JerryG
Super Member
/forums/images/graemlins/smirk.gif
Grade 8 shear bolt confirmed!
- Thread starter SacandagaBrad
- Start date
The grade is the number of hash marks + 2.
grade 2 = no hash marks.
grade 4 = two marks.
grade 5 = three marks.
grade 8 = five marks.
Each "grade" has corresponding standards for hardness and tensile strength and the tolerances on the those parameters.
The exact torque required to shear off a given grade of bolt depends on the configuraiton of the shearing mechanism and can vary widely with the diamater of the shaft the bolt is through, the hardness of the material of the shaft and sleeve, how sharp the edges are, the looseness of fit in the hole, the gap between the shaft and the sleeve, and lots of other things.
Then there are all the manufacturing tolerance on all of the above, corrosion, lubrication, paint, vibration, pulsation of the drivetrain torque and implement, and metal fatique to try and take into account.
Trying to estimate the inertia ise even more complicated. Its the diffence in rotational inertia (mass times radius times rpm's) that the shear bolt has to absorb. If a very light implement is stopped quickly while a heavy drive shaft, coupling, transmission, engine, etc continue to spin on the other side, the inertia of all of those heavy components exerts a lot more torque on the shear joint than the rated torque output of the tractor in a static situation.
Bottom line: it is not really possible to design the shear bolt. The engineer has to make an estimate and try it. Then, depending on how often it shears in a nuisance situation vs. how often it fails to prevent damage to the remainder of the drivetrain, you either increase or decrease the grade, diameter, or some other paramater of the shear setup.
The manufacturer's experience is what counts in this situation. A manufacturere who has been making and selling the type and design of equipment in question for a number of years will be in the best position to have the best estimate.
In this specific case, it does sound like they just made a mistake, had a typo in their document, re-used the wrong part number, etc.
- Rick
grade 2 = no hash marks.
grade 4 = two marks.
grade 5 = three marks.
grade 8 = five marks.
Each "grade" has corresponding standards for hardness and tensile strength and the tolerances on the those parameters.
The exact torque required to shear off a given grade of bolt depends on the configuraiton of the shearing mechanism and can vary widely with the diamater of the shaft the bolt is through, the hardness of the material of the shaft and sleeve, how sharp the edges are, the looseness of fit in the hole, the gap between the shaft and the sleeve, and lots of other things.
Then there are all the manufacturing tolerance on all of the above, corrosion, lubrication, paint, vibration, pulsation of the drivetrain torque and implement, and metal fatique to try and take into account.
Trying to estimate the inertia ise even more complicated. Its the diffence in rotational inertia (mass times radius times rpm's) that the shear bolt has to absorb. If a very light implement is stopped quickly while a heavy drive shaft, coupling, transmission, engine, etc continue to spin on the other side, the inertia of all of those heavy components exerts a lot more torque on the shear joint than the rated torque output of the tractor in a static situation.
Bottom line: it is not really possible to design the shear bolt. The engineer has to make an estimate and try it. Then, depending on how often it shears in a nuisance situation vs. how often it fails to prevent damage to the remainder of the drivetrain, you either increase or decrease the grade, diameter, or some other paramater of the shear setup.
The manufacturer's experience is what counts in this situation. A manufacturere who has been making and selling the type and design of equipment in question for a number of years will be in the best position to have the best estimate.
In this specific case, it does sound like they just made a mistake, had a typo in their document, re-used the wrong part number, etc.
- Rick
Junkman
Super Member
Is that a ASTM or SAE standard?????? Low carbon steel or medium carbon steel? Quenched and tempered or cold rolled? /forums/images/graemlins/confused.gif
Egon
Epic Contributor
Informative well written post Rick. Keep them comming.
Egon
Egon
PineRidge
Super Member
<font color="blue"> The grade is the number of hash marks + 2.
grade 2 = no hash marks.
grade 4 = two marks.
grade 5 = three marks.
grade 8 = five marks.
</font>
Junkman were you asking me personally or was this posters information correct? /forums/images/graemlins/grin.gif
grade 2 = no hash marks.
grade 4 = two marks.
grade 5 = three marks.
grade 8 = five marks.
</font>
Junkman were you asking me personally or was this posters information correct? /forums/images/graemlins/grin.gif
</font><font color="blue" class="small">(
grade 5 = three marks.
grade 8 = five marks.
)</font>
A small correction : the Grade 8, which is a medium carbon alloy steel has 6/six marks. and these are designated by SAE standarts.
Concerning the standarts; People usually ask if the science is universal, why are there many standarts even in the same field. This many standarts is because the engineering itself is an approximate science with many emphirical or semi-emphirical constants/variables determined by the experiements. One group of engineer makes an experiment and they find a result and another group of engineers find another result. Although these two results are close to each others this causes different standarts born in the same field. Another reason is in the use of measurement unit system; hence DIN of Germans using SI and SAE of USA using lb-inch system. Also, the standarts sometime also take the other factors from other fields. For example, fuel consumption standart (or CO2/CO production of an engine) also uses air pollution/environmental engineering limits. Standarts set the bounds (either upper or lower limits) and any standart of fuel consumption by engine can only set upper limit. It says "you can't burn more than this amount of fuel in a this kind of engine" - but, it doesn't say "you can not burn less than this amount of fuel in your enginer". If it says the latter, then I can think of only that an oil boss who wants to sell more oil is involved in setting that standart.;-).
grade 5 = three marks.
grade 8 = five marks.
