Any Bolt mettalurgists out there? - Woodworking Talk - Woodworkers Forum
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post #1 of 21 Old 01-21-2020, 12:30 PM Thread Starter
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Any Bolt mettalurgists out there?

I'm getting mixed information on bolt grading and hoping someone here knows the skinny.

I'm comparing 8.8 grade and 12.9 grade. I'm hearing that while 12.9 bolts may have higher tensile strength, they might be more brittle and thus prone to shear from a side load instead of deform so that in some applications, the 8.8 might actually be the better choice.

Does anyone here know the real poop in bolt grades?
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post #2 of 21 Old 01-21-2020, 02:48 PM
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There are no published shear strength values for ASTM standards. All specifications are based on tensile and yield.

Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.
Yield Strength: The maximum load at which a material exhibits a specific permanent deformation
Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set.

A softer bolt will stretch more and bend a little further before it fails. However, a harder bolt will resist that stretch and bending moment far longer than a soft bolt. Therefore, as far as "strength" is concerned, the higher grade bolt will almost always outperform a lesser grade bolt. So unless you have a critical application that calls for a special high strength bolt, the extra cost is usually unwarranted. Your application dictates the bolt size, grade, and torque value.

There are scores of websites that can explain bolt properties. If you have an application that may be critical, it would behoove you to contact better reference than this forum. Fastener failure can hurt or kill you (or someone you love).

Another $000,000,000.02 worth of advice,
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post #3 of 21 Old 01-21-2020, 03:08 PM
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As is often the case, your inquiry made me even more curious than usual. So I contacted my engineering staff here and got a bit more lowdown.

I knew there was no published shear strength for standards, but there is a "rule of thumb". It is stated that shear strength of a bolt should be approximately 60% of the published tensile strength. Therefore a metric 8.8 grade bolt (sized below 16mm), which has a minimum tensile strength of 800 Mpa (116,030.16 PSI) would shear at around 480 Mpa (69618.096 PSI). A 12.9 Mpa bolt has a minimum tensile of 1220 Mpa (176,945.994 PSI) would shear at around 732Mpa (106,167.5964 PSI).

For SAE size comparison a grade 5 bolt is 120,000 PSI (72,000 shear) tensile and a grade 8 is 150,000 (90,000)PSI.

The higher grade bolts are far superior to the lower grade.

By the way, bolts are rarely used for shear load. Most structural designs rely on friction of the bolted materials for joining. The higher the fastener tension, the more friction imparted on the joined materials. Shear loading is usually unwanted and if a joined connection fails in shear load, it is usually attributed to loose fasteners.
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Another $000,000,000.02 worth of advice,
Mark

Last edited by Shop_Rat; 01-21-2020 at 03:13 PM.
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post #4 of 21 Old 01-21-2020, 03:11 PM
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I've never heard of grade 12......

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Originally Posted by Quickstep View Post
I'm getting mixed information on bolt grading and hoping someone here knows the skinny.

I'm comparing 8.8 grade and 12.9 grade. I'm hearing that while 12.9 bolts may have higher tensile strength, they might be more brittle and thus prone to shear from a side load instead of deform so that in some applications, the 8.8 might actually be the better choice.

Does anyone here know the real poop in bolt grades?
In all the truck repairs and custom work I've done since 1965, I've never found a need for any fastener of a higher grade than 8. This includes spring shackles and other high shear applications in suspension systems.
but, I'm no expert for certain. What is your application for the grade 12 bolts?

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #5 of 21 Old 01-21-2020, 04:13 PM
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Grade 8 is a SAE grade. Grade 12.9 is a metric grade. Apples and Oranges.
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post #6 of 21 Old 01-21-2020, 05:35 PM
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Are they equivalent?

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Grade 8 is a SAE grade. Grade 12.9 is a metric grade. Apples and Oranges.

apples OR oranges that taste like apples?

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #7 of 21 Old 01-21-2020, 07:28 PM Thread Starter
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The application are the bolts that hold car seat slider rails to the floor of the car and to the underside of the seat. Presumably, it’s important that the seat stay put in a wreck.


