High Tensile steel bolts

[oldbutnotslow]

I have just replaced an alternator-tensioning bolt with yet another one of 8.8 hardness sold as high tensile. How hard is this in the scale of hardness?

 

Grant

 

😬 183 BHP of black and stone chip excitement. 😬

here

[Bozz]

Depends on the size but roughly between 117 to 302HV Vickers (BS3692).

 

Hope this helps.

 

Bozz

 

It's not MANGO it's TANGO (McLaren Orange and Black 1.6SS)

[oldbutnotslow]

Thanks Bozz, not much! What I was looking for was "is it hard, very hard or made out of tissue paper!"

 

Grant

 

😬 183 BHP of black and stone chip excitement. 😬

here

[Bozz]

Hardish 😬

 

Bozz

 

It's not MANGO it's TANGO (McLaren Orange and Black 1.6SS)

[Simos]

Bozz, you're not a management consultant are you ??

 

If not you've missed your calling

[Paul McKenzie]

8.8 is 'normal'.

 

12.9 is 'high tensile'

 

Paul

[rj]

4.6 to 4.8 is normal.

 

8.8 is when you start marking them harder than soft ...

 

 

/r

[CageyH]

Would a titanium bolt help?

Would this be less susceptible to shear?

[oilyhands]

10.9, 11.9 and 12.9 are prgressively tougher, 8.8 might be deemed hard but relatively speaking they are cheese, use a 10.9 since 11.9 and 12.9 are not always available in common siuzes/head configurations.

 

Oily

[susser]

From what I've seen on OBNS's bolt it's not shear but tensile failure.

The load on that bolt is a kind of single shear as it only needs a single shear to fail, but there are 2 shear planes, (Could even be 0.5 shear ) but it seems there's a lot of tensile stress as the spacer tries to rock.

Maybe welding the spacer to the bracket would reduce the tensile load as the spacer could no longer rock, and it would be in true single shear, if you see what I mean.

Kind of type stuff

[oldbutnotslow]

Thanks guys another trip to the fastener factor will be undertaken tomorrow to get a 10.9 Interestingly the second failure is within a mm of the same sheer point as the first bolt

 

I've checked the alignment which appears ok albeit from a fairly cursory inspection. Ive never used a lock washer stupid as it may seem as it is not supplied in the 'kit' from CC. I believe that I have therefore over tightened the bolt in an effort to compensate. I have not used Locktight on the premise that it would be a bugger to get the broken part of the bolt out should it break again. Add to this a perhaps an over tight fan belt and all the portents for another failure look to be there particularly when you consider that there is considerable leverage pressure from the over tight fan belt on the bolt as the thread inserted in the engine block is in the ratio of around 3 if not 4 to 1 with that passing through the bracket and spacing bobbin.

This time I've put a splt washer on the bolt and have the fan belt adjusted more generously.

Thanks for the help

 

Grant

 

😬 183 BHP of black and stone chip excitement. 😬

here

[Bozz]

As already said I think....

 

from a tensile 'strength' point of view Grade 4.6 is the lowest and 14.9 the highest and the 'hardness' increases accordingly. In 'our' industry 8.8 is considered to be 'high tensile'.

 

I remember reading somewhere that the higher tensile grades are much more sensitive to over tightening, so beware.

 

 

Bozz

 

It's not MANGO it's TANGO (McLaren Orange and Black 1.6SS)

[Griff]

Hardness is not really relevant. The gradings relate to tensile (breaking) stress and the percentage of tensile stress where the bolt yields. For a 4.6 grade the "4" relates to a tensile strength of 400N/mm² and the "6" relates to yield occuring at 60% of tensile. So yield strength of the 4.6 is 400 x 0.6 = 240N/mm². For little difference in cost and useable properties an 8.8 bolt has a yield of 640N/mm² or more than 2.6 times greater.

 

When you get into the realms of 10.9, 12.9 or 14.9 be very careful as these can become more brittle - one reason why they aren't used much on cars. I would suggest you check you're using the right size bolt (sounds stupid but if somebody else replaced it before you, did they use the right replacement?) and thick washers (thin washers may bend causing the bolt to loosen). Also don't lubricate the bolt as this will cause it to slacken for sure. To lock the fixing, preference is for a second "lock" nut. Nylock is OK but will still shake loose and spring washers are worst of all. If possible, can you use a fine thread? Standard is metric course pitch but a finer pitch has less of a gradient so is less likely to undo itself under vibration.

 

Also sounds possible you might be bending the bolt, trying to compensate by overtightening it? This can cause another mode of failure known as "bottlecapping" but that normally occurs right near the head of the bolt where it connects to the shaft. Have a close look at the quality of the bolts too - there should be a small radius (to relieve stress concentrations) where the head joins the shaft but we see some dreadful cheap imports that very badly made!

 

Good luck.

 

It's not pink, it's medium red violet... 😬