PTO Augger...what sort of sheer pin?

[bkuhn]

I purchased a used pto driven post hold digger and wondered if someone could suggest what sort of sheer pin i should use? The tractor is 35hp/29 pto hp. The guy at the farm store suggested using a grade 2 bolt but so far that seems to sheer at the first sign of a small rock. Sure seems like the augger (12" by the way if that is important) could handle much more and not break 3 times while digging the first hole, but maybe i'm giving it too much credit. I rented a tiller a few weeks ago and they gave me grade 5 bolts to use. But maybe that's different because of the different implement.

Any suggestions would be appreciated. Thanks all.

Brad

 

[Farmerford]

Brad:

Although shear pins protect the entire drivetrain, they are sized to the weakest component. That is, they should break just before the weakest component is damaged.

In PHD the weakest component is usually the PTO shaft because it is so long and therefore will twist and bend fairly easily.

My Bush Hog brand has a 1 3/8" diameter (I think) input shaft and the shear pin is a "special" 3/8" pin. My Kubota B2400 with only 18hp at the PTO will easily shear a pin at any speed much above idle if the auger hangs on a root. I later learned from Bush Hog that the only difference between the PHD model the dealer sold me (rated for a 9" auger) and the next size up (which is rated for a 24" auger) is the strength of the PTO shaft.

Had I purchased the PHD with more capacity, the only difference would have been a stronger driveshaft and the shear pin would have been upgraded from the weaker "special" pin to a stronger one. I say "special" pin because the PTO yoke that attached to the gearbox was drilled so that only the special Bush Hog proprietary shear pin could be used. The stronger PTO shaft would have been drilled to allow a different and stronger shear pin to be used.

I say all this to point out that the rather long PTO shaft inherent with PHD's is probably the weakest link and the component that will be first to fail if you upgrade the shear pin from a Grade 1/2/3 bolt to a Grade 5 or 8. It also points out that the long PTO shaft has a very low torque limit, particularly if the shaft is one of the smaller sizes.

At one point I considered using a Grade 5 bolt and, if the shaft were damaged, I would upgrade to the stronger shaft. But I changed my mind. I did drill the yoke so that I could use hardware store Grade 1/2/3 bolts rather than the hard to find (and expensive) Bush Hog proprietary pins. The bolts seem to shear as easy as the Bush Hog pin, so I suspect the Bush Hog pin has about the same tensile strength as a Grade 1/2/3 bolt. I just carry a box full of 3/8" Grade 1/2/3 bolts and take a break for a beer each time I break one.

 

[rankrank1]

I use grade 2 bolts in my ancient post hole digger of unknown brand in order to protect the PTO shaft as it definitely appears to be the weakest link.

Safety Note: I have found that wrapping the shear bolt area on the PTO shaft with a layer or two of duct tape prevents the pin from being flung when it shears when digging. Helps protect barn siding, windows, or more importantly anyone else who might be in the area that is not seated on the tractor.

 

[Slacker]

I use grade 2 bolts in my ancient post hole digger of unknown brand in order to protect the PTO shaft as it definitely appears to be the weakest link.

Safety Note: I have found that wrapping the shear bolt area on the PTO shaft with a layer or two of duct tape prevents the pin from being flung when it shears when digging. Helps protect barn siding, windows, or more importantly anyone else who might be in the area that is not seated on the tractor.

I've done something similar, but the reason was to not lose the self locking nut that holds the shear bolt in. I guess it works as a saftey device also.

 

[davidseaquist]

I had a linebacker Jr Post hole digger for my old tractor bx22. I was told to use a grade 5 bolt. I still sheared the bolts in my rocky soil. David

 

[VistanTN]

You don't say, but if the hole for the shear pin has been stretched/elongated, it is important to tighten it down in the direction of normal contact. If not, the auger can slow/stop and the shaft keep turning -- even a small amount will act like a scissors and cut the pin.

Another thought you might want to slow the engine speed down a bit, not to stall the engine, but just dig slower and with less torque on the shaft. OR slow the lowering depth rate down. I could dig at 540 rpm and drop the auger as fast as it would dig in FL sand, but in TN rock I have to slow down both a bit (or spend a LOT of time replacing grade 2 bolt shear pins).

 

[mboulais]

You don't say, but if the hole for the shear pin has been stretched/elongated, it is important to tighten it down in the direction of normal contact. If not, the auger can slow/stop and the shaft keep turning -- even a small amount will act like a scissors and cut the pin.

Another thought you might want to slow the engine speed down a bit, not to stall the engine, but just dig slower and with less torque on the shaft. OR slow the lowering depth rate down. I could dig at 540 rpm and drop the auger as fast as it would dig in FL sand, but in TN rock I have to slow down both a bit (or spend a LOT of time replacing grade 2 bolt shear pins).

I will second all of that. I almost never run a PHD at full speed to drill a hole. 540 equates to 2500 rpm for me and I will usually run it at 1500 rpm. I have a grade 5 bolt in mine and the 1/2" bolt holding the auger (only had 1 in) was grade 2. It sheared the 1/2" bolt.