Bolt torquing

[chippie]

Can anyone shed any light on the difference between a high tensile bolt ( say an 8.8 ) and a stretch bolt of the same diameter?

I understand that stretch bolts are a one off torque tighten and throw away after use, but I haven’t heard of ordinary ht bolts having to be replaced…..

Typical example of stretch bolt application, front brake callipers carrier mounting bolts, torqued to the required setting on assembly at the factory, but need replacement once removed…It is assumed that the tightening process takes the bolt to its plastic limit on elasticity, so retightening on reuse risks failure….


Discuss…,

[huffo]

Grade 8.8 is simply the tensile stress capacity of the steel used to make the bolt. I believe it’s 560N/mm2.

Stress = force / area
Strain = change in length / original length

The stress-strain curve for most steels begins to level off once yield strength has been achieved. A gross simplification is that the stress-strain curve is a fairly constant gradient line up to the yield point, then it goes almost horizontal to the point of failure. In practice this means that the bolt stretches in direct proportion to the load applied until it reaches yield, whereupon it’s length rapidly increases with barely any increase in load.

If you remove the load before the bolt has yielded, it will go back to it’s original length. However, once stretched beyond yield it’s permanent and it won’t go back.

Increasing the torque on a bolt beyond yield will stretch the bolt a little more but won’t make very much difference to the clamping force. By designing all the bolts to be torqued up beyond their yield strength, the designer can be more confident that all the bolts are applying the same clamping force.

If a bolt is not torqued beyond yield, then theoretically the clamping force is directly related to the torque and the thread pitch. In practice, friction makes the clamping force vary a bit. This is why some designs call for the bolts to be torqued beyond their yield point, because then the clamping force is better known/controlled.

I don’t know what grade of steel is used for stretch bolts, and it could be different in different applications.

You don’t want to use a bolt with different yield strength to what the manufacturer intended in a stretch-bolt situation because you will be altering the clamping force.

If you re-use a stretch bolt, the risk is that it will not stretch at all until you have exceeded the load it was previously stretched with - which is pretty close to it’s failure load.

Cheers
James

[chippie]

Thanks for the enlightenment, certainly gave me something to think about….

[wightdiag]

By designing all the bolts to be torqued up beyond their yield strength, the designer can be more confident that all the bolts are applying the same clamping force.

Ahh so that's why they use torque to yield on things like cylinder heads. Great info thanks!

[Noberator]

Can you feel the force?


May the Force be with you.


Remember.


I'll get me coat.

[OldGit]

A lot of bolts are pre-loaded with threadlock type stuff - these are, or should be, always replaced. Brake caliper and yoke bolts are often this type, certainly Audi caliper bolts are included in the brake pad kits when of the threadlock type and are mandatory change on the caliper yoke.

Most 'stretch bolts' are made with a reduced section, which is the 'stretch' bit, this is to prevent damage to the threads and the head seat, these are usually torqued as the first stage to tighten the fixings, then turned by a specified angle to carry out the stretch. this is normally within the 'elastic' range of the material, moving into the 'plastic' stage can leave the fastener liable to failure.

[chippie]

So to further the discussion, I’ve just watched a video on replacing front discs on a Passat B7 presented by Autodoc.


During the reassembly process, it looks like they reuse the caliper bolts and torque them to 75NM……..


Does anyone else refit the orig fastenings? I thought these were a ‘one shot’ fixing?


I just happen to need to swap out the discs on my Passat soon prior to MoT….

[OldGit]

Some do, some don't - Merc. don't, Audi do, JLR do & don't... Volvo mostly do but don't with some of them on the old Focus based models...

Just to throw something else in, Audi wheel bolts are fitted dry, the rest of VWG are lube'd (there's a specific product to use) but both variants of mechanically the same part are torqued to the same setting.

In the absence of access to the workshop instructions, if it has any type of threadlock (blue or red/pink) or any sign of a reduced shank, I'd replace them, if they're just 'dry' bolts, then re-use should be fine.

The same goes for the use of copperslip or similar on brake parts, if it's not lube'd from new and not indicated in the workshop instructions, it (generally) doesn't need it, the only exception would be to reduce friction where the caliper yoke is corroded enough that the pads may bind and cleaning them up would lead to 'chattering' under braking due to poor fit.

[chippie]

OldGit, OG for short…😁

The only guidance I have on the bolt replacement is that from a Haynes manual….

I think the only time I’ve ever swapped bolts was on a Pinto head…

I usually work on ‘if it comes undone ok, it can go back ok….’ Unless of course there are signs of wear or physical damage…

So my next job is front discs on my Passat….😁

[OldGit]

Well the other thing you could do is ring the dealers and ask if they have the bolts in stock, if yes, then they are regularly sold = change them. if not.....

[valhalla]

May 20, 2022 22:20:39 GMT 1 OldGit said:

Well the other thing you could do is ring the dealers and ask if they have the bolts in stock, if yes, then they are regularly sold = change them. if not.....

The other thing to bear in mind with VAG is that the fixings that secure the calipers are often fitted with 12-point multi-spline female sockets, so it always pays to replace them if you are keeping the car long enough to need to work on those calipers again in the future. The splines are often knackered after corrosion has set-in, and the bit slips a bit, so it's an insurance against finding that you cannot move them in the future.

[chippie]

thnx guys,

I know the rears are the 12 point Star fixings, and are not a stock item (had to order them for the previous Passat....) the fronts are just hex head 21mm a/f, so their removal shouldnt be troublesome.

I managed to swap out the rear discs easily, no need to remove the caliper carrier.....😀

A job for next week....l