# Shear pin/ shear bolt theory...?



## d3500ram (Feb 5, 2017)

Educate me on shear pins/ shear bolts. I will refer to either or both going forward as "shear bolts" or just "bolts" unless there is a distinct difference. 

In my post "http://www.snowblowerforum.com/forum/craftsman-snowblowers/116802-1450-shear-bolts-did-not-shear-when-needed.html " it was mentioned by a couple of members that there are shear bolts for the auger, but none for the impeller. 

When dissembling the front housing the shear bolts for the auger are obvious, and there were what looked like shear bolts for the impeller as well as a roll pin. I feel that if the snowblower had some type of shear protection for the impeller I would not have sustained the damage that is shown in the images posted.

Kudos to everyone here! (I am not sure if I am wiser or more confused...LOL) but in the learning and researching here as well as call to mfgr's tech support I am finding out that there is no shear protection on the impellers!! Is this in fact true? 

I am super paranoid of trying to keep a newly purchased snowblower from sustaining the damage that my last one incurred. I inherited a very similar model to the one shown from a co-worker; he has very similar damage to it from an object while blowing his similar gravel drive way. If I had at least road base for my driveway I would be less concerned, but I have gravel driveway that I police thoroughly each fall for objects. I have no control over rocks and such thrown back in from the county plow that maintains the gravel road in front of my house. 

Are shear bolts/ pins available on certain model snowblowers for both auger AND impeller protection. If not, why not?

Thanks.


----------



## unknown1 (Dec 13, 2015)

We need to stop meeting like this ;-)

Historically, the majority of manufacturers had shear bolts ( or pins ) on the augers but all they had on the impellers were roll-pins or regular bolts.
That was to protect the auger gearbox and the augers themselves. I guess the assumption was that impellers didn't need them and that the belts would slip or the engine stall.
A couple of manufacturers introduced shear pins on the impellers too. Honda was one.. there may be others. It was believed that this was design integrity.

In more recent years... more manufacturers are adding them to their impellers too.
Whether that is because of design integrity or because their metalwork is getting cheaper and cheaper and therefore needs more protection depends on the manufacturer and your mood. ;-)
You will see them on the 3-stage MTD machines (under various brand names).. lots of opinions why that is.


----------



## Kiss4aFrog (Nov 3, 2013)

:smiley-signs009: What he said.


----------



## unknown1 (Dec 13, 2015)

stuart80112 said:


> We need to stop meeting like this ;-)
> 
> Historically, the majority of manufacturers had shear bolts ( or pins ) on the augers but all they had on the impellers were roll-pins or regular bolts.
> That was to protect the auger gearbox. I guess the assumption was that impellers didn't need them and that the belts would slip.
> ...


I don't believe that Toro is on your shortlist but wanted to add this for completeness.
Toro claim that their gearboxes are so strong (on some models) that they don't even need shear pins/bolts for the augers
That's covered in this short video: 





Even though the guy from Toro marketing said they don't have shear pins... this next Toro model certainly does... confused yet?
Presumably different model years are being discussed in each video. I believe the top video is the most current situation. Not certain.


----------



## d3500ram (Feb 5, 2017)

I do not see a Toro track drive model. I would consider one if they made tracks. I really think that this style of motion will be better than tires (with chains) for my situation. When (if) you come up to take the Husq motor I can show you the driveway situation.


----------



## unknown1 (Dec 13, 2015)

d3500ram said:


> I do not see a Toro track drive model. I would consider one if they made tracks. I really think that this style of motion will be better than tires (with chains) for my situation. When (if) you come up to take the Husq motor I can show you the driveway situation.


Right... I was just being anal about getting the most complete answer I could on shear pins for the next person who comes by. 
The Toro gearboxes are/were a selling point for some people. Everyone has their own reasons why they love (or hate) particular machines.


