Go BottomDumb shear bolt questions
 | kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon 
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2004-06-04 87719
I was chipping some damp material and the tractor started lugging so I shut it off to clean out the blower chute. This got me to thinking about protective devices for the pto driveline. My rotary cutter has a slip clutch, but it appears there is no safety device on the chipper. If it had a shear bolt where would it be? Could a cheap shear bolt be added easily? Are they available off the shelf? I know a slip clutch would be around $100 but for this application a shear bolt should work fine and be cheaper. The chinglish chipper manual leaves lots to be desired.
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 | DRankin
Join Date: Jan 2000 Posts: 5116 Northern Nevada
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2004-06-04 87722
Near as I can tell, a shear bolt for a tractor implement is just a standard grade (non-high strength) bolt.
I have seen special (weakened) bolts for certain applications such as low powered snowblowers. ....
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 | AnnBrush
Join Date: Mar 2004 Posts: 463 Troy OH 
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2004-06-04 87726
Here is a good African (where people usually get by with less) standby: Standard mild steel (flux coated) welding rods make excellent shear bolts as the metal is soft and will shear easily, they are always available and usually very cheap. ....
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 | yooperpete
Join Date: Jan 2004 Posts: 1413 Northern Michigan 
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2004-06-04 87729
Most of the shear bolts that I've seen are about 3/16 diameter or larger. Which is bigger than flux rod. If you can't find brass or if they don't hold up, I use round head square neck bolts (i.e. stove bolts). They are softer than any other steel bolt available. ....
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 | shortmagnum
Join Date: Nov 2003 Posts: 848 Wisconsin 
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2004-06-04 87730
Ken, your teacher would tell you, "No shear bolt question is a dumb shear bolt question." :-)
My 5' cutter uses 1/2" dia. bolts so even the soft ones are pretty stout. Maybe the chipper isn't subjected to the extreme forces that the brush cutter is but it seems that it might be easy to accidentally throw in a rock or something.
Dave ....
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2004-06-04 87731
Ken, it's not a dumb question.
Unfortunately a chipper, by design, is a tough thing to protect. It has a VERY heavy fly-wheel which would transfer most of the shock load onto the chippers balde before the driveline even felt it.
The only protection which is practical on a PTO-powered chipper is the internal PTO clutch unfortunately.
Best of luck. ....
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| kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2004-06-04 87737
Thanks, guys. With Murf's response I guess I won't worry about it. This chipper has a pretty big 175 lb. flywheel so there's lots of inertia there. One time it lugged on me it stalled the tractor so it seems that the pto clutch doesn't slip much either. Lesson learned - don't try to chip for at least 12 hours following a good rain :) ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2004-06-04 87742
Ken, I can't believe that the dampness has much to do with it.
We have used ours in pouring rain and it made no difference at all. A chipper is a pretty simple beast and the different manufacurer shouldn't change anything, a flywheel and a blade are the same in Chinese as they are in Finnish.
If anything I would think the water would lubricate the blade and soften the wood making the cutting even easier.
Best of luck. ....
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| kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2004-06-04 87745
I wouldn't have thought the dampness would hurt either but it sure seemed to. The blower chute plugged three times in the morning following a rain but as the afternoon came and stuff dried out there was no more plugging. I suspect it has something to do with the fir needles. They don't have much surface area to catch the airflow and when they are soaked they weigh more than normal. Whenever it plugged there was tons of fir needles in the chute. Chipping wet deadwood with no needles wasn't a problem. ....
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 | TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2004-06-04 87753
You'd think that there be some protection for the pto drive and clutch in event the chipper jammed. Maybe the theory is that the chipper would tear itself apart first.
There was a discussion here where a person with a rotary cutter blade jammed into the ground. The pto clutch burnt up before the tractor could be shut down and the cutter has a slip clutch. ....
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| kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2004-06-04 87755
That's sort of what I was thinking. The fact that a chipper jam caused the tractor to stall indicates that the chipper won't tear itself apart so something else has to give. Maybe that something else would be expensive to fix. Which brings me back to my original question, how can I add a shear bolt to the pto driveline? I know you can buy add-on slip clutches but this situation shouldn't happen very often so a cheap fix is appealing. ....
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 | harvey
Join Date: Sep 2000 Posts: 1550 Moravia, NY
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2004-06-05 87769
Ken do the expensive brands have shear bolts? Shear bolts are there to work if there is a sudden shock load, AHHHH if you dropped a big piece of rebar in it thinking it was a branch.
The size of the bolt to install would /should be the more important question. Too small always shearing too big what was the point.
Put it in your PTO driveline, usually where the pto shaft attaches to the unit.
Remove the square stock and set screws if it has them. (Maybe it only has a bolt thru it and you already have one.) Drill an EXACT fit this I think requires letter drills for the size bolt you want to use. If the size is bigger than the key way I'd drill thru the set screw hole thru the key way. If not bigger, drill away from keyway area. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2004-06-05 87771
The pto shaft for my blower has a shear bolt in the coupler to the tractor. I think it's more or less like Harvey described--a small bolt through a flange that allows the collar to open and release the shaft when sheared. I'm still working on the supply of shears that came with the blower so I've just replaced them without thinking about them. I think they're 6mm and fairly soft since they're intended to protect the blower. Harder ones would take more power bit I don't know if the hardest ones available would take the full pto power. In comparison the shear in the output shaft of my auger is a 5/16" grade-2. It's much larger but maybe softer but the coupler on the pto wouldn't accommodate near as large a bolt.