)</font>
A small correction : the Grade 8, which is a medium carbon alloy steel has 6/six marks. and these are designated by SAE standarts.
Concerning the standarts; People usually ask if the science is universal, why are there many standarts even in the same field. This many standarts is because the engineering itself is an approximate science with many emphirical or semi-emphirical constants/variables determined by the experiements. One group of engineer makes an experiment and they find a result and another group of engineers find another result. Although these two results are close to each others this causes different standarts born in the same field. Another reason is in the use of measurement unit system; hence DIN of Germans using SI and SAE of USA using lb-inch system. Also, the standarts sometime also take the other factors from other fields. For example, fuel consumption standart (or CO2/CO production of an engine) also uses air pollution/environmental engineering limits. Standarts set the bounds (either upper or lower limits) and any standart of fuel consumption by engine can only set upper limit. It says "you can't burn more than this amount of fuel in a this kind of engine" - but, it doesn't say "you can not burn less than this amount of fuel in your enginer". If it says the latter, then I can think of only that an oil boss who wants to sell more oil is involved in setting that standart.;-).
</font><font color="blue" class="small">(
Not quite sure what you mean, but their gear box and erroneous Gr 8 shear bolt were high enough quality to stop a 24 hp diesel tractor cold in its tracks operating at full PTO speed, without damage to their implement. The shear bolt isn't only to protect their gearbox, it is also to protect whatever tractor it's connected. I would hate to think my weakest link was somewhere deep in the PTO transmission. That repair would probably cost more than the implement! )</font>
Under normal condition, any new component of gearbox, of PTO transmission system and of the engine is stronger enough than these shear bolts. So, the shear bolt is always the weakest link in a PTO+gearbox+engine+attachment system. But this is for normal quality systems. If your gearbox is low quality or if your engine is old or ill somewhere at its components, we may not always say this surely that the weakest point is the shear bolt.
Ps: By the way, shear bolts are ductile and therefore, their ultimate shear loads which they can resist is high enough. If Grade 8 bolt is already on your system and if you don't want to change it, you can add a small notch in your bolt. This will reduce the shear load it can resist. You can make a few experiements by changing the depth of that notch in the bolt to find exact shear load for your special system (it's special because your system isn't new anymore.) Your this special bolt with a certain notch will be somewhere between, say, Grade 2 and Grade 3.
Not quite sure what you mean, but their gear box and erroneous Gr 8 shear bolt were high enough quality to stop a 24 hp diesel tractor cold in its tracks operating at full PTO speed, without damage to their implement. The shear bolt isn't only to protect their gearbox, it is also to protect whatever tractor it's connected. I would hate to think my weakest link was somewhere deep in the PTO transmission. That repair would probably cost more than the implement! )</font>
Under normal condition, any new component of gearbox, of PTO transmission system and of the engine is stronger enough than these shear bolts. So, the shear bolt is always the weakest link in a PTO+gearbox+engine+attachment system. But this is for normal quality systems. If your gearbox is low quality or if your engine is old or ill somewhere at its components, we may not always say this surely that the weakest point is the shear bolt.
Ps: By the way, shear bolts are ductile and therefore, their ultimate shear loads which they can resist is high enough. If Grade 8 bolt is already on your system and if you don't want to change it, you can add a small notch in your bolt. This will reduce the shear load it can resist. You can make a few experiements by changing the depth of that notch in the bolt to find exact shear load for your special system (it's special because your system isn't new anymore.) Your this special bolt with a certain notch will be somewhere between, say, Grade 2 and Grade 3.
AndyMA
Elite Member
- Joined
- Oct 11, 2000
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- 3,652
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</font><font color="blue" class="small">( The grade is the number of hash marks + 2.
grade 2 = no hash marks.
grade 4 = two marks.
grade 5 = three marks.
grade 8 = five marks
)</font>
I think what Junkman was getting at is that many of us have not heard of SAE Garde 4. I personally am familiar with Grades 0,1,2,3,5,7 & 8.
I've attached a chart which shows the various SAE & ASTM grades, their markings and the materials they are made from.
Andy
grade 2 = no hash marks.
grade 4 = two marks.
grade 5 = three marks.
grade 8 = five marks
)</font>
I think what Junkman was getting at is that many of us have not heard of SAE Garde 4. I personally am familiar with Grades 0,1,2,3,5,7 & 8.
I've attached a chart which shows the various SAE & ASTM grades, their markings and the materials they are made from.
Andy
Attachments
MajMac392
Gold Member
- Joined
- Jan 8, 2002
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- 406
- Location
- Southern Maine
- Tractor
- Cub Cadet 7234, John Deere X300 Riding Lawn Mower
Interesting posts. I guess I can't count because I "knew" a Grade 5 had 3 Hash marks and I thought a Grade 8 had 5 hash marks. Obviously not. Thanks for setting us straight.
Junkman
Super Member
Andy.......You are correct..... that is what was running through my tractor mind..... When I originally went looking for it, I found more different standards and types than I had previously known of. I am soooooooo old that I remember the days when Chrysler Corporation had all the bolts for their automobiles marked with the Chrysler logo on the head of them and it wasn't the current 5 point star....