Bolted Cantilever Beam Load 2

[tetwin11]

I want to have what is essentially a cantilever beam bolted to a plate with a single bolt passing through the plate and into a tapped hole in the beam, as crudely depicted in the image. There is to be a downward load at the end of the beam. I am really unsure of how to analyze this as to what loads it will support.
Any suggestions?

http://files.engineering.com/getfil...-a8d3-a00929a72a64&file=bolted_cantilever.jpg

 

[hokie66]

The lever arm is the distance between the bolt centroid and the bottom flange centroid. Then you need to check the plate for bending and the tapped hole for pullout. The whole thing is unstable torsionally unless there is something else preventing rotation.

 

[corus]

Friction prevents rotation, and shear. You'd need to check that there is enough clamping force to prevent shear though. In theory the bolt shouldn't take any load so there also must be sufficient clamping force for the top and bottom of the beam to produce reaction forces to balance the bending moment produced by the load at the end of the beam.

 

[Cockroach]

This is a classical cantilever deflection problem. Obtain the support reaction and moment at the left hand side as the result of load. Then analyse your bolt under shear and normal load as a result of the shear and bending.

You will need to look at the equations for thread shear in order to ascertain the strength of the bolt. For this you will need both the Box and Pin threading geometry. Again, this is a textbook computation and not difficult.

There is lots of website literature to guide you through the calculation. It is not hard at all. You give no detail other than a layout of the problem, the best I can do.

Good luck with it.

Regards,
Cockroach

 

[tetwin11]

Thanks for the replies.
With a fixed end, I can analyze the cantilever beam for deflection/stress/etc and find the reaction at the supported side. However, I am unsure of how this will be affected by preload on the bolt.
I made a sketch of what I'm thinking (in 2D):
Preload will create a distributed pressure on the surface of the beam. The moment will cause shear (not depicted) and a distributed normal force. I'm treating it as if it is one piece because the entire surface should remain in contact.
Does that make sense?

 

[dhengr]

Tetwin11:
You would do well to use pdf or jpg to transmit attachments, many people can’t read other formats, so you eliminate them from the discussion and potentially helping you. I can’t see your second attachment from 17JAN12, because you used png.

You don’t seem to have a very good handle on the relatively simple problems you are asking about. What is your technical experience level? Your being honest with us about your experience level and the full details on your design problems allows us to understand your problem better and to know at what level to start trying to help you. Remember that a CAD program does not make you a designer or and engineer. You would do well to get yourself a few text books on Engineering Mechanics, Strength of Materials, Machine Design, maybe a M.E. design handbook or two, etc., etc. And then do some digging and homework on your own, that will stick with you longer than just coming here for the quickest answer. Maybe it would be a good idea if you took a few first year collage engineering courses on some of these subjects, if you are going to be doing this kind of design regularly.

You will probably not get full joint contact in the joint you showed in your first sketch. You have not located your bolt in the most favorable location, and you may want to consider more than one bolt for stability reasons. You have to show your sketches in more detail so people can understand what you are trying to do, what your beam really is, what supports the bolt head, etc. And, you shouldn’t draw free body diagrams and leave out some of the loads or forces, i.e. shear not shown, is it really a point load, is there a uniform load too? This just confuses you and leads others to think you don’t know what you are doing. Hokie pretty much said everything, but your exact details may cause some slight variations, and you must fill in the details. Talk to your boss or a senior engineer about these problems, they should know what you know and don’t know, if for no other reason than to keep you and the company out of trouble. That local interaction and mentoring is also much more rewarding when you actually find a knowledgeable teacher.

 

[tetwin11]

dhengr, thank you for the feedback. I do have the educational background in ME and am an early career engineer, but do often lack the communication skills required... I am new to the workforce (and actual design work) and new to the forum. Thanks for the feedback.

 

[hokie66]

I could read your sketch, and it seems to show that the preload from the bolt causes uniform precompression. That is a big assumption and depends on a lot of variables. Corus is correct that friction due to the clamping force will resist rotation and sliding, but structural engineers like myself don't count on it.

 

[GregLocock]

More generally the only engineers who think that one bolt is a secure joint are either desperate (bicycles) or electrical, or presumably, in the case of electric bikes, both.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[lisa247]

Hi, the solution to me depends mainly on one factor, the plate that the bolt is attaching to - what material is it made from? What shape and size is it? What is the plate attached to and how is it attached? I would start by figuring out what will fail first, the bolt, the bar, or the plate - or perhaps even the thing that the plate is attached to.....

Lisa

 

[dvd]

Consider a counterbore in the arrangement if you must have a single bolt.

Ask for some help from your colleagues at work, or your boss.

It is better to discuss a design while it is still on paper than when there are real parts being thrown in the scrap bin.

 

[rb1957]

how much load is being applied ? 10 lbs ? 10,000 lbs

will tightening the piece against the wall damage the wall (finish) ?

are you limited to a single pt attachment ? does this have a special design consideration ?? it can work, but most people will recognise it for what it is ... a cheap, "crappy" joint.

several fasteners in shear would be the structurally preferred design.

 

[imcjoek]

several fasteners in shear would be the structurally preferred design.

I'm not of the structural persuasion and I may be about to question semantics, but I'd like some clarification anyway.

Strictly speaking, wouldn't the preferred design be fasteners in tension, with the shear taken by the friction between mating surfaces?

 

[rb1957]

nah, "we" hate to use friction for shear loadpath ... too variable.

and what's wrong with split infinitives ? "to boldly go ..."

would you rather "several fasteners in shear would be the preferred design, from the structural perspective" ... 'cause there could be design limitations that drive the single fastener approach.

 

[lisa247]

If there was more than one fastener in this case they would no longer be acting in shear, there would be a heel and toe effect putting one or more of the fasteners in tension and a point load pushing against the plate, or whatever its attaching to. There isnt enough information for us to be able to give anymore answers!

 

[imcjoek]

@rb

You may infinitely split your infinitives where you please.

I was more questioning the practice of allowing and/or wanting the threaded pointy side of the fastener to contact the surfaces of the clearance hole. I had always presumed that to be bad form, IE: flywheels often (but not always) employ dowel pins of some kind.

So you're saying the typical gusset plate one may observe joining members of a truss bridge has the bolts loaded in shear? On the surface, this strikes may as a bit scary, as who's to say all the clearance holes have the same exact diameter, and therefor that all the bolts share the shear evenly? I thought the idea of the giant plate and three dozen bolts was to insure sufficient clamping such that the members cannot slide relative one another.

I'm not suggesting what you say is wrong. Just probing for other opinions so I (or somebody else reading this) may learn something.

 

[rb1957]

i agree a shear spigot would be a good way to carry the shear.

my business it's more about shooting rivets, so you get hole fill and effective shear transfer. or interference fit bolts. bolts in clearance holes, yes, you're relying on the friction of the clamped faces ... untill something slips. or the preload relaxes.

"the practice of allowing and/or wanting the threaded pointy side of the fastener to contact the surfaces of the clearance hole" ... i think this is pretty universally recognised as bad, or Bad, practice ... it can work but it won't work well. "fasteners in shear" would bear on the shank, not the thread.