----------



## Spectrum (Jan 6, 2013)

The worm drive represents a gear reduction of about 10 or 12 to 1. The auger is moving much slower but has proportionally more torque. Combined with helical design it has much more potential to jam and damge itself. Here shear pins have real value.

The impeller is running much faster and with speed comes force. The effect of force when something bad happens is instantaneous and stalls the engine. By then the impeller may or may not be damaged. Shear pins may help in a fraction of cases but overall they are probably just another failure point.


----------



## snow-shovel (Aug 13, 2015)

The reason for auger shear pins/bolts is to protect the auger. When you hit something if there is no shear pin, the tines of the auger will be bent. I would rather replace shear pins than try to straighten auger tines or replace the auger.

Same reason for shear pins on the impellers. If not shear pins, the blades on the impeller will bend. They are even harder to straighten than auger tines.

When something is picked up in the chute, there is a lot of momentum by the spinning engine, impeller, gear box, and auger. That much metal does not stop instantaneously and something will give.


----------



## unknown1 (Dec 13, 2015)

Let me try to expand for anyone who cares.

I think Spectrum's point is that the shear pins often can't help on the impeller... there is so much angular momentum because the impeller is heavy and moving fast... if it suddenly hits an obstruction and stops instantaneously, the damage will happen and you can't stop it. The amount of kinetic energy goes from huge to zero almost instantaneously and the damage is done whether the shaft is still trying to drive it or not. I say "almost instantaneously" because there is a finite length of time spent bending some metal on the impeller or the housing or both.... I think that was snow-shovel's point when you said "something will give". Disconnecting from the shaft buys nothing in those cases because the impeller already has lots of kinetic energy when the obstacle appears (whether the shaft continues to drive it or not) and it is decelerating rapidly as it bends the metal on the impeller and/or the housing. Notice it is already bending and will keep doing so until it comes to rest. The shock transferred back to the engine will either stall the engine or (I guess) the belts may slip if they are worn or loose. If the shear bolt on the impeller were to break you have not gained much because the metal is already bending as the impeller is decelerating. Best case scenario.. the impeller does not bend but just stops without damaging the impeller or the housing. For that you need a strong impeller and a strong housing...if the housing is thin and weaker than the impeller.. the housing will buckle outwards or even explode a hole. If the housing is strong but the impeller is weak.. the housing will survive but the impeller will be bent or buckled. If both the impeller and the housing are inherently weak.. they will both bend buckle snap or burst.
Back in the day, impellers were extremely heavy and rigid compared to the housing metal.. so you'd expect the impellers to survive and the housings to suffer. These days.. everything is flimsy so everything suffers. The only good news with flimsy impellers is that they carry much less rotational kinetic energy because they are lighter.
I like to think of this as two big spheres where one crashes into the other. If both are made of polished steel.. nothing gets damaged. If both are made of play-doh, they both get mushed, if one is steel and the other is play-doh the soft one will suffer. However a shear pin cannot stop the two balls colliding. The energy is already in the system.

There are some scenarios where something might get partially trapped and dragged around the impeller housing on a merry-go-round instead of suddenly stopping. In this case however, the rock (or whatever) would probably be ejected out the chute anyway and so a shear pin failing doesn't buy you much there either even if the shear pin could detect it was happening.

The only case I can think of where the shear pin can have a real chance of helping with rotational blockage is if something gets trapped between the impeller casing and the impeller in such a way that it does not get ejected and does not bring the impeller to an immediate halt. That would tend to have a grinding effect and wear out circles on the impeller casing sheet metal. In that case a shear bolt could help if it could detect that situation and break. The thinner the sheet metal.. the more helpful that would be... that was my hidden message when talking about the "cheaper and cheaper metal" in modern day snowblowers. You are probably saving the impeller casing rather than the impeller itself in this case (it all depends on the relative strength of the housing metal compared to the impeller metal in other words which is steel and which is play-doh). You often see these scratches on the inside of impeller housings. A shear pin could have helped. This whole problem could be alleviated if not totally eliminated by having tighter tolerances between the impeller and the impeller housing by adding what is commonly referred to as an "impeller kit". Is it really so expensive to add a few chunks of rubber and a few bolts into the design of a snowblower compared to (say) hand warmers or battery powered chute manipulators? 

