Calculating a moment from a circular bolt pattern
Calculating a moment from a circular bolt pattern
(OP)
I am designing a large structure that rotates on a large bearing. In order to order a bearing I have to have the horizontal loads, vertical loads and a moment. I have been designing trusses and transfer towers for conveyors, and so normally getting these loads would be straight forward; but as expected the bolt pattern in circular, and this has me wondering how to do this. I have the horizontal loads, and the axial loads. I have the vertical reactions at the bolt pattern, but how do I us the reactions in determining a moment? These reactions are positive and negative around a neutral axis, and so I know there is a moment in this bearing. My first approach was to calculate the moment based on the individual bolt reactions x their distance (horizontal not radial) from the neutral axis. Is this correct? I thought this would work since I would consider the compression/tension of a square structure the same way, and use that to determine overturning, and other stability issues as well as sizing the anchor bolts. Does this work for a circular bolt pattern as well? The bearing is 15' in diameter, and has 48 evenly spaced bolts. Can someone point me in the correct direction to figure this out?
Thanks
Thanks






RE: Calculating a moment from a circular bolt pattern
the typical appraoch is to assume bolt load is proportional to distance from the neutral axis. you could also assume that one bolt is in compression and the rest are in tension, as though the bolt group is pivoting about the one bolt, possibly two bolts if they align wiht the axis of the moment.
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
"Design of Monopole Base Plates"
Calculate the bolt group moment of inertia and determine bolt loads. I will upload and repose this document if you cannot find it.
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
What large structure, how large, what loads? Don't keep so many secrets, you're asking for help and ideas, but want us to waste our time guessing at what you're talking about. Is this a building, is it a big piece of mobile equip., these kinds of questions influence anything I would say about it. As a first shot, I would find the worst moment induced by your structure above, considering wind, earthquake, variable DL & LL's, etc., multiplied by a 1.5 load factor, and your bearing system must support at least this moment. Any righting moments which would lessen the overturning moment should be limited to DL which CAN NOT be changed. Now on a 15' bolt circle with 48 bolts, assume 4 or 5, maybe a few more, bolts are resisting the tension all equally with a lever arm of 14.75'; all just to get a feel for the bolt size and magnitude of the bolt forces. You can not design a 15' thrust bearing stiff enough in the way it reacts a moment to really consider all the bolts loaded in proportion to their distance from the center. Most of your stiffness must be built into the structure above.
On this design problem, I don't think I would be thinking about bolt tension on a thrust bearing, as for example on a large crane or backhoe. I would be looking for a big enough radius track and roller system so that what I was talking about was a max. compressive load of P and a min. compressive load of .25P or some such vs. the 1.5(overturning moment) less any right moment. I just read your last post and now have an idea what you're talking about. I would not likely consider bolt tension in normal operation, only in short term maintenance situations, but maybe not theneither. I'm gona post this and you chew on it, try this approach, and post back. And I'll think about your last post a bit more.
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
A sketch or drawing would be worth a 1000 words, but have a look at this site for designing a monopole base it might help:-
http:/
RE: Calculating a moment from a circular bolt pattern
I assume you're putin-me-on with that description. I do know what a stacker/reclaimer is, and of course, depending on who you work for or who made this particular one, there are several variations on the theme. I'm usually a stickler for more info., a better description, and sketches, and usually recite the old saw, 'a sketch is work a thousand words.' But, your description turns this expression on its ear. Your description must be based a thousand drawings. I'll reread your last post after I pick my mind up off the floor and get it back in place, but obviously a general arrangement drawing and a few of the assembly drawings at the different levels would be very helpful in deciphering your description and the load path. Knowing the magnitude of the loads, type of equip., etc. is helpful in laying some of the groundwork, but I suspect you have painted quite a different picture than most thought they were chatting about. I've worked around stacker/reclaimers, never actually designed one, but have done a lot of design on that type of equip. Let me try to digest your last post, and you reread mine, and think on it a bit, I think you are asking the wrong question, and over complicating the problem, which admittedly is a complicated piece of machinery. Who do you buy these bearings from? You don't pick them out of a std. thrust brg. catalog. In general what does this detail look like on other machines of your own design, a someplace to start from? In fact, this thinking is probably what has you feeling you musta shot a bull someplace in your reaction and moment calcs., yes?
