Everything you need to know about SHEAR BOLTS

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’s Standard Handbook for Mechanical Engineers

Fasteners: Making the grade - A Technical Discussion
(www.rockcrawler.com)

@Bulldog: That was my problem, everyone's "experience" kept giving me
either conflicting information or they could not explain why there was a
need for a specific "bolt". Additionally they could NOT identify what a
"shear bolt" was.

So what I found was that there is no such thing as a graded "shear bolt".
There are only "shear pins" which are non-threaded pin that have stress
relief groves cut into them.

The miss-information that is perpetuated by almost everyone is the term
"shear bolt". Like I said, it does not exist. It is a term used to identify a
"bolt" that is used in an application to provide "shear relief" for certain
tolerances which is identified by the "grade" of the bolt.

If I used a grade 2 bolt for my snow blower then I would only be using a
bolt that is providing 2,174 lbs of working force (1,567 lbs if it was
threaded) which is far lower than the 3,177 lbs the snow blower can
handle. It may work but you are not getting the most efficiency out of
your implement and you may spend more time replacing bolts when it
is unnecessary.

Lastly I do not believe that all bolts used for "shear relief" should be grade
2, but I also believe you should never put a higher grade bolt into an
implement when a specification calls for a specific grade (i.e. replacing
a grade 2 with a grade 8).

Sammy3700 said:

I agree with BD only grade 2 in a pto shaft. That being said I have 0 experience with snow blowers. If we get snow today it is normally gone tomorrow.

Click to expand...

The application I was investigating was specifically for a snow blower and the reference material for the bolt needed was from the manufacturer.

Regarding PTO shafts, I would look up what the manufacturer recommends for the bolt
used. The tolerances must be known. There is something to be said for being "safer than
sorry" but I remember using my tiller and when I hit rocky ground if I sheared a weaker
bolt every time I hit a medium sized rock I would never have gotten the job done.