Everything you need to know about SHEAR BOLTS

[kfgriffi]

I know this is a long post but I am a believer in not trusting anyone
because they posted something on the internet. So I am providing
all details and references so you don't have to believe me you can
see how the figures were calculated.

If you don't want to read what led to discover the shear bolt data you
can scan down to TECHNICAL DETAILS. If you don't want to read all
the technical details you can scan down to BOLT FINDINGS.

First a little description of what led to me to discover all this information.
We had a snow storm that dropped about 3 inches of heavy sleet (i.e.
snow that melts to rain and refreezes before it falls to the ground) the
other day and I went out to shovel it and it was very heavy. I then
tried to use my ATV with a plow and it could not even bite into the sleet
and ended up going up on top of it. When you walked on it you did not
even compress it and leave footprints. So this was very dense heavy
snow.

Took out my Kubota B2710 with front mounted B2782 SnowBlower
and proceeded to clear the driveway. It worked beautifully! I kept the
RPM up to 2,400 and drove slowly to not force too much sleet into
the second stage so it would not be over worked. But alas the inevitable
happened, I broke the shear bolt that connects the stage 1 auger to the
stage 2 fan and I did not have another one. Tried a bolt I found in my
collection and limped along and finished clearing the sleet. Then I started
my search for a proper shear bolt to replace the one that broke.

Firstly, THERE IS NO SUCH THING AS A GRADE 8 SHEAR BOLT! I asked
people that were supposed to be knowledgeable in this area and got
answers like "a grade 8 shear bolt is softer metal", "a grade 8 shear
bolt has stress lines" and "you should never use a grade 8 shear bolt
as it will wreck your machine". Even though the Kubota manual states:
SHEAR BOLT 1/4" NC X 1 1/4" GRADE 8.

TECHNICAL DETAILS:
A grade 8 bolt must meet specific technical details in order to be identified
as a grade 8 bolt. They are:
1) Medium Carbon Alloy steel
2) Quenched and Tempered
3) The head will have 6 lines denoting grade

There is no such thing as a "shear bolt" when referencing graded bolts.
A "shear bolt" is something that is created usually be cutting relief
groves in a pin and this is used in walk behind snowblowers or lawn
tractor implements. When referencing tractor implements that require
larger tolerances the item needed is a "bolt" and it is used in a way
that it provides shear relief (i.e. it will fail before something more
expensive) at certain tolerances.

In the case of my Kubota B2782 SnowBlower the dealer provided a
picture of a grade 8 bolt that was FULLY THREADED (i.e. from head to
end of bolt).

In order for me to find out if I could get this locally I needed to determine
what force would be required for this bolt to fail. This calculation is
defined as:
SF = SC in KSI * SA

SF = Shear Force needed to cause failure
SC = Shear Capability of bolt
SA = Surface area

SURFACE AREA: This is the circumference of the bolt where the single
shear force will be applied to cause failure. This is
calculated by using: Pi * r2
SHEAR CAPABILITY: This is calculated by using 60% of the tensil strength
of the bolt.

For a non-threaded grade 8 bolt the tensil strength is 150,000 pounds.

A grade 8 bolt with the dimensions of 1/4 x 1 1/4 the surface area
is: Pi * (.25 / 2) ^2 (i.e. Pi times radius (.25/2) squared). This equals
a value of: 0.049087385

The shear capability of a grade 8 bolt is: 90,000 ksi

So the SHEAR FORCE needed for a grade 8 bolt is: 4,418 lbs.

A grade 8 bolt that is threaded has a diameter of 0.212 (i.e. the
diameter at the lowest point of the thread).

So the SHEAR FORCE needed for a grade 8 threaded bolt is: 3,177 lbs.

This is a difference of 1,241 lbs of shear force. Also note that a grade 5
non-threaded bolt actually has more shear force required than a grade
8 threaded bolt (i.e. 3,534 lbs. vs. 3,177 lbs.)

