Functional thread feature 3

[cougfan]

Is the thread pitch line the functional feature for mating surfaces of threads on a bolt and nut? Is this where contact exists? Also, am I correct when the tendencey for a thread to back off is releted to pitch(or lead in a single thread)? Sounds basic, but I am a Material Science Engineer. Thanks for your help!

 

[Cockroach]

In theory, the pitch diameter is the diameter of an imaginary cylinder which would pass through the threads at points were the width of the thread euals the width of space cut by the surface of the cylinder. It equals the diameter of the screw thread at one-half the thread depth.

So in essence, the pitch diameter is the contact points between screw and nut. Contact are on the sloping sides of the thread and not the crest and root.

Yes, this is related to both pitch and lead in a screw system. But the tendency to back off is also due to the preload torque given to a thread in order to achieve make-up. Depending on your nut-bolt system, you can eliminate backing off by using locking nuts, lock washers, even threadlok. (i.e. high strength glue)

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[MechNorth]

Another option for securing threads is to use a SpiraLock tap (nut, blind hole, or thru-hole). It is extensively used in aerospace applications and others where vibration is an issue. It may take a bit of practice to get cutting parameters right if you are automating the tapping. Check out their site for more info...

http://www.spiralock.com

 

[Screwman]

Whenever I hear Pitch Diameter and functional surface together I think of GD&T and the fallacy of using pitch as a datum. Pretty tough to use an imaginary diameter as a datum.
The contact area of the thread flanks is not a point contact at the pitch, it is a flank contact that varies depending on the relative major and minor diameters of the male and female threads. Take a look at the ANSI or ISO basic thread standards to get a better idea of the overall variables that go into making up the mating surfaces. It is a really complex situation of spiral helixes.
The pitch of the thread has a pretty much trival effect on loosening. Clamp load, bearing surface friction and thread flank friction far outweigh the effect of the helix angle on loosening. If the parts are tight enough and the fasteners have a good L/D ratio and the joint retains clamp load you will not have loosening problems. If one of those is not present you will have to use some auxiliary feature or device to prevent loosening, changing to a lower thread helix angle won't do it.

Dick