Everything you need to know about SHEAR BOLTS

[KeithG]

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)

 

[zload]

Nice job on the research, chart posted and write up. Link just went to the main page so I looked up the article as below:

Fasteners

 

[Grouse Feathers]

Must be another engineer, I think my Marks is behind me in the bookcase.

My BX5455 blower calls for a grade 2 bolt for the fan, and I have both fully threaded and partially threaded bolts. Without pulling the shear bolt, I did replace two of them this week, I believe it doesn't matter which bolt I use. It looks like the shear point is located on the threaded portion of the bolt on either the fully threaded or partially threaded bolt.

On the BX5455 the fan shear bolt holds two flanges together so there is a single shear plane. I think there are a several variables involved in single plane shear, including surface finish, the compression force supplied by the shear bolt and bolt clearance in the hole. I think there is a lot of SWAG and safety factor in the specifications for single shear, shear bolt.

 

[Tooljunkie]

A grade 8 bolt 1/4"x1-1/4" is threaded all the way. So to get around this, simply buy a longer bolt with the shank long enough to fill the shear zone with unthreaded part. Cut off unnecessary threads.

A grade 5 bolt will bend before shearing which may or may not wedge a piece in the drive part of the blower. Thus making repair more difficult.

 

[gssixgun]

Interesting and informative read

I actually tracked down the Manual for my Farm King to double check my bolt specs after reading it..

Honestly a popped shear bolt is a rare occurrence for me, when I bought the Blower it was 1 year old and the guy handed me a baggie full of bolts and washers and pointed to where the Auger shear bolt was

I got the impression that he had been popping bolts from the price I got on the blower and when he said he bought "Stronger Bolts for it"

I tracked down the manual and found that the PTO shear bolts were Grade 8 but the Auger bolt was supposed to be a Grade 2 NOT the Grade 8's he had handed me
I bought the right bolts in 2013 (5) sets LOL still have 3 left


Thanks for the write up

 

[skeets]

All I know about them, is they always seem to shear at the most inopportune time!!

 

[Bulldog]

Never had any snow blower experience but from all my past experience on the farm all shear bolts are grade 2. Anything above that you stand a chance of damaging the implement. A friend of mine tried a grade 8 in his phd and twisted the drive line out.

 

[Sammy3700]

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.

 

[KeithG]

@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).

 

[KeithG]

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.

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.

 

[William1]

From my own limited knowledge, shear strength is also based on bolt diameter. If the bolt specified is on the small side (like a 1/4") then a grade 8 makes sense. Kick it up to a 5/16 and a weaker bolt is called for, given the forces the same.
As was pointed out, higher grade bolts tend to snap and not bend, hence a grade 8 bolt being a better choice.
A shear notch is only needed when the bolt (or pin) used is stronger than the designed maximum load and it needs to be weakened/have a designed in fail point. The advantage of doing is is with a notch, once failed, removal is easier due to the deformity often being less than the widest diameter. This is also why a fully threaded bolt is often used.

Too often, I have seen people repeatedly over load equipment, break a part and decide that the part was poorly designed and make a new, stronger part. Only to find they then break something else, often a more expensive and harder to service component.

Hint, if you shear bolts, it may be because you are running the rpm too low. Most implements are intended to be run at the engine operating speed or a specific PTO speed. Under load, the speed will drop and that under load rpm is your target. So in this case, if you are blowing snow, watch the rpm, if it drops under load, increase it. If the load decreases and your rpms go up, lower the engine speed.

 

[CaveCreekRay]

How about grade "Chinese" bolts?

Those shear on me driving them in with a Bosch Drill-driver. Slightly harder than wood. Not as hard as good plastic.

Be safe out there.

Ray

 

[Bulldog]

KeithG, I get what your saying. Like I said before I have no experience with snow blowers at all. The only thing we ever had that needed shear pins was a square baler and post hole diggers. When we needed them we went to our local hardware and bought grade 2 bolts. Only time we had them brake was trying to feed to much hay in the baler or hitting a big rock with the phd.

From the way you describe the specs in your manual none of this applies to a blower.

All I know is this, a fast way to spend (depends on length and size of drive line) $500 is by using a grade 8 bolt for a shear pin in a baler or phd.

 

[William1]

How about grade "Chinese" bolts?

Those shear on me driving them in with a Bosch Drill-driver. Slightly harder than wood. Not as hard as good plastic.

Be safe out there.

Ray

Yup. Nothing makes me madder than predrilling the correct diameter, running in a lag bolt only to have it snap just as the shank begins to enter. You know you are not applying too much torque, you kow it is because it is just a cheap piece of hardware.

 

[Grouse Feathers]

Yup. Nothing makes me madder than predrilling the correct diameter, running in a lag bolt only to have it snap just as the shank begins to enter. You know you are not applying too much torque, you kow it is because it is just a cheap piece of hardware.

A little bees wax on the threads will make all the difference especially in hard woods. The bees wax will not run and cause finishing problems if you are building something that will be finished. Bees wax will also help in wet treated wood and for long lag screws in soft woods.

 

[L4740]

Yup. Nothing makes me madder than predrilling the correct diameter, running in a lag bolt only to have it snap just as the shank begins to enter. You know you are not applying too much torque, you kow it is because it is just a cheap piece of hardware.

There may not be anything as a grade 8 shear pin, but there is also no such thing as lag bolt. It is a lag screw.

A lag screw was a metal fastener used to fasten a stave to a barrel.

A bolt has fine threads and are used with nuts. A screw has coarse threads that is used in wood.

Just sayin'

 

[Bulldog]

Couldn't resist

 

[coachgeo]

Great info. added link to here in the ULTIMATE readme sticky at the top of Service, Repair & Maintenance section

 

[FTG-05]

Another good paper on fastener tech:

http://www.pirate4x4.com/tech/billavista/NutsandBolts/index.html

 

[bcp]

NASA on common lock washers.

The whole article here:

NASA Fastener Design Manual-98 page PDF

Bruce