Torque vs. axial force on UNC threads
Torque vs. axial force on UNC threads
(OP)
I am trying to locate a formula for determining the axial (linear) force generated by the torque used on a 1/4 -20 UNC screw.
I can derive that force for Acme and ball lead screws, but the pressure angle is different than the UNC thread. I can also derive the clamping force or tension/preload in a screw that is seated, but this existing application (misapplication) is used for inserting large PWA's into a motherboard, similar to a lead screw.
I can derive that force for Acme and ball lead screws, but the pressure angle is different than the UNC thread. I can also derive the clamping force or tension/preload in a screw that is seated, but this existing application (misapplication) is used for inserting large PWA's into a motherboard, similar to a lead screw.





RE: Torque vs. axial force on UNC threads
MA = FM (0.16 · P + 0.58 · d2 · ?G)
where
MA is the assembly torque in N m
FM is the preload in kN
P is the pitch in mm
d2 is the pitch diameter in mm
?G is the thread friction coefficient
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Torque vs. axial force on UNC threads
T=kPD where T=applied torque, k=the empirical "nut factor", P=the axial force developed, and D=the nominal diameter (e.g. .164 for No.8-32 UNC).
A very broad rule of thumb for unlibricated, plated steel or CRES fastners is that k=.15.
If you have a routine need to do this kind of calculation you may want to get TORKSENSE for about $120 per seat. This program contains many metal and non-metal materials of many different alloys and grades (provides yield points). You enter the percentage of the .2% offset yield you wish to develope. It also includes numerous lubricants and finishes. It's a good deal for the money. A Google search will turn up the webpage address.
Tunalover
RE: Torque vs. axial force on UNC threads
See thread31-14960 for additional "nut factor" background...