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Thread: Square Steel Tubing Calculation (and related design questions)

  1. #1
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    Square Steel Tubing Calculation (and related design questions)

    Let me state up front that I'm not an engineer. I've come to this forum looking for some advice/assistance.

    A good customer has put me in a position that I don't care to be in, however his business is too valuable to refuse. We custom build a machine (to his specifications). He has a need to raise and lower the entire machine 36". The machine is enclosed in a steel 4" square tube "roll cage" (for lack of a better description). He wants us to use the vertical square tube supports to build a lift assembly. In other words, he wants to use pneumatic cylinders to lift the machine. In fact, he has suggested that we use pneumatic cylinders located inside of tubing that slides inside of a larger bore tube to raise the machine (he does not want to use hydraulics). The machine weighs 6,100 lbs. and is near uniformly weight distributed. (The reasons he is not figuring this stuff out himself is too complicated to get into in this forum.)

    Now, I know from research that if I am using four air cylinders to lift 6,100 lbs (and adding a fudge factor of approximately 1,500 lbs), each air cylinder bore will need to be 5". Thus, his 4" vertical square tube will not house the air cylinders. Here is what I propose, but I want to run it past the experts first:
    • Replace his 4" vertical square tube with 7" square tube (0.188" thick wall to allow 6" square tubing to slide up/down inside tube)
    • Use 6" square tube (0.188" thick wall) as the "extension leg"
    • Install the 5" bore air cylinder (outside dimensions = 5.5" x 5.5") inside both tubes; connect cylinder and rod ends to the 7" and 6" tubes


    Now for my questions:
    • Will the thickness of the 6" and 7" tube be sufficient for the weight it must support?
    • How many inches or feet of overlap should I have for the 6"/7" tube so I don't have to worry about the machine being wobbly or unsafe when it is elevated? (Something tells me it should be at least 33% overlap... probably from my days working as a carpenter... lol)
    • Would you use a double acting air cylinder to control lowering the machine?
    • Would you use hitch pins to hold the machine in the elevated position once it has been raised?
    • Check valves on the air lines with a manual ball valve to depressurize (for lowering the unit)?
    • Is there an easier way to do this?


    If this is off topic for this forum, please direct me to the correct forum. All assistance is greatly appreciated. Thank you.

  2. #2
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    Welcome to the forum and I kinda feel a little for you in your plight with the customer.

    Hate to say it, but the description you have given is nowhere near enough information to offer an informed decision.

    Just for a tiny start, you have not mentioned...
    Lateral loads from any direction.
    Bracing of the fixed "legs." (Tube overlap will count for squat if legs are not braced to carry lateral, rotational, axial or shock loads)

    Gotta say also, I would be against air for such a weight (THREE TONS). Air is compressible and Hydraulic fluid is not. It would be nuts (in my opinion) to proceed with air just because the customer does not want hydraulics. Who pays when the air system turns out to be to unreliable and even dangerous after x-thousands of dollars have been spent getting to that point?

    At this stage I will not get to the other questions as you need (in my opinion) to stand up to the customer and tell them they need an Engineer to design the system and then you can build it.

    If the customer balks then you would do well to recommend your nearest and greatest industry-competitor to take on the job.

    Just one old fart's, been-there-seen-that-didn't-like-it opinion.

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    Administrator Kelly Bramble's Avatar
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    Thinking about it - will be back...

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    Quote Originally Posted by PinkertonD View Post
    Welcome to the forum and I kinda feel a little for you in your plight with the customer.

    Hate to say it, but the description you have given is nowhere near enough information to offer an informed decision.

    Just for a tiny start, you have not mentioned...
    Lateral loads from any direction.
    Bracing of the fixed "legs." (Tube overlap will count for squat if legs are not braced to carry lateral, rotational, axial or shock loads)

    Gotta say also, I would be against air for such a weight (THREE TONS). Air is compressible and Hydraulic fluid is not. It would be nuts (in my opinion) to proceed with air just because the customer does not want hydraulics. Who pays when the air system turns out to be to unreliable and even dangerous after x-thousands of dollars have been spent getting to that point?

    At this stage I will not get to the other questions as you need (in my opinion) to stand up to the customer and tell them they need an Engineer to design the system and then you can build it.

    If the customer balks then you would do well to recommend your nearest and greatest industry-competitor to take on the job.

    Just one old fart's, been-there-seen-that-didn't-like-it opinion.
    Thanks, Pinkerton. I suspected this "modification" was complicated enough to require an engineer. Here are my responses to some of your very valid concerns:


    • I don't know how to calculate lateral load. The only lateral loads will be from wind. This unit will be located outdoors; it is being leased out to contractors. The machine looks like a big box on stilts. The legs are currently about 75" long with the machine located mostly above the 60" mark (up to approximately 144"). The client wants to be able to lower the unit 36" (for transportation; can't lay it on its side due to the internal components of the machine).