My blower shaft is a generic one so ones that already have a shear bolt can be bought. As Harvey mentioned the hole size and alignment is pretty important. If the drill size is wrong or a hand drilled hole wobbles a shear won't work as expected.
I would be aware that slip clutches and shear bolts do slightly different jobs. Shears are better at protecting against high shock loads and slip clutches better at protecting against continuous high loads. ....
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| kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2004-06-05 87786
The chipper is driven by a one piece splined shaft. The pto driveshaft connects directly to the chipper and there is no square key stock or set screws or keyway. If I were to install a shear bolt it looks like an adapter of some sort would be required to fit between the chipper and the pto driveshaft. Do some pto shafts come with shear bolts already installed?
I am not very worried about big shock loads since the operator hand feeds material into the chipper and should not feed rebar, but when the blower chute jams the chips and fir needles compress into a hard mass that drags on the flywheel. The tractor then lugs and sometimes stalls. Maybe a slip clutch would be better for this application.
Harvey brought up a good point about what other brands do so I downloaded a BearCat pto chipper manual. It doesn't mention any sort of pto driveline protection, but the troubleshooting manual does address the clogging discharge chute and lugging/stalling of the tractor. They don't seem to be concerned about it though. ....
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| harvey
Join Date: Sep 2000 Posts: 1550 Moravia, NY
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2004-06-06 87825
Had not considered that type of input shaft... I have not looked at any chippers at chippers in detail. But I think back to the baling hay days with flywheel balers The shear bolt on your chipper should/needs to be between the flywheel and the rest of the machine.
As far as plugging and stalling many farmers have been there and done that on blowers filling silo. No real permenant damage done. Unless you keep trying to clear the plug by slipping the clutch...
I'm glad it's you out in those pines getting ate up by bugs. I spent lotsa time in Alaska and a couple of days in Washington and can only immagine the size of the skeeters this time of year... ....
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 | blizzard
Join Date: Jul 2003 Posts: 282 Central Maine
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2004-06-06 87831
Ken,
I think shear pins/slip clutches are to protect the attachment from too much torque, wheather from overload or shock load. The 'protection' they give the tractor is only incidental to this. As your tractor is bogging down, but there is no shock load, a shear pin weak enough to break at the available torque of your PTO would might be very frustrating, and an inexpensive slip clutch might not last long. Either would tend to reduce the total available power to your chipper, if sized to prevent the problem you mention.
However, my Puma snowblower's drive shaft has shear pin protection built into the universal joint. It seems sized about right for my 24 PTO HP, in that as the tractor starts bogging the pin will shear. Part# 6545. This takes a standard 1/4 X 2-1/2 NC bolt and Nylock nut. You might look into it, but Puma parts are premium priced. Otherwise, have you tried feeding bare (branchless) wood in occasionally to help clear the needles? Spraying the inside of the chute with ArmorAll or PAM? Or maybe just shutting down on a schedule to clear the chute? I don't think you'll hurt your tractor if this only happens very occasionally, but stalling under full load won't do it any good, either. There's a link to Agri-Supply slip-clutch driveshafts below...
Good Luck, and Good (rainless) Weather,
bliz
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Link: Some-Clutches
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| kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2004-06-06 87836
It's sounding like it's not much of a problem. Now that I know it can happen I can identify the symptoms and shut things down before it stalls. If I just avoid chipping when the needles are wet it will probably never happen again. Feeding an occasional dry branch would work in theory, but when it lugs down in the middle of a 30 foot douglas fir, branches and all, it's not easy to pull the tree back out.
Blizzard, thanks for the link and Puma info. It probably would be a good idea to spray some PAM in there each time the chute is off.
Harvey, we really don't have many (any?) skeeters where we are. Certainly nothing like Alaska. It's really pleasant working in the woods this time of year. I've probably spent a good 50 hours out there in the last six weeks and haven't been attacked by a single bug. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2004-06-07 87914
Ken, although I have never tried it I do know several people who have installed remote control 'kill switches' for when they are working alone, but away from the operator's seat, things like PTO winching, or running a log-splitter.
They sell remote controlled solenoids, similar to the keylees entry units for your vehicle, for 'switchless' installation of high-power amplifiers, fog lights, etc. These units are inexpensive, the good ones are only $50 the cheapies are $25 so even as an experiment it's not very painfull.
They use the radio controlled solenoid to activate the electric fuel shut off on the engine. Pushing the remote control button does the same as the key, but works from up to a 100' away.
Cheap insurance I suppose.
Best of luck. ....
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| DRankin
Join Date: Jan 2000 Posts: 5116 Northern Nevada
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2004-06-07 87919
I got pretty used to mosquitoes when we lived in Alaska and I got worried the first summer we were in Nevada when I saw a swarm of them here.
My wife, who was born here told me not to worry.... those weren't skitters, they were buzzards. ....
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