With an auger, the situation is different. As far as the gears in the gearbox are concerned, when the auger stops this is equivalent to stopping the brass gear dead in its tracks. However the worm gear keeps spinning and chews into the brass circular gear removing its teeth. To prevent that, if the auger shear pins snap and if the augers free spin relative to their shaft (or more accurately the shaft free spins relative to the stopped augers).. this allows the shaft (and therefore the brass gear) to keep rotating in unison with the worm gear. Therefore damage is avoided. AS far as the augers themselves are concerned... they should not bend if the shear pin snaps because the inner shaft will turn freely. In this case, using the sphere analogy, the shear pin was successful in preventing the two spheres from crashing together both for the gearbox and for the augers themselves.

With a super-strong gearbox (Toro?) when the circular gear stops, it is presumably strong enough to also stop the worm gear and either the engine will stall or (perhaps) the belts could again slip if worn or loose. In that case the shear pin is theoretically unnecessary as far as the gearbox is concerned. However, as far as the augers are concerned you could have an issue if the engine does not instantly stall. Toro's assumption must be that it will. I think that is a dangerous assumption if you have a powerful engine (say 13HP) and in that case you'd be safer to have shear pins on the augers to protect the augers even if the gearbox does not need to be protected because it's super-strong.

Is anyone still awake here? If so.. this next part is where I think impeller shear pins could be more useful.

All the above has talked about angular momentum and rotations and forces around the impeller shaft. But what would happen if an obstacle managed to impart a shock that has a component parallel to the shaft? This could happen if the rock jammed against the angled fins of the impeller which is not a perfect disc. It needs the fins to throw the snow. Forgetting all the rotational effects which were discussed above, now concentrate on the forces parallel to the shaft. This can be either forwards (pushing the shaft towards the auger gearbox) or backwards (pulling the shaft towards the engine). If there was a shear bolt and if it failed, it could prevent these forces that are trying to punch the shaft one way or the other. If these forces are allowed to exist then you can imagine various types of damage to the auger gearbox and its seals as the worm gear is suddenly pushed forwards or pulled backwards. Does anyone buy that? What I just described is a situation where I think an impeller issue can damage the gearbox too. That type of damage could be eradicated with a fast failing shear bolt that prevents the forward or backward forces happening on the impeller shaft. We don't usually talk about impeller issues impacting gearboxes but I believe it's possible. The other type of damage that can be caused by these forces can happen at the other end of the shaft. Examples might be the impeller housing bulging outwards and backwards towards the engine and/or damage to any other parts like bearings/bushings in that area.

-----------------

For the more mathematically-inclined readers, in the case of an impeller, the kinetic energy will be 1/2 I w**2 ( half I omega-squared ) where omega is the angular velocity... notice that energy goes as the square of that angular velocity (which is fast for an impeller) and I is the moment of inertia and related to the mass of the impeller (which is large for some machines.. especially older machines). For a flat disc the moment of inertia is 1/2 m r**2 (half m r-squared) where m is the mass and r is the radius of the flat disc. If the impeller is heavy (m) and especially if it's radius is large (r) things scale up rapidly with radius-squared. An impeller is not a flat disc but the approximation is still useful. You could treat it as being a cylinder and get similar results. It's this rotational kinetic energy that exists with or without a final connection to the shaft and will potentially ultimately lead to rotational impact damage if either the impeller or the housing or both are weak. (play-doh spheres crashing with other spheres).

