grades of steel for bolts

[robmar]

what is the difference between 10.9 and 12.9 grade bolts in terms of metal specification

 

They are to be used as engine mount bolts (to chassis) on my busa if that makes any difference...

 

rob

[Guest]

Generally the higher the scale the stronger they are.... BUT its not that simple, as hardness can also bring brittleness. For some applications a lower grade is preferable as they have some stretch or flex before failing. So if you have been advised to get 10.9s then it does not necessarily mean that 12.9s are better in that application.

 

We use American Grade 8 bolts on skateboard trucks, and over years of use I know that higher rated grades, even titanium ones, are more likely to break.

 

I'll leave it to an expert to advise what is the best grade for your particular application.

 

 

Edited by - Stridey on 16 Jan 2014 09:17:02

 

Edited by - Stridey on 16 Jan 2014 09:21:39

[oilyhands]

An 8.8 bolt has a breaking strain of around 150,000lbs/sq in. I am sure they are suitable for you application.

 

Oily

[robmar]

got sent this info by another member offline http://www.blatchat.com/t.asp?id=194931 thanks

 

the application if you can imagine is similar to a K except there is no rubber bush between the engine mount and the chassis IYSWIM

 

rob

[Birkin S3 ZA.]

your mild steel bolts 4,8 is 07oM20 steel, your 8,8 bolts have more carbon 080M40 grade.

Once you get to 10,9 there made from 1% chrome moly steel depending on heat treatment can be from 800/1200 MPa.

12,9 from 1 1/2 % chrome moly and Nickel with some Molybdenum as well 817M40

900/1550 MPa depending on treatment.

Chris.

 

 

 

[charlie_pank]

Quoting oilyhands: 

An 8.8 bolt has a breaking strain of around 150,000lbs/sq in. I am sure they are suitable for you application.

 

Oily

 

 

That's sounds fine at first reading but they're probably mostly going to be in shear. What's the surface area of the parts of the bolt involved in the shear?

 

As mentioned above, as the bolts are 'tougher' they can be more likely to suffer a brittle failure. Also, it's estimated that 90% of failures are fatigue rather than stress failures, so failure can occur well below 150,000lbs/sqin. :(

 

Edited by - charlie_pank on 16 Jan 2014 13:46:52

[juleslid]

While we're at it, what about stainless steel bolts?

Like the idea of them, but are they likely to be more brittle, or have other undesirable traits?

Cheers,

Jules

 

[AdamQ]

What's the surface area of the parts of the bolt involved in the shear?

 

Assuming I've understood the question correctly, that's the area of the coss-section of the bolt at the point where the two parts it's holding together meet (and are trying to go in opposite directions), i.e. pi × r × r. To complicate matters slightly (or maybe it simplifies it?), depending on the configuration, some of the shear load might be taken by friction between the joined components owing to the clamping action of the bolt so the bolt will not be taking the entire load.

 

as the bolts are 'tougher' they can be more likely to suffer a brittle failure

 

I don't think that's quite the right way round is it - see Stridey's post - very generally (and for a given material), a stronger bolt is more likely to suffer a brittle failure, but a tougher bolt is less likely to suffer a brittle failure (almost by definition as toughness could be considered to be the direct opposite to brittleness).

 

As you say though, fatigue rather than strength and toughness is likely to be the governing criterion ...

 

Adam

[oilyhands]

IME stainless is much more brittle, has less tensile strength and is much more likely to suffer from fatigue.

 

Oily

[Birkin S3 ZA.]

Stainless looks the part but don't use for anything where you need strength holding critical parts together, engine mounts included there to soft.

Chris.