Countersunk Cap Screw - Critical Fasteners

[Ampto]

Hey all, Patrick From Eureka Racing Team, Car is progressing slowly, but hopefully surely for this years Australian competition in December

I have an issue with clearance for bolts inside my upright design to attach the brake rotor. I have suffice clearance if i attach the rotor with countersunk cap screws, but I'm confused with rule T11.1.2 which states flat heads are not permitted, but it doesn't say flat head cap screws or what i call countersunk cap screws

Does the rule apply to countersunk cap screws?

Otherwise i'll have to change a few things, not a huge amount of work but just annoying as really like the way it is.

Cheers Patrick

Eureka Racing Team
The University of Ballarat

[Francis Gagné]

T11.1.2
The use of button head cap, pan head, flat head or round head screws or bolts in critical locations
is prohibited. These locations include the driver’s cell structure, and driver’s harness attachment.

Note: Hexagonal recessed drive screws or bolts (sometimes called Socket head cap screws or Allen
screws/bolts) are permitted.


My interpretation is that your bolt is not in a "critical location" as defined by this particular rule. SO it does not apply to you. Anyway, neither does it imply that countersunking is prohibited, as long as the bolt used is not one defined explicitly even in "critical locations". It seems that your design is compliant with the rules, however, as usual this is only my opinion and is totally officious.

[Michael Royce]

Patrick,
If I was the official, I would disagree with Francis and say that attaching a brake rotor WAS a "critical location".

However, I would agree with him that the only way to be sure is to write to the FSAE-A Rules Committee.

To be sure, do NOT go to the competition without either fixing it or getting the OK from the FSAE-A Rules Committee. It is not something you want to be changing at the competition!

A general word of advise to everybody, STAY AWAY from these fasteners when you are designing your cars. Don't even have them in your shop! The reason they have been banned in "critical" locations is that is that the amount of material between the bottom of the recess for the Allen wrench and the underside of the head can be very thin and hence the manufacturers of the fasteners have said publicly that they are not to be used for certain usages - I think the word was "structural".

[Francis Gagné]

I re-read his post, and I confused his meaning of coutersunk cap screws.

I thought he meant a SHCS (example http://www.mcmaster.com/#91290a326 ) which sits in a countersunk hole. If he indeed related to a flat head socket cap screw (FHSCS) (example http://www.mcmaster.com/#91294a240 ) then yes I agree they are not good practice in high strength application (even McMaster warns you so...)

Michael, it seems IMHO that the definition given of "critical location" is wider than that given in the rule. Yet the T11.1.1 is much more specific. Maybe it should read : ...includes, but is not limited to, ...? or am I missing something big here?

Anyway in the case of FHSCS in structural/safety related applications would fall under the Good Engineering Practices rule A1.3 I guess.


Oh, and Patrick, There are other common ways to attach a brake rotor than threaded fasteners that does not take much place or mass http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Drew Price]

Originally posted by Francis Gagné:
I thought he meant a SHCS which sits in a <STRIKE>countersunk</STRIKE> counterbored hole.


Fixed.

[PatClarke]

Ampto,

Submit a request for clarification to the Australian Rules committee.

Cheers

Pat

[Ampto]

Thanks guys, I've emailed off a request for clarification.

In the mean time I'll try and convince the brake guy to go to floating rotors rather then fixed so i don't need to fix the rotor to the disc hat with fasteners.

I don't think strength of the bolts will be an issue as it is fastened with 6 x M8 which is quite conservative. Interestingly my VTR1000 uses button heads to fasten the rotors, and there are about M4 or M5 in size :S

[Kevin Hayward]

Ampto,

You have raised a pretty interesting situation. Clearly the brake disc attachment is critical, however the likely loading condition would indicate that it is extremely unlikely that the bolt would be loaded excessively in the direction that would cause failure of the bolt. It is essentially a transfer of shear force through the friction contact of the disc to the mounting face (or it should be). Does that then make the bolt critical?

That being said I would suggest two things:

1. Floating discs are pretty easy to implement and probably have a wider range of suitable calipers to choose from for the average FSAE car.

2. Get a rules clarification if you want to stay with floating caliper (or at this stage if it is even an option). Send plenty of pictures and adequately describe your design and its intent. Clearly communicated clarifications will save you time.

Kev

[Bemo]

Originally posted by Francis Gagné:
T11.1.2
The use of button head cap, pan head, flat head or round head screws or bolts in critical locations
is prohibited. These locations include the driver’s cell structure, and driver’s harness attachment.


Well I'd say it is pretty clear, that with the use of the word "include" it is not meant that these are the only locations, the rule referres to. The first sentence is pretty absolute. These bolts are forbidden in ALL critical locations - without exceptions. And wether the attachment of the brake rotors is critical or not is something you can hardly argue about.

If I was to scrutineer the car, I definitely wouldn't accept it.

[atm92484]

Pfffft...back in my day we had a set of Keizer wheels that needed to come apart for re-sealing. Low and behold the original bolts seized to the magnesium centers.

Our solution: use a countersink to remove the damage and install countersunk bolts.

No one died.

[Bemo]

Especially for wheel centers I have personally witnessed (and I'm not the most experienced guy) several examples for failures which lead to dangerous situations. You can ask Pat about the wheel from Paderborn, I think at FSG '09 or '10.

You should not underestimate this topic and be really careful with this type of bolts. Just because there is an example of the usage without problems doesn't mean it isn't critical at all...

[Ampto]

Just an update,

The Rule Committee considers the rotor connection critical, which i 100% agree with.

i even got a definition of what they consider a critical fastener, any bolt which is a fully loaded bearing. This is of course open to your interpretation.

The only type of socket head cap fastener which is allowed is just the standard type in critical locations

Its ok, no biggy, now i can tell the brake guy to go floating like i originally wanted haha.