bolts and nuts

[dian]

why do they have the same pitch?

sounds weird? well, "overpitching" the nut apparently yields 25% more fatigue strength. so why is this not a stadard feature, at least where it matters, like automotive applications? you gun them down anyway, right?

(it is well known that the loading of thread flanks is highly overproportional, causing the screw to often fail near the first thread and creating a huge stress concentration degrading the endurance limit even more. mj and unj threads do exist as an attempt to improve the situation.)

 

[strokersix]

That's a highly refined design when you are after the very last bit of performance.
Example: Spacecraft where weight is critical and cost not so much. Or perhaps high end racing. I think this added complexity is tough to justify otherwise.

A counterbore to bury the first thread is commonly done to reduce the tendency to pull the first thread. Also adds a bit more stretch to the fastener to "stiffen" the joint.

 

[Joe Henderson]

Sort of a cheap thread lock.

 

[Rickyb]

Keep in mind that to benefit from the finer pitch the tolerance of the screw and mating part have to get tighter. For strength you are looking at the shear strength of the screw along the pitch line. A little float in the pitch diameter has a significant impact on that shear strength. If you think about it in the extreme, 100 tpi or 0.010” pitch, a thou or two float of the pitch diameter would have a very significant impact.

BTW, in the context of strength and fatigue we are talking about cap screws not bolts.

 

[UptownSport]

FoMoCo used interference threads on wheels of 3/4-1ton PU/vans- I think in the early 90's
I assume it was in lieu of using left hand threads.

I heard low numerical tolerance eliminates the use of coatings- If Zinc thickness on each surface are .005", and the tolerance were .008", then the fastener would be too large to thread
.008 - (.005 +.005) = -.002"

 

[Doozer]

why do they have the same pitch?

sounds weird? well, "overpitching" the nut apparently yields 25% more fatigue strength. so why is this not a stadard feature, at least where it matters, like automotive applications? you gun them down anyway, right?

(it is well known that the loading of thread flanks is highly overproportional, causing the screw to often fail near the first thread and creating a huge stress concentration degrading the endurance limit even more. mj and unj threads do exist as an attempt to improve the situation.)

Yeah, I read the same book.
I try not to be an engineer all the time
because it annoys people. Short answer
is the benefits are small and 99% of the
time, it is not that critical.

-Doozer

 

[dian]

glad you are annoyed.

i would see a benefit e.g. when being able to use a 10.9 screw i a 12.9 application without all detrimental issues like toughness or hydrogen embrittlement and associated coating problems.

 

[Doozer]

I am not annoyed and you are not annoying.
But the potential is real.
You seemed to have drawn conclusions that
the facts do not support.

-D

 

[EPAIII]

Not from what I am reading about this here. If the screw/nut was not torqued to full spec, then the bolt/screw would not be stretched and then, yes it may function as a thread lock.

But this seems to be a way of obtaining a more even distribution of the stress on each of the threads that are engaged. So, the bolt/screw WOULD be fully torqued and it would be stretched to the point where it's thread would match the "overclocked" thread in the nut.

This, of course, assumes that the nut or the surrounding material in a tapped part would not deform, but it will. But the nut will deform to a lesser degree than what the bolt/screw will. And probably less so for a tapped hole in a part that is even larger than a nut. But there will still be some deformation.

Sort of a cheap thread lock.

 

[john.k]

I did a job once where 1"UNC tiebolts had to have double width nuts ......when I priced the nuts ,they were like $5 each,and there were hundreds of them.....so I rang around and one of the scrappies had a lot of 24mm double nuts with the plastic lock in them for like 10c each........so I retapped the 24mm to 1"UNC....no one ever noticed the difference,even though it was obvious if you looked.