bolt clamping load

[jklingel]

I THINK that is the term I am after. I need to locate a table that will tell me, basically, how much load a certain grade/thread pitch/diameter bolt will take before it fails, assuming it is properly torqued, etc. I don't care how it fails, I just need to know how much it will take before some kind of failure occurs. Specifically, I am interested in what a grade 8, 1/2" NF thread bolt will carry compared to a 9/16" bolt, same quality. I have searched, and am not positive I have found what I need, and the engineering sites/forums give me a headache. One table I found gives me a clamp load of 18,250 lbs for the 9/16 bolt, but I don't know if I understand clamp load correctly. Is clamp load the same as "rip it apart load"? Thanks. j

 

[russlaferrera]

Try www.k-tbolt.com/bolt_chart.html K-T bolt company

 

[dxdexter]

Bolt data taken from my "Statics and Strength of Materials" textbook.

1/2"- 20 UNF minor diameter = 0.4387"
9/16"-18 UNF minor diameter = 0.4943"
Grade 8 minimum tensile strength = 150,000 psi

Since : Stress = Force/area

Area of 1/2" UNF = 3.14159 x (0.4387/2)^2 = 0.15116 in^2
Area of 9/16" UNF = 3.14159 x (0.4943/2)^2 = 0.191898 in^2

Therefore minimum tensile load should be:
Force = 150,000 x 0.15116 = 22,673 lbs for 1/2"-20 UNF
Force = 150,000 x 0.191898 = 28,784 lbs for 9/16"-18 UNF

 

[dxdexter]

I just found a table for you, it seems like the data is the same as my calculations. The table also show both tensile and shear forces for Grade 5 and Grade 8 bolts.

Link: http://www.rockcrawler.com/techreports/fasteners/index.asp

 

[jklingel]

Dx, et al: Thanks for the info, and even crunching the numbers for me. My question now is this. Will the bolt fail due to the tensile strength of the material (shank failure?), or will the threads fail first (thread deformation/shear?)? I.e, which is the weaker link in the failure process? I took a stab at the proper nomenclature for those failures, btw. I have never had a bolt fail, unless I have had a rusty one that snapped the bolt in the minor diameter while twisting it off. However, I don't know if that is because of corrosion or if shank failure is the weak link. Thanks again. john

 

[dxdexter]

Dx, et al: Thanks for the info, and even crunching the numbers for me. My question now is this. Will the bolt fail due to the tensile strength of the material (shank failure?), or will the threads fail first (thread deformation/shear?)? I.e, which is the weaker link in the failure process? I took a stab at the proper nomenclature for those failures, btw. I have never had a bolt fail, unless I have had a rusty one that snapped the bolt in the minor diameter while twisting it off. However, I don't know if that is because of corrosion or if shank failure is the weak link. Thanks again. john

This is not an easy question to answer since there are many variables. You would need to know the tensile strength of the material threaded into and the number of threads engaged and then do some ciphering.

When using a nut we generally look for a minimum of two threads showing above the nut so there is no question between the installer and inspector weather there is 100% thread engagement. This is the criteria we use when inspecting bolted connection in structural steel construction (in Canada). The nut should also be the same grade as the bolt.

Generally speaking when properly designed, a bolted connection should fail in tension across the bolts minor diameter rather than stripping the threads.

 

[jklingel]

Dex: Thanks once again. A last question is this: Is it easy to clarify for a novice the issue of tensile strength (22K, as you give) vs clamping load (14K, from a table)? Is the clamping load the minimum load needed before a bolt "feels" an added load, and then the tensile strength is the load above which permanent deformation will occur? (If I understand what I have been reading, a properly torqued/lubed/designed bolt will not "feel" any additional load until said load exceeds the internal force that tightening the bolt exerts on the bolt shank. Reality?) In other words, tightening a bolt properly pre-loads it, but that pre-load is nowhere near failure load. Correct? Too complicated to get into on a forum? Thanks again. john

 

[dxdexter]

Dex: Thanks once again. A last question is this: Is it easy to clarify for a novice the issue of tensile strength (22K, as you give) vs clamping load (14K, from a table)? Is the clamping load the minimum load needed before a bolt "feels" an added load, and then the tensile strength is the load above which permanent deformation will occur? (If I understand what I have been reading, a properly torqued/lubed/designed bolt will not "feel" any additional load until said load exceeds the internal force that tightening the bolt exerts on the bolt shank. Reality?) In other words, tightening a bolt properly pre-loads it, but that pre-load is nowhere near failure load. Correct? Too complicated to get into on a forum? Thanks again. john

I can only speak to the installation of bolted connections with regard to structural steel and not those in machinery, but the concept is similar.

Simply put: bolts are normally pre-tensioned (tightened) to 70% of their specified minimum tensile strength by what is referred to as the "turn of nut" method. The bolt is brought to a "snug tight condition" in which the plies are brought into contact. The nut is further tightened a predetermined number of degrees to achieve the pre-load. In order for a bolt to carry any additional stress (force per unit area) then the forces within the bolt would need to exceed the pre-load.

A fastener will have an elastic range and a plastic range. The fastener will return to its original dimensions if stresses are within the elastic range.Once it goes beyond the elastic limit and enters the plastic range ,permanent deformation, or set, will occur. Within the plastic range the material will begin to yield and then reach its ultimate stress before the breaking point is reached.

Any more technical than that is beyond my pay grade.

