Rule of Thumb - Numbr of Thds 2B Full Strength
Rule of Thumb - Numbr of Thds 2B Full Strength
(OP)
Back in my first year of work, one of the senior engineers (who threatened to pummel me if I ever again pointed out that his BSME was dated 12 years before my DOB) said that the rule-of-thumb for a fastener to be sufficiently engaged was that 5 threads was considered full strength.
I'm wondering if anybody's heard this and could validate it.
Certainly in a critical application I'd take the time to calculate the thread loading.
I'm wondering if anybody's heard this and could validate it.
Certainly in a critical application I'd take the time to calculate the thread loading.





RE: Rule of Thumb - Numbr of Thds 2B Full Strength
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
If you want more than a rule of thumb, then there is a useful formula, which basically balances the shear and tensile strengths of the bolt. The formula is more or less a ratio of twice the effective shear area of the bolt to the effective tensile area of the bolt. You can find it in the Machinery's Handbook, as well as other Machine Design References.
If you don't have these references, check out the set of on-line class notes at this web site: http://www.utm.edu/departments/engin/lemaster/machine_design.htm
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
The 1.5 * major diameter for a blind hole is due to the presence of the hole chamfer and the incomplete point threads on the screw. If you subtract them, you get the 1.0 factor.
For threads beyond a nut (nut = through hole), I have seen the rule of thumb to be 2 threads. Also, there is a specification, DIN 78 Protrusions of bolt ends, which requires 2P (P = thread pitch) screw end length protruding beyond the nut for a free-spinning nut and 3P for a top-constricted prevailing torque nut. These protrusion lengths include incomplete threads on the screw, so the actual full-thread beyond the nut is ~ 1P.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
Steel: 1 * (Major Thread Diameter)
Aluminum: 1.5 * (Major Thread Diameter)
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
addresses that the different materials
affect the ratio. If you are using high
strength bolts and AS36 material you might
want to consider using a higher ratio than
the 1 times the diameter of the bolt.
Or to be rediculous if it were high strength
bolt and plastic, the simple rule does not
apply. The material strengths of the two
part must be examined. The simple rule applies
for similar strength material. How many times
were you pummeled? The five thread rule
makes no sense as pitch of thread is not constant
in ratio to the diameter.
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
Bickford gives detailed formulas for cases of differing relative nut and bolt strengths. Basically the minimum shear strength in the threads must be greater than the tensile strength of the bolt.
For the case of nut and bolt of approximately equal strength the equations are fairly simple. The shear area of the threads is pi/2*(nominal pitch diameter).
Ron
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
I still like the 1 * PD for engagement one full thread stickout myself as it’s proved itself for many a year.
http://www.hexagon.de/dose/dose1e.htm
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
Just the other day I think I heard another one.
The rocker shaft on some Chrysler V-6 are becoming famous for breaking. The reason seems to be that the bolt that clamps the shaft to aluminum head goes is short, and is threaded into a tall cast boss on the head, subjecting the boss to bending and tension. The entire boss cracks after a while, then the now-cantilevered rocker shaft cracks a little later. Rappety tap.
Shops are effecting a decent repair by threading the hole deeper, buttering the broken boss with epoxy, and putting a new shaft in with a longer bolt in that position.
If the hole had been tapped deeper and a longer bolt used then maybe the boss would not have been subjected to the same loading. I'd like to see a broken boss to see if the cracks started from the threads (tension?) or an outer surface (bending?).
RE: Rule of Thumb - Numbr of Thds 2B Full Strength
Apparently, all you ever wanted to know about threads, including tensile area, and--most importantly--thread shear area, is right here:
http://www.utm.edu/departments/engin/lemaster/Machine%2...
Enjoy!
--Steve