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After searching here and the web... I could really use some engineering help with part of my boom project. There is a length of square tubing that can be extended up to 4 feet. It slips into the main tube with about 3/16 to spare. The present plan is to secure the inner tube at various positions using a removable pin.The maximum load at L should be 500 Lbs. The average load will be more like 300 Lbs.Im concerned with the strength of the inner tube when it is extended. The specs of the inner tube are highlighted in blue.I could increase the strength of this tube by welding in a full-length piece of vertical bar stock. Is that overkill? Can someone here calculate the load capacity at L ?Many thanks,Oasis Attached Images
Reply:can the boom have more than one mount for the triangle bracing?....say for instance the triangle come from the top for the main tube and have a tube or piece of square coming from the bottom like you have now to attach to the extension at various points as you extend it out?....it would kinda make it an arrowhead shaped jib boom once its finished?...if not, i would think a thicker tube for the main and sleeve it at the end to prevent the opening from splitting or getting out of square and then making the inner tube out of double walled square...the jib i use at work all night long is mondo huge and it's only rated for 650lb max...i'll get a pic tomorrow night for you to see how big it is just to be rated for that load on a regular basis...
Reply:Oasis,I suggest doing a quick search on "truck-mounted crane" or "jib crane" using your favorite web search tool. You should find a bunch of hits on cheap cranes sold on E-bay as well as some on Harbor Freight. I bought a cheap HF jib crane, and it has similar load ratings to what you're trying to build. Pay a visit to HF if you can, and check the dimensions on the tubing. this should tell you if you're in the right ball park. If you can't find a crane to inspect, let me know and I'll take some quick measurements on mine when I have a few minutes to spare.It looks like your crane is going to be fixed to a column or beam of some kind. Make certain that the crane mount is also rated for the applied load.Railin93, that may be why the jib crane you use has a seemingly low load limit. The limit may be the method that the crane is attached to the floor or wall, not the crane itself. Cranes, particularly those with long booms, need a surprising amount of mass at the base in order to keep from upsetting.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:More thoughts, Oasis, your 1/4" reinforcment would be a good idea, but you'd need to split the tube in half, and weld the bar in place along the entire length or at the least use several long stitch welds in order to transfer the load effectively to the reinforcement. A couple tack welds at either end won't work and you'll end up with a false sense of security; until they pop. You're probably better off using tubing with thicker wall. You could also run 2 cables and winches. One to lift the load. and the other to support the weight on the boom. I'll try and attach a drawing of what I'm suggesting. Attached ImagesBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:A 500 lb load will put 500lb x ? amount of stress on the vertical shaft.
Reply:You are basically trying to do the same design as my engine hoist. It's buried in the shop, so I can't get measurements easily off of it. The outer tube does have a stiffening strap made of solid material on to to form a truss somewhat like A_DAB_will_do posted. I'm pretty sure the material on my hoist is much heavier than yours however and I believe the tube is larger. It's rated at 500lbs fully extended As others have mentioned, 500 lb's 4'+ out on a lever equates to some 2000 lbs+ trying to rip the mounts out. You need to be sure the post can take that sort of bending force as well as the mount..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Thanks,The boom will be attached to present hydraulics for doing the lifting. I do plan to sleeve the end of the main tube with ¼ plate, and yes I can beef up the main tube and frame all I want. Im concerned with the cantilever capacity at point C. Im sure it will handle 200 Lbs. Id like to know what the maximum cantilever specs of my material is. Ive looked at and used other booms. Im replacing one I have right now that is much to heavy duty and cumbersome. A_DABThank you for the wench idea and the drawing.I know that trussing the top would add a lot. So, I now have another option of doing this using a chain and turnbuckle to accommodate the extension, see sketch. fyi - The extension will only be used occasionally and not be repositioned more than a few times per day.Also, to add the internal bar stock
I thought I would: insert the bar stock, reach in and weld the ends, then drill a ¼ hole every 5 along the top and bottom centerline of my tube and then stick weld the bar to the tube at each point.Which option(s)? Overkill? Attached Images
Reply:The problem with a center bar is not just the welds you have mentioned, but with the cross holes you will need to drill to make this adjustable besides all the way in or all the way out and the general idea. You'd be better off with thicker wall tube than a center suport. Beams that need to resist deflection either get taller, or add more material to the top and bottom flanges, not thicker webs (although that often happens at the same time) A thick center brace will be applying loads to the weakest unsupported part of the tube..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I can take a look at stresses in movable part if still required (later when I get home)...Just some observations:1. This is not cantilever - this is a beam since both ends are free.2.Welding a bar inside movable beam - no use since there is no real connection between two.For this we have to take a look at a couple of things:1. What is the length of movable beam inside of stationary beam?2. Are you sure about properties of material? - this is no ordinary steel.3. Are you sure about 500#? This means you have to apply the load at practically zero speed. Is this the case?We would assume - no holes - dramatically weakens the member...