 

[jklingel]

Excellent. I'll see about getting you a pay raise. j

 

[jklingel]

(Pardon me if this shows up twice. Wrote in, forgot to log in, logged in, and it appeared to have evaporated. Chitina. Again, then....) Not to whip a dead horse, but to make sure that what I am reading here and elsewhere is getting into the calcium impregnated, once-cartilagenous spheroid on my shoulders. Take two identical, 10 lb tensile strength, perfect bolts, and use each to clamp two plates together. Bolt A is loose, while B has a 7 lb preload. When a 6 lb static load is applied to each, A will feel it all, while B will feel little to none. At 9.9 lbs, both will feel the same load, and at 10.1 lbs, each will snap. OK so far? Now get new bolts, same setup, and apply a dynamic 6 lb load to each, having the load cycle between 6 and 0 lbs. Bolt B will not feel much, but bolt A may just fail because of the repetitive stress/relaxation cycles. True? This makes sense to me, anyway. NOW, what happens when a dynamic, on/off load of 8 lbs is applied to each? I assume that each will start hurting, but that A will again fail much sooner than B. Finally, I promise to not ask any more bolt/load questions. "Trust me. I have a hammer." Thanks. j

 

[MXtras]

Here - look at this:

http://www.futek.com/boltcalc.aspx

Excellent posts, DXDEXTER. Good info and well composed. Kudos to you, my Canadian friend! You canuks might not know how to read E-bay shipping information, but you sure know your nuts!

Scott

 

[jklingel]

MX: Played w/ the torque/load calc table; excellent. Thanks. j

 

[jklingel]

Futex calc'r is screwy

On second notice... I got looking at the info that the Futex bolt torque calculator gives out, and I can not trust it. The results are very inconsistent and therefore unreliable. EX: A 1/2" bolt, in NC and NF, returned recommended torques of 1.x and 13.x in-lbs. Etc. Bummer. j

 

[dxdexter]

Take two identical, 10 lb tensile strength, perfect bolts, and use each to clamp two plates together. Bolt A is loose, while B has a 7 lb preload. When a 6 lb static load is applied to each, A will feel it all, while B will feel little to none. At 9.9 lbs, both will feel the same load, and at 10.1 lbs, each will snap. OK so far? Now get new bolts, same setup, and apply a dynamic 6 lb load to each, having the load cycle between 6 and 0 lbs. Bolt B will not feel much, but bolt A may just fail because of the repetitive stress/relaxation cycles. True? This makes sense to me, anyway. NOW, what happens when a dynamic, on/off load of 8 lbs is applied to each? I assume that each will start hurting, but that A will again fail much sooner than B. Finally, I promise to not ask any more bolt/load questions. "Trust me. I have a hammer." Thanks. j

If you were to apply a dynamic load to a fastener, which exceeds that of the pre-load, then the joint would fail long before the fastener would fatigue.

Lets take an example of a cylinder head. The bolts apply a clamping force to the head and gasket. In order to overcome the clamping force applied by the fasteners, the cylinder pressures would need to lift the head from the block and thereby blowing the gasket.

Just curious, what exactly are you trying to figure out? If you present the exact problem I may be able to help.

As a side note: using torque to estimate tension within a bolt is an inexact method. It does not take into account friction within the fastener, be it within the threads or at the mating surfaces. Using a torque wrench is merely a low cost "convenient" method.

 

[jklingel]

Just curious, what exactly are you trying to figure out? If you present the exact problem I may be able to help.

I am trying to understand why a pre-loaded bolt will take more static load than a loose one, if it does. I understand how the pre-load prevents a bolt from "feeling" sub-pre-load forces, and how repetitive on/off loads will stress (and fail) a loose bolt before it will do so to a pre-loaded one. But, I don't see how the pre-load effects the static load-carrying capacity of the bolt. Does it?
Thanks for hanging in there w/ me, too! I'll get it sooner or later.... j

 

[dxdexter]

I am trying to understand why a pre-loaded bolt will take more static load than a loose one, if it does. I understand how the pre-load prevents a bolt from "feeling" sub-pre-load forces, and how repetitive on/off loads will stress (and fail) a loose bolt before it will do so to a pre-loaded one. But, I don't see how the pre-load effects the static load-carrying capacity of the bolt. Does it?
Thanks for hanging in there w/ me, too! I'll get it sooner or later.... j

Are you referring to shear or tensile load situations?

I'll have to do a little research of my texts but my first instinct is to say the fastener will fail at the same tensile load weather it is preloaded or loose assuming you apply a single steady load until failure. Cyclic loading is however a different story. A bolt with no pre-load will fail sooner due to increase fatigue under repeated loading and unloading. A pre-tensioned bolt will receive less variation in cyclic load and therefore less fatigue and longer life.

If you are referring to shear resistance due to static loading of a connection, then the connection will take a higher static shear load if the bolts are pretensioned mainly due to the fact that you are adding the friction present between the members due to the bolt clamping forces plus the shear resistance of the fastener.

A connection with both shear and tensile loadings will combine the explanations above.

I hope this helps because I'm not sure where to go from here.

 

[jklingel]

Dx: http://www.garagejournal.com/forum/images/smilies/bounce.gif EXACTLY what I was looking for. Thanks. This all makes perfect sense, now that I have read about the importance or pre-loading. Yes, I was referring to tensile load, but thanks for adding the info on shear as well. Now I am a 100% knowledgeable bolt engineer, and will seek employment thereof.