Understanding Bolt Capacity
Understanding Bolt Capacity
(OP)
I need to hang an overhead fixture that weighs approxiamtely 500lbs. My company currently uses 4 Ø1/2-13 UNC grade 8 bolts for this purpose. I came up with a torque spec from the Machinery's handbook of 128ft-lbs to torque the bolts down. I am wondering how much capacity is left for the bolt to support a load when you consider the preload you're applying. The preload is 15325lbs for the 128ft-lbs torque. The proof strength is 120000psi so for a nominal Ø of 1/2" would it be 120000 X .19625in^2(cross sectional area of .5in bolt) = 23550 (bolt capacity) - 15325 (preload) = 8225lbs(capacity of bolt remaining)? So then that bolt, torqued to recommended spec of 128ft-lbs can then support a load of 8225lbs? If anyone can explain I would be greatly appreciative.
Thanks
Thanks





RE: Understanding Bolt Capacity
This has confused the heck out of me too.
The best way to understand this is to look for bolt strain. If your fixture remains clamped to whatever it is mounted to, the bolts see only the initial tension force. If you load your fixture to the point where it separates from its mount face, then the bolts stretch (strain), and the bolt tension increases above your initial tension.
Go back and read up on bolts in your machine design textbooks. It is counterintuitive, but not complicated.
This is good design practise. A loosely tightened bolt will stretch under load, and it will experience metal fatigue. A properly tightened bolt sees a constant force.
RE: Understanding Bolt Capacity
The following statement was presented as lesson number one when it come to bolting.
"If a screw or bolt is preloaded (tightened) beyond the working load encountered, changing stresses will not affect it"
RE: Understanding Bolt Capacity
If I understand the posts from drawoh and unclesyd correctly, the preload established a level which working load must EXCEED before the bolt is affected. Am I on target?
I should caveat this by asking if this thread is only referring to tension loads on the bolt. At least that is what I am gleaning from it.
Thanks!
RE: Understanding Bolt Capacity
RE: Understanding Bolt Capacity
http://www.boltscience.com/pages/basics2.htm
RE: Understanding Bolt Capacity
If we preload the bolts to 100 lbs, then hang a load of 20 lbs, the tension in the bolt is still 100 lbs? Howe about increase the load from 20 lbs to exact 100 lbs, now what is the tension in the bolt?
RE: Understanding Bolt Capacity
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RE: Understanding Bolt Capacity
RE: Understanding Bolt Capacity
RE: Understanding Bolt Capacity
If two plates are bolted together the preload is obtained by compressing the two plates and elongating the bolt (the change in dimension of each will obviously depend on the modulus of the materials used). When a load is then applied, adding additional tension in the bolt, the bolt does elongate. However, with the bolt increasing in length, the plates are not compressed to the same degree. Therefore, the tension the bolt received from compression of the plates is less than that received during preload.
So an additional force must always (even if less than preload) increase the stress in the bolt (because it will elongate), but as it lengthens the force from the plates compressing decreases, so it does not "feel" the entire load.
Does that make sense? Am I on the right track?
-- MechEng2005
RE: Understanding Bolt Capacity
RE: Understanding Bolt Capacity
I am playing with a model flange in CosmosWorks. The flange is 100mm OD by 12mm thick, and it is aluminium 6061-T6. I have attached it with six M4X0.7 A2 stainless steel screws torqued down to 3000N each. I assumed that the screws contacted a 9mm diameter. I then created a second model, identical, except that there is a 15000N force on it.
The opposite face is fixed in all directions. By rights, it ought to be fixed only in axial compression. I put 9mm pads 0.05mm high on the top face for the screws to act on.
Flange with M4 screws
Flange ith M4 screws and 15000N load
The deflection at the bottom of the screws is around 6.5μm when the flange is not loaded. When the flange is loaded, the deflection reduces a bit to around 6.1μm. I am trying to interpret a colour gradient here. This makes sense. The external load should pull the flange back a bit.
The M4 screw has a pitch diameter of about 3.6mm. For a 12mm length of A2 stainless at 3000N, I estimate it will stretch about 18μm. I can put the calculations up here if you want.
For a lot of practical design, the deformation of the flange under load is not significant, and unclesyd's crude rule of thumb is valid. Interestingly, this is quoted in my Machine Design book (V.M.Faires) as the lead in for the elastic analysis I am sure you are recommending. This does make a good case for the clamping force grossly exceeding the external load.
Obviously, if we are clamping something soft like a gasket, this assumption is not valid.
RE: Understanding Bolt Capacity
Could you explain how my example is different than MechEng? I think we're saying the same thing. An additional load will increase the stress because of the stretch, but it won't cause the bolt to "feel" any additonal tensile load unless it is greater than the preload.
