Bolt Strength

[Nev62]

I just changed a Ball Joint and the bolts were stuffed (2 streched and one snapped). The number on the orignal bolts is grade 10. I replaced replaced these with 8.8s.

Question, apart from the 10 indicating an automotive bolt, is there any difference in tensile strength between 10 and 8.8 (should I be worried?).

[-Scott-]

A grade 10 bolt is stronger than a grade 8, but any bolt can be snapped if used inappropriately.

The biggest killer of bolts is lubricating the thread, then torquing up. Torque settings are usually for dry bolts, when the bulk of the tightening torque is used to overcome friction in the threads, and the rest becomes tension in the bolt. If the threads are lubricated and the bolt torqued up to the same level then less torque is used to overcome friction, and more becomes tension in the bolt.

Of course, some people have no idea what a torque wrench is, so they take a large spanner and make it as tight as they can.

Were you the one who installed the dead bolts?

Cheers,

Scott

[Nev62]

A grade 10 bolt is stronger than a grade 8, but any bolt can be snapped if used inappropriately.

The biggest killer of bolts is lubricating the thread, then torquing up. Torque settings are usually for dry bolts, when the bulk of the tightening torque is used to overcome friction in the threads, and the rest becomes tension in the bolt. If the threads are lubricated and the bolt torqued up to the same level then less torque is used to overcome friction, and more becomes tension in the bolt.

Of course, some people have no idea what a torque wrench is, so they take a large spanner and make it as tight as they can.

Were you the one who installed the dead bolts?

Cheers,

Scott

The bolts have been in and out a few times (thanks to dead CVs and CV boots). The last time my son gave me a hand and did them up (with what I don't know).

Will the 8.8 bolts be ok?

[ISUZUROVER]

The number on the bolt head is the tensile strength of the bolt in Newtons/square mm/100. So to get to units you can deal with easily (kg/mm2), multiply by 100 and divide by 9.81 (or 10 if you are being rough).

So a grade 10.8? ("grade is not really correct for metric bolts) bolt is about 110kg/mm2 and an 8.8 bolt is 90kg/mm2. So an 8.8 bolt is about 82% of the ultimate tensile strength of a 10.8 bolt.

What size bolt is it? The larger the bolt the less the difference will matter. It is likely if they were stretched they had been overtightened, or tightened too many times.

Out of interest, calling these bolts grade 8 and grade 10 is not really correct, because an imperial grade 5 bolt (3 marks on the head) is roughly equivalent to an 8.8 metric bolt, and a grade 8 (6 marks on the head) is roughly equivalent to a 10.8 metric bolt.

[Nev62]

What size bolt is it? The larger the bolt the less the difference will matter. It is likely if they were stretched they had been overtightened, or tightened too many times.

Out of interest, calling these bolts grade 8 and grade 10 is not really correct, because an imperial grade 5 bolt (3 marks on the head) is roughly equivalent to an 8.8 metric bolt, and a grade 8 (6 marks on the head) is roughly equivalent to a 10.8 metric bolt.

Bolts are 20mm long, unsure of the diam but old bolts used 12mm spanner new heads are 13mm. These are the 3 bolts that retain the upper ball joint.

[bogged]

After seeing a dudes front end fall off his Paj up Murray Sunset because if incorrect bolts, I would use the correct bolts.

Lucky he was only doing 15k's at the time (eric has the video of it all happenin!), but if it happened 1 hour or so earlier when we was sniffin along 120+ it could have been fatal...

[Madmac]

A grade 10 bolt is stronger than a grade 8, but any bolt can be snapped if used inappropriately.

The biggest killer of bolts is lubricating the thread, then torquing up. Torque settings are usually for dry bolts, when the bulk of the tightening torque is used to overcome friction in the threads, and the rest becomes tension in the bolt. If the threads are lubricated and the bolt torqued up to the same level then less torque is used to overcome friction, and more becomes tension in the bolt.

Of course, some people have no idea what a torque wrench is, so they take a large spanner and make it as tight as they can.

Were you the one who installed the dead bolts?

Cheers,

Scott

interesting info, i wasnt aware that there was different ratings for hi tensile bolts, ive always just thought of mild steel and hi tensile. for example when i did my bodylift, i simply took the original bolts to my local boltmaster store, and said i want the same only 50mm lomnger in hi tensile. might get the originals out tommorow and compare the markings with the ones they sold me. is there an australian or international standard for marking hi tensile bolt strenths, on bolts?

[ISUZUROVER]

Bolts are 20mm long, unsure of the diam but old bolts used 12mm spanner new heads are 13mm. These are the 3 bolts that retain the upper ball joint.

Sounds like they are M8 bolts (or at least around an 8mm diameter). If they are in tension and the thread root diameter is 6mm, the old bolts would be able to take about 3 tonnes and the new ones 2.5 tonnes. If in shear, 4 tonnes and 3.3 tonnes. That is a big difference, especially if there are 2 or 3 bolts per side. If the original bolts were grade 10 then I would get some grade 10 bolts (the bolt shop should be able to order them).

