Oversize pitch diameter.

[diamondjim]

Does anyone have any information about a threaded
hole with an enlarged pitch diameter being used.
Like if you had 29 of 30 and the last had an enlarged
pitch diameter, could the 30th hole be used?, how much
load decrease would be expected? an obvious candidate
for an insert either helicoil or keensert? My first
thought is that the internal thread would be stronger
since it is on a larger diameter. Anybody tested anything
like this? I know you need a value like .002 larger or
whatever. Just assume it is .002 larger than the guage
diameter that would throw it out of spec. We are looking
at a 1.5 inch diameter thread with a 6 pitch.

 

[Kenneth]

You might want to take a look at thread404-11447 for a (somewhat) related question.

You can use the same formulas referenced in that thread (found in Appendix B of FED-STD-H28/2B -- available for free at

http://astimage.daps.dla.mil/docimages/0001/34/77/H28-2B.PD9

-- start reading at about page 56) to readily calculate the (theoretical) load required to shear or "strip" an internal thread at a given pitch diameter, length of engagement, material shear strength, etc. This calculation is based on the geometric shear area of the internal thread at minimum material condition (equal to the area of that thread which is intersected by a cylinder with a diameter equal to the minimum major diameter of the mating external thread over the length of engagement) and shear strength of the parent thread material. You can do these calculations with both an “in-tolerance” pitch dimension and your measured (assumed?) "oversize" pitch to quantify any effect. As you surmised, if your length of engagement (and shear strength of your internally threaded member) is adequate (including the effects of your oversize pitch) to develop the tensile strength of the mating external thread, any strength reduction will indeed be very minimal (and moot).

If you don’t know the shear strength of your materials, two possible sources of free online information are MatWeb (usually “typical” values) at

http://www.matweb.com/

or in MIL-HDBK-5 (usually statistically adjusted values to a given confidence level) at (warning huge download!)

http://astimage.daps.dla.mil/docimages/0001/73/51/MILHBK5H.PD5

or (smaller download; pick your chapter/material)

http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml&br=0.

Alternately, you wish to use an applicable material specification “minimum guaranteed” value.

Try to get an actual pitch diameter measurement for your calculations. Consider "proof loading" if you have the equipment/capability. Hope everything checks out OK. Don’t think I’ve ever seen a 1.500-6 UNC helicoil or keensert…

 

[diamondjim]

Kenneth,
I am familiar with the h28 handbook and
machineries handbook and understand the
formulas. I have not heard or seen any
reference to oversize pitch diameter, only
that it is scrap if does not meet the
standard. I am talking about .002 beyond
but it might as well .010 as the value
is moot. It either meets the standard or
not. My gut feeling is to get an oversize
bolt for this one hole. You all know about
liability. Paint this hole and the bolt
to be certain that the two are matched may
not relieve one of liability problems. If I
were making it for myself, I would have no
problem with the decision, but I flinch at
the liability problem otherwise. I think I have
answered my own question. Thanks for your
input.

 

[Kenneth]

My misunderstanding. Thought you were just looking for a way to calculate the effect of an “oversize” pitch on load carrying capability. As described above, the H28 formulas (which include pitch) can be used to do that. The (legal) question of strict conformance to specification (and any related liability) is another matter entirely, as you have noted…