RE: Understanding Bolt Capacity
I am learning here, this topic is fantastic.
I guess the bolt "does" feel some load even it is less than the preload as you have pointed out in saying "An additional load will increase the stress because of the stretch,..". An increase in stress and length, isn't that mean a increase of load otherwise would not occur?
RE: Understanding Bolt Capacity
RE: Understanding Bolt Capacity
Are the displacements absolute values? That is not intuitive, and not helpful when trying to understand both tensile and compressive displacements/forces/pressures that are present in bolted joints.
jaydigs,
Everything in your post from 11:55 today is wrong. If the bolt is pretensioned to 100, and there is an external force of 20, then the bolt will have a tension > 100. This value will depend on the stiffness ratio of bolt to joint. If the external force has exceeded the pretension, then the joint has gapped, the bolt force equals the external force, and the relative stiffnesses are irrelevant because the "springs in parallel" model no longer applies.
RE: Understanding Bolt Capacity
http://www.boltscience.com/pages/basics7.htm
RE: Understanding Bolt Capacity
My displacements are absolute values, given the limitations of my FEA model, as noted.
The reduction in screw deflection under load actually indicates a higher stress on the screws.
RE: Understanding Bolt Capacity
OK, so for a 20lb external force the bolt tension is >100 but <120, the actual value depending on relative stiffnesses. In terms of bolt failure then, it seems fairly cumbersome to try to calculate the actual load on the bolt, what is the rationale for saying if the design load (working load x safety factor) is less than preload you're good? When I was thinking the bolt didn't see any additional force from an applied load it made sense, but now, if the applied load does add a certain percentage to the bolt tension it seems you would need to determine this actual value. So how could I make sure my 20lb external load was ok?
RE: Understanding Bolt Capacity
The rationale for keeping the design load less than the preload is because it is a simple concept, not that it is highly accurate.
RE: Understanding Bolt Capacity
Lots of concepts are simplied. This one is pretty close to reality, as long as the joint is rigid.
I will have to re-run my model above for bolts that are large, relative to the joint. The above model is a good representation of a housing. It does not model a pipe connection very well.
RE: Understanding Bolt Capacity
Thanks for your help in explaining this. I'm sure a lot of people will benefit from this thread since bolted fasteners is not given enough time in a conventional ME curriculum. I know a lot of engineers use fasteners everyday without understanding completely the analysis that goes into specifying them.
RE: Understanding Bolt Capacity
Welds were not covered very well in the curriculum I took but these real life topics (such as bolts) rarely are.
RE: Understanding Bolt Capacity
However, I have re-confused myself on this matter...
The logic from my OP still holds as one argument. The bolt is preloaded, causing the clamped members to compress slightly. Then, a force is applied. This force will increase the stress in the bolt, causing it to elongate. However, as it elongates, clamped memebers are allowed to decompress. This reduces the stress caused by the preloading. For the notation I will use:
P = Preload force
F = Applied force
A = Effective cross-sectional area of bolt
S = Stress in the bolt
1 = State of bolt with only preload
2 = State of bolt with preload and applied load
It makes sense then that:
S1 = P/A
S2 < (P+F)/A
However, in trying to determine a value for S2 I came across the following difficulty. I believe the amount of applied force required to elongate the member to a point where the effective preload is zero should be the same as the preload. The energy used to preload the bolt is effectively stored in the clamped members. The amount of energy required to release this energy should be the same as was required to compress them. Therefore, if F = P then the clamped members are no longer compressed. This would mean that basically there is no preload and the only relevant force is F. However, if there is no preload at that point then P2 = 0, and from F = P we find that F = 0. So the force required to elongate the bolt and release the preload is nothing! How is a force of zero elongating the bolt?! The argument contradicts itself!
Can anybody see where I went wrong?
I think the error must be somewhere in the logic that the force required to elongate the bolt and release the compression in the clamped members is the same as the preload. If this were true, assuming both the elongation of the bolt and compression of the clamped members are linear, with change in length/thickness proportional to the force, then the stress in the bolt at anytime until F>P would be constant, since any increase in F would reduce P by the same amount... However, I don't see how the logic in the storage of potential energy in the clamped memebers is incorrect...
Or perhaps this is just a case of slight variations in assumptions I have made (and are commonly made in theories) such as linear stress/strain, negligible reduction of area, etc, cause the model to fail to produce accurate results. Or maybe there are other effects that occur experimentally that I am not accounting for in this argument?
Anybody who has thoughts or could point me to some other information or flaws, it would be appreciated!
