Max Shear Stress of Bolts 2

[Gatorman]

I need to know what the max shear stress of a 3/8-16 x 1 UNC Grade 2 bolts. I have 8, 4 per side, used to lift 2500 pound machine. Is there a online calculator or a formula to figure this out?

 

[rmw]

Fasteners are not generally intended to be used in shear to transmit load, although they can up to their stress limit.

Fasteners are designed to attach by clamping forces, a lifting attachment to the part being lifted with enough force to enable the friction between the lifting attachment and the part to carry the load.

This was demonstrated in ME lab, and is one of the few lessons I actually remember from the old school days.

If it is necessary to lift this load with the bolts in shear, then I hope you find your answer. If not, check the friction loads generated by the clamping forces, and the bolt torques necessary to accomplish this, without over stretching the bolts.

Also, if it is absolutely necessary to do this with the bolts only, why not consider a higher grade bolt? Why not do this in either case?

rmw

 

[eg9arh]

to calculate the shear load of the bolt, you simply multiply the dia of the bolt by the length of engagement into the material (be it a housing or a nut). This gives you the area of the thread under load. If a bolt (or screw) is to shear, it will fail across the thread.
Just use this area in the general equation:
in your case, just assume the load is acting across the cross section of the root of the thread, and devide the load by that area.
The max shear loads can be found in the back of most bolt/screw producers catalogues.

Hope this helps

 

[Gatorman]

I am useing carrage bolts to bolt D-Rings on the sides of an air compressor. I have 2 D-rings per side and 2 3/8-16 x 1 carrage bolts per D-ring. The total weight of the machine is 2500 pounds. What grade do you think I should use? A Grade 2, 5, or 8?

 

[eg9arh]

I dont think your going to have much of a problem, the stress the bolts are under is really low, go up a grade to 5 if you want to be realy safe, but i doubt the bolts are going shear through. Could you not secure a double strap under the compressor as a fail safe?

 

[Gatorman]

I do not know about that. I just wanted to know if there was a chart or calculation to run that would back me up when quality control checks the machine and tells me that I am wrong in having those bolts. Thank you for your help!

 

[CoryPad]

Gatorman,

There are three potential failure modes: tear-out, bearing yield, and fastener fracture. [blue]eg9arh[/blue] gave you one (bearing yield). You should perform a full analysis of the joint. An available reference is the NASA Fastener Design Manual, available at:

http://gltrs.grc.nasa.gov/citations/all/e-4911.html

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SuperStress]

I need to disagree with rmw when he says, "Fasteners are not generally intended to be used in shear to transmit load..."

Most joints in aircraft construction are shear joints because they are more structurally efficient than tension joints. Fasteners perform just fine in shear applications.

There also seems to be some confusion over whether you are talking about bolt shear (loaded transverse to the axis of the bolt) or thread shear (loaded parallel to the axis of the bolt).

8-3/8" Grade 2 bolts should be more than capable of lifting 2500 lbs. in any orientation, but the selection of bolts may depend on the consequences of the failure. Are there people that are endangered in any way if this compressor falls? Are there building codes or other regulatory requirements that specify a particular factor of safety for these installations?

Without seeing the actual geometry of the attachments, it's impossible to know if there are any eccentricities or other misalignments which will result in higher fastener loads.

SuperStress

 

[SuperStress]

Having thought about my last post, I should qualify my statement about loading fasteners in shear. I am specifically referring to using gripped fasteners.

It is true that you generally don't want to load any fastener in shear through the threads unless you can help it.

SuperStress

 

[rmw]

Gatorman,

How does your "D" ring attach to the compressor. If the "D" ring mounting has a base plate through which the bolts pass, the bolts serve the purpose of clamping the base plate of the "D" ring to the compressor. Then the friction forces govern, and they will greatly exceed the shear strength of the bolts, assuming the base plate has enough surface area.

If in fact, you do have the "D" ring attached in such a way that the bolts are in true shear then your concern is valid, and the answers given above pertain to your situation, not that I like all of them.

I would not walk under any load that depended solely on bolt shear strength to support it. It us just something I carried away from the ME lab that day. We clamped it, we could not overcome the friction. We loosened the bolts so that the bolts were only in shear, and we snapped it right off with ease. Seeing is believing.

Also, how do you know if someone previously stressed the bolts beyond their elastic limit on the last lift, or how do you know one of the bolts isn't a counterfeit from XXXX. Both these concerns would also be valid for the same fasteners used in a clamping situation, but one would have to overcome the friction of the clamp and break all the bolts at once to drop the load. Not that that type of thing doesn't happen.

