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Thread: Please Help Me With Structural Analysis!

  1. #21
    Technical Fellow jboggs's Avatar
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    Dave is right. But I am curious, since you haven't told us yet, what level of precision are you after? I see vertical linearity of the rails, vertical position of the lift, lateral deflection of the column, and rotary positon of the turntable as concerns. Have you actually defined those requirements?

    You're using linear bearings (because you like them and are comfortable with them which is fine), but the structure you are putting them on doesn't appear to have the stiffness, rigidity, and precision normally associated with the use of those bearings.

  2. #22
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    Assume the linearity to be taken are of in the fact that they are precision shafts and I will be using the loosest possible tolerances on my linear bearings, Vertical Positioning precision will be achieved through my VFD controls, and also through limit switches, vertical positioning needs to be accurate to an inch or two. Lateral deflection of the column, I do see a reason to be concerned, neglect it. Rotary positioning: needs to be accurate within 3-4 degrees, spring loaded plunger at base will be the positioned, also, a limit switch will ensure that actuation can only occur in a stationary locked position.

    On the contrary, I think using flat stock that is bolted together achieves higher precision than a structural beam. Also I think with the way the axial load and moment load is applied it will be quite rigid, (the "C" shape will also be secured by cross bars on the way up making it look more like a square tube in some spots)

    Can you guys help me with my machine design calculations? here is what I have:

    900lb load being lifted by a #80 chain (1 inch pitch) Drivin Sprocket on top of structure has a 3.35"OD and 9 teeth, and total stroke length is about 72", I would like to accomplish the lift in about 15 seconds

    running torque is 900*(3.35/2)/12 = 125 ft-lbs
    Desired Running RPM's = (72in/15s)*(1rev/9in)*(60s/1min)= 32RPM's

    Take a standard 3phase Motor with "X" HP @ 1750RPMs and I get a gear ratio of roughly 50:1 that needs to be achieved
    125ft-lbs/50=2.5 ft-lbs of required drive torque... use a FOS of 2 and settle at 5 Ft.-lbs. of drive torque
    now Drive torque for a 2 HP @ 1750 RPMs is To=5250*2/1750=6 Ft.-lbs...

    To achieve the 50:1 I plan on putting a 25 tooth #40 sprocket on the same shaft as the #80 Driven gear, The 25 tooth will then connect with a 10 tooth (2.5:1), the 10 will be on the same shaft as the output of a 20:1 worm gear... the input of the worm gear will C-face mount to the 3 phase motor..

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  4. #24
    Technical Fellow jboggs's Avatar
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    You answered my questions. Go for it.

    One last suggestion:
    "the "C" shape will also be secured by cross bars on the way up making it look more like a square tube in some spots"
    Why not just use a square tube?

  5. #25
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    Quote Originally Posted by wderrick View Post
    Sprocket on top of structure has a 3.35"OD and 9 teeth
    Wes, a couple of things here. Pitch Diameter is more important than OD.

    I would go for as large a tooth count as possible for wear and smoother operation. Can you actually buy a 9-tooth 1" pitch sprocket?

    A 9-tooth sprocket only has 4-1/2 teeth in mesh and at 1" pitch will make for a pretty jerky motion and a lot of exercise on the link-pins (wear??).

  6. #26
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    Quote Originally Posted by jboggs View Post
    Why not just use a square tube?
    Wes has unlimited funds, so why not fabricate a square tube?

  7. #27
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    Quote Originally Posted by jboggs View Post
    You answered my questions. Go for it.

    One last suggestion:
    "the "C" shape will also be secured by cross bars on the way up making it look more like a square tube in some spots"
    Why not just use a square tube?
    I like the C-shape because it gives me a sexy way to mount all my parts. Also I plan on utilizing the inside of the "C" as a chain safety guard.


    Mr. Pinkerton, thanks for the tips about my sprockets, I got part specs of mcmaster: http://www.mcmaster.com/#roller-chain-sprockets/=ie2bhg
    The reason why I chose 9 tooth was because more teeth means a larger diameter sprocket and therefore more torque required to drive my load... perhaps getting hardened teeth could solve my problem? or perhaps a combination of both that and more teeth?

  8. #28
    Lead Engineer RWOLFEJR's Avatar
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    I took a quick cruise through this and have to admit that I skipped over some of the details because the design seemed to be changing faster than I can read and I'm actually pretty busy lately.

    But... As near as I could tell from all this is that your new design will look nothing at all like what you posted in your original post. Given that... maybe a fresh sketch of something closer to where you're gonna go with this would be something you'd like to consider sharing so that others might be able to offer you some possible additional tweaks?