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
That's about what I expected after rereading your post; and yes they all serve the same function, but my 'variations on the theme' meant design detail variations btwn. different manuf's. And, yes you should be careful what info. you put out into the internet ether. That's what makes this forum such a difficult means of exchange on these kinds of problems, because the details you and I should be able to point at and sketching over should be held fairly close to your vest.
I'm not sure I would, or that you can, find the moment on that 15' dia. thrust brg. by looking at some bolt reactions from above. I would do gross overturning and righting moment calcs. While I can't see what you are looking at, those bolt loads may be worst case loads from a number of different or other conditions, not directly indicating the moment you are looking for. Furthermore, you are probably not really looking for a moment, as we typically think of them, you might be better to give the brg. people (13000ft.-kips/15ft.) = 867 kips, up on one side and down on the other side of the brg. and applied over a 4 or 6' arc length. This arc length will be a function of your structure arrangement and stiffness just above the outer race and will most likely align with the axis of the reclaimer boom. Obviously, there can be secondary moments from wind or EQ, maybe additive to the above and possibly perpendicular to the boom axis. But as a first shot, assuming the 4 or 6' arc len. above, the 867k is only really applied to 6 or 8 bolts in that arc length. Bolts nearer the N.A. are loaded much less by this load, or are there to take the cross moment. But, of course, all bolts must be able to take this load as this reclaiming level slews. This is obviously a very indeterminate problem, but hone in on it, in steps. Won't those bearings come to you in 6 pieces, 6 or 8' arc lengths which dove tail together to make the whole brg. That kinda matches with my above thinking, maybe one arc length has to take this entire moment reaction.
RE: Calculating a moment from a circular bolt pattern
BA
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
BA
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
Half the bolts (24) taking most of the load (that's 12 in back) sounds about right, and half of those (that's 6 or 8, by gully that's what I said abv.) sounds about right too. I'd size those 6 or 8 to take the full load at yield. The 18" N.A. shift doesn't surprise me either, without your programs, I guess that's just some sorta Mc/I +/- (-P/A) shifting the N.A. Again, the brg. people will want to know what your structure is above the outer race, (a 15' dia. ring beam?) because you provide the system stiffness, they can't. What percentage of your moments and load are DL & LL? How do you account for vibration and fatigue? Is there a industry design std., or what stds. do you design to? Where are you located?
If your boss (or another senior engineer) is a good practical design engineer, you should involve them in these questions. There is no reason for a young engineer to feel embarrassed by these kinds of questions when you've never seen them before, he should understand this. He knows your equip. much better than any of us do, and can give you much better, and to the point, guidance than we can. Ask him to be your mentor, he knows your operation far more than we do, history, std. details and ways of handling various conditions. He certainly has an interest in your getting it right, and will want you to do well. It's much better if you can both be looking at the same plans, specs., suppliers drawings, etc., and be able to draw sketches, etc. as your questions come up. This can really be a rewarding working and learning relationship. With your apparent enthusiasm and general understanding of a mighty complex structure, I can't imagine that he wouldn't want to help you and work more closely with you. He wants you to do as much as you can on your own, but at the same time he doesn't want you getting in trouble for lack of an intelligent question or two.
Idrive... a simple line drawing free body diagram, with loads and a few dimensions would meet BA's needs, I suspect.
BA... There are probably 20 or 30 assembly drawings and sub-assembly drawings of that monster at that particular level, along with several arrangement drawing and piece details. Basically, it's a 20x30 or 40' box structure, or space frame, about 6 or 8' high; with a 200' conveyor/loader boom hinged to one of the 20' sides. The whole thing rotates on a 15' thrust bearing.
RE: Calculating a moment from a circular bolt pattern
If it is on levelling nuts and there are a good number of bolts (at least 8) then you can model the ring of bolts as an equivalent ring.
Then using stress =M/s and P=stressxarea you get the result that P = 4M/ND.
This may also be a good starting point if you do have a grout bed.
