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Calculating a moment from a circular bolt pattern
3

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

RE: Calculating a moment from a circular bolt pattern

i guess the moment is in-plane, putting tension and compression loads on the bolts ?

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

(OP)
Yes the moment is the in-plane. The reactions I have roughly 75% of the bolts in compression and the rest in tension. A neutral axis can easily be identified. Someone here suggested that I simply use the bolts with the highest tension and compression, set them at the diameter of the bolt group, and calculate the moment. I don't think that would be correct since it would not take into account the rest of the reactions ni the bearing.

RE: Calculating a moment from a circular bolt pattern

Google or search in this forum for

"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

i'd probably neglect the moment the bearing could react too.  btw, when i said "bolt load is proportional to distance from the NA" i was thinking of a bolt pattern with one bolt size.  if you have a mixture of bolt diameters, then it is better to say "plane sections reamin plane", so that bolt extension (=strain? =stress) is proportional to distance.

RE: Calculating a moment from a circular bolt pattern

(OP)
I was able to find it. Thanks a lot. Looks like I have some reading to do.

RE: Calculating a moment from a circular bolt pattern

(OP)
I believe what the bearing people want is the moment that the bearing will be exposed to. All bolts will be the same size though.

RE: Calculating a moment from a circular bolt pattern

if your structure rotates on the bearing, doesn't the bearing see the applied loads ?  [i thought that the bearing was an alternate loadpath, so neglecting it is conservative]

RE: Calculating a moment from a circular bolt pattern

(OP)
Well, this structure does ride on the inner race of the bearing with all of the torque being applied to the outside race and into the concrete foundation as the structure slews. The moment that the bearing is exposed to is the force of the structure above in an unbalanced condition. The structure has a 200 ft boom that reclaims product from a pile that the stacker boom above it has previously made. The structure is balanced, but under some conditions the structure can become unbalanced. Example; during high winds, seismic events, or the boom needing to be removed for any reason. If the boom is removed then some of the counterweight is removed as well, but enough weight remains to produce a moment in the bearing. I have the reactions that the bolts connecting the structure to the bearing would see as if connected directly to the foundation. I figured this would show me what point forces would be translated from the structure through the bearing, and then to the bolts. Once I have the moment in the bearing I can get the bearing ordered, and then use the same forces to design the anchorage of the structure.

RE: Calculating a moment from a circular bolt pattern

The moment of inertia of the bolt pattern is the sum of the distances from the neutral axis squared. Then the stress in an individual bolt is + or -Mc/I. Then add the axial force per bolt.  

RE: Calculating a moment from a circular bolt pattern

(OP)
Thanks for all your help.  

RE: Calculating a moment from a circular bolt pattern

The distances are perpendicular distances from the NA, as you mentioned. Otherwise you are calculating the polar moment of interia of the bolt group using radial distances.

RE: Calculating a moment from a circular bolt pattern

Idrivetrains:
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

(OP)
Sorry, I suppose more information would help. This is a stacker/reclaimer. The feed conveyor comes in at the top of this machine at 80' above grade. A stacker boom with a 116' boom is at 65' and counter balanced to 95% of DL and material load. The stacker booms sits on a bearing to allow it to slew and luff in order to control the stacking of the material which is wood waste destined for a boiler. The bearing is connected to a torque arm that sits at 45' and functions as a separator between the stacking assembly and the reclaiming assembly. Just the below the torque arm is another bearing connected to a platform (about 35' from grade) holding the winch and mast that controls the elevation of the reclaim boom. This bearing isolates any torque forces from translating from the stacker boom to the reclaim boom. However all vertical and shear forces are translated down through the central tube structure that supports everything from this platform to the bottom of the stacker boom's tower. The platform is then supported by a large plate work weldment about (20') that serves as the structure holding the reclaim boom. It is 6' deep about 12'wide and about 60' long with about 27' behind the centerline of the entire structure. The boom then extends out 203' from a connection support hanging below the weldment that allows the boom to scrape material from the pile, up a ramp and into chutework within the supporting weldment. This chutework contains the motors and drive assemblies for the reclaim boom. The material then drops through chutework in the top of the foundation pedestal (at 11' above grade) and onto a conveyor that sends it to another part of the power plant. The weldment platform supporting the reclaim boom is sitting on its bearing on top of this foundation pedestal. The loads I am seeing are in the neighborhood of 13000 k-ft of moment at the base. 400 kips of shear, and 1300 kips of vertical load. I am checking the moment because our bearing size choice is too small for this application. However, I believe I have mis-calculated the moment, and thus my question. Other machine structures of this nature are getting about the same vertical and horizontal loads, but their moment is nearly 1/2 of the amount I am getting. I am in the process of checking loads, their placement, and a whole other list of items to see if I have missed something or possibly increased a load that should not have been. I do appreciate the assistance because it has allowed me to look at this from another direction. Sorry about the grammer and punctuation, but I am in a crunch on another project that I am working on at the same time. Cheers

