Nut Bearing Surface Area

[nctexan22]

A bolt circle was machined .200" too small on a valve bonnet (top flange). Production saw fit to just remachine the bolt circle at the correct dimension ontop of the smaller bolt circle, and as a result there is now 14% less nut bearing surface area (see attached sketch). How should I approach determining if this will be detrimental/cause plastic deformation of the bonnet material (316-30ksi) or nut material (A194 Gr. 2H-) at the reduced area surface contact section?

Force due to valve bore pressure equates to 30,630 lb. There are 8 studs so each should see 3828 lb. Dividing by the reduced amount of surface area (.545 in^2) gives 7023 psi. Would I then need to break down the area further to isolate the reduced section and evaluate pressure at that point, comparing it to yield strength of the material?

Thanks for any pointers.

 

[desertfox]

hi nctexan22

You need to attach the sketch

desertfox

 

[nctexan22]

Sorry took me a sec to figure it out...

 

[desertfox]

Hi nctexan22

I think your approach is reasonable, your taking the flat area of the nut and taking away the extra bearing area that is lost by the error correct?
desertfox

 

[CoryPad]

Aren't the fasteners preloaded? Those forces should be much higher than the applied forces.

 

[desertfox]

Hi nctexan22

Corypads right its your pre-load after the bolts are tightened that you need to consider when doing your calculation.

desertfox

 

[nctexan22]

Desertfox, correct I am accounting for the area lost due to the machining area.

I know there are many discussions and threads concerning preload, and I am still working to grasp the concept entirely. From what I infer, you want to preload your fasteners to provide enough force to overcome the loss in clamping load caused by an externally applied force (in this case pressure, and to seat the particular gasket you are using). This amount of preload needs to remain below allowable stresses prescribed for the fastener as well, though "overstressing" the fastener may be better than providing too little preload.

Aside of preload calclulation methodology, I will assume a preload of 200 ft-lbs for a 3/4" stud, producting 60,000 psi in the stud and 18,120 lb of compression per stud (per Garlock tables). This is above the required calculated preload force in the attached spreadsheet "Fpl" which is equal to Wm1, or 85,900 lbf/8 = 10,737 lbf per stud, and below 2/3 yield for the stud. So then I take the 18,120 lbf, and divide that by the reduced amount of surface area available to the nut, .545 in^2, resulting in a pressure of 33,247 psi. Dividing the area into 6 segments would I be able to assume that if they were all equal, they'd share an equal pressure distribution, or 5,541 psi per surf. area segment?

 

[desertfox]

Hi nctexan22

I am a little confused by your post however this is what I was trying to convey:-

Yes your pre-loaded bolts should have a higher load then the external load exerted on top flange of the valve bonnet which you seem to have according to your spread sheet.
Now your concern I thought was the reduced area under the nut face as it may cause the clamped material to yield so you simply divide your reduced area of clamped material into your bolt pre-load if that resulting stress exceeds the yield stress for the material then its no good.
In your spreadsheet your bolt yield stress is given as 10500 lb/in^2 yet your allowable is 70000lb/in^2.
I can see if your spreadsheet is correct and the bolt yield stress is a typing error, then yes you could tighten those bolts more, however you need to check the yield stress of the material your clamping and I can't see a yield stress figure for that material.
No its not intended that you yield the bolts or increase the clamping load to compensate for the reduction in area under the nut because increasing the load would not help.
I am not sure why you want to divde the stress figure you obtained into segments perhaps you could explain further.

regards

desertfox

 

[unclesyd]

I'm not at my computer and this thing has no calculator/

I don't think that the loss of 14% of the nut bearing area is going to affect the amount of preload you can safely develop in the fastener or yield the flange.
There are a lot of highly stressed fasteners in slots or elongated bolt holes.

Check your 2H nuts to see if they have a landing area on one face or machined full face.

Is this a valve for in house use or for the market?

 

[CoryPad]

The calculation you need is the fastener preload (18 120 lb) divided by the contact area (0.545 in[sup]2[/sup]), which gives you a contact stress of 33 200 psi. The allowable contact stress for materials usually is ~ equal to the ultimate tensile strength due to the nature of localized forces on solid bodies, so you are not in danger of overloading the bonnet.

 

[nctexan22]

Desertfox,

My concern is the reduced surface area under the nut. The surface pressure exists due to preloading force distributed evenly across the total nut/bonnet contact surface area. So at any point under the nut, even where there is less bonnet material touching the nut due to the enlarged hole, the surface pressure would be uniform, correct (F=PA)? I was initially thinking that there would be localized areas of contact stresses greater than any other area of contact under the nut, thus why I was considering dividing the geometry. Also bolt yield is 105 ksi, and I was taking the allowable as 2/3 that or 70 ksi.

Unclesyd,

The use of studs/nuts in slotted holes had entered my mind as well, and I agree with you that a 14% reduction in surface bearing area does not seem to be terribly significant, I just want to be sure it is ok.

Corypad,

The bonnet material is AISI 4130 NQT, with a tensile of 103,000 psi, so you are right I should be just fine with the reduced area in comparison with tensile. Desertfox was indicating that the contact stress should be compared with yield however. So which is it for allowable contact stress?
Of course even in comparison with yield (75 ksi) I should be ok.

 

[desertfox]

Hi nctexan22

I would just compare the 33,247psi with the yield of the material which you have given as 75ksi, so as you stated its well inside the limit even if you use the tensile strength.
The reason I said use the yield was I had no idea what the material was for the bonnet untill today so it was just airing on the cautious side.

desertfox

 

[CoryPad]

Allowable surface pressure is similar to bearing stress. These localized contact stresses must be close to the ultimate tensile strength for the parts to begin gross plastic deformation. You can read more about this, and see data, in MIL-HDBK-5 and VDI 2230.

 

[nctexan22]

Desertfox, CoryPad, UncleSyd, thanks very much for your input I really appreciate it.