Metallurgist opinion needed please
 

OK,
Most of us know that a grade 5 is not as strong as a grade 8, BUT, 8's are more for vertical pull/torque and not for shear or horizontal torque.

In question, a (2" x 1/2), bolt at the head, which would would give more strength mounting a shoulder belt? (Bolt is threaded in approximately 2"s.)

DV

While the tensile strength of graded fasteners is specified, their toughness (ability to absorb energy) is not. However, I would think that the Grade 8 is preferable in this instance if it is torqued to specifications. As long as the (sandwich) compression of the belt bracket is effective, the Grade 8 should work well.

A quick Google search yielded the following web site that recommended Grade 8 bolts for stock car racing applications.

http://www.stockcarracing.com/howto/...ion/index.html

Bolt grading
 

DV, To begin, the bolt grades describe the working stress, yield stress and tensile (axial direction) properties of the bolts. see attached chart from a bolt vendor. I am not a seatbelt expert and I have wondered the same question as I am planning my seat belt mounts. The answer to your question will depend on the design of bolted joint, ie. how the seat belt is being bolted to the frame, I have seen many different solutions on these cars. But in general bolted joints need to have strength in both bending (shear) and axially. For seatbelts, if you take look at modern car designs, the typical 3-point system is set-up whereby the bolted joints are set-up to provide most but not all of the strength in SHEAR or in other words, in a crash the 3-point seatbelt loads when transferred to the bolts are trying to rip the head of the mounted bolt off in shear. (many seat belt buckles are bolted to or part of the seat base frame). The bolted joint needs to withstand alot of "G" loading in a crash beyond 10 G's in many cases (depends on the design scenario since, Force =mass x accel). I have noticed most aftermarket and some OEM seatbelt hardware is Grade 5 for street applications but I dont know what the professional class racers are using as far as bolt grades. Others who do know should add to this thread.

Are there any bolt stress scenarios in which Grade 5 would be stronger than Grade 8, or is Grade 8 always stronger and you are just trying to decide whether Grade 5 is enough?

Doug,
My thinking with my limited knowledge (as shown already) is that the grade 5 MAY be better than an 8 strictly for shear! ??? Now "kitcarb" etc. has me doing some hard research- thank you guys. Will read them all!
DV

doug,

I'm reaching a long way back to my college days, so cut me a little slack if I don't get all of this exactly right. Engineers consider many properties when evaluating the appropriateness of a metal for a particular job. Those properties include hardness, toughness and britlleness among others. Grade 5 bolts don't have as much tensile strength (resistance to pulling) as Grade 8, but they are typically tougher and less brittle than Grade 8. So, a Grade 8 bolt is better for an application that will not be exposed to continuous on and off stresses (fatigue) and needs to be as strong as possible for occasional stresses. But a Grade 5 bolt might be better for an application where fatigue is more of a concern. So, it's not as simple as saying Grade 8 is always better than Grade 5. Given the way seat belt bolts are used, I used Grade 8 in my car.

The devil's in the details... do you want it to bend, or do you want it to break. Either grade could work with a properly designed connection.

To a degree, you're splitting hairs here between grades 5 & 8. The nominal compositions of each could be identical/overlapping. Same for the mechanical properties. Hard to say what composition you'll actually GET if buying bolts at the hardware store, tho economics dictate that the manufacturer will use the cheapest route possible: either the lowest alloy content that's heat treated to its limit, or a higher alloy content with a more mild heat treat... whatever gets them the strength they need for the lowest cost.

Yes, the Grade 8 bolt is marginally stronger, but this strength comes at the expense of ductility (elongation)... so it will be less ductile/more brittle (~2% difference).

A given bolt material could be heat treated to either Grade 5 OR Grade 8 requirements.

Hardness, yield & tensile strength only tell you part of the story. For a static application with axial loading, grade 8 should provide the highest strength. For the application in question, I'd look at toughness... since dynamic/impact-type loads would be of primary concern. Specifically, I'd look at the Charpy V-notch values at room temp. Granted, Charpy values are primarily an indicator of notch sensitivity, but they also provide an indication of general toughness.

If you're looking at shear-type applications, I'd try to design the connection so that all threads closest to the load application are engaged, thus minimizing notch sensitivity in the threads.