Reply:I just did a quick number crunch on the bending stress of the extendable beam. At 4 feet out, with a 500 pound load it comes out to almost 38 ksi. I agree that the steel numbers on the table don't look right. Plan on 36 ksi steel. Adding the internal vertical piece would reduce the stress by about 20% assuming that it can be installed in a way that does not allow it to twist or buckle. Doing a rough check, 1/4" wall is just over 18ksi. A top member would improve your numbers but also makes it a FEA type analysis to figure out exactly what your stresses will be. The pivot pins may be a problem. I can check those out if you put in the rest of the dimensions. The pin holes for boom length are no big deal since they will run through a semi stress free area of the pipe.ReneLast edited by zerepener; 01-19-2010 at 01:18 PM.
Reply:Seems like it's done. Just a question to zerepener - did you calculate as a beam with 2 loads? Since no internal length known ?...
Reply:I just did a cantilevered beam. The max bending stress is going to be a function of the boom out only. This is because of the point loading where ther outer tube supports the boom. The stress on the top of the boom where it contacts the inside of the outer tube may be an issue depending on how short the extendable boom is. That also applies to the bottom of the outer tube where the boom contacts on the bottom. I don't expect those contact points to be a problem. Rene
Reply:Thanks guys,The frame is fully supported (sorry_ not shown). I considered point “C” to be the fixed end of the inner tube. Point “L” is the free end. The plan is for 1’ of the inner tube remaining in the main tube at the full 4’ extension. There will be a midway position with 2.5’ in and 2.5’out, and then fully retracted with 4’ in and 1’ out. I plan to shim the last 1’ of the main tube so that there will be minimal play and close contact during heavy lifting.Sorry about the chart. I gave engineering by numbers a try for this problem but I failed- that’s why I asked. Rene- Many thanks, that’s just type of information I was looking for. I will upgrade to a structural steel tube of ¼” wall 2 ½ x 2 ½ @ 7.11 Lbs/Ft. It sounds like I may also want to add the truss. Although I might be able to simply stay within weight to length limits. I generally know the load weights.If it’s not too much to ask – What is the maximum static load for a cantilever beam of ¼ wall 2 ½ x 2 ½:At 1' length?At 2.5’ length?At 4’ length?This really helps,Oasis
Reply:I think Ive got it!Someone pointed me towards online deflection calculators. I found a simple one that works and appears to be pretty accurate. Its just little conservative according to my shop tests. Its for a variety of materials and dimensions being fixed at one end, load to the other (diving-board) giving the flex in decimal inches.Link: http://easycalculation.com/mechanica...ular-beams.phpHeres what it looks like: Attached Images
Reply:That 2.5" beam is a bit stronger than the one that you originally had planned. The max loading using a safety factor of 2 (18 ksi) is 824 pounds at 4'. That is a number that is totally dependent on boom stickout. At 1 foot, the max is 4 times 824, at 2 feet it is twice and at 3 feet it is 4/3. You still need to figure out if the pivot points can take that kind of load.
Reply:Thanks again,I really appreciate you sharing your expertise. You helped me dodge the dreaded bullet of bent steel.Many happy returns,Oasis
Reply:Oasis, last thought, but you need to be mindful of the fact that those deflection calculators are based on static loads. The moment you swing that jib crane, or set the load to bouncing, you're talking about a dynamic load, and the math and physics involved changes. If this is a possibility in whatever application you're builiding this boom/jib crane for, then I advise caution. At the very least, some proof load testing with the max weight you intend to move, and some added safety precautions during the test.respectfully,A_DAB_will_doBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Well put A_DAB, I've left out many important elements to my project. The boom extension was my primary concern. It is the weakest link. That's a good thing. I just didnt want it to be too weak. The worst-case scenario should be 5 feet of bent tubing, which can be easily replaced. Overloads will be avoided via pressure relief valves, which will limit the involved hydraulics.I'm the sole operator at a safe distance, no overhead... I'll do plenty of safe testing and include some labeling for load rates when I paint her up. I'm looking forward to that point!Thanks again,Oasis |
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发表于 2021-8-31 23:59:11