# Gantry design ?



## pineyfolks (Mar 2, 2013)

Waiting for it to warm up to start on a gantry crane for my shop , the I-beam I have is 10' long the overall height will be 9' 6".  My question is how long do I have to make the base of the legs to be safe?  Is 5' enough? I don't want to make them longer than I have to so it will be easy to get around in the shop. But I don't want a crash! Its going to be 1/2 ton cap.


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## Ray C (Mar 2, 2013)

Bill,

The dimensions are really based on how it's used.  If you can give some idea on how you intend to use it, I can help with some basic guidelines.  As always, they are guidelines and any gantry used beyond it's design parameters is a dangerous thing.


Ray



pineyfolks said:


> Waiting for it to warm up to start on a gantry crane for my shop , the I-beam I have is 10' long the overall height will be 9' 6".  My question is how long do I have to make the base of the legs to be safe?  Is 5' enough? I don't want to make them longer than I have to so it will be easy to get around in the shop. But I don't want a crash! Its going to be 1/2 ton cap.


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## pineyfolks (Mar 2, 2013)

Mosty I'll be using it to unload my truck , load and unload my machines and weld table, things that are just to heavy for one old guy. Its just my hobby shop . I'll be using a chain hoist and trolley. my upright post will be 5" OD  1/4" wall tubing with bracing to the casters forming a frame like the HF ones. The I beam was off a 1 ton jib crane.


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## Ray C (Mar 2, 2013)

level ground or slant on your driveway?


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## pineyfolks (Mar 2, 2013)

Its level its going to be used inside.


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## Uglydog (Mar 2, 2013)

I am in the same spot.
Have been thinking about a build summer 2013.
I've been looking at this link as a source for build information.

http://www.beacontechnology.com/gantrycranes/

Also, consider looking at the last edition of "Home Shop Machinist" they devoted an article to several considerations.

Remember this will be above your head, I hope you are a good welder. 

Or are you going with bolts? If that's the case remember that each hole weakens the material. That's why riveted bridges, while once the standard, fell out of favor. The holes became a limiting factor.


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## Ray C (Mar 2, 2013)

If you stick to 1/2 ton on level ground and if the distance between the wheels is 5', you'll have some limitations.   You'll need to lift the weight, drive the truck out, lower the weight as low as possible then, move the gantry.  In other words, keep the weight low as much as possible while moving it.  Of course, don't let it swing back/forth.

For gantries that are to be moved with the weight at higher elevations, you need considerably more leg length.

Now the other factors are the strengths of your connections and placement of the struts etc...  Without knowing the materials, its hard to know how much twisting is going on.


Ray




pineyfolks said:


> Its level its going to be used inside.


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## pineyfolks (Mar 2, 2013)

Alot of good info there Uglydog. Thanks  ,Fitting and Welding will be no problem but Thanks for the word of caution to those less experienced.


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## November X-ray (Mar 2, 2013)

I had a similar situation, and what I did, while only a level concrete floor, is use ratchet straps to tie the load to the outer bottom frame in a criss-cross fashion to keep the load from swinging with the load as low as possible. I would never try to move a load on a rough surface with a gantry crane.


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## Ray C (Mar 2, 2013)

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.


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## Uglydog (Mar 2, 2013)

Ray C said:


> 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,


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## pineyfolks (Mar 2, 2013)

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.


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## Ray C (Mar 2, 2013)

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



Ray C said:


> 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.
> 
> ...


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## pineyfolks (Mar 2, 2013)

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


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## Uglydog (Mar 2, 2013)

Ray C said:


> 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.
> ...



Alot of physics for me to chew on. I've got a mouthful right now. But, please keep it coming.


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## Ray C (Mar 2, 2013)

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




Uglydog said:


> Alot of physics for me to chew on. I've got a mouthful right now. But, please keep it coming.


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## HMF (Mar 3, 2013)

Temporary Wooden Gantry that can be taken down:

http://www.hobby-machinist.com/show...-Assemble-VN-Milling-Machine?highlight=gantry


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## Uglydog (Mar 4, 2013)

Ray C said:


> 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


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## Ray C (Mar 4, 2013)

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




Uglydog said:


> 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.
> ...


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## Tony Wells (Mar 4, 2013)

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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## pineyfolks (Mar 5, 2013)

Thanks Tony , I'm open to all ideas. I like to see what others are using.


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