Calculating a moment from a circular bolt pattern 3

[Idrivetrains]

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

 

[rb1957]

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.

 

[Idrivetrains]

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.

 

[LSPSCAT]

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.

 

[rb1957]

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.

 

[Idrivetrains]

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

 

[Idrivetrains]

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.

 

[rb1957]

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]

 

[Idrivetrains]

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.

 

[Splitrings]

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.

 

[Idrivetrains]

Thanks for all your help.

 

[Splitrings]

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.

 

[dhengr]

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.

 

[Idrivetrains]

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

 

[desertfox]

Hi idrivetrains

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://www.risatech.com/downloads/risatower/Technical_Manual_1.pdf

 

[dhengr]

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?

 

[Idrivetrains]

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.

 

[Idrivetrains]

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.

 

[dhengr]

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.

 

[BAretired]

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