rodgling
|
| posted on 22/3/13 at 10:18 AM |
|
|
Rotor bolt grade
Anyone know what grade of bolt I should use for attaching hats to rotors? (5/16 UNF).
I would use a Wilwood or AP bolt kit but they both supply Torx head and I have to use a socket cap for clearance.
I'm leaning towards 12.9 but would this be too brittle, is grade 8 or 10.9 a better choice?
10.9 socket cap seems impossible to get hold of so if 12.9 is safe I'll use that.
|
|
|
|
|
britishtrident
|
| posted on 22/3/13 at 10:59 AM |
|
|
The torque between disc and bell should be transmitted by friction alone so as long as the bolts are properly tight the bolt grade shouldn't
matter that much.
8.8 would do the job as would 12.9.
If you use 12.9 just be careful not to over torque.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
rodgling
|
| posted on 22/3/13 at 11:16 AM |
|
|
Thanks BT.
Wilwood specify a torque of 25 ft-lbs, which is what you would use for a 12.9 bolt. And I've just spoken to Hi-Spec who told me they use grade
8.8...
Decisions... now I'm leaning more towards 8.8 to avoid the brittleness concern.
|
|
|
rdodger
|
| posted on 22/3/13 at 12:50 PM |
|
|
Won't 12.9 rust a lot quicker?
|
|
|
rodgling
|
| posted on 22/3/13 at 01:28 PM |
|
|
Superficially, probably 12.9 will rust a bit, not a big concern though.
I've found some M8 bolts which are grade 8.8 - that's 0.06 mm bigger than 5/16", can't see any reason why I shouldn't
use these instead? (the hat and rotor are unthreaded, I will be using an all-metal locking nut on the other side).
|
|
|
snapper
|
| posted on 22/3/13 at 01:42 PM |
|
|
Metric and UNF thread pitch is different
DON'T BODGE BRAKES
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
|
|
|
rodgling
|
| posted on 22/3/13 at 01:48 PM |
|
|
Yes, obviously - I'm not suggesting using a UNF nut on a metric bolt!
I mean using an M8 nut & bolt through the unthreaded holes in the hat & rotor.
|
|
|
mcerd1
|
| posted on 22/3/13 at 02:07 PM |
|
|
quote:
Originally posted by rodgling
Yes, obviously - I'm not suggesting using a UNF nut on a metric bolt!
^^ he means the torque settign won't be right - as it depends on the thread pitch !
-
|
|
|
rodgling
|
| posted on 22/3/13 at 02:09 PM |
|
|
Sure, but I can calculate/look up the correct torque setting easily enough. The only thing I can see would be having 0.06 mm less hole clearance
around the bolt, which doesn't strike me as an issue?
|
|
|
rodgling
|
| posted on 22/3/13 at 03:07 PM |
|
|
Well, in the end I was able to find some 5/16" grade 8.8 bolts so job done.
Unfortunately they are fully threaded (no shank) which is not ideal but given that HiSpec use 8.8 M6 bolts and mine have a minor diameter of more than
6mm I think this will be fine.
[Edited on 22/3/13 by rodgling]
|
|
|
mcerd1
|
| posted on 22/3/13 at 04:04 PM |
|
|
quote:
Originally posted by rodgling
....in the end I was able to find some 5/16" grade 8.8 bolts....
are you sure they are 8.8 - as thats a metric grade ?
(ISO 898 is the spec)
UNF and UNC should be to BS 1768, in grades P, S, T, V or X (A or B are possible, but much weaker) - this should be marked on the head of the bolt
(the nuts should be marked 0, 1, 3 or 5)
(ok thats Hex head bolts - but I'm sure the grades are the same...)