The bolts that came out were 8.8 - not 100% sure if they’re original, but appear to be. Some have suggested that in a wreck the seat rails could be subject to shear and a bolt that deforms may be better than one that’s brittle and breaks. Others have suggested that higher strength bolts should be used.
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post #8 of 21 Old 01-21-2020, 07:41 PM
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Quote:
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The application are the bolts that hold car seat slider rails to the floor of the car and to the underside of the seat. Presumably, itís important that the seat stay put in a wreck.


The bolts that came out were 8.8 - not 100% sure if theyíre original, but appear to be. Some have suggested that in a wreck the seat rails could be subject to shear and a bolt that deforms may be better than one thatís brittle and breaks. Others have suggested that higher strength bolts should be used.
One of my jobs I did for about 3 yrs is as a driver for a wrecker company and one of the contracts was for the county and state in where we work the accidents. Luckily I dont remember my dreams when I sleep but I work at some really bad accident scenes and the few times I seen where the seat was detach it ripped out of the floorboard and pieces of the floorboard was still attach by way of bolts to the seat. So what ever bolts the automotive industry uses to attach the seats then I would whole heartily go back with them.
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post #9 of 21 Old 01-21-2020, 09:26 PM
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That's what rusty floor panels do .......

Quote:
Originally Posted by Quickstep View Post
The application are the bolts that hold car seat slider rails to the floor of the car and to the underside of the seat. Presumably, it’s important that the seat stay put in a wreck.


The bolts that came out were 8.8 - not 100% sure if they’re original, but appear to be. Some have suggested that in a wreck the seat rails could be subject to shear and a bolt that deforms may be better than one that’s brittle and breaks. Others have suggested that higher strength bolts should be used.

It all depends on the direction of impact whether the bolts will be in shear or tension. A frontal would be tension, a side would be shear, HOWEVER it's not quite that simple. I've made a few custom seat attachments for various hot rods and I worked with automobile seating in another life. The seat belt attachments take the brunt of the tension loads ..... IF they are being used. The seat itself, probably weighs around 75 lbs or so, the human occupant weighs more than that.

If an unoccupied seat comes out of the floor pan in a frontal, it would have to be VERY rusty. The backing plates on seat belt attachment bolts are supposed to survive the collision, rust or not.

You can go to the dealer and get OEM bolts OR have them read the specs for you OR look them up online. Grade 8 SAE is what I've used in my truck and I try to use the OEM bolt holes.

Seat belts work because they restrain the occupant from careening into the instrument panel and or through the front windshield. I've seen actual accidents first hand where the unbelted occupant almost went through the windshield, but for the plastic laminate, she would have. I myself, was in an unbelted frontal collision, my fault and not paying attention to the vehicle in front. My head cracked the glass and when I braced for it, I wrapped the steering wheel almost around to touching the steering column. I was very fortunate. The car in front was dead stopped and turning left, a '59 cat eyed Chevy.
So much for that.
Also remember there are 4 bolts, not just one, and for the seat to come loose all four would have to fail.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #10 of 21 Old 01-22-2020, 10:06 AM Thread Starter
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Also remember there are 4 bolts, not just one, and for the seat to come loose all four would have to fail.

On the car in question, each rail has 7 bolts holding the rail to the seat and 4 bolts holding the rail to the floor. That's 22 bolts per seat!

Maybe that's enough?
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post #11 of 21 Old 01-22-2020, 10:51 AM
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On the car in question, each rail has 7 bolts holding the rail to the seat and 4 bolts holding the rail to the floor. That's 22 bolts per seat!

