maximum stability angle 3

[Bert2]

HI,

I'm trying to calculate the maximum stability angle for a set of paired trailers with a load and COG above them.

from what i can rem you form a triangle from the COG of the unit down to the veritcal COG of the trailers then..??

is it a midd point on one of the trailers ???

ive attached a sketch to help explain.

does anyone know of a quick calc to get the angle?

im not interested in the ratio of satbility just the angle.

regards.

 

[desertfox]

hi Bert2

You need to draw a force triangle of the mass vertically down and the load acting at right angles to the mass, then the reaction R which must pass through the C of G of the component mounted on the trailers.
I have uploaded a sketch which might help.

desertfox

 

[Bert2]

Thanks

desertfox great help.!

 

[desertfox]

hi Bert2

Your welcome,the only thing to point out is that the angle and the magnitude will vary with the horizontal load through the C of G.
the horizontal load through the C of G is usually determined for a given velocity of trailer going round a curve ie the inertia force m*v^2/r

m= mass

v= velocity

r= radius of curvature of bend

desertfox

 

[Bert2]

after studying the froce triangle,

this is a fine solution if you know the force 'P'

however i dont so should i assume it ?

if the load is 215Te @ 0.1g corner force ?

 

[desertfox]

hi Bert2

To estimate the horizontal force you could take moments about the outer wheel ie mass*g*5160/2 = P*5144 transpose for P.
Alternatively if you know the maximum velocity the trailer is likely to see and the tightest raduis of curvature it might go round you can work it out from the inertia force.
see this file below go to page 10.

desertfox

 

[Bert2]

thanks desertfox im going through the clacs now really need to look at my old college notes.

once again many thanks.

 

[desertfox]

hi Bert2

Your welcome

 

[dhengr]

Bert2:
You shouldn’t be asking this kind of question the way you have, on this forum, when you seem to have so little understanding of the problem that you wouldn’t know if you were getting good advice or not. Free advice may only be worth what you pay for it, and it can get you in trouble if you don’t know what you are doing. You must understand the problem, not just run some numbers through a formula somebody gives you, which you don’t seem to understand; and I doubt that your college notes will help much either. Mostly, you need a mentor at the place you work, your boss, a senior engineer or some such. And, you shouldn’t be embarrassed to ask for help from them, because you could get in a bunch of trouble with the way you’re going at this. I can assure you that the manufacturer of that cylindrical pressure vessel would be very nervous about the people handling their product if they heard these kinds of questions and interactions about and pertaining to the movement of their product.

With a 215 ton vessel, its C.G. at 16.88' above grade, on mating saddles which bear on bolsters with spherical center plates and constant contact side bearings; which finally bear on side by side trailers, I wonder if you don’t actually have four trailers in this move. Maybe they are just using some existing bolsters and don’t need the articulation of the center plates. And, I suspect you are not actually dealing with a centripetal force problem. They just don’t move these loads at speeds which would make this an issue, or I’d like to be there with my video camera to record the carnage. But, that’s the formula you have been given and are trying to plug some numbers into, and seem oblivious to the fact that you may be going about this the wrong way.

You need to get with your superior to better define the problem, most bosses want to be helpful with their junior employees, if for no other reason than to keep you from getting them in trouble. Then you need to get with the trailer manufacturer and/or the trailer owner/operator, and ask them what the trailers can do and what they can handle. The more likely reason for the question you were asked is that someone wants to know if they can run this system on a road with some side slope, to be determined, by you. Obviously, you could roll the load over, to the low side, but more likely you will start overloading the low-side trailer and its tires and this will dictate the controlling angle of the resultant of the weight. This resultant in the force triangle is all weight (not mass) with a negligible centripetal component. And, on fairly rigid trailers it should fall well inboard of the center of the lower trailer. However, your sketch indicates trailers which can be adjusted vertically with their hydraulic suspension, and that is how the trailer operator will handle this problem. They will adjust the suspension so that the decks of the trailers stay in the same horiz. plane, and your original question goes away.

