Hi grade bolt into softer material

[scarecrow55]

I have a hi strength bolt (12.9) going into a cast steel body - (no I can't change it), if I work out the engagement length based on the relative UTS of the materials I get a factor of 2.4 x the engagement length of the correct "nut" so I can tighten it right up.
All well & good but surely there must be a limit, infact 2.4 seems a bit excessive - If you use the UTS for wood you'ed get an engagement length of a few feet but I bet it wouldn't work.
Anyone know the limits? could I work on say the forces on the first 5 thrds or so?
Thanks

 

[CoryPad]

The ratio of engagement length:nominal diameter does vary depending upon the materials used. A factor of 2.4:1 when using a 12.9 bolt and cast steel does not seem excessive at all.

In elastic analyses of steel:steel joints, the first 5 threads react only 86% of the preload, so you would need to use more than that. Six threads react 92%, while seven threads react 97%.

However, the engagement length calculations are based upon fracture, not elastic (yield) strength. During yielding, the preload is "transferred" to more of the threads.

If you want to use actual calculations, try VDI 2230 [faq725-600].

Regards,

Cory

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

 

[dimjim]

I am curious as to which formula
you used to calculate the length
of engagement. What load are
you putting on the bolt and what
is the applied load?
2.4 seems like a high number to
me. Machinery's Handbook under
Bolts and Nuts has a section on
Working strength of bolts and right
after it has formulas for Stress
Areas and Length of Engagement of
Screw Threads.
Is this the same that you used?

 

[scarecrow55]

Thanks Guys. Dimjim I used a formulae from

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

I've seen the same one elsewhere, it could have been Machinery's

 

[dimjim]

It looks like the same information.
Will you ever torque or tensile the
bolt to its full tensile load?
Rearranging the equation, what length
would be required for your maximum
working loads assuming a 2 times
factor of safety against 75 percent
of the proof load?

 

[scarecrow55]

I'm utilising , & need, the full tension & recomended torque by Unbrako for this bolt. This will give me a SF of about 1.3 on slip between the joint. I think that will bring the bolt stress to near 0.75 Yield.
I'm going to do some pull tests today to see what fails first.

 

[dimjim]

I think the clamping load that they recommend is
75 percent of the proof load and not 75 percent
of the yield strength.

 

[BillPSU]

One limiting factor for high length engagement of bolts is the ability to manufacture the thread in the part. The drag on a tap with a 2.5 to 1 engagement to diameter ratio is approaching the limit of normal taps in steel. An interrupted thread tap may be required to manufacture the hole and increased minor diameter may also be needed. If this is a blind hole there are another set of manufacturing issues.

 

[scarecrow55]

FEEDBACK
I torqued 3 bolts into the casting with the thread length as calculated in my opening post and all three failed by tensile failure of the bolt at the torque figure that would load the bolt to yield ( it kept giving until the bolt broke). So the calcs seem OK for that particular instance.
The threads in the hole and on the bolt look visually OK, but I expect there is a little deformation.

 

[Screwman]

Make sure that you use a hardened washer under the head of that SHC screw or you will likely have quite a bit of embedement into the bearing surface unless it is hardened.

Also watch out for the potential for stress corrosion failures if you have an environment where there is a possibility of corrosion. PC 12.9 was taken out of the SAE standards because of it's high propensity for SCC failures in the field. Not to say it won't work, but there is a much greater likelyhood of problems than with a PC 10.9.

 

[unclesyd]

Aside from all the good advice posted above there is one other consideration especially in your connection and touched on by CoryPad that needs to be addressed.
Your need to make sure you have good quality threads cut to the maximum depth possible in order help maintain maximum contact on the first few threads.

http://www.spiralock.com/technology/about_technology/torque