Rule of Thumb - Numbr of Thds 2B Full Strength 1

[Binary]

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.

 

[CoryPad]

Assuming the thread form is a typical 60 degree variety, and screw and nut threads are steel, then the typical rule of thumb for axial thread engagement 1 * the thread major diameter.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SJLeemans]

I've always heard it was 1.5 * the Maj Dia of the screw for a blind hole; and a minimum of three threads showing for a thru hole. Of course, this all depends upon materials, etc.

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

 

[CoryPad]

SJLeemans,

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.

 

[brengine]

This is what we use...
Steel: 1 * (Major Thread Diameter)
Aluminum: 1.5 * (Major Thread Diameter)

 

[diamondjim]

I like Ken Bolen's answer in that it
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.

 

[arto]

Shigley's "Mechanical Engineering Design" ch. 6-9 sez "3 full [nut] threads are all that are required to devevelop full bolt strength"

 

[Hukari]

A good rule of thumb for fastener thread length of engagements is that there is no general rule of thumb only, I suppose, rules that apply to limited applications.

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

 

[chili]

Shigley is the god of materials engineering, but we always have gone with one diameter of engagement....it takes the guess work out of the equation.

 

[unclesyd]

Here is paper relating to the pullout/stripping of threaded fasteners that might be of interest.
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

 

[Tmoose]

I believe rules about 1X diameter for steel and 1.5 dia for iron and ??X diameter for aluminum are for >>BOLT<< strength. I have a few personal experiences with situations when the parent material strength has been the limit.

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?).

 

[Boothby171]

Well, well, well,

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 Design/Notes 28.pdf

Enjoy!

--Steve