You are doing serious engineering now, so get with it and find a good advisor/mentor. A majority vote (team work) on a bunch of bad advice doesn’t result in a good engineering solution. Desertfox usually gives darn good fundamental advice with good sketches and examples. But here, I suspect he read your question quickly and thought ‘that’s the old standard centrifugal force on a curve problem’ we had in our first dynamics class.

 

[desertfox]

Hi Bert2,dhengr

dhengr your right I did take it as a centrifgal force on a curve problem, however as you rightly point out, the post doesn't define any horizontal force, which I assumed that the OP knew, but hadn't included it in his question and I didn't give any clue how he could calculate it, I just gave a force diagram.
Its only when the horizontal force was not known I gave two options on how to find it in my third post, the first being taking moments about the outer wheel which would actually cover the trailer if it was on a slope, or alternatively if it was a centrifugal force problem on a curve.
If it is a slope problem then I quickly did this calculation assuming that the trailer's are connected rigidly and no sliding takes place.(see attached file)
I am not so sure that the OP is a junior engineer, I think an older engineer just a bit rusty if its something one hasn't calculated in a while.
Anyway the answer I get is just over 26.5 degrees to the point of no return.

desertfox

 

[Bert2]

@ dhengr ;

i understand your point of view but i think you have taking this a little too seriously as this is all theoretical work im doing just to brushing up on the engineering for it, and is has no physical form present or future.

i actual found my old engineering file on the trailers themselves and found a solution to the intial question,
however you also have to consider the hydraulic layout for the trailer and as you correctly pointed out there is adjustibility in the trailers however i neglected this and was just looking at the problem statically.

Therefore you should stop jumping to conculsions that aren't what you think considering you didnt have the full details...true that i didnt give them all but i didnt need to as this is theoretical.

Next time ill stop and think if i even want to post a question here if that is the reply im going to get.

@ desertfox

again great advise and notes which i read and are identical to my old college notes which i was just getting a better understanding of the physics around this subject on a whole.

my end solution is very similar to you last pdf.

many thanks.

 

[desertfox]

hi Bert2

Your welcome, don't be put off posting, sometimes we get people asking for advice and we just try to take care that people don't run off with "a little knowledge is a dangerous thing" so to speak.
So I believe dhengr's post was in the best interest of safety.

regards

desertfox

 

[electricpete]

@ desertfox again great advise

There is also a button "Thank desertfox for this valuable post" which you can use to acknowledge valuable contributions.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[dhengr]

Bert2:
I get my exercise by “jumping to conclusions” warranted by OP’ers who don’t provide “full details” or misleading sketches, “too seriously.” My original post was in the ‘interest of safety’ but also because I didn’t think you asked the right question or were being offered the right answers. And, in good part, that was your fault. I didn’t mean to hurt your feelings, but I most certainly did mean to get your attention and make you think a bit more. The sketch you provided would lead many engineers to assume you were talking about some serious, real world, engineering. If you intended what you now are suggesting, just a simple question about rigid body rollover, your sketch should have looked like desertfox’s sketch with his 26FEB, 5:12 post, not the one you provided; or your question should have clarified that your actual sketch was to be used for dimensions only, and that all else about it was to be assumed to be one big rigid body, with infinitely strong tires. Otherwise, how would we be expected to know that. Then, if you study desertfox’s sketch and calcs. for a minute and see that the weight cancels out of the equation, and if your sketch is to scale, you might come to the conclusion that you could get your angle approximation by drawing the resultant through the C.G. and your tipping point (desertfox assumed the edge of the lowest tire) and measure the angle right on the drawing. 5144 vertical, 2580 lateral, what’s the angle? But, that is not the answer to the real world problem that your sketch shows. Desertfox says “trailers are connected rigidly” and “26.5 degrees to the point of no return.” And, there is a whole bunch of real serious engineering and a trailer operating manual to explain those two statements and where the load resultant can actually fall on the trailer deck.