    A little thing that grabbed my attention was the debate over structural vs. weldment etc. and there's nothing says you can't machine a beam or piece of sqaure tubing to make it truely square? Cheaper to square up a beam than to bolt one together from material that will also need to be machined to make it true?

    Have fun...!
    Bob

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    here is a what I've got right now: Assembly chain.PDF ... I imagine its hard to get the full picture... the stuff going on at top is speed reduction using sprockets (50:1)... I have struggled finding a worm system that didn't limit my HP to a point below where I was willing to go, so I abandoned it and went with a break motor instead. I am confident that the system you see will work, but I am not happy with it... it is over budget, lol. I was extremely frustrated doing the power transmission design and calculations... It appears that all worm boxes have extremely low allowed input HP not to mention they are only 80% efficient.

    The three main areas I see that could improve things are:

    1. the structure, I do not understand why you say it would be cheaper to go with tubing? I looked it up on discount steel.com and I can definitely save money by getting bar stock opposed to getting a 8x4 tube and machining it. also, why would I have to machine true my bar stock. I remember being taught in school that flat stock has extremely good tolerances, especially compared to tubing!?

    2. Power transmission, this should probably be #1, BRAKE MOTORS ARE EXPENSIVE!!!!!.... I want to go back to some other form of brake (worm GB) can someone explain to me why worm Gear Boxes have max HP and why is it so low!!!

    3. the linear shafts, in particular those Pillow Block Sleeve Bearings are expensive @ 1.5 diameter. On the flip side I've heard people mention using V rollers... how would those support with a chain?

  10. #30
    Technical Fellow jboggs's Avatar
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    A few random thoughts:
    > Are you absolutely sure that the joint at the bottom of the column can take that moment load? My gut tells me you need stiffeners.
    > So you plan to bolt the whole thing up? No welding? Not sure how that saves money.
    > In my experience, the raw materials involved in a fabrication were always less costly than purchased items and labor. So I've always operated under the guidance that "steel is cheap compared to failure".
    > How are you going to guarantee the alignment of the column with rotary axis of the turntable? A slight misalignment can cause significant "wobbling" at the top as it rotates.
    > Frankly, the whole structure strikes me as inadequate. (You wanted honesty, right?) It may stand up ok if everything is in perfect alignment, but I don't want to be standing near it with a 900 lb load at the top of the stroke when it starts to deflect just a little. Didn't you say in the beginning that you were concerned about forktruck hits?
    > Have you thought about what would happen in the case of failure? You need to insure that the possibility of human injury is well understood and minimized.
    > Is your footprint size limited? I would lean toward a larger turntable if possible.
    > Have you looked at jib crane structures? That's almost exactly what you are building here. They come pre-engineered for larger loads. Could you buy one and modify it? The difference is that they have a fixed center structure with an outer frame that rotates around it. That means the main moment load at the bottom is absorbed by a fixed structure rather than a rotating bearing.
    > How much horsepower are you thinking you need? It shouldn't take much.

    The attached sketch uses 2.5" Osborn Loadrunners in a similar application. Bishop-Wisecarver also has a similar line.
    Attached Images Attached Images

  11. #31
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    Quote Originally Posted by jboggs View Post
    > Are you absolutely sure that the joint at the bottom of the column can take that moment load? My gut tells me you need stiffeners.
    I didn't notice it when I saved the picture, but I must have had those gussets turned off. There are 3 more gussets, two in the front, one in the back.

    Quote Originally Posted by jboggs View Post
    > So you plan to bolt the whole thing up? No welding? Not sure how that saves money.
    I sort of agree, bolting it up is nice, but its expensive, this could be one of the things I change as I try to save some money. If I do that, I will also decrease the size of those huge bar stocks from 1/2" to 1/4" or at least to 3/8 (because I don't need to tap into them anymore...) only problem is that I was planing on using set screws to position my bearings on top, could I just press fit those instead you think?


    Quote Originally Posted by jboggs View Post
    > In my experience, the raw materials involved in a fabrication were always less costly than purchased items and labor. So I've always operated under the guidance that "steel is cheap compared to failure".
    I am not sure if I undrstand, could you explain further because it sounds like good advice.

    Quote Originally Posted by jboggs View Post
    > How are you going to guarantee the alignment of the column with rotary axis of the turntable? A slight misalignment can cause significant "wobbling" at the top as it rotates.
    Valid point. but what choice do I have? The only assurance I can give is that I am aligning the axial load with the axis of the turntable. Yes one option is to go with a second, larger, slewing bearing around the outside of my turntable structure... but I am over budget and it could very well work as is, so I will field fit a backup plan if it is inadequate in this regard.