Maybe that's enough?
And probably 3 bolts holding the seatbelts to the car. Not sure how that fits in to the mix; I just thought it must be relevant somehow :)
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post #12 of 21 Old 01-22-2020, 10:58 AM
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If it's to hold a seat in a car, do these bolt holes already exist as from the manufacturer? Or are you modifying something? If the later, either make sure those bolts go into the frame somewhere or you reinforce the floor pan. Otherwise they could just rip out. I've looked into putting buckets in place of a bench into my plymouth and that's the advice I got from a bunch of old timers. I'm presuming some of them learned the hard way.
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post #13 of 21 Old 01-22-2020, 11:35 AM Thread Starter
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And probably 3 bolts holding the seatbelts to the car. Not sure how that fits in to the mix; I just thought it must be relevant somehow :)
Actually, there's a bracket that holds the seatbelt to the underside of the seat. That bracket is held on by 2 bolts that are part of the 7 bolts holding the rail to the seat.
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post #14 of 21 Old 01-22-2020, 11:42 AM Thread Starter
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If it's to hold a seat in a car, do these bolt holes already exist as from the manufacturer? Or are you modifying something? If the later, either make sure those bolts go into the frame somewhere or you reinforce the floor pan. Otherwise they could just rip out. I've looked into putting buckets in place of a bench into my plymouth and that's the advice I got from a bunch of old timers. I'm presuming some of them learned the hard way.


They're going into their original locations, I'm not modifying anything. When I took the seats out, some of the bolts were a little rusty and some of the heads were stripped, so I decided to replace them all.


I've been able to find zinc plated bolts in 8.8, but the 10.9 and 12.9 I've found are all black steel which would be prone to rust and I don't want that.
The bolts I removed were stamped 8.8, but I'm not 100% sure they're original. I just want to make sure that whatever I put back is strong enough.
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post #15 of 21 Old 01-22-2020, 11:53 AM
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You're kinda overthinking :) . Which isn't a bad thing, but still.


Grade 12.9 is stronger that 8.8...in every way. If heat treated correctly, they don't get "brittle". They may be less likely to stretch than their 8.8G counterparts, but that's ok.


8.8 was original equipment, right? Use those if you're worried about "brittle"...if you want more overall strength, use the 12.9. In an accident, there are SO many other things that are more important to worry about.


I'm not trying to downplay your concerns, but...really...use whichever one you want, it'll be fine.
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post #16 of 21 Old 01-23-2020, 03:46 PM Thread Starter
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You're kinda overthinking :) . Which isn't a bad thing, but still.
I hear ya. The ones I took out were 8.8 and I was planning to replace them with 8.8 when a guy on the car forum said they should be 12.9.

I think the main concern is safety. I'm wondering though --- in a crash bad enough to dislodge the seat, in a small car with no airbags, there might be bigger problems....
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post #17 of 21 Old 01-23-2020, 03:55 PM
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Take a walk round a wreckers yard, I think you may discover the bolts holding the seats down would be the least of your problems with a lot of cars on the road today.
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post #18 of 21 Old 01-23-2020, 04:22 PM
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Quote:
Originally Posted by Quickstep View Post
I hear ya. The ones I took out were 8.8 and I was planning to replace them with 8.8 when a guy on the car forum said they should be 12.9.

I think the main concern is safety. I'm wondering though --- in a crash bad enough to dislodge the seat, in a small car with no airbags, there might be bigger problems....

You can remove the "might" part. There would be much bigger problems. Even so, I wasn't trying to "nay-say" your concerns...they are legitimate concerns, but the difference between 8.8 and 12.9 is....well, I don't want to use the word negligible, but it's kind of like the difference between getting hit by a train with 50 cars or one with 100 cars...there's a difference, but the result to the car is the same.
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post #19 of 21 Old 01-23-2020, 08:13 PM
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Take a walk round a wreckers yard, I think you may discover the bolts holding the seats down would be the least of your problems with a lot of cars on the road today.
The most important of a car is the nut that holds the wheel.
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post #20 of 21 Old 01-23-2020, 08:25 PM
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Quote:
Originally Posted by Quickstep View Post
I'm getting mixed information on bolt grading and hoping someone here knows the skinny.

I'm comparing 8.8 grade and 12.9 grade. I'm hearing that while 12.9 bolts may have higher tensile strength, they might be more brittle and thus prone to shear from a side load instead of deform so that in some applications, the 8.8 might actually be the better choice.

Does anyone here know the real poop in bolt grades?
This may help on shear strength and more:
https://www.eurocodeapplied.com/desi...ign-properties


If not, try Googling "shear strength of metric bolts" - I go over 200K hits on it.
.
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