All we have to go by is what you give us in your question and your sketch, which most people fail to provide. I find many of the questions on these forums to be lacking in sufficient information and detail for a meaningful response, so I don’t mean to be picking on you alone. No one is trying to discourage you from asking questions. As I said earlier, I would (your boss would) much sooner have you ask than do it wrong and hurt someone or something; thus desertfox’s “a little knowledge is a dangerous thing.” When you frame a question, you must step back and ask yourself; if I knew nothing about my problem or what I needed to know (as is the case with the people who will be responding here) have I given them enough info. so they will understand all the important details, and they can’t misinterpret what I have asked or what I really want.

The reason a local advisor/mentor is so important is that they are looking at the same drawings and info. you are, and can ask questions at the moment, and draw you the sketch desertfox finally did, to send you off in the right direction, or to assure you that you’re going in the right direction. Here, you ended up about 10 posts later, before desertfox provided you with the right sketch and simple calc. for what you really wanted. And that is through no fault of our own. Actually, he did give you all you should have needed in his first two posts, if you really read the full meaning of the posts. I think the confusion came up because of the way the question was asked and the sketch that was provided with the question; and the fact that you were struggling with determining ‘P’. This would quickly be cleared up when you saw the expression on your advisor/mentor’s face with the original question. I suspect desortfox has done some serious mentoring over the years, he gives darn good advice, and gets right to the point, once he knows what the question is. I’ll give him a star. And, that’s why the way you ask the question is so important. There are a lot of smart people here, all willing to help once they understand the problem and what you really want, otherwise we’re just sending electrons back and forth.

 

[LSPSCAT]

As an aside, I have found that at many places we lack truly qualified, knowledgeable elders to seek advice. I have worked in both the defense and construction/engineering industries and I have yet to really work under someone who had such a great working knowledge of engineering that I felt comfortable relying on them for advice. There were very good practical people with lots of hands-on knowledge in rigging/concrete formwork/iron-work, however, when it came to true-design work and calculations I feel like the engineering was lacking.

So, my only thoughts from the above digression is that some of us really may not have anywhere else to turn. I have spent weekends researching and analyzing work problems to check, double check, and triple check my thought processes in just these situations. I have sought advice and recieved very cursory responses..."oh yeah that looks good". I have never had a manager really go through my calculations to truly understand what I am doing. Some of there approvals may come from experience, but not all, you can definetely tell when your manager is in over his or her head. This is most noticable if you ever ask about solid models and FEA. Maybe I am in a unique position, however, I doubt it. As I said I have worked for some well known companies and have been extremely dissapointed at the quality of leadership and older engineers. I wish it was different, but for the most part I feel as if I am on my own. I am older now so I feel confident enough through past design work things have always worked out, but it would be nice to engage in some true face-to-face, meaningful, engineering discussions.

 

[desertfox]

Hi dhengr

Thanks for your kind comments

desertfox

 

[Bert2]

LSPSCAT ,

Sounds similar to my situation.

 

[drawoh]

Bert2,

I asked a similar question to this in the Structural Engineering forum, thread507-255357. The responses may be useful to you.

At some point in your analysis, you may have to inform your boss that you are not qualified to do it, and that it is time to bring in a consultant. It may be the math. It may be the practical understanding of what will go wrong in the field.

JHG

 

[dhengr]

LSPSCAT & Bert2:
I am not suggesting you should not ask here, but I’m saying that it is a sad state of affairs if this is the only place you can go with all your questions and for your guidance. The “face-to-face, meaningful, engineering discussions” and interactions, as you put it, can be very rewarding for both partes in the exchange. And, I am sorry for you fellows and gals if you have not had this experience and type of guidance. Us older fellows and you younger guys have a lot to offer each other, and these forums are one means to do that, but in many respects they are a poor substitute for that more intimate interaction with someone you trust and respect. There aren’t many, real, dumb questions, but you have to have studied the problem deeply enough so as not to ask a dumb question in a dumb way. You should not feel dumb or embarrassed for not knowing the answer to a problem you have never seen before and your advisor has faced a hundred times. But, you should know enough to know how to frame the problem and your questions in a meaningful way so you don’t look silly. If this does happen on occasion, we have a quick exchange (a laugh) and move on, because we’ve come to understand each other. Of course, your own self study is important, it’s part of the game if you care about your profession, we have all had to do that along the way. Certainly the fact that you spend weekends researching and analyzing work problems speaks highly of your interest in bettering yourself and your care for your profession.