There's several ways to skin this cat...

Easy answer - neither one.

Use AN. Available at www.PegasusAutoRacing.com .

I also recommend that you read Carroll Smith's series of books Prepare to Win, Engineer to Win, etc. for a detailed explanation of WHY.

Circle Track Tech Article- How the Pros do it
 

Take a look at these 2 articles. Seems the pros use Grade 8 fasteners. Here are two articles I found with useful info from the racing community in CIRCLE TRACK.

http://www.circletrack.com/safety/ct...ide/index.html

http://www.circletrack.com/safety/ct...unt/index.html

Based on my research so far, I am leaning towards the Schroth 4 point belts for my Cobra for street duty only. If you want to see the R&D that goes into their belts, check out their web site too, cool stuff.

Bill

This questions comes up a lot, and there's no clear cut answer.

As mentioned above, Carrol Smith's, "Nuts, Bolts, Fastners, and Plumbing Handbook" is an excellent read. You'll learn more about fastners than you thought there was to learn. He wrote in easy to understand terms, so even I could learn from it. http://www.amazon.com/Fasteners-Plum...0&sr=1-1-spell

In his book, he states that grade 8 is for clamping only. It should never be placed in shear - even double shear - because it can fracture. The 8's should only be used where there's a dowel to prevent lateral movement.

He goes as far as to say, "There is no place on a race car for a Grade 8 fastner." Not sure I agree with that statement, but there it is.

As far as the seat belt goes, I think if you manage to break a grade 5 or 8, it won't matter. The belts would have cut you in half already.

Wow, I had no idea there was so much to this.

It gets better. ;)

http://link.brightcove.com/services/...id=29739765001

The really important part (pertaining to correct bolt installation) starts at 2:15 into the video.

Rodger

CSX4068
I have used Pegasus racing products for years, (lot of pieces for the DV's came from Pegasus) but I never gave a thought to AN bolts... Went to Rodger's site. Watched the video on AN for the HOMEBUILDER.... thought that was strange until I realized he was talking about homebuilt AIRPLANES!

BASIC information that all of us should know!

'VanGilder says that the seatbelt mounts should also allow the belt to swivel. This keeps the belt in a straight line between the mounting point and the buckle. In the event of an accident, the force of the driver's body against the belt is distributed evenly." "Circle Track"
**
Bob, Carrol Smith's writings are nothing new to me, but I do believe a lot of his writings have been upgraded over the years.

** This is one of the reasons I use shoulder bolts, so the belts can swivel at impact.
DV

Randy,

Smarty freakin pants....

i was gonna say that...:p

whats going on brother? havent seen ya around lately

Jason

Have you considered "Race Bolts"?


http://www.racebolt.com/

ohekk,
No, never heard of them before. Another good reason to be informed. Tubular bolts-hmmm?

At this point since I have them in stock I think I will go with 8's on the top with proper shoulder and threads. I have tapped the first contact (frame), after the bolt is screwed in place and properly 'set' I will then put some type of lock nut in place. Thinking now that the lap belts will have 5's, and use 8's on the shoulder belts.

Comments?
And thanks to everyone that contributed to my keeping my ignorance to hopefully tolerable levels!

DV

I have to say if you are in an accident that shears off a good size grade 5 bolt or grade 8 bolt I think you are dead in impact anyway so why worry.

The hardened bracket that the webbing is attached to might give way before the bolt does? You are only as strong as the weakest link.

jams,
I build a lot of custom cars for my customers, (street rods, cobras and total customs). Liability is a major concern, let alone the 'agony' of a possible death or serious injury due to saving a $1.99 on assembly! Hard enough to sleep as it is! "Just do the best that you can and leave the results to providence."

Love your avatar!
DV

This place has a great selection of about anything you can imagine. Quality? If it was good enough for my plane, I figure it's good enough for my car.

http://www.aircraftspruce.com/

Go through the "Product Index" and look around. You'll be amazed at what you will find under the "Electrical" and "Hardware" tabs.

Here's the section on AN bolts.

http://www.aircraftspruce.com/menus/ha/bolts.html

After knowing about all of this "Right Stuff", you'll never go to the local hardware store again!

Rodger