having said that weve been having a few issues with UNC and BSW bolt grades recently 
[Edited on 22/3/2013 by mcerd1]
-
|
|
|
rodgling
|
| posted on 22/3/13 at 04:11 PM |
|
|
Well, they are sold as such. TBH a lot of UNF socket caps sold in the UK seems to be sold as class 12.9 rather than grade whatever
High Tensile Cap Head Bolt 5/16"' UNF Thread 1"' Long Fasteners Workshop Car | eBay
|
|
|
mcerd1
|
| posted on 22/3/13 at 04:42 PM |
|
|
sold as maybe - but whats actually writen on the bolt ? (all bolts must have the grade and maker marked on them)
but I guess that someone could be making weird hybrid bolts with metric material and inch sizes 
alot of bolt suppliers seem to know less than they should about the grades
we had one try and tell us that a grade 8.8 was the same steel as S355 and the old BS916 bolts
when you look it up BS916 is not that far off a S355 (still not the same mind) but then S355 isn't a bolt grade at all !
and the nearest ISO898 bolt material is somewhere between a 4.6 and a 4.8 
-
|
|
|
rodgling
|
| posted on 22/3/13 at 04:48 PM |
|
|
Is it very unusual for imperial sized bolts to be made with metric material specifications? It doesn't sound unreasonable?
Don't know what's marked on the bolt, I will check when they arrive.
|
|
|
Not Anumber
|
| posted on 22/3/13 at 04:54 PM |
|
|
Rotor ? thought this was a thread about the new Locost helicopter for a minute there.
|
|
|
rodgling
|
| posted on 22/3/13 at 07:12 PM |
|
|
Just read the receipt and they're rebranded Demon Tweeks bolts (at twice the price), grade 12.9. Bugger. Back to square one.
|
|
|
MarcV
|
| posted on 25/3/13 at 12:13 PM |
|
|
Can't really tell why you wouldn't want to use 12.9 instead of the 8.8s ? I know the 8.8 is more prone to bending, but the 12.9 needs such
a lot more force before breaking than a 8.8 that is should always be a better choice, no?
Should probably also not be that difficult to calculate the actual load on these things? Take the brake torque (for example assuming 70% bias on the
front, 1.1g deceleration and 1.0 friction coefficient of tyres). Along with the disc bolt radius you will find a force between hat and disc, which
probably is loading 2 bolts at most?
shear strength of the different bolt grades is well known.
|
|
|
rodgling
|
| posted on 25/3/13 at 12:30 PM |
|
|
Marc: I asked Hi-Spec about 8.8 vs 12.9, and they thought that 12.9 would be more brittle (which was my concern too) - apparently they use M6 8.8,
even on much larger systems. So I think it's very safe to say that M8 8.8 will be strong enough. I suspect as BT said, either would be fine.
I have to use socket caps for clearance reasons (otherwise I'd use Wilwood/AP/etc supplied bolts). In the UK, these are only available in
ungraded or 12.9. I've ended up ordering some class 8.8 bolts from Ebay in Germany - this seems to be the easiest answer.
Also, it turns out that in M8, I can get a bolt with a shank of a suitable length, whereas in 5/16 I can't. So M8 it is.
In principle the hat and rotor won't move relative to each other, so there can't be any shear load at all? Or is my lack of mechanical
engineering background showing...?
|
|
|
Grimsdale
|
| posted on 25/3/13 at 01:36 PM |
|
|
To comment on a few misconceptions in this thread:
I don't understand the impression that 12.9 bolts are brittle. They are inherently ductile.
The strength of a fastener is primarily controlled by the heat treatment, not the composition, so an 8.8 grade bolt could be made from the same raw
material as a 12.9 bolt.
The corrosion resistance of the bolt is not affected by the strength of the fastener.
Socket head cap screws are generally made to 12.9 grade as the reduced cross section of the head leads to a weaker design. It's rare to find
anything else.
[Edited on 25/3/13 by Grimsdale]
|
|
|
rodgling
|
| posted on 25/3/13 at 02:04 PM |
|
|
I'm no expert and please correct me if I'm wrong - but my understanding is that higher grade bolts are stronger (12.9 is about 50%
stronger than 8.8) but more brittle.