-------------------------

In all cases, the stronger your components, the stronger your shear bolts can be. In the limit, you don't need shear bolts at all. Conversely, the weaker your components, the weaker your shear bolts have to be. I believe this is probably the root cause for stories we hear where people are breaking 15 or more shear bolts in a typical season with certain machines from certain manufacturers. Components and metalwork are getting weaker over time to save costs. In the limit, or close to it, your shear bolts need to be as weak as matchsticks. At that point the machine is clearly not fit for purpose.. Another possibility for breaking all those shear bolts is a reckless operator who is just slamming the machine into frozen drifts and not being "reasonable". Or some combination of the two.

---------------------------

To try to answer one of the initial questions "Are shear bolts/ pins available on certain model snowblowers for both auger AND impeller protection. If not, why not?" I've already mentioned Honda and MTD 3 stage machines. So I'd prefer to rephrase that and ask "Is there an argument for always using shear bolts in both places?" Can you live with that?
I think there IS an argument for using them everywhere. However they can't be too strong or they won't fail properly. Neither should they be too weak or they will fail unnecessarily.
In the majority of cases, (excluding some Toro models) the auger gearbox is known to contain a weak brass gear. So we certainly need shear bolts on the augers. However, I also tried to show that an impeller blockage can transfer shock parallel to the impeller shaft which can also do damage. For that reason I think there is an argument for having them on the impeller too to prevent those forces. That's why I think it makes sense to have them in both places.

However there's another question that wasn't asked.. that is... "If I have shear bolts in both places am I protected from all forms of damage"
For that I'd say the answer is no you are not protected. The rotational kinetic energy of the impeller will always potentially cause problems if it suddenly encounters an obstacle on the impeller with or without a shear bolt. The likely damage will be to the impeller itself or to the impeller casing or both. This is especially true as metal gets thinner and weaker.

Your question also effectively asked "For those designs that do not have impeller shear bolts...why not?"
Obviously no one can mind read the thought processes of the designers. I can only guess that they didn't feel that the risks justified the additional cost. In other words, they either don't buy my argument that the forces parallel to the shaft can exist, or they don't think the effects are worth bothering about because the parts are not too vulnerable to forces in that direction, or they simply didn't stop to think about it or finally cost. Who knows except the designers themselves?

I'd like to introduce two other questions... "Why on Earth are there brass gears in auger gearboxes in the first place?"
There's only two possibilities I can think of.... First...if brass is somehow substantially cheaper than a more hardened metal. Second... if the brass gear is intentionally a sacrificial part to save some other part of the system. I can't imagine what that gear is trying to save and apparently neither can Toro. So I guess it has to be a cost issue. I can't think of any other explanation.

Finally.. "Why have an auger gearbox at all?" If the throwing mechanism was not at 90 degrees to the auger shaft you wouldn't need a 90 degree gearbox. In the case of Bobcats, the throwing mechanism is on an axis parallel to the augers and therefore there is no auger gearbox to worry about. The Bobcat's ability to throw snow and slush is very impressive. Is the perpendicular throwing impeller inherently cheaper to build? I have no idea. Is it believed that the 90 degree turn allows the snow to be thrown off to the side better? I'm not convinced when I look at the performance of a Bobcat... and even if that were true... it would simultaneously put into question every single-stage machine that exists today. In any case, when the snow leaves the impeller area, it is traveling tangential to the impeller and therefore vertically. That does not affect the ability to throw left, right, forwards or backwards. That is all handled by the position, curvature and length of the chute.
The only possible explanation for the current impeller design that I can think of is cost. But when I look at the chain-driven Bobcat I find it hard to believe that the Bobcat design would be that much more expensive to implement in today's machines... but I guess it must be.

I'm always open to corrections, alternative explanations or alternative theories.


----------



## jermar (Dec 10, 2014)

Thanks Stuart. Wow! Lots of effort in this epic post.