[ISUZUROVER]

is there an australian or international standard for marking hi tensile bolt strenths, on bolts?

Yep - SAE (society of automotive engineers) and ISO standards. The basic rule is the higher the number (for metric) and the greater the number of radial lines (for imperial) on the bolt head the stronger the bolt. Ungraded bolts are usually about equivalent of grade 3-4 (less than half the strength of a 8.8 metric bolt).

http://www.americanfastener.com/techref/grade.htm

http://euler9.tripod.com/bolt-database/22.html

[Bush65]

Imperial bolts use SAE strength grades, eg grade 5 or grade 8.

Metric bolts use ISO property classes, eg class 8.8, class 10.9 or class 12.9 (there are other lower classes, but these are the only high strength classes).

The class is not a decimal number, but 2 separate numbers deliminated by the point.

The 1st number (8, 10 or 12) is 1/100 minimum tensile strength of the bolt material (higher number = higher strength). Multiply 8 by 100 to get tensile strength of 800 MPa.

The 2nd number (8 or 9) is 1/10 ratio of min yield stress/min tensile stress.

Generally, 10.9 is the highest class for hexagon head bolts stocked in Australia and even then the range of sizes is not as great as for class 8.8. Unbrako socket head cap screws are mostly class 12.9.

Metric nuts also have strength classes, eg 8 or 10, which designates the tensile strength of the nut material (higher number = higher strength). The ISO standard covers class 12 nuts, but good luck to you if you can buy any from stock.

The ISO standard for nuts specifies dimensions that ensure that 90% of bolts will break during tightening before the thread strips - if a class 8 nut is used with a class 8.8 bolt or a class 10 nut is used with a class 10.9 bolt.

Regarding tensioning.

The usual tightening torques specified for bolt used in machinery that is intended to be dissassembled and re-assembled are determined to achieve a tension in the bolt that is approx 65 to 75% of the bolt proof load (proof load is approx the yield strength), when the threads are lightly oiled (as usually supplied).

Installed bolt pre-tension, (not tightening torque) is the value that is used for the design of bolted joints, particularly for fatigue design. The problem facing designers, is how to get the bolts installed to the correct pre-tension.

The most reliable method is to directly measure how much the bolt stretched when it was installed. This method is use for critical applications, but is not practical for most.

Tightening torque is not a very accurate way to achieve a design bolt tension (accuracy is in the order of 25%, because of friction between mating threads and bolt heads or nuts). Specified tightening torque is not permitted for fully tensioned bolts (tensioned to approx yield) in building structures for this reason.

With small dia bolts, the torsional shear stress induced while the bolt is being tightened, will usually lead to failure before the tensile strength of the bolt is reached. The torsional shear stress reduces to near zero after tightening ceases.

In my experience, most bolt failures are due to fatigue, which results because the bolts are tensiond too low, or the joint surfaces are not prepared correctly. Unless the joint is grossly overloaded.

Edited to make corrections.

[ISUZUROVER]

Great info John. I was taught that the number after the decimal point was the modulus of elasticity, is that correct?

[Bush65]

Ben,

No, modulus of elasticity is the ratio of stress/strain and for all practical purposes is the same for all steels (stainless steels are a little lower 195GPa vs 205GPa).

I wrote the previous post from the top of my head and made an incorrect statement regarding the 2nd number (now corrected).

The 2nd number is 1/10 of the ratio of min yield stress to min ultimate tensile stress.

Multiply both numbers together will give 1/10 of the minimum yield strength. For an 10.9 bolt the minimum yield strength is 10x9x10=900MPa.

The second number is a good indicator of how brittle (or ductile) the bolt is, a lower number indicates less brittle. This is what lead me to make the incorrect statement before.

[J Top]

This is all relevent to the std bolt manufacturers numbers on the bolt heads.
The vehicle manufacturer's use their own numbering system, the common numbers being 4, 7 & 9.
An 8mm bolt with a 4 on the head has a torque figure of either
1.25 pitch 9 ft-lb
1.0 pitch 9 ft-lb
An 8mm bolt with a 7 on the head has a torque figure of either
1.25 pitch 15 ft-lb
1.0 pitch 16 ft-lb
An 8mm bolt with a 9 on the head has a torque figure of either
1.25 pitch 22 ft-lb
1.0 pitch 23 ft-lb
An 8mm bolt, class 8.8 has a torque of 16 ft-lb
An 8mm bolt, class 10.9 has a torque of 23 lb-ft
This can be used to give an idea when replacing bolts of class's required
J Top

[Bush65]

This is all relevent to the std bolt manufacturers numbers on the bolt heads.
The vehicle manufacturer's use their own numbering system, the common numbers being 4, 7 & 9.
snip...J Top

J,

That is not a system that all vehicle manufacturer's use. I may be wrong, but I suspect it is a JIS (Japanese) standard.

Generally bolts and screws used in machinery or vehicles, are pre-tensioned to between 65% and 75% of the bolt proof load. Proof load depends on the bolt property class and diameter.