    Quote Originally Posted by jboggs View Post
    > Frankly, the whole structure strikes me as inadequate. (You wanted honesty, right?) It may stand up ok if everything is in perfect alignment, but I don't want to be standing near it with a 900 lb load at the top of the stroke when it starts to deflect just a little. Didn't you say in the beginning that you were concerned about forktruck hits?
    > Have you thought about what would happen in the case of failure? You need to insure that the possibility of human injury is well understood and minimized.
    > Is your footprint size limited? I would lean toward a larger turntable if possible.
    > Have you looked at jib crane structures? That's almost exactly what you are building here. They come pre-engineered for larger loads. Could you buy one and modify it? The difference is that they have a fixed center structure with an outer frame that rotates around it. That means the main moment load at the bottom is absorbed by a fixed structure rather than a rotating bearing.
    The Turntable is rated for an axial load of 30,000lbs and a moment load of aroundddd 180,000 in-lbs. and I think I calculated my static moment load to be no more than 28,000 in-lbs (probably more like 20,0000... so ya, FOS of at least 6.4 is adequate if you ask me.

    Now that being said, I think the idea of using a mounted base like that of a jib crain sounds pretty damm good, and I did look into that. I went with the turntable because it is easier. Also keep in mind that I have dual shafts, opposed to a jib crane has one arm that goes out. its a lot harder to share an axis of rotation like that. Someone should make mounted bearings with 2 mounts 180 degrees apart! I would be all over that!


    Quote Originally Posted by jboggs View Post
    > How much horsepower are you thinking you need? It shouldn't take much
    This is copy and pasted from earlier:

    900lb load being lifted by a #80 chain (1 inch pitch) Drivin Sprocket on top of structure has a 3.35"OD and 9 teeth, and total stroke length is about 72", I would like to accomplish the lift in about 15 seconds

    running torque is 900*(3.35/2)/12 = 125 ft-lbs
    Desired Running RPM's = (72in/15s)*(1rev/9in)*(60s/1min)= 32RPM's

    Take a standard 3phase Motor with "X" HP @ 1750RPMs and I get a gear ratio of roughly 50:1 that needs to be achieved
    125ft-lbs/50=2.5 ft-lbs of required drive torque... use a FOS of 2 and settle at 5 Ft.-lbs. of drive torque
    now Drive torque for a 2 HP @ 1750 RPMs is To=5250*2/1750=6 Ft.-lbs...

  12. #32
    Lead Engineer RWOLFEJR's Avatar
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    Couple quick things...
    F.Y.I. the part number for the turntable you called out in an earlier post wasn't for the 30,000 lb. gadget... It was a lighter one. Better be sure you're ordering the right thing and take the extra cabbage into account. And yep you're right about rectangular tubing being very expensive... Just bought some for wheelie bars for my tractor cause I wanted to be able to lift the weight of the tractor with them and also wanted them to look nice. 3 x 7 x 3/8 wall... Big $$$ Big overkill...

    I was thinking more like a W Beam... Stout and one of the lowest per lb. things going.

    I'm assuming your figuring on using cold rolled steel bar when you say the material will be super nice. Good luck with that... I've never worked with a piece of cold rolled bar that didn't have some banana to it. Sure you might be able to pull it into position but the easiest way to deal with cold rolled stock is to dust it off.... otherwise you'll be trying to deal with maybe being off by as much as ??? 1/8... maybe 1/4 inch over ten feet due to banana and twist etc. You could get lucky...? Hot rolled bar is actually usually a little straighter but you don't have the sharp edges and not quite as pretty a finish. Regardless... going with what your latest sketch shows I'd use a wide flange I-beam and dust it.

    I was also considering something more like Boggs was thinking. (Dave's gonna like this...) Tubing... Neat stuff...!
    Thinking maybe a ring gear shrunk to the mast and rotate the entire rig...? Maybe whip up a sleeve to run up and down the mast? Key the mast? Maybe drive it like a radial arm drill post?

    Thinking out loud...

    Good Luck!
    Bob

  13. #33
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    Boggs,
    although I am not ready to trash the turntable Idea... Assembly.PDF
    I was intrigued by finally having something to model off of, and I decided to try another prototype... its not done yet, but I am about to call it a day here. Hopefully you can get the picture of what I am doing. The anchored and welded structure is solid round stock 2" diameter. (its actually cheaper to get that than 2"-1/4 tubing). The only think that would be cheaper would be sch40 pipe- can i get away with this? I know how to do the analysis... its knowing how to model the forces thats the trouble... the lower platform is supported by a thrust bearing which you can't see hardly, and is sheathed with a flanged impregnated bronze bushing... the upper platform is the same setup except I have two sheaths, and the thrust bearing is right on top of the 2"stock. When I am done the motor will face mount connect out the side to a worm box that will sit right on top. the output shaft of the worm will go out the back of the structure and the output shaft will be parallel with the driven shaft (connected via sprocket and chain)

    I am also going to make changes to the "turntable" design. Trash current structure and go with a simple 3x3 tube. Trash the homemade GB and somehow resort back to a worm gear.

