Understanding Bolt Capacity 7

[jaydigs]

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

 

[drawoh]

jaydigs,

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.

JHG

 

[unclesyd]

Here is a statement that I encountered many years ago that goes with what was posted above.

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"

 

[debodine]

This is an area that still confuses me a bit too. Yes my header says electrical but I like to learn in all areas. )

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!

 

[jaydigs]

I guess you can look at is as the preload is the same as the clamping force. So, say you preload a bolt to 100lbs (for simplicity). You have 100lbs of clamping force. If you hang something that weighs 20lbs from this, the tension in the bolt is still 100lbs, you've just taken away from the available preload. If you hang something 120lbs, you have 120lbs of tension on the bolt then. In this way, if you keep working loads below the preload you don't have to worry about additional stresses. This sound correct?

 

[MintJulep]

Think of the bolt as a spring.

http://www.boltscience.com/pages/basics2.htm

 

[kslee1000]

jaydigs:

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?

 

[CoryPad]

jaydigs is not correct. If you have an initial precompression of the joint and pretension in the screw, then add a tension force to the system, there will be an increase in tension in the screw and the joint will have a decrease in compression. A bolted joint behaves like springs in parallel. The Bolt Science link above describes this, as do other references like NASA RP 1228:

http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?1990/RP-1228.html

 

[jaydigs]

The bolt science article was very helpful! Just to recap, as an analogy to the bolt, special plate, and bracket mentioned in the article...my example of a bolt with a 100lb preload would not see any increase in bolt tension from the additional 20lb external load. Only after exceeding the 100lb preload with say 120lbs external load, would the tension increase and it would NOT be 120lbs, rather something mush less as determined by relative stiffnesses. Calculating the exact load is cumbersome and maybe not so precise, so as a design principle, never allow the working load to exceed the bolt preload (including a safety a factor). Am I tracking now?

 

[CoryPad]

No.

 

[MechEng2005]

I was intrigued by this thread and think I have some handle on it now...

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

 

[CoryPad]

Yes

 

[drawoh]

CoryPad,

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 6.5[μ]m when the flange is not loaded. When the flange is loaded, the deflection reduces a bit to around 6.1[μ]m. 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 18[μ]m. 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.

JHG

 

[jaydigs]

CoryPad;

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.

 

[kslee1000]

jaydigs:

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?

 

[Dinosaur]

It seems to me a free-body-diagram will show the bolt load does not increase until the new load exceeds the preload. What changes is the amount of preload the bolt exerts against the face of the original material.

 

[CoryPad]

drawoh,

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.

 

[CoryPad]

For an explanation of bolt tension vs. joint compression vs. applied external forces, look see the series of pages here:

http://www.boltscience.com/pages/basics7.htm

 

[drawoh]

CoryPad,

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.

JHG

 

[jaydigs]

CoryPad;

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?