-- MechEng2005
RE: Understanding Bolt Capacity
http://www.boltscience.com/pages/basics5.htm
I think you may be confusing preload with clamp load. Preload is the force carried by the bolt. Clamp load is the load carried by the joint materials. With no external force clamp load and preload are equal. The external load is transfered to the bolt through the joint material. As the external load is increased the clamp load is decreased, but the bolt load is increased and the bolt elongates the same as the joint materials decompress/expand. The clamp load can go to zero but the bolt load continues to increase; ulitmately to failure.
Ted
RE: Understanding Bolt Capacity
I'm still not sure I have a firm grasp on the qualitative results, but I'll keep trying to wrap my head around this and let you know if I come up with anything...
-- MechEng2005
RE: Understanding Bolt Capacity
Picture two U-shaped plates held together by a single bolt. The upper plate is connected to rigid support and the lower plate is connected to a load. The bolt connects the flat portion of both U-shapes to each other and is pretensioned.
The pretension load in the bolt acts on the plates holding them together (clamping force). In order to put more load in the bolt, you need to separate the plates by overcoming the force holding them together (i.e. the pretension load / clamping force).
The only way to add more load directly to a pretensioned bolt would be to weld a bar directly to the bolt head and pull on it. This way the load goes directly into the bolt without having to over come the clamping force between the plates.
I hope this is helpful
RE: Understanding Bolt Capacity
In general terms, and ignoring the effect of stiffness of the clamped parts:
If you apply an external load that is less than the pre-load, the tension in the bolt is equal to the pre-load.
If you apply an external load that is greater than the preload, the tension in the bolt is equal to the applied load and you have joint separation.
In other words, you have to overcome the pre-load in the bolt for the tension to change.
tg
RE: Understanding Bolt Capacity
That has been discussed previously. However, if you have a preload of "P" and add an applied load, "F", the bolt elongates (reducing the clamped load). For the bolt to elongate you MUST have increased the stress/force acting on the bolt. So the rule the if F<P the bolt doesn't seem right. Maybe it is accurate as a rule of thumb, but not exact.
I have been looking at this and done some calculations. I have simplified the problem to be a bolt going through a round tube. This way, when the bolt is preloaded, I can determine the compression of the tube (since I know the cross-sectional area, where-as with clamped members the behavior would be different depending on bolt head size and how the load is transfered through the plates at the connection). Then, I looked at what happens when a force is applied and set the length of the bolt equal to the length of the tube and solve for the clamping force. If everything is correct in my calculation, the force acting on the bolt when a force is applied is ALWAYS greater than just the preload, even when the force is much smaller than the preload.
As an example, I used a 1/2"-13NC bolt going through a tube with OD 1", ID 17/32", a free length of 2", Preload of 7,344 lb, Applied Force of 1,000 lb. Both the bolt and tube have a modulus of elasticity of 29E6 psi.
The clamp load ends up at 6,547 lb. So the total force on the bolt (clamp load + applied force) is 7,547 lb.
Increasing the applied load to 7,000 lb gives a total load of 8,767 lb. So the effect of the applied load is definately less, but if it were assumed that it had no effect and the load was just equal to the preload, there is a difference of almost 20%. Not too bad for a "rule of thumb", but not really good approximation either.
So clearly, the total load is greater than the preload, but less than the preload plus the applied load.
Again, this assumes a bolt running through a tube which is basically useless in practical applications (nevermind that stress concentrations, sticking of materials, etc were ignored). I do not intend to imply that what I have done is quanitatively accurate, but I do think it is qualitatively reasonable.
-- MechEng2005
RE: Understanding Bolt Capacity
Ted
RE: Understanding Bolt Capacity
We are both right. (How's that for diplomacy...) My point is a practical rule of thumb, based on the fact that in most bolted joints, the stiffness of clamped parts is much greater than the stifness of the bolt itself. Of course you increase load in the pre-loaded bolt whan adding external tension, but not by much, until the joint has separated.
I believe your tube/bolt example is not in agreement with the literature (and my comment) because there is not enough difference between the bolt and tube stiffnesses, as compared to a conventional joint.
I suggest you read Shigley Mechanical Engineering Design, Section 8-4 thru 8-7 (in the 8th ed.), esp. the computation of bolt and member stiffness.
What I've done is to read it and understand it once, then I typically follow the simpler rule of thumb mentioned above...
tg
RE: Understanding Bolt Capacity
Ok, we agree. I happened to be sitting next to Shigley (the book, not the person). I am glad that I went through my simplified example, as I now have a greater appreciation and understanding of what is being discussed in the book. Thanks for the info.
-- MechEng2005