    • "Upper legs" are supported by horizontal 4" square tube top and center (approx. 60" from base) on all four sides of the machine. I didn't know if the "lower legs" would require bracing since the lift is only 36".



    • Client is very much opposed to hydraulics since it will be leased out (often in environmentally sensitive areas). He is providing the air system with each machine so there will be some level of control over the equipment reliability and air quality.

  5. #5
    Technical Fellow jboggs's Avatar
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    daigoro,
    As one who has been designing systems like this for a long time, allow me to add my comments to Dave's - "ditto". You are heading into deep waters in which you will need a good map. Not that what you want to accomplish is all that difficult. It just has an enormous number of possible failure modes and your design must account for each one.

    They aren't all mechanical in nature. Lifting that kind of load, you want to control its motion very strictly. You want it to move when you tell it to, and how you tell it to. And you don't want it to move unless you tell it to. That's the control system. Sounds simple, but ... maybe not so much.

    Your reply above just multiplied the number of potential failure modes: "it is being leased out to contractors". This means that neither you nor your customer will have control over how this thing is used, or misused. Let's see, I have 3 tons on stilts several feet up in the air operated by unknown individuals. How many ways can this go wrong? A perfect setup for unintended usage and resulting damage or injury. Plaintiff's lawyers can easily find their way through so-called "protection clauses" and aren't real picky who they sue. Anybody that touched it will do, especially if they have any assets.

    I would strongly suggest three steps: (1) engage the services of an engineer experienced in designing similar systems, (2) plan on building and testing a few prototypes, and (3) involve some of these "contractors" in its development to get their thoughts and input.

  6. #6
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    daigoro,

    In addition to all cautions and advice, please consider that because the thing stands and operated fine as it is, by raising it you will be adding serious extra loads to the structure. It may be very stable now, but 6-feet in the air will add enormous extra loads to the structure's basic stability.

    The air-lifting idea is just plain nuts regardless of who or where it is operating and by whom. The client needs a whack (virtual of course) to the side of the head and let the Engineers decide what is best to achieve the desired end results and it will NOT be compressed air for raising it. A competent design and subsequent manufacture will provide a completely sealed and environmentally secure hydraulic system. You do not see cars spewing hydraulic brake-fluid all over the landscape - do you?

    The course you and the client MUST take for this is clear and that is to do as JB suggests. Get an "EXPERIENCED" Engineer to design this. It is NOT a trivial issue of just raising it up on legs.

    I now back away from further discussion on this topic as this needs no more from a DIY approach and MUST be managed by an Engineer.

  7. #7
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    I appreciate everyone's input. It is as I suspected. I will be having a lengthy discussion with the client next week regarding this issue. Many thanks to all.

  8. #8
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    Quote Originally Posted by PinkertonD View Post
    daigoro,

    In addition to all cautions and advice, please consider that because the thing stands and operated fine as it is, by raising it you will be adding serious extra loads to the structure. It may be very stable now, but 6-feet in the air will add enormous extra loads to the structure's basic stability.

    The air-lifting idea is just plain nuts regardless of who or where it is operating and by whom. The client needs a whack (virtual of course) to the side of the head and let the Engineers decide what is best to achieve the desired end results and it will NOT be compressed air for raising it. A competent design and subsequent manufacture will provide a completely sealed and environmentally secure hydraulic system. You do not see cars spewing hydraulic brake-fluid all over the landscape - do you?

    The course you and the client MUST take for this is clear and that is to do as JB suggests. Get an "EXPERIENCED" Engineer to design this. It is NOT a trivial issue of just raising it up on legs.

    I now back away from further discussion on this topic as this needs no more from a DIY approach and MUST be managed by an Engineer.
    Thank you.... just a clarification... (and not that it makes any difference to the discussion) the existing unit is already on legs... the client just wants to be able to lower it 36" and raise it back to it's original height.

  9. #9
    Lead Engineer RWOLFEJR's Avatar
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    Some thoughts...
    Consider a screw drive...?
    If that doesn't appeal to you or them at least go air over oil cylinders for solid movement.
    The drive lugs can be located outside of the tubes... Best if can be inboard or not outside of box footprint, so out of harms way?

    Air will bounce without the added stability of oil cylinders. Screw would be best and lowest maintenace etc. If the foot print is large... and as you say these are already out there just need to be able to lower for ease in hauling... then might not be all that necessary to reinvent the posts?

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