BOLT FINDINGS:

........................Size.....Size....Surface............Shear......Tensil.......Shear
......................(diam):.(len):.....area:..........capability:.strength:..Failure
...................................................................................................(lbs):
Grade 8
not threaded 0.25 1.25 0.049087385 90,000 150,000 4,418
Grade 5
not threaded 0.25 1.25 0.049087385 72,000 120,000 3,534
Grade 2
not threaded 0.25 1.25 0.049087385 44,400 74,000 2,179

Grade 8
threaded 0.212 1.25 0.035298935 90,000 150,000 3,177
Grade 5
threaded 0.212 1.25 0.035298935 72,000 120,000 2,542
Grade 2
threaded 0.212 1.25 0.035298935 44,400 74,000 1,567


Bolt Technical Details:



References:
Fastenal Technical Reference Guide: Fastenal Company Engineering Dept.
S7028 Rev. 9

Mark逞ｴ Standard Handbook for Mechanical Engineers

Fasteners: Making the grade - A Technical Discussion
(RockCrawler 4ﾗ4 and Offroad Magazine | Covering the world, one rock at a time!)

 

[dfkrug]

Interesting info.

My local rental yard, which deals with broken shear bolts all the time, uses Grade 8
bolts cuz they shear cleaner than lower grades. The equipment calls for Grade 2 bolts,
but those are more malleable, so they often are harder to remove after shearing.

That's what they tell me, after lots of experience.

 

[Mitkof]

In old days with the Fergie 40 and posthole auger, grade 2 bolts were too expensive. I bought a sack of 20penny spikes. POP, stick a new one in and bend it over. Sometimes I could use the same spike twice, as they were long enough. Do not think I would ever use a Grade 8 CAT bolt for equipment shear protection. IMO, my old wimpy attachments would be totally destroyed before any grade 8 bolt would shear and protect it.

 

[kfgriffi]

I am not claiming that you should use a higher grade bolt, but I just wanted to determine
what the shear tolerance was for the snow blower I was using .

The interesting thing I found out was that if I used a grade 5 bolt, which was not threaded
at the point where the shear force was being applied, then I would actually be using a
STRONGER bolt than the threaded grade 8 bolt.

If the manufacturer says the correct application is to use a bolt which will provide 3,177 lbs
of resistance before failing then all I wanted to know was what could I use to obtain that
without going over and risking serious damage to my machine.

This information also means that given a known tensil strength for a bolt, you can determine
if it is applicable for your equipment.

Thanks.

 

[JimRB]

Some interesting reading if one is going to get curious about bolts, nuts, washers and plating. http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf

 

[RoyJackson]

Interesting thread...
I always go with what's called out in the implement manual. Should there be damage, if a non-specified grade bolt is used, any warranty claim would likely be denied. I'm relying on the engineering/design staff doing their job.

 

[Eagleview]

Don't forget to DOUBLE the shear strength of ANY bolt you use as you are NOT shearing it in ONE place as the testers have , but in TWO places as it passes right through the shaft (if it is the type that goes right through with a nut on the other end).
So if a bolt is rated at 150,000psi , it will take near double that to shear it in the TWO places .

 

[RickB]

Interesting thread...
I always go with what's called out in the implement manual. Should there be damage, if a non-specified grade bolt is used, any warranty claim would likely be denied. I'm relying on the engineering/design staff doing their job.

A shear bolt can be any spec bolt a machine manufacturer specifies as correct to provide shear protection at a particular location in a particular machine. A shear bolt can be of any diameter, length and hardness.

 

[PILOON]

I went with the theory that the harder the more brittle the metal will be.
When I used hardware cheap bolts they tore rather than sheared making them difficult to drive out. Also they were rounding the edges of the holes as the wanted to elongate and bend B4 shearing.
Grade 5 bolts became my standard and long enough that no threads are in the shear zone. (threaded zone would be weaker than the body)
The result is that following each shear incident the pieces simply fall away and the center is an easy push out.
Also the mating faces are still nice and square after many dozen 'incidents'. (gravel drive)
(I would estimate that I have sheared probably 20/year for about 8 years or so.)
Shearing incidents vary with the years due to timing and depth of freeze before first snows. Also the base build up is a major factor.

 

[Mace Canute]

Thanks for taking the time to post the information.