RE: Calculating a moment from a circular bolt pattern

Idrivetrains:
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

(OP)
I have a model, and I will try and get a picture of it uploaded here so everyone can at least see it, and maybe follow my rambling description. I will see what I can get done. I may have more time to figure it out later tonight. We are working with Rotek and Kaydon. I picked one of the Kaydon bearings out of their catalog for a preliminary design size and capacity. Stacker/reclaimers are new to me, but when given a project I always barrel right into it. This has been no different. I will think about what has been said, it has been helpful. Maybe I am asking the wrong question. It would not be the first time. BTW Most people can't get me to talk very much, but I seem to go overboard when it comes to writing. Thanks again for the help.

RE: Calculating a moment from a circular bolt pattern

(OP)
I don't know if this will work, but here is a couple of pictures of the model to give an idea of what it looks like. I have put them into a word document, but again I am not even sure this will work. I can't get a better view since I am sure my boss would not appreciate me sending out details of his new machine. I think these have enough detail to give a visual to my description, but not enough to get me into hot water. Nearly all stacker/reclaimers work on the same principles though, and there isn't too many variations on this theme. Hope this helps. I may be overthinking this.

RE: Calculating a moment from a circular bolt pattern

Idrivetrains:
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

Why don't you try to present your problem in a clear, concise way we can all understand?  A simple line drawing with forces would be helpful and much easier to understand than all the verbiage which has been presented to date.

BA

RE: Calculating a moment from a circular bolt pattern

(OP)
The reactions at the boundary conditions I set up, bear out what you have said. The "bolt" reactions alternate between really high and really low loads with of course compression at the front and tension at the back. Only about half of the bolts are taking the majority of the loads, and the half at the front and back are taking the greatest amount of load. What I also found is that the neutral axis is about 18" behind the geometric center of the circle. I ran the DL case and then multiplied the reactions of the bolts by their distance from the neutral axis, added everything together and got roughly 13500+- k-ft. I found the original moment by putting the entire structure on a single node with all its DOF fixed. When I ran the DL case again the moment reaction was 13700+- k-ft. I figure, since they were close to each other I am on the right track. At this point I have only run unfactored cases to check may balance, and to come up with the forces to give to the bearing people. I have also run single live, wind and seismic unfactored load cases in order to get Not to Exceed loads and send them to the guys designing the foundations. I figured I would get this question answered before I started running the LRFD loads cases to finish the design process. Your explanation really seems reasonable, and I think you right about the delivery of the bearings, although I have not had experience with that yet. It's good I have the weekend to absorb this before getting back to work. I really appreciate your help.

RE: Calculating a moment from a circular bolt pattern

Duh!

BA

RE: Calculating a moment from a circular bolt pattern

Just my two cents, but I think you should be summing moments about the geometric center of the bolt group to get the net moment.  This is no different than taking a cut in a member to find internal forces (the forces always act at, or about the centroid of the section).  The fact that you have  line of zero bolt force that doesn't coincide with the geometric center means that you have axial load in combination with moment.  I would analogize to building a P-M curve for a concrete column section.  You assume different strain patterns and determine what the axial loads and moments are at those assumed strain conditions.  When you do that, however, you are always taking moments about the centroid of the section, not the neutral axis depth.

RE: Calculating a moment from a circular bolt pattern

Idrivetrains:
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

It really depends if it is on a grout bed or on levelling nuts.

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

(OP)
dhengr,
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

(OP)
csd72,

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

why to the reactions alternate so much ?

RE: Calculating a moment from a circular bolt pattern

Idrivetrains:
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

Idrivetrains,

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

ditto

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