bending 4x4x.250 sqtube

   / bending 4x4x.250 sqtube #1  

muddstopper

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Apr 11, 2006
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Location
western NC
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Ventrac, Steiner
Been trying to figure out how much weight I can lift using 4x4x0.250 sq tube. All those online calcultors and formulas just confuse me so I will ask here.
The tube is going to be used as the root boom of a knuckleboom loader. Tube will be 8ft long with a fixed pin at one end and cylinder attachment point approx, 49in from the fixed pin. The other end of the boom will have another 8ft piece of tubing (dipper boom) attached with pins with one end of the cylinder attached somewhere between the 49in clyinder location of the first boom and then attached to the outer boom. Max weight my rotator will handle is 4400lbs, altho I expect less than half that max load at any given time, but that weight could be suspended at the end of the 16ft long boom. With the load weight at full extention I am afraid I might bend the root boom at the cylinder mount location. Bascily what I need to know is just how much weight can a 4x4x0.250 sq tube support at a 4ft extention from middle of the tube. . I have a drawing, but cant figure out how to get it to post from autocad.
 
   / bending 4x4x.250 sqtube #2  
Take a screen shot of it.

You may want to bridge it on the top like a boom pole on a tractor, which are typically rated for a ton or two with much smaller material.
 
   / bending 4x4x.250 sqtube #3  
Take a screen shot of it.

You may want to bridge it on the top like a boom pole on a tractor, which are typically rated for a ton or two with much smaller material.

^ Xcellent advice.
 
   / bending 4x4x.250 sqtube
  • Thread Starter
#4  
Lets see if this screen shot stuff works. KNUCKLEBOOM.png If this worked, you should see some multi colored lines. The sq yellow box is the side mounts for the base that the boom attaches to. The red lines are the boom in its upright and lowered position. The blue lines are the hydraulic cyl, light blue line is with cyl fully extended. the pic just showed the mounting points not the entire boom which would be about 47 inches past the cylinder mounting point. As you can see, I am not an expert in autocad and I was just drawing this up to determine where to drill the holes for the boom and cyl mounts.

As a note, I just ordered the Valby R3 rotator to build the grapple with. Its actually rated for 6600lbs instead of the 4400lbs that the R2 is rated for. R2's where out of stock and the R3 was on sale for only $70 more than the R2. $800, no tax and free shipping from Construction Equipment and Material Handling Industrial Equipment - Summit Equipment Outlet just in case someone is interested. I think he said there where 3 left.
 
   / bending 4x4x.250 sqtube #5  
Cant really tell whats going on. Looks to me like the boom has no bending moment at all because they hydr. cylinder is supporting all the weight out at the end. Can you hand sketch a diagram and then post a photo of that? Draw it with the boom in its worst case position for bending.

Or can you find a similar knuckleboom with google images and post that?
 
   / bending 4x4x.250 sqtube #6  
Will this help?
 

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   / bending 4x4x.250 sqtube
  • Thread Starter
#7  
SA, charts like that just confuse me. I dont understand what they are saying, altho to someone in the business, they might make perfect sense.

SoDo, here is a pic of a knuckle boom, Mine will be a much smaller scale. Take away the cab and downsize everything else, the root boom is pretty much how I intend to build mine.trailer-mount-knuckleboom-loader.jpg My drawing is only to show the mounting points of the root boom cylinder, the actual boom will extend about 47 inches past the cyl mounting points. With the boom mounted on a pin at the base the cyl will act as a fulcrum and would be the place where the boom would have its most stress and where it would mostly bend . Fulcrum is almost in the exact middle of the boom. What I am trying to figure out is how much weight can I put on the 4x4 sq tube before it starts to bend. The tube will be re-enforced in that area with 1/2x6 in plate and that plate will serve as the mounting point for the dipper boom cylinder for the outer boom. Very similar as in the picture. My boom is planned to be 16ft in total length. Estimated max weight to be applied to end of 16ft boom will be around 2000lbs. I dont know the formula for factoring in the leverage, but am guessing actual stress at the root boom cylinder mount to be approx 12 times actual lifted weight, or 24000lbs. That estimated weight would only apply when and if the boom was fully extended, which wouldnt be the perfect way to make a lift anyways. What I need to know is will the 4x4 tube support 12 tons before bending and if it wont support the weight, what size tube do I need to use. The 1/2x6in plate should make the tube stronger, but I dont want to factor in that extra support in my calculations. My thoughts being make it safe enough, than add bracing for insurance.

SA, I may of just had a light bulb go off. Looking at your chart, I didnt have a clue as to what Kips meant. After a search I found a online calculator. Using the online calculator and figureing out my tube at 4 ft has a kip factor of 20. Using the calculator that translates to 20,000 lbs. It would seem I am very close to my estimated max load in that area. If you can verify that I am on the right track that would be a big help.
 
   / bending 4x4x.250 sqtube #8  
Lets see if I can help. The charts says 4" x 4" by 1/4" wall thickness is good for 13,000-pounds in a 6-feet span. But that is a uniform load. Generally these charts for point load in the center of the span you cut the uniform load in half. So it would be good for 6,500-pounds.
 

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   / bending 4x4x.250 sqtube
  • Thread Starter
#9  
Lets see if I can help. The charts says 4" x 4" by 1/4" wall thickness is good for 13,000-pounds in a 6-feet span. But that is a uniform load. Generally these charts for point load in the center of the span you cut the uniform load in half. So it would be good for 6,500-pounds.

Ok, you lost me. I cant see where you are seeing a 6ft span or a load value of 13,000lbs.
My root boom is 8ft in length so I would assume that would be the span and the cyl rod end (fulcrum) is pretty much centered at 47 in. one way and 49 in. the other end. Using a 8ft span according to how I am reading the chart would suggest a kips of 9.9 or 9900 lbs. Less than half of what I am looking for. I'm probably reading the chart wrong.

And I now see where the 13000lb comes from a 6 ft span having a kips of 13=13000lbs. Also if taking half the 8ft span at 9900llbs/2 would only give me about 4950lbs, way off from where I need to be.
 
   / bending 4x4x.250 sqtube #10  
It has to b a lot stronger for point load than uniform. But you guys are on the right track for shooting from the hip,,,, sorta. Those charts have accepted safety factors in them. If you could find a similar chart for point loading it would apply better.

==========================

2000 lbs on the end will deflect 1.8" at the end, and .33" at the cylinder attach point. 2,000 lbs won't cause a permanent bend (60,900 psi stress vs 74,000 psi strength). So Mudd your gut feel for 4x4x.250 is pretty good. I would not have guessed it to be that strong. But you want a safety factor of 3x or 4x,,,, and all you have is 1.2x.

If you add the truss over the top your strength will increase by about 3 or 4x, then you will have the safety factor you need. You could use 3/16" flatbar for the truss. It would look nice with a 3" x 1/4" flatbar truss with a peak at the center, but the peak is more "design" not necessity.

Your cylinder mounts probably should be independent of the strengthening elements. They should be set to fit your cylinder ends. The bracing can be its own unit on the topside.

If you choose a design you like the engineering can be done on that.
 

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