Bolt Science

Thank you very much for the link. It is an excellent reference.

 

Indeed - a great site and one of the guys there, Bill Eccles is also a very helpful person. The site really does open your eyes into the wonderful world of bolted joints - something we all take for granted all too often, but the science behind it is quite amazing.

Paul

 

Hello from Germany,

sorry, but I think, the column 9.8 should read 8.8, or am I completely wrong ? I'm a Euro mechanical engineer, but still willing to learn and have to admit, that U.S. - specs are not my daily business. I don't want to hi-jack this thread, but it's now too late over here ( 1.00 a.m. ) to do any research

Best Regards

Martin

 

So how would you chose a bolt spec? Take the chassis bolts that secure the a-arms to the forks. As I track my car I am going to replace all 16 of these this winter. My options are:
- use OEM bolts at about $20/each for the 14 shorter and $40/each for the 2 longer
- go to a metric fastener store and buy the same length in 8.8?
- ditto but 10.9?
Philip

P.S., the bolts are 12x1.25. Spec is 7kgm, or approximately 50 ft lbs

 

Philip; I assume you are talking about your '77 308 GTB listed in your profile.
Aren't the bolt heads marked with the spec ?
I have replaced all my suspension bolts on my 308 GTB some years ago with OEM bolts, which now wear a Cavallino :-| instead of the more useful specs.
but I still have the old bolts which are 10.9

If you can source the correct dimensions in 10.9 this is the way to go. No 8.8 please ! But at the time when I replaced my bolts - in 1997 - I had to recognize, that 10.9 bolts with fine metric thread were not much cheaper at the fastener shop than those OEM bolts.

Best Regards

Martin

 

Martin
Yes, thanks.
Philip

 

Don't want to be a party pooper and I know there is a lot of folklore....but a word of warning, the values in the excel table are for dry conditions.

Bolts are better tightened oiled and this means less torque for higher loads. If you put these higher 'dry' torques on (even slightly) oiled bolts then you run the risk of failing the bolt.

Here's a table for oiled bolts. Sorry it's in German, I hope it's clear enough.

Vorspannkraft = bolt load (N), Anziehdrehmoment = tightening torque (Nm)

Happy screwing guys

 

Here's the table (i hope) it was too big first time
Image Unavailable, Please Login

 

Hi Richard, greetings to Switzerland

I feared that , but to spread this issue further; these tables are only 'circa' - values for a first orientation and preselection. Perfect evaluation or calculation of tightening torques also requires knowledge about the thread surface treatment, about how many components are clamped between the nut and bolt, the material from which the clamped components are made of, and much more.

Best Regards from Germany

Martin

 

Martin
greetings to Germany. Have you also got thick snow??

Quite agree with you about bolts, although I don't believe there's much point in trying to get too accurate.

Maybe this will bore the socks of everybody, but as an example of what an effect friction has... recently at work we've been testing the slip torques of a gear on a shaft with a standard tapered joint held on with an M10 bolt. At first it was nitrided - slip torque 150 Nm. Then we removed the nitriding (it shouldn't have been there) and tightened it up, degreased and dry - slip torque 300 Nm. Then we oiled it and the slip torque went up to 400 Nm.

It surprised me anyway..

sinking under a snow drift,
Richard

 

Richard, I would like to send you a PM. What's your native language - if there isn't any in Switzerland : -) , german or french ?

Best Regards

Martin

 

English (German also OK)