just say about torque? (Force x Distance). Long base legs = a long distance that produces a torque somewhere. Guess what? Base legs too long put too much stress on the bottom struts.

Long story short (and I'm sorry for rambling on) this is a classic problem where basic mechanics meets-up with material strength... and if there's any time I don't feel bad about over-building, this is it.

Ray

Alot of physics for me to chew on. I've got a mouthful right now. But, please keep it coming.
 
Tell you what... I promise to draw and post some diagrams of a teeter-totter and show how to calculate the stress on the beams and the stress on the pivot bolt in the middle. It's easy really and only need multiplication and division -and of-course a table that shows the properties of bolt and beam characteristics. Please though, give me a couple days to work on it because I really need to finish the little upgrades in my shop and show the steps on how I got the lathe leveled out.

BTW, Uglydog, as I recall, you're making a bicycle frame. Knowing these kinds of things really helps figure-out where the frame needs reinforcement and where you can get away with using light/minimal materials. Those are more complex problems and it's harder to come-up with exact numbers and figures but the guidelines will at least show you what areas are critical and which are not. Learning about free-body diagrams is a very, very useful and practical thing.


Ray


Alot of physics for me to chew on. I've got a mouthful right now. But, please keep it coming.
 
BTW, Uglydog, as I recall, you're making a bicycle frame. Knowing these kinds of things really helps figure-out where the frame needs reinforcement and where you can get away with using light/minimal materials. Those are more complex problems and it's harder to come-up with exact numbers and figures but the guidelines will at least show you what areas are critical and which are not. Learning about free-body diagrams is a very, very useful and practical thing.


Ray

Ray, I look forward to learning. I've got alot of it to do.

I got my Gisholt to .016 on a test bar at about 14 inches Sunday afternoon. Was hoping for better.
I look forward to any coaching you might give.

Daryl
MN
 
Haven't forgotten... There are two important concepts called "Constrained Motion" and "Unconstrained Motion" which defines when to consider either torque or force as the primary concern. I'm thinking of the best way to use the same diagrams to get both points across. ... Still noodling on the "lesson plan" so to speak.


Ray


Ray, I look forward to learning. I've got alot of it to do.

I got my Gisholt to .016 on a test bar at about 14 inches Sunday afternoon. Was hoping for better.
I look forward to any coaching you might give.

Daryl
MN
 
I've got a shop built rolling gantry I could get you some photos of, and any dimensions you want. I have a fixed lift ring in the center that I hang a hoist on. It is easy to build, and bolts together. It is 3 pieces. Around here, 2 3/8 and 2 7/8" pipe is common, so that is what the legs are, with plates on the bottom for the metal casters. I have had over a ton up on it comfortably. It did require cutting, welding and some machining, but nothing that can't be done with basic tools.
 
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