fine thread bolts

So, what's the preference for using fine threaded bolts in some cases, like wheel bolts, etc.

My 48 loader cylinder flew apart today when the 4 bolts holding the front cylinder together finally decided to give up. The studs welded to the casing are fine threaded.

I'm wondering if there's an advantage to replacing them with fine threads, or are coarse OK? I assume I need to get some grade 8 1/2" bolts and cut the heads off before the local welder repairs it.

Thanks.

Fine threads are less likely to work loose and will exert a greater force than coarse threads will at the same torque.

Thanks. I've always wondered.

Fine threads leave more of the shank intact and therefore the bolt has a greater effective cross-sectional area. This results in a higher clamp load for the same amount of bolt stretch.

Brendon is correct regarding the cross-sectional area. A fastener loaded in tension will always fail in the threaded area (assuming it doesn't have a reduced-diameter shank). So since fine threads leave more effective cross-sectional area of a fastener, a fine threaded fastener will be stronger than a coarse threaded fastener of the same diameter.

And here's an interesting and surprising fact that goes against common belief - With all things being equal (same material, hardness, thread finish, thread accuracy, degree of lubrication, etc.), the amount of preload (fastener tension) resulting from a specific torque on a fastener is, for all practical purposes, only dependent on the diameter of the fastener. Fine threads give you only about a 3% more tension compared to coarse threads when torqued the same amount. For example, you would get the same clamping force from a fine-thread bolt torqued to 100 ft-lb as you would from a coarse-thread bolt torqued to 103 ft-lb. Not enough difference to matter in the real world. This is because almost all of the torque exerted when tightening a fastener goes into overcoming friction - about 50% of the torque is used to overcome the friction between the face of the nut/bolt to the member, and 40% is used to overcome friction of the threads. So only about 10% of the tightening torque actually goes toward putting tension in the fastener. However, since a fine threaded fastener can handle more tension, you can torque fine threaded fasteners to a higher level and gain more clamping force.

Against common belief meaning the opposite of in the first place, the fine VS coarse thread issue is a loaded topic can we say?

Taking a 1/2 inch bolt, we have 13 TPI VS 20 TPI which theory states is a 65% advantage in generating preload value. We aren't ever going to see that in the real world taking into account friction upon the nut face under the greater preload itself, let alone the greater thread length contact area.

But to have 65% vanish into 3% advantage - I'm just not buying that one today. Since the pundits for the most part want this discussion to not exist in the place and won't go on record (at least any credible ones that I could find quickly) as to a real world value, I'll suggest a more realistic 10 to 20% advantage in preload for fine over coarse as a very rough rule of thumb. 65 we know it's just not going to be. At 3 percent, the BS meter is pegged if not outright broken. End of the day, fine does make more preload than coarse (common belief), but the advantage is no where near theoretical due to increased friction losses mainly.

No valid reason exists that those same greater losses can't be compensated for with a bit more twist. But here be dragons as they used to say and you are entirely on your own.

However, lubricated VS out of the box dry torque values for achieved preload are consistently 50% LESS. So if you want more clamping force, a bit of oil DOES make a real meaningful difference. All published torque values for assembly can be safely assumed to be lubricated values just in case you really needed to see that in print somewhere.

Thank you, gentlemen.
I done lernt something today.

Bob

Ditto. VH

Lee, the 3% advantage was taken from "Mechanical Engineering Design" by Joseph Shigley. In that text, Shigley develops a relationship between bolt torque and bolt preload that simplifies to:

T=K*Fi*d

where T = torque, Fi = initial preload force (tension in the bolt), d = bolt diameter, and K = torque coefficient which is derived from a mathematical relationship between the mean diameter of the thread, the bolt diameter, the thread angle, the helix angle of the threads, and the coefficients of friction between the threads and between the washer face of the nut or bolt and the clamping surface.

Shigley has calculated the K factor (based on a reasonable coefficient of friction of 0.15) for a range of bolt sizes and thread pitches, and here are a few:

1/4-20 NC - 0.210
1/4-28 NF - 0.205 (about 2.4% difference)

1/2-13 NC - 0.201
1/2-20 NF - 0.195 (about 3% difference)

7/8-9 NC - 0.194
7/8-14 NF - 0.189 (about 2.6% difference)

(Those values are where I got the "3% difference".)

Shigley suggests using 0.2 as a realistic value for most practical applications. And the point is that although there is an advantage of using fine threads to gain more clamping torque, the advantage is actually quite small. I've certainly been wrong before and you would point me toward some technical data or studies to factually show this analysis is wrong, I would greatly appreciate it.

But all this is based on an average coefficient of friction of 0.15 which is greatly affected by a number of factors, including lubrication.

Torquing a bolt is actually a fairly crappy way of trying to obtain a desired preload (which is really the goal). But it's easy and quick. In the structural world, there are things like crush washers where preload can be visually determined. And the "turn of the nut" method is still one of the most reliable methods of achieving desired preloads because it eliminates issues of friction.

Thanks Dan for the source of the info posted, was wondering. Back in skool I had to show my work when doing math problems - I wonder if Shigley ever shows his because just exactly where he gets the numbers from is the most important part.

I wouldn't be blaming you even if I had a more concise source as those are Shigley's numbers, you are just doing a book report on it in essence, and saving everyone hours and hours of reading it for ourselves, I for one am grateful. For now I think we can agree to disagree and still be gentlemen and I do appreciate the comment as to that conduct here.

Ultimately it won't be settled for me until I take two bolts to a block of cast iron and torque them to the same bolt stretch measurement numbers and note the torque required to get there. Probably not gonna happen anytime soon since I have many other things to do a bit more pressing, winter is just right around the corner.

Lee, I apologize for not referencing the source of the information in my first post. As an engineer, I get criticized for explaining how to build a clock in response to a inquiry about what is the time. So in some groups I tend to abbreviate which helps prevent the eyes of people from glazing over.

Anyway, regarding Shigley, his development of the torque/preload formula is quite rigorous and begins with basic principles of physics, so from my viewpoint, he did "show his work".

If you do perform your experiment with measuring bolt stretch with respect to torque, take lots and lots of sample readings because I suspect your results will be all over the spectrum and that the best you can hope for is to get a reasonable average from a large number of readings.