----------



## 10953 (Sep 4, 2017)

i have a new toro and taught when seeing the spacer on the auger bolt that is was a shear bolt. IT isn't, the bolts and the auger are black colored marked grade 8 with a spacer used to take up the fact of a longer then needed bolt,the impeller is like wise but uses a grade 5 yellow colored bolt again it's a 2 inch long one were a 1 1/2 would work 
i've had toros for 30 years all didn't use shear bolts or pins never had a auger box fail, even when a machine picked up a Belgian paving stone hidden in a plow bank that simply jammed and staled the machine
that i personally think proves toro's claim of simply using better hardened gears works


----------



## unknown1 (Dec 13, 2015)

87 powershift said:


> i have a new toro and taught when seeing the spacer on the auger bolt that is was a shear bolt. IT isn't, the bolts and the auger are black colored marked grade 8 with a spacer used to take up the fact of a longer then needed bolt,the impeller is like wise but uses a grade 5 yellow colored bolt again it's a 2 inch long one were a 1 1/2 would work
> i've had toros for 30 years all didn't use shear bolts or pins never had a auger box fail, even when a machine picked up a Belgian paving stone hidden in a plow bank that simply jammed and staled the machine
> that i personally think proves toro's claim of simply using better hardened gears works


Agreed... when two polished metal spheres collide... nothing breaks. However, if people are willing to buy compromised play-doh components.. there's lots of manufacturers willing to sell them. I hope Toro stick to their guns and that the market plays well for them for doing so. Sadly, I think consumers get distracted by the gadgets on the dashboard instead of checking what's under the hood. No news there!


----------



## 10953 (Sep 4, 2017)

yep all the bling makes for more sales. like buying a corvair thinking one was buying a corvette c 2 
i'll try to get some shots of the bolts i found toro had used on the new 928 since i swamped them out for proper length of the same grades, 
? i have other then bulk prices,why would they spend the money for steel spacer used on a 2 inch bolt when a 1 1/2 fits dead on,unless that grade 8 marked bolt really is a shear bolt


----------



## unknown1 (Dec 13, 2015)

stuart80112 said:


> Agreed... when two polished metal spheres collide... nothing breaks. However, if people are willing to buy compromised play-doh components.. there's lots of manufacturers willing to sell them. I hope Toro stick to their guns and that the market plays well for them for doing so. Sadly, I think consumers get distracted by the gadgets on the dashboard instead of checking what's under the hood. No news there!


I just picked up a damaged 13HP Husqvarna today from the OP (thanks!).
No-one uses horsepower as a specification any more but old habits die hard. 
13HP engines are difficult to stall... and that got me re-thinking about Toro.
It's because of the damage to this machine and his Craftsman that the OP asked about shear pins in the first place.

The Toro strong gearbox design basically says "we have a super-strong gearbox so we don't need shear pins" .

To recap... 
If you have a strong gearbox it does not need to be protected with auger shear pins however there is an additional safety requirement that the engine must "stop" to protect the augers.
When an obstruction stops the augers, the engine wants to keep them moving. The gearbox is now super-strong... so it is no longer a weak link in the chain (it doesn't have a brass gear).
But who wins the battle between the Augers (where the metal can buckle or break) and the engine?

I can think of a few ways to ensure that the augers never lose the battle.

1) Only ever use engines that will stall "easily" which ensures that the augers win the battle.
2) Allow high-powered engines that are difficult to stall but change the opponents in the battle by re-introducing shear pins into the augers... but Toro say they don't do that.
3) Allow high-powered engines that are difficult to stall but design a detection system that can spot that the battle is occurring and command the engine to stop. 

