Bolt connecting PTO shaft to rotary cutter

[Roscoe]

Gday Flakrat, Firstly there are a few reasons why a shaft can break.

It may not be the original shaft for the mower and somebody has replaced it with a smaller diam one.

Any wear in the UJ's, centre splines and shaft splines where it slides over the tractor PTO can set up radial shock loads in the shaft.

If the PTO shaft is not a nice sliding fit over the input shaft on the mower then you are wasting your time getting any sort of bolt to work properly.

For a shear bolt or pin to work as designed then its very important for the bolt or pin to be an exact fit in any hole that it goes through.
If the bolt is allowed to move at all either radialy or in an axial plane then it takes more force to "break" it.

Mild steel or grade 2 bolts are commenly used as shear bolts but they are just bolts.

Correctly designed shear pins and bolts are normally only available as