For pre-tension in that order and lightly oiled threads, tightening torque in Nm is approximately 0.2 x bolt dia (mm) x pre-load (Newtons)

[Nev62]

So in short, I should replace the 8.8 rated engineering bolts with 10 rated automotive bolts. Thanks guys, bofore this I thought there were just comon bolts and hi-tensile bolts.

[Rainbow Warrior]

A grade 10 bolt is stronger than a grade 8, but any bolt can be snapped if used inappropriately.

The biggest killer of bolts is lubricating the thread, then torquing up. Torque settings are usually for dry bolts, when the bulk of the tightening torque is used to overcome friction in the threads, and the rest becomes tension in the bolt. If the threads are lubricated and the bolt torqued up to the same level then less torque is used to overcome friction, and more becomes tension in the bolt.

Of course, some people have no idea what a torque wrench is, so they take a large spanner and make it as tight as they can.

Were you the one who installed the dead bolts?

Cheers,

Scott

I understand where you are coming from, but I've also seen many bolts die from lack of lubrication too

I'm also guilty on not using a torque wrench other than engine assembly bolts.

[-Scott-]

A grade 10 bolt is stronger than a grade 8, but any bolt can be snapped if used inappropriately.

The biggest killer of bolts is lubricating the thread, then torquing up. Torque settings are usually for dry bolts, when the bulk of the tightening torque is used to overcome friction in the threads, and the rest becomes tension in the bolt. If the threads are lubricated and the bolt torqued up to the same level then less torque is used to overcome friction, and more becomes tension in the bolt.

Of course, some people have no idea what a torque wrench is, so they take a large spanner and make it as tight as they can.

Were you the one who installed the dead bolts?

Cheers,

Scott

I understand where you are coming from, but I've also seen many bolts die from lack of lubrication too

I'm also guilty on not using a torque wrench other than engine assembly bolts.

I'm a hack. I rarely use my torque wrench, and when I do I "allow" for my belief that it "trips" low.

I often lubricate threads, then don't lean on the spanner as hard. I also tend to use a lot of spring washers.

Scott

[ISUZUROVER]

So in short, I should replace the 8.8 rated engineering bolts with 10 rated automotive bolts. Thanks guys, bofore this I thought there were just comon bolts and hi-tensile bolts.

If the original bolts had 10.9 on the heads, then it wouls be best to replace them with the same. If they just had the number 10 on the heads, then they are probably made to the JIS (japanese) standards and you need to find the equivalent ISO standard.

[Nev62]

So in short, I should replace the 8.8 rated engineering bolts with 10 rated automotive bolts. Thanks guys, bofore this I thought there were just comon bolts and hi-tensile bolts.

If the original bolts had 10.9 on the heads, then it wouls be best to replace them with the same. If they just had the number 10 on the heads, then they are probably made to the JIS (japanese) standards and you need to find the equivalent ISO standard.

Just the 10

[ISUZUROVER]

So in short, I should replace the 8.8 rated engineering bolts with 10 rated automotive bolts. Thanks guys, bofore this I thought there were just comon bolts and hi-tensile bolts.

If the original bolts had 10.9 on the heads, then it wouls be best to replace them with the same. If they just had the number 10 on the heads, then they are probably made to the JIS (japanese) standards and you need to find the equivalent ISO standard.

Just the 10

I am not so up on Japanese standards - where was your car made?. If the bolt is JIS F10T Then it is roughly equivalent to a class 10.9 metric bolt. However it could be some other automotive standard I am not aware of.

F8T(800～1000N/mm2>ã€

[Nev62]

[quote="ISUZUROVER"]I am not so up on Japanese standards - where was your car made?. If the bolt is JIS F10T Then it is roughly equivalent to a class 10.9 metric bolt. However it could be some other automotive standard I am not aware of.

F8T(800～1000N/mm2>ã€

[Mark2]

Bolts are 20mm long, unsure of the diam but old bolts used 12mm spanner new heads are 13mm. These are the 3 bolts that retain the upper ball joint.

Sounds like they are M8 bolts (or at least around an 8mm diameter). If they are in tension and the thread root diameter is 6mm, the old bolts would be able to take about 3 tonnes and the new ones 2.5 tonnes. If in shear, 4 tonnes and 3.3 tonnes. That is a big difference, especially if there are 2 or 3 bolts per side. If the original bolts were grade 10 then I would get some grade 10 bolts (the bolt shop should be able to order them).

I've always been led to believe the shear strength of most steels is roughly one third of the tensile strength??? Also depends on whether its loaded in single or double shear.

[ISUZUROVER]

I've always been led to believe the shear strength of most steels is roughly one third of the tensile strength??? Also depends on whether its loaded in single or double shear.

AFAIK the shear strength of most bolts is about 2/3 of the tensile strength, not 1/3.

However, in a tensile application the bolt will break at the root of the splines, but in a shear application, the load should be on the full width section of shaft.