*Does anyone know if Toro machines have a detection system (3) or are they simply relying on the engines to stall (1)?
*I think it is the latter but I'm not sure

For completeness, one possible battle detection system (from Honda) is described here: http://www.snowblowerforum.com/forum/1053802-post1.html


----------



## 10953 (Sep 4, 2017)

( *Does anyone know if Toro machines have a detection system (3) or are they simply relying on the engines to stall (1)? )*
*very good point STU! *
*when my 824 powershift found that hidden paver the motor didn't stall, but the belt sure started slipping fast, what I personally saw helping was in the design of the idler tension assembly, it only allowed just enough tension to hold the belt tension in normal use, but didn't when the auger stopped moving, instant slip/burn along with the loud noise of rock and steel.*


----------



## 10953 (Sep 4, 2017)

here is the bolts toro is using, the black ones came off the wheels the other grade 5 from the auger and impeller, why so long with spacers on all 6 ? 
one thing that has me wondering about is the auger shaft bearings, they are die cast and hollow where they thread and hold to the auger housing. tapered 2 step down to the shaft , almost as if they will shear off if the auger takes a good shot . they also had short flange bolts going into them , i know 1/4 inch isn't much but the way they went in the bearing would split with a bolt 1/2 way though 
should they shear or break,it's a lot of work to split the housing from the tractor to remove the assembled auger/impeller so new can be installed,


----------



## unknown1 (Dec 13, 2015)

87 powershift said:


> ( Does anyone know if Toro machines have a detection system (3) or are they simply relying on the engines to stall (1)? )
> very good point STU!
> when my 824 powershift found that hidden paver the motor didn't stall, but the belt sure started slipping fast, what I personally saw helping was in the design of the idler tension assembly, it only allowed just enough tension to hold the belt tension in normal use, but didn't when the auger stopped moving, instant slip/burn along with the loud noise of rock and steel.


You didn't explicitly say it... but it sounds like the answer is that they do not have a "battle detection system" and did not explicitly re-introduce shear pins/bolts either.

That can buy a lot of grief with a powerful engine as you discovered ;-) I still think that the robust gear boxes are preferable though.... but in this case, a brass gear could have become a self-sacrificing "protection" to save the augers. I'm not sure which I'd prefer... trash the gearbox or trash the augers? Neither sound promising. I think Toro should have shear bolts in there anyway just to give that added protection. Maybe those auger bolts really ARE shear bolts but they just don't call them that. A bolt that snaps at the right time is a shear bolt whether you call it that or you don't. In fact... even a grade 8 bolt acts like a shear bolt if all the other parts are grade 10.. whether you call it a shear-bolt or you don't... it will break and save the grade 10 components.

I avoided this point in the earlier posts because I didn't want us to get lost in the semantics of whether a bolt is a bolt or is it a shear bolt? The OP may have been thinking this way too when he was willing to just call them "bolts". 

If it's over-sized with a spacer collar.. it will act like a shear bolt whether we call it that or we don't... it then just becomes a matter of the metallurgy, the implied necessary breaking shear force and the cross sectional area of the bolt... I didn't want to go there but here we are. ;-) It turns out you don't even need a spacer collar for this to be true... but the collar is a big clue in the intended design.


----------



## unknown1 (Dec 13, 2015)

A couple of farewell observations.
There are many ways to skin a cat. Whenever you have two things spinning and one stops, there are several ways of allowing this to happen without dire consequences.

With bigger machines, like farm equipment, they use a "slip clutch" which allows the shaft to keep spinning even if the work tool jams and hits an obstruction. That slip clutch is acting like a shear-bolt... but it doesn't snap, it just slips.

With helicopters you have the opposite problem. If the engine stops you want the rotors to keep going so that you can auto-rotate to safety. They use another solution but this time they call it a "free-wheeling gearbox". 

Finally.. if you want to see what can happen when all that rotational kinetic energy gets out of alignment (like running a machine after your impeller has been buckled and is out of balance), check out this video. If the energy in the impeller is sufficient and if you trigger any resonant frequencies in the impeller housing this is the kind of end result that can happen. In this video, the resonant frequencies are caused by an interaction between the rotors and the helicopter as it bumps and bounces on the ground. At first it starts off looking innocent enough.. but when the resonant frequencies amplify they destroy the machine. With helicopters this phenomenon is called ground resonance. Anyone want to volunteer to be the test pilot? If the front rotor had behaved like the rear rotor I'm not sure he would have walked away from this one.