From the Steel Designer's Handbook:
http://books.google.co.uk/books?id=df7R9rT3nmUC&pg=PA200&lpg=PA200&dq=8.8+12.9+brittle&source=bl&ots=GZ9e9dct8E&sig=Bdwz1
mnsItEmU_besMCLThvy-qU&hl=en&sa=X&ei=RlhQUYDDCai70QX7joDYDA&ved=0CDYQ6AEwATgK#v=onepage&q=8.8%2012.9%20brittle&f=false
|
|
|
Grimsdale
|
| posted on 25/3/13 at 02:37 PM |
|
|
You're sort of right, but it's more a case of 'less ductile' rather than 'more brittle'. The bolt is still ductile
and as such will plastically deform before overload failure.
The link you've provided relates to hydrogen embrittlement. This can occur with 10.9 and 12.9 grade bolts that have been poorly electroplated,
without a satisfactory post-plating baking step, which is needed to drive off hydrogen which can get absorbed by the steel.
It can happen, but it's a manufacturing defect, not an inherent property of the material. If you're really worried about it, use an
unplated bolt.
Andy
|
|
|
MarcV
|
| posted on 25/3/13 at 08:37 PM |
|
|
Oke, just a quick sanity check;
Stopping your car at say 800kg, 70% at the front, 1.1g deceleration gives a brake torque of about 500Nm per front wheel.
Bolt radius for the disc is about 80mm, so that's 6.25kN if it were a single bolt
Single shear strength of M8 8.8 (or 5/16" grade 5)
25kN on the shank, about 16kN on the thread
For 10.9 / grade 8:
33kN on the shank, 21 kN on the thread.
Base on these figures it really makes sense to buy the bolts with shanks (shouldered bolts, is that correct, we'd call them borstbout). Then the
load applied on max braking is 25% of the breaking strength of M8 8.8 and should be fine.
Dave, as far as I can see, on these discs the bolt is the only connection between disc and hat. Therefore the full brake torque / force is transferred
through these bolts (in single shear) from disc to hat.
|
|
|
Grimsdale
|
| posted on 25/3/13 at 09:39 PM |
|
|
MarcV
A bolt has a shank and a threaded region. A screw is a fully threaded bolt. A shouldered bolt has a shank of larger diameter than the thread.
Clear as mud, but hope it helps (plus most people get it wrong anyway).
Andy
[Edited on 25/3/13 by Grimsdale]
|
|
|
rodgling
|
| posted on 25/3/13 at 11:47 PM |
|
|
quote:
Originally posted by MarcVBase on these figures it really makes sense to buy the bolts with shanks (shouldered bolts, is that correct,
we'd call them borstbout). Then the load applied on max braking is 25% of the breaking strength of M8 8.8 and should be fine.
Dave, as far as I can see, on these discs the bolt is the only connection between disc and hat. Therefore the full brake torque / force is transferred
through these bolts (in single shear) from disc to hat.
Thanks for the numbers, I will indeed be using shanked bolts, stupid not to really. 12.9? 8.8? Not fully decided yet, but your numbers make me
confident that either would be easily enough. Andy: I take your point about brittleness, looks like self-coloured 12.9 is the best choice?
Disagree about the shear force - my thinking is: the bolt clamps the hat to the rotor, placing the bolt in tension along its length. Under braking,
the friction between the hat and rotor prevents them from sliding against each other, NOT the bolt resisting a shear force (it would only see a shear
force if the rotor slips against the hat?). So I think that the bolt only sees the tension along its length? Similar to how wheel bolts work I guess.
|
|
|
MarcV
|
| posted on 26/3/13 at 07:30 AM |
|
|
Andy,
Even more confusing are the difference between languages. For us a bolt (bout) is defined as a threaded fastener which is torqued from the outside of
the head (ie hex head), where a screw (schroef) is torqued from the inside (ie torx, philipshead).
Dave,
With the braking forces estimated now you could well be right. M8 8.8 with 17kN preload should resist about 10kN of braking force before the disc will
slide relative to the hat. Ofcourse the preload changes over temperature considerably, but if it wouldn't hold, the bolts should still survive
well.
Overall no reason to foresee problems with 8.8 and these are best available (at least overhere)...
|
|
|
