A little guidance in regards to the strength of this assembly.

[andrewkeisler]

Well this is a structural question, but as I scanned through the structural forum, I noticed mainly building related questions and answers. I figured I would give the mechanical forum a shot!

My question is in regards to the picture attached to my post. The bolt that secures the shaft to the plate is made of a much stronger material then the shaft itself. Shear load is applied to the radius area of the shaft. Is it safe to assume the shaft is as strong with the bolt fastened into it as the plate and shaft made as one solid piece?

If the bolt is stronger then the material it is replacing, I don't see it being a weaker design...

I don't have a degree in engineering so please excuse my ignorance and confusion.

Thanks, Andrew

 

[Tmoose]

The screw head configuration limits the torque that could be applied to it, and the resulting clamping force.

if the loading is cyclic in nature:
- to avoid fatigue the clamping force needs to be greater than prying force resulting from applied load.

http://session.masteringphysics.com/problemAsset/1008098/4/yf_Figure_11_30.jpg

- the clamping force needs to be great enough to prevent motion of the interface

 

[andrewkeisler]

The shaft and plate are made of 7075-t6 aluminum. The bolt is made of alloy steel (160psi min.) I assume the 7075-t6 threads would limit the amount of torque far before the screw head configuration. Is that safe to assume?

I also have configured 4 smaller bolts into a circular pattern around the larger bolt in the center. I didn't show them in the picture because I didn't think they would be relevant.

Thanks, Andrew

 

[rb1957]

not sure i get the question ... i don't see the interaction between the strength of the bolts and the strength of the shaft ?

the shaft has to carry the loads to the reactions (the bolts). the bolts have to carry these loads into the plate (supporting everything). if there's a single bolt, you'd expect the off-set moment to come out as a couple between the edge of the shaft flange and the bolt (in tension). if there's a pattern of bolts, then the base for the couple will be bigger (and the loads smaller). so the critical bolt is carrying tension and shear. i'd make sure the the holes in the shaft (for the bolts) weren't tapped at the interface ... so you really want to have a shear spigot on the CL (close tolerance hole) to carry the shear and tapped (clearance) holes to the bolts reacting tension.

clear as mud ?

does the shaft sit in a recess on the plate ?

Quando Omni Flunkus Moritati

 

[izax1]

If this is a pure shearing load, you can do a quick analysis to find the weakest point. The shear stresses will be a function of Shear modulus (G) times the Shear area. Although the bolt will have a higher Shear modulus due to stiffer material, it will have a smaller area. So much for the quick analysis. But you would also have to consider SCF due to the radius in the shaft. I would advise you to discuss the problem with someone with adequate background.

 

[andrewkeisler]

My question is in regards to the strength of the shaft around the area of the radius. Is the shaft weaker with the bolt fastened as the same diagram as one solid piece? I'm wondering if you take a weaker material, drill and thread it, throw a stronger material inside of it, does it make it stronger, weaker, doesn't really change, or too many variables to give a solid answer? Obviously, in my situation, the stronger material doesn't go all the way through the shaft, so I could picture the scenario working a bit different then one that does, especially in different loading types (ie bending).

A bearing slides on the shaft up to the point of the radius, that is why I assume the raduis area would be the point of shear.

The other 4 bolts are 1/4 20's. They thread into the shaft 3/8 of an inch via a 90 degree 1.125 inch diameter degree bolt pattern.

The shaft presses into the plate around .2 to .25 of an inch.

To give you guys a scale, the large bolt in the center is 1/2 inches in diameter...


Thanks for the answers guys! I'm trying to be as clear as possible, but sometimes it's a bit hard to type it out! lol

 

[prex]

If your worries are about shear strength, then for sure the weakest section (in the largest diameter part) is at the bottom of the bolt and is of course the section of the shaft with a hole. So there is no contribution of the bolt to the shear strength.

prex
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[LittleInch]

I think it's very difficult to say. I'm still not totally clear what forces the bolt is experiencing and from where. If you could show the final condition with the bearing and where the loads are on that bearing we might get a better idea. Having a much stronger bolt will mean that your bolt shouldn't fail, but it could easily mean that loads become more concentrated elsewhere on the plate or shaft that otherwise they would not be. Also the potential exists to strip the treads on the shaft or pull the bolt through the sheet unless you are very careful with the torque values. Although good for load distribution, the fact that the shaft is recessed slighly into the plate makes it more difficult to see where the failure might be.

If the plate and shaft was one piece then you be able to model it easier and add some more re-inforcement on the shoulder / taper it in. I can't see any fundamental flaws in this design, but where it would fail is not clear until you run some numbers and do checks on buckling and shear in all the components. If vibration or shock loading are involved then it gets much more complex...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rb1957]

having a strong bolt only means that the critical section as far as bolt loading is concerned will be the threads in the shaft; i don't think (like prex) that the bolt material affects the shear strength of the shaft ... the holes drilled in it do. of course there are other things to consider (like thermal effects). primary would be which holes are clearence (and so ineffective in shear) and which close fit; i'd suggest a shear spigot (an untapped dowel) and a pattern of screws (for tension loads).

does the shaft sit in a recess ?

Quando Omni Flunkus Moritati

 

[chicopee]

Show us, in your sketch, the shear force acting on the shaft.

 

[BrianE22]

If you cut off the section and do a free body analysis, some of the shear will be taken up by the aluminum and some by the bolt. The difficulty is figuring out how it is distributed between the two. The shear modulus of the bolt is higher than the shear modulus of the aluminum so my guess (just a guess) is that the bolt will handle a significant percentage of the shear. That means you're not losing much in shear strength.

Another consideration though, is that the bolt, in tension, will see combined shear and tension loading. The shaft will see combined shear and compression loading. You should look at the magnitudes of the stresses from each of those loads (applied shear and bolt preload) and evaluate the combined stresses for both the bolt and the shaft.