----------



## 10953 (Sep 4, 2017)

NO THANKS on being the volunteer pilot . suffering a small bird hit to the prop on the jabiru was scary and costly enough.


----------



## unknown1 (Dec 13, 2015)

87 powershift said:


> NO THANKS on being the volunteer pilot . suffering a small bird hit to the prop on the jabiru was scary and costly enough.


Hey that jabiru bird blender sounds fun. Did you turn off the engine, trim the ailerons and land or what? Or was it a gentle hit and just give the plane a bunch of shakes. What was the outcome? Just change the shear bolt and you're good to go. ;-)
----------------

Just another day at the office for the pilots. 
I'm guessing this might be a servo controlled remote system without a pilot. Doing these kind of tests on or close to the ground with a real human would be a bit reckless. But the only way to test ground resonance outside of a design simulator is to be on or touching the ground. It was all tied down (which makes matters worse when the chains start tugging) and there are multiple different camera angles on youtube so this self-destructing chinook(?) was certainly *intentional*. Notice the little white pole that lets them measure the deflections after recording the results on this next video. You see the resonance better from this angle too. A used chinook costs about $3M these days. 






Wait... I forgot to turn on the camera.. can we try take 2?

If anyone is watching this and has decided that all helicopters are death-traps... A real-life chinook has a landing gear and is not nailed to the ground like this. You only get ground resonance when touching the ground (hence the name). The landing gear would let the helicopter move and alleviate some of the problem. Also, as soon as a pilot detects this happening (if they are landing) he/she could take off again to a hover if they have sufficient rotor RPM and the problem would immediately stop (assuming that the rotors are not out of balance or some other out-of-spec issue due to a bad hard-landing). If you detected it starting to happen on engine run-up for a take-off with lower rotor RPMs you'd just shut down the engine and engage the rotor brakes to avoid it developing any further then abandon the flight and get the machine checked out. This test *intentionally *nailed the chinook to the ground and let it develop presumably to measure the problem. In this test you can hear them shut down the engines but they did it way too late *intentionally*. A real pilot would not let it get that far before shutting down and applying the rotor brakes.


----------



## d3500ram (Feb 5, 2017)

stuart80112 said:


> ...Historically, the majority of manufacturers had shear bolts ( or pins ) on the augers but all they had on the impellers were roll-pins or regular bolts. That was to protect the auger gearbox and the augers themselves. I guess the assumption was that impellers didn't need them and that the belts would slip or the engine stall...


 Makes sense about protecting augers and gearbox... but as I have experienced on the Husq and your soon- to- own Crafts, the damage that can occur to impellers and housing with a high-torque engines can be quite devastating.



stuart80112 said:


> ...In more recent years... more manufacturers are adding them to their impellers too.
> Whether that is because of design integrity or because their metalwork is getting cheaper and cheaper and therefore needs more protection depends on the manufacturer and your mood. ;-)...


 Regardless of the reason, to me I would think that any added fail-safe "fuse" called a shear bolt would be desirable. 



stuart80112 said:


> ..You will see them on the 3-stage MTD machines (under various brand names).. lots of opinions why that is.


Perhaps I should have looked at cheaper MTD type blowers knowing that there is a fail safe built into them even though they are cheaper models while factoring in that I will need to replace them every couple of years (<-- sarcasm...LOL)



Spectrum said:


> The worm drive represents a gear reduction of about 10 or 12 to 1. The auger is moving much slower but has proportionally more torque. Combined with helical design it has much more potential to jam and damge itself. Here shear pins have real value.....


That makes a lot of sense especially the "real value" as you allude.



Spectrum said:


> ...The impeller is running much faster and with speed comes force. The effect of force when something bad happens is instantaneous and stalls the engine. By then the impeller may or may not be damaged. Shear pins may help in a fraction of cases but overall they are probably just another failure point.