Now, I must also say that any answer I give is in the realm of knowing absolutley nothing about aircraft construction except to hope that it was done right upon getting into one of the things. My experience is with machinery, and often heavy machinery. And, I have seen some awful big stuff dropped.

rmw

 

[enginebob]

Put it this way. I have a chain hoist, from some manufacture. It has one 3/8 bolt in shear, holding the chain to the hook. This hoist is rated at 1/2 ton. 1000lbs.
With your 2500 lbs load, each set of 4 bolts will be carrying 1250 lbs. I'd say you will be very safe. Just make sure what ever sling you use is not slung too close, as that will greatly increase the stress.

 

[enginebob]

Forgot to mention. Grade 8

 

[jmw]

There is a calculator at

http://www.globalspec.com.

I don't know if this answers your question though. Goto

http://www.globalspec.com/resources

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SuperStress]

rmw,

It's interesting how different things are in different industries.

In aerospace, not only are most of our joints in shear (think fuselage lap splices), but bond strengths and friction are routinely ignored in any joint that also has redundant fasteners.

Boeing design practice is that the fasteners have to be able to take the full design load (to ultimate) without any contribution from the bond.

Then again, I've designed a few parts for aircraft that have bolts that cost $531 apiece (1" diameter, 6" grip length, Inconel 220 ksi). Not exactly the kind of thing you see in tooling applications.

SuperStress

 

[jetmaker]

SuperStress,

One important thing you forgot to mention about aircraft joints is that most, if not all, of the shear loaded joints use fasteners in high quality holes with good fits.

jetmaker

 

[rmw]

Superstress,

Is what you are referring to like a lapped skin joint on a aircraft outer skin, using rivets or such for attachment??

rmw

 

[SuperStress]

rmw,

Yes, outer skin lap joints are riveted. Much of the rest of the structure is as well. Wing and fuselage stringers and frames are attached to skins with rivets or Hi-loks in shear joints, floor beams attach to the side-of-body with shear joints, and so on. In fact, I'm a lot more hard-pressed to come up with examples of tension joints in airplane structure.

Jetmaker is correct as well; high quality holes with tight tolerances are used virtually everywhere, and fastener quality is extremely high (strength, dimensional stability, runout, etc.).

You hardly ever see a fastener on an airplane with a strength of less than 125 ksi, and the standard for most primary structure is 160 ksi tension/95 ksi shear. As I posted before, many of the bolts in critical structure are titanium (for lighter weight) or Inconel (for higher temperature performance and strength).

SuperStress

 

[rmw]

SuperStress,

That helps with something that has been residing in my 'hmmm-box' for years, now. (That is the place in your brain where, when you see something that does not jive with what you think you know or understand, you think, 'hmmm' and plant it back to be dealt with in the future).

I already related the ME lab where we learned about the advantages of using friction in a tension joint situation to get much higher strength than just the shear stress of the bolt(s) alone, but as I have boarded many (way too many) airliners, and saw the rivited joints, I filed a 'hmmm' away in my 'hmmmm-box'.

So, my question is, along with the shear stresses on the rivets or Hi-loks, do they produce any tension, or if they do, is that used in the joint strength calculation?

In my world, I still see, from time to time, and when I began in the engineering world with my slide rule, I used to still see a lot of boiler drums and pressure vessels of rivited construction. I drive by an old rivited construction receiver tank at a natural gas metering station near my office every single day of the week.

While this method is no longer in use to any extent that I know of, at least, I did understand that the cooling of the red hot rivet after it was installed and peened into place, tensioned the joint at that point, and that was as good as a bolt tensioned to a particular stress by stretching.

So, it fit within the parameters of the lesson learned in the ME lab, and, other than the antiquity of it, does not go into my 'hmmm' box. (Notwithstanding the fact that caulking riveted joints was also a common maintenance practice with such vessels.)

I do make the observation that if that tank were to be replaced with a new modern welded tank, one weld seam that would barely be visible from the roadway where I pass, some 20-30 ft away, would replace a whole lot of rivets used to join the lapped ends of the current pressure vessel shell.

I guess, as part of dealing with the 'hmmm' situation in my mind, I assumed that there was some combination of factors, like the need to maintain the lapped skin joints of an aircraft tightly together that prevented the use of threaded fasteners in tension on much greater centers than the riveted joints. I also assumed that in the manufacturing process, a joint of a given length could be riveted in much more rapid fashion than with a worker twisting a wrench to tighten a bunch of screws.

