ABSTRACT
Internal threads are usually chamfered or countersunk. While removing the feather edge of the thread reduces the possibility of cross-threading, it also reduces the amount of thread shear area in the length of engagement. Based on experimental results, Alexander found that for a nut with a 90° countersink, the length of threads in the countersink contributed only 40% of the strength of an equal length without the countersink. Therefore, countersinks can be accounted for by modifying the length of engagement used to calculate thread shear areas as follows:
LE' = LENom - 0.6(Dcmax - Dlmin)
where
LE' = effective length of engagement (in., mm) LENom =nominal length of engagement (in., mm) Dcmax = maximum countersink diameter (in., mm) D 1min =minimum minor diameter (in., mm)
(26)
BBL=(S,)(As) BSL=0.6(S,)(AS,)(C1)(C2)
NSL=0.6(Sn)(ASn)(C1)(C3)
where
BBL=boltbreakingload(lb,N) BSL=boltstrippingload(lb,N) NSL=nutstrippingload(lb,N)
(27) (28) (29)
Thefactorof0.6inthestrippingloadequationsdenotesthecommonratiobetweenshear strengthandtensilestrengthforductilemetals.ComparisonoftheresultsofEqs.(27)-(29) indicateswhichfailuremodewillprevail.Thevariablesarethenmanipulateduntilthedesired conditionofboltbreakageisachieved.