Your post and experience are certainly frustrating and disappointing, really a sad commentary if it is universally true. Maybe some companies with an established product line may not be doing much new design and engineering, just doing the same-old, same-old, as long as it continues to sell. Riggers and contractors don’t do real much complicated design and analysis, but have a wealth of experience if they’ve been in the business very long. Maybe some of these people have become bit complacent, from the engineering standpoint, because their particular business relies so heavily on experience and then some seat-of-the-pants engineering. Furthermore, the liability belongs to the company, so they may care a bit less. Maybe the fact that they trusted you to do the right engineering calcs. should be taken as a feather-in-your-cap.

In my office we do engineering for these types of clients and it’s my _it in the wringer if we do something wrong, so we try to do real thorough engineering. Check your calcs., you bet I would on the first few jobs with each different building material, or when I knew you were facing a situation new to you. Then, I would come to trust your judgement, and know that you weren’t bashful about asking when you weren’t sure. If you brought me an 18" WF, when my more experienced gut told me it should be a 24" WF, I’d tell you to go back and look at it again, without a bunch of calcs. on my part. And, at first you’d be amazed, just as I was, how did he know that, and why didn’t I see that? I have had mostly good experiences on both side of the mentoring relationship, but a few bummers too.

Look to professional associations, and join a few; ASCE, ASME come to mind. Look to materials or product specific organizations or associations and their literature, AISC, ACI, ASM, AWS for example. Your company might even pay for some of these. Most of the more experienced people at any of these places are more than willing to share their knowledge and experience. Look to engineers who work for companies other than your own if there is such a dearth of knowledge and experience within your own company. Talk to your old college profs. for help or for ideas of people in your area who might work with and help you. It costs you lunch and a beer once in a while, and may even lead to a more meaningful job, because you’re showing more interest and initiative than most. Maybe you should look for a job with a consulting engineering, structural engineering, office if you want design work. Most of the senior people there are kinda forced to stay current and relevant if they want to continue to practice.

As for FEA, there’s not much doubt that I can’t keep up with you, in fact that’s where I come to you for the same kind of help I have given you in other areas, in fact that’s why I hired you. I actually taught a couple courses in computer applications in structural engineering in about 1966-68, we punched our own cards and carried them over to the main frame computers. I even have several FEA text books from that era. But, I’ve also never had a building fail because I used a slide rule to do the design at that point in time. I know what I want modeled, I may have a better feel for structures at this stage in our development than you do, and can probably give you some ideas on that account. I can tell you where I think we want a tighter mesh, etc., but you know far better than I do, how to actually do it. I just can’t kept up, because I don’t use it often enough, so I’m not nearly as proficient as I assume your are. In that respect, I may be “your manager in over his head.” Just as I should be patient with you, you should be with me, and this is possible if there is a good mentoring exchange going on.

 

[LSPSCAT]

dhengr,

I appreciate your insight and thank you for your in-depth discussion. I have enjoyed being on my own and also hated it. I certainly do think the loss of good senior mentors and professionals will eventually hurt this profession in the United States. FEA is not the answer either, I have used it succesfully as backstop/7th member of the team but most of my questions get back to theory and how things have evolved.
Some of my frustation also comes from the quality of the current textbooks as compared to some older texts. I can pick up texts from Timoshenko on Structual Analysis/Strength of Materials and it is all right there. Calculations fully worked out, differential equations solved, and for the most part they are completely worked out with even some friendly banter added in as discussion. My favorite, now somewhat dated, Blodgett's "Design of Welded Structures" was almost a converation on design rather than a strict textbook. He explained what the thinking was and how he got there mathmatically. Take a textbook from the last 20 years...can you find anything like this? People joke that we do not understand the fundamentals as well as we should, has it occured to them that the older textbooks engrained the math in the text rather than giving excerpts and then the answer...with no math on how they got there?! I am finishing my graduate work now and really enjoy engineering and learning and hope to develop a more positive attitude towards the "system".

I think I partially question the education system and also the economy/business for some of what goes on.