Am I any where close to the mark, or do I have to repeat Aerospace 101? I appreciate your coming back to me and enlightening me on a topic that has been a curosity for me for a while.

You must remember, that in my paradigm, I deal with Boilers and Turbines that operate at just under 4000 PSIG, and as you can imagine, in the case of the turbine, for example, the bolts that clamp the two turbine shell halfs together are massive. Sometimes several inches in diameter, 4-6-8", and are tensioned to discrete stretch values, accomplished by heating (much like the rivet situation) or torqueing (sp?). In the old days, we knew the thread pitch, and calculated how far the nut had to twist once it had begun to tension to get the amount of stretch needed. Today, they use fanciful hydraulic torque wrenches. We used to use slugging wrenches, one guy operating the sledge hammer, and another one standing on the wrench to keep it from jumping off as it was struck with the sledge hammer. We have come a long way baby!!!

Many a time, in the old days, I had the task of mounting a dial indicator on a 6" bolt (stud) and measure the elongation to verify the tensioning, whatever the method used.

Leaving such a job, and getting aboard an aircraft with riveted construction really stirred up the 'hmmm-box' I have found this discourse very interesting.

I guess I am going to have to get the 'star' button hot to show my gratitude.

I still would not walk under the aforementioned engine being supported by one 3/8" bolt of any grade. But, that is just me.

rmw

 

[SuperStress]

rmw,

Thanks for the star! Back at ya!

In airplane lap joints, the rivets produce no appreciable tension, and friction in the joint is completely ignored. Rivets are never allowed in tension joints; the only tension they see are from secondary loading. Some older planes (early 737s) used a cold-bonding process in addition to the fasteners, but there were problems with disbonds and subsequent fatigue failures along the laps (e.g. Aloha incident in 1988).

Modern lap design utilizes sealant in the lap to maintain the integrity of the pressure vessel, but relies entirely on the fasteners for the required strength. In fact, if the rivets are over-bucked, they can squeeze out the sealant creating leaks. Most longitudinal laps on Boeing planes are 3 rows deep, which also helps minimize any secondary rivet tension (tension-bearing couple) caused by the eccentricity of the joint. Joints are designed so that the lower rows in the outer skins are critical in fatigue, in order to facilitate inspection (you don't have to tear out the interior to see the upper row, inside skin).

In order to minimize weight, specific shear fasteners have also been developed (lower head height and shorter threads) when rivets can't provide adequate strength. These are used in thicker joints, usually starting around .080". I'm sure that doesn't sound very thick to you ME-types, but realize that the external skins on many commercial transports are more like .040" in the chem-milled pockets (away from fastener rows).

Similar in principle to the hot-installed rivets you mentioned, tension bolts on airplanes are torqued to around 90% of their yield strength to prevent bolt fatigue.

The pressure inside the airplane cabin is around 8-9 psid, with the usual design goal of maintaining an 8000 ft. cabin at cruising altitude. Quite a difference from the 4000 psig boilers you are used to!

SuperStress

 

[rmw]

SuperStress,

Thanks for that.

You know, as an engineer with turbine experience, and specifically gas turbine experience, since one particularily scarey ride on an unmentioned airline (the flight was made while leaving a gas turbine training session in upstate New York, so you can guess the airline) where I could hear things in the engines that I did not want to hear, I have always instructed my travel agents to never seat me anywhere near the engines, wherever they might be. I just did not want to know. I took note of the Pensacola incident where many of the compressor blades ended up in the passenger cabin.

One of my (then) co-workers demanded off a flight in Los Angeles once many, many years ago, (only to be met at the gate by the FAA, the CAB, and who knows who else, wanting to know why he wanted off, to which he explained that he saw something in the engine start up that he did not like.) That flight, an overseas flight, landed in Hawaii for refueling, and when it did, something went wrong, and it put the nose gear up into the passenger cabin (747) and killed a passenger. Engine related?? Neither he, nor I know, but he sure was glad he wasn't still on that flight later that day. (We usually used our FF miles to upgrade to first class when possible. The last thing we wanted to use them for was to travel somewhere else.)

So, I solved my engine fobias by taking the cowards way out, but you have given me another area (I did take great note of the Aloha incident, since I flew, and still fly a lot of 737's) to obsess on.

Just joking!!! I always figured if I survived the auto ride to the airport, the dangerous part of the trip was over. The air travel system in the world is a tribute to the integrity and expertise of your (and my-the turbine part) industry.

Thanks for helping me understand your process better.

rmw