Installation torques for small screws 3

[Tunalover]

Folks-
I have search high and low for recommended installation torques for unlubricated No. 0 and No. 00 stainless steel screw threads. Does anyone have numbers with well-documented sources?


Tunalover

 

[diamondjim]

Tunalover
What are you using these for?
What material are they mating with?
Is the application static, dynamic,
or what? How long are the parts
or clamping length?

 

[btrueblood]

The standard formula T = 0.2FD should still be valid for those threads, if lubricated (Shigley, Mechanical Engineering Design, 4th ed., McGraw Hill, 1983). Without lube, the coefficient goes from 0.2 to roughly 0.8 (one of the reasons for using lube is to get rid of the "roughly" part).

My Machinery's Handbook, 19th ed., gives tensile stress area for a size 0-80 UNF thread as 0.00180 sq. in. No listing for size 00 threads, though, sorry. Sizes below "0" are listed in standard UNM (Unified Miniature Threads), in mm units.

Take allowable proof stress for the material (typ. 80-90% of yield), multiply by Atensile to get F in lbs., plug into equation using D in inches, and calculate torque (in-lb).

That equation works in about 90% of all cases. As diamondjim notes, though, there are a lot of other things you must consider, even if they can be neglected in final analysis.

 

[Tunalover]

diamondjim-
We are using the 0-80 screws for engagement with either tapped irridited AL6061-T6 or with 0-80 SST nuts. In all cases the screws are unlubricated SST.

For the 00 screws, they engage tapped irridited AL6061-T6. Does the length matter?

btrueblood-
I have well-documented, authoritative installation torques for larger unlubricated SST screws and will test the formula you are providing. If the formula jives with the values for those screws, then THANKS! Stand by. I'll let you know how it comes out.


Tunalover

 

[diamondjim]

Tunalover,
as to the 00 screws
90 96 112 120 threads per inch?
I think you need to list some
grade or bolt or more about
the strength or clamping force
to calculate the torque.
I assume the aluminum is about
70000 psi tensile?
Screws about 50000psi tensile?
I would break about 10 of these
and see what torque figure that
is and back off accordingly to
about 85 percent of that value.
I guess your application is secret.
If you change manufacturers of
the screws, I would test the
breaking strength again. These
small screws have no class
assigned to them but I do think
they have a grade assigned.
I think they would vary from
one lot to the next.
I would use a friction factor
of around .14 for the irridited
threads to calculate the torque
value as defined above.

 

[Screwman]

There are a lot of variables that are going to make calculation very difficult in this case. One of the things that we have found is that on parts in this size range there is a significant size effect and normally accurate calculated results aren't always supported by reallity.

Yes, length matters greatly.

Best way is to test. Take a statistcally valid sample size and duplicate the actaul assemblies precisely (use all new components each time). Torque until you get failure in a component (it may not always be the screw breaking). Now take 75% of the mean failure torque as your nominal seating torque. this is a quick and dirty way to maximize the available strength of your particular joint components.