RE: Calculating a moment from a circular bolt pattern
Here is a sheet showing the reactions at the pinned boundary conditions. The axis is 7.5degrees off of the page centerline, but I have marked them so it should be easy to see how the reactions bear out what we have been discussing. Normally our company does conveyors and transfer towers. They are all square, pretty easy, and straight forward, and I have been doing them for four years (however, I was hired here 4 months ago.) This is the first time the company has moved into this type of thing. The company took a look at what the market offered, and thought we could come up with many improvements. That's when this hit my desk. Our senior engineer is retiring, and is spending less and less time in the office, but of course I will get with him and go over this. The only other structural engineer was out of the office last week, and so I figured I would put it out to a place that I have been reading and following for some time. Figured I would get some outside views, ideas, opinions, and perspectives from a different angle than what was available to me right at this moment. I have, and appreciate all that has been put forth. I have sized the bolts based on the few that are most heavily loaded. Since the machine has a rotating requirement of 270 degrees I am going to design all the bolts based on the highest tension/shear shown in the calculations since most will eventually see those forces. The included picture shows only the tension reactions at the nodes. I was able to get the information to the bearing manufacturer using your suggestion of giving them the reactions that the connection would be subject to. Hindsight being what it is I should have done that in the first place, since they know more about their bearings than I do, and should know the capacity of their design. We were able to order the bearing and it was only slightly bigger than my original estimation. Thanks for your help. I will now know where to start and what to figure out first when we go to expand this to a larger conveyor set up. Cheers
RE: Calculating a moment from a circular bolt pattern
The base will be on both leveling nuts and 4 inches of non-shrink grout. This is really the only way we can provide a flat enough surface for the bearing. Thanks for your help.
RE: Calculating a moment from a circular bolt pattern
RE: Calculating a moment from a circular bolt pattern
I don't understand the bolt force values either (in kips?), and I'm not sure we are talking about exactly the same thing. I understand the 7.5° or 12" bolt spacing, but not the 7.5° offset, I don't really understand the two different kinds of bolts and exactly how they interrelate, or why are they are so close together to need the offset. And, I certainly don't understand the wild variation in bolt forces, from one bolt to the next as you move around the circle. Assuming the back is at 12:00 noon; I would expect the bolts from 11 to 1:00 to take 60% of the tension from the boom moment, and those from 10 to 11 & 1 to 2:00 to take another 20%, give or take. In the front your structure just sees an arc shaped bearing force (compression from the ring beam) and the values are symmetrical about the boom axis. Their brg. sees the concentrated roller loads too. Then I'd superimpose the DL righting moment, and at this point the bolt forces should be pretty symmetrical; and finally I'd superimpose any lateral load moment you want to find a max. bolt force. I would expect that max. will happen at only two approx. symmetrical places on your bolt circle to the top race, but can happen anyplace within the 270° swing arc on the lower race. I think your sketch shows all of this, at once, and I can't quite digest that so easily, in one step. I want to see that each of these makes common sense, then I'd superimpose them. Furthermore, I want to see that any variations in your structural system stiffness and the ring beam stiffness above the upper race is not radically influencing the bolt forces or loads on the brg., that will not be good for their brg. That ring beam really has to be almost as rigid as your conc. pedestal and grouting. If the upper structures stiffness is significantly influencing bolt forces you may want to study and adjust that.
Try to get that senior engineer to continue to work with you for a while, and not just on this project, but rather, as you learn the ropes. Maybe part time, maybe on call, etc. Even to the point that you get your boss to let you buy him lunch or a beer periodically so you can learn from him and pick is brain on company history and methods. Obviously, that depends on what he has to offer, since you're all just starting out on this equip. That can be a wonderful interaction and quite helpful to you.
RE: Calculating a moment from a circular bolt pattern
Perhaps you believe that you have explained your problem in a way that everyone and his dog can understand. Well I, for one haven't a clue what the problem is.
When I look at your diagram, I see that every second dot in the upper area of your diagram has a circle drawn around it. Are we supposed to know what that means? I do not.
Starting at the top center, the dot has 30.6 (no circle). Moving clockwise, we have 126 (circled), 29.4 (no circle), 121.3 (circled) and so on.
What do the numbers and circles represent and why the hell haven't you explained in before now?
I believe that the issue you are attempting to solve is relatively simple but I also believe that you owe this forum a proper explanation of your problem. So how about starting now?
BA
RE: Calculating a moment from a circular bolt pattern