    As far as the turntable spec being wrong, that was an old one I was originally looking at, I beefed it up to the strongest one which is rated at 30000lb axial

    thanks for all the input guys.

  14. #34
    Technical Fellow jboggs's Avatar
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    You asked for elaboration on "steel is cheap compared to failure". That goes along with my previous statement that in my experience the costs of fabrication labor, machining time, and commercial items usually far outwieghs the cost of the raw steel used in a machine. Related to that, only after a few harrowing experiences does one really learn to strongly consider the possible results of some unexpected failure.

    It does appear to me that your device carries a significant potential for human injury in the event of some failure, either structural or mechanical. That's not a slam on your design. Its a result of the job itself. I cannot emphasize enough the importance of taking steps to prevent such events.

    That's why whenever I encounter a decision related to the cost of a certain structural arrangement I force myself to also consider the risk associated with that same arrangement. If 1/4" plate is good enough, and its failure includes some risk of harm, I'll go for 3/8". If a 6" fillet does the job, I'll go for 8". If the rating for #40 chain is just above my anticipated load, I'll go for #60 or 80. When something you designed fails, you don't want your defense to be that you "saved money" by building it too weak. Picture that conversation with your boss, or worse yet, a jury.

    Bottom line: In the long run, steel is cheap.

  15. #35
    Lead Engineer RWOLFEJR's Avatar
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    Shoot for saving the cabbage by minimizing the number and complexity of the components. That's the key... Strong and safe for small dollars = win. Thing is... as has been said... you can't cheat on the steel.

    2" bar doesn't look like enough to me. (this is without any calculations... just age) I don't get involved much with our purchasing here and I'm not up to date on prices but let's just say you're going to pay about a buck a lb. for your bar... 10.68 lb. per foot... ten foot... $106.80 for the bar. I just gave a yell over the wall to our purchasing guy and he say we're paying about .56 lb. for ERW tube. Thing is we buy truck loads / mill runs. So let's say from a warehouse you're gonna pay... even 1.50 (very high for ERW... should be able to get D.O.M. for that $) But say you have to pay the buck and a half. And let's say you go with 8" x 1/4 wall... Something nice and stout... that's 20.69 lbs. per foot... ten foot... $310.35. So you now have a serious post vs. a twangy bar at an extra cost of $203.55. But in all likelihood your difference in cost will probably only be about a hundred bucks.

    Drop the wall down to .120 & about half the weight and you'll still have a much better post.

    Now I'm assuming that this is equipment for your in house use. Maybe I'm wrong there? If you're developing this gadget intending to market it then yes I can see where you'd need to get worked up over every penny. But for in house use it's a no brainer to over do it and feel good about it's ability to handle the job. So what if the unit costs 5,000 to build insted of 4,000... close enough. But yes if you're gonna market it then you have to trim all the fat you can to make it something people will pay and you can make a buck on.

    Relax and try to forget about it if you can for a day or so. Sometimes I find that walking away from project for a spell can help see things a little differently the next time I look at it. Also no matter what you settle on, there's a good chance after it's done you'll think of things you could've done a little differently. It's always easier to critique somebody's design than to come up with it yourself. Don't take any of this the wrong way. Just offering suggestions to improve it?

    Keep plugging away at it and something will pop to mind that'll give you the warm fuzzy feeling to go with it.

  16. #36
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    my reason for being stingy about going with a bigger diameter has to do with finding bearings that work for me. If I go bigger than 2" I am pretty much left with using crane slew bearings, and I would have to buy 4 @ around 500 each... they are expensive because they are the "4 points of contact" bearings. That being said, whats beautiful about this current design is the fact that it is reinforced the entire way up the beam, at the bottom it is reinforced by gussets and then above that those shafts will help reinforce it. According to my calculations, bending won't occur until well over 1000lbs of well pinpointed concentrated side force and shearing would be almost double that.

  17. #37
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    Quote Originally Posted by wderrick View Post
    bending won't occur until well over 1000lbs
    In my old humble opinion, bending is the least of your worries with that single central bar. Lack of torsional rigidity is going to be alarming with linear bearing-rods wracking and jamming the bearings. Just an opinion, mind you and I have been wrong before.

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