 In my case, the engine did not stall and the impeller sustained major damage. This happened to my Crafts as well as an inherited Husq blower form a co-worker that I passed on to member Stuart- both machined were re-baged units from a common manufacturer and had nearly identical damage from a foreign object enter and getting past the auger. I still think shear pins at the impeller would have been desirable... unless...

...the shear pins would not be able to be designed to keep the impeller in motion without constantly needing to replace them. That is: would the force of the thrown snow constantly be snapping the pins which is equal to the designed shear when an object enters? Or simpler put- the force of the snow is nearly equal to the force of a foreign object such that the shear pins do not know under which load to break and snow force is nearly equal to a foreign object force and in both cases the pins would break? If design specs were such, would it be that an operator would be replacing shear pins almost constantly under normal snow throwing operations because snow or object forced on an impeller were nearly the same? (not sure if what i am trying to say is making sense??)



stuart80112 said:


> Let me try to expand for anyone who cares....


 I do and with all due respect (<-seriously) ... I need to either stay up much later and open another bottle of Scotch and thoroughly dissect your mini dissertation to fully comprehend (or get up very early and make the coffee extra strong...to fully digest... but it would need to dedicate time... and to be quite honest, rotational physics in college was many years ago LOL!



stuart80112 said:


> ...I'm always open to corrections, alternative explanations or alternative theories.


 I still theorize that regardless of machine quality would benefit from a shear pin at the impeller UNLESS design specs for shear would not allow snow to be thrown by it without breaking: i.e.- snow force is equal to foreign object force and it would break in either case not allowing a snow blower to properly toss snow because the pins would always be breaking. In order to design a shear pin to operate to throw snow would be too strong to break when a rock or other object would bind the impeller, thus making it worthless to be effective... perhaps?



jermar said:


> Thanks Stuart. Wow! Lots of effort in this epic post.


 Ditto!



87 powershift said:


> yep all the bling makes for more sales. like buying a corvair thinking one was buying a corvette c 2 ...


 Hey, hey... 'Vairs were cool cars regardless of what Ralph thinks ...LOL (wish I still had the 2 we had when I grew up... would be neat to restore them for posterity.)

Bottom line as far a shear pins in my opinion: should be in place unless they break under normal operating conditions and if making them not break under normal condition they would be too strong to give way when they need to. Lot of good physic talk here and I do not know the right answer... i only know that it would be cool to get on a time machine that also throws snow and go back and sip whiskey or roll a few with Mr Sir Isaac to get in side his mind....LOL


----------



## unknown1 (Dec 13, 2015)

To d3500ram..

My final conclusion in that giant post was... yes.... shear bolts in all places!

The problem with the shear bolts on the impeller... the old-school impellers were so heavy you could hardly pick them up with one hand... they were more like engine flywheels.
Therefore, when you engage the augers, the forces involved to accelerate that impeller would need to be pretty strong and so the shear bolts could not afford to be too weak.
That's a bit like your point about throwing snow involving strong forces... but even more so.. they really were very heavy indeed.
I'm beginning to suspect that's why they only ever used roll-pins..... the shear bolt would need to be almost unbreakable anyway to accelerate that monster impeller.

The ones you've seen on your Husqvarna and Craftsman are way lighter than in the good-old-days... that's why they got bent when those monster 13HP engines hit a rock.


----------



## 10953 (Sep 4, 2017)

stuart80112 said:


> Hey that jabiru bird blender sounds fun. Did you turn off the engine, trim the ailerons and land or what? Or was it a gentle hit and just give the plane a bunch of shakes. What was the outcome? Just change the shear bolt and you're good to go. ;-)
> ----------------
> 
> had to shut it off and glide in the last 1/2 mile, garbage dump chicken better known as a sea gull so knowing aviation you know how much prop was missing and how much other damage it suffered
> yes i needed a new clean set of skivvies .


----------

