Grade 8 shear bolt confirmed!

SacandagaBrad · Dec 20, 2003

In an earlier post I described an instantaneous stall due to a shear bolt not breaking on a Woods SS52 snow blower blowing snow....

Well, I feeling better and got out to check the factory installed shear bolt on the drive shaft. It is a Gr 8 /forums/images/graemlins/mad.gif I picked up a handfull of Gr 2 and Gr 5 to try.

I encourage anyone with a new Woods snow blower to check the drive shaft shear bolt to make sure it is not a Gr 8. Easy enough to do without removing the bolt, you can see the head of the bolt clearly. If you've got 6 hash marks on the head, it's a Gr 8! Get it out of there. A TC24 at 2500 RPMs and little snow load could not shear it. That's asking for trouble IMO.

I just called my dealer to alert him to the problem, he was surprised and will check all new ones going out the door from now on. Seemed appreciative of the notice.

I would be interested to hear the results of your inspections. As pointed out by Rick in the earlier post, Woods indicated to him that the Gr8 spec in the manual was an error. Apparently they get their drive shafts from an outside source, who probably just used Woods spec without questioning it.

Anyway, if this saves anyone some serious damage to your tractror, it was worth the post.

Brad

art · Dec 20, 2003

You should remove the shear bolt and make sure that the pieces are lose or free to shear!

Junkman · Dec 20, 2003

I just talked with a friend that has a Woods also and he was outside doing a repair. Seems that his also has the grade 8 bolt and as a result, he broke a weld in the impeller. Didn't know why until I told him of this post. Then he looked at the shear bolt and saw the reason. Wonder how long this problem has been going on for? Also wonder why Woods didn't do a safety recall notice with a new 15 cent bolt!!!!!

SacandagaBrad · Dec 20, 2003

Junkman,

I'm wondering those same things myself. As I said before, my dealer was surprised and apparently had never checked or heard of the problem. He is both a NH and Woods authorized dealer.

I gave her a test run down a scraper bank using a Gr 2. Lots of chunk ice and some small stones from the road, no problems. That's my dad on the machine - he's much better at running tractors than digital cameras /forums/images/graemlins/wink.gif

Brad

nomad · Dec 21, 2003

Lower Grade shear bolt confirmed!?
But, this also means that the quality of its gearbox of snowblower too is lower.

For ex., Grade 5 bolt is a medium carbon steel while Grade 8 is a medium carbon alloy steel. Shear capability of Grade 5 is lower than Grade 8. So, using lower grade shear bolt is like a fuse for a lower quality gearbox. Grade 2 is for lower quality cheaper gearboxes.

So, I guess Woods were probably told by their supplier that their gearbox was so good quality that they could resist a shearing force of even 4000-5000 pounds which is the shear capability of Grade 8. Isn't there any saying like "Tell me your shear bolt I will tell you its quality of your machine."

Egon · Dec 21, 2003

Wouldn't the diameter of the shear bolt also be considered in the design calculations.

Egon

JerryG · Dec 21, 2003

Yes, it is a fuse for the gear box, PTO shaft and also the transmission. You use the softest bolt that will do the job, not the hardest bolt that it will take. Most but not all shear bolts are grade 2. All you are doing here is trying to stir up trouble.

SacandagaBrad · Dec 21, 2003

( So, I guess Woods were probably told by their supplier that their gearbox was so good quality that they could resist a shearing force of even 4000-5000 pounds which is the shear capability of Grade 8) )

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! I wonder what would have happened if I had a 40 hp tractor.

Brad

PineRidge · Dec 21, 2003

I have a 4 to 5 year old Woods snowblower on the back of my 40 HP New Holland. It has grade 4 shear bolts in the PTO and the sprocket assembly and they don't slow my engine down at all. When I hit a golf ball sized stone they immediately shear. That's why I always carry a zip lock bag of them in the toolbox.

Junkman · Dec 21, 2003

How many hash marks are on a grade 4 bolt?????

JerryG · Dec 21, 2003

/forums/images/graemlins/smirk.gif

keeney · Dec 21, 2003

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

Junkman · Dec 21, 2003

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 · Dec 22, 2003

Informative well written post Rick. Keep them comming.

Egon

PineRidge · Dec 22, 2003

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.


Junkman were you asking me personally or was this posters information correct? /forums/images/graemlins/grin.gif

nomad · Dec 22, 2003

(
grade 5 = three marks.
grade 8 = five marks.
)

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.;-).

nomad · Dec 22, 2003

(
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! )

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 · Dec 22, 2003

( 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
)

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

MajMac392 · Dec 22, 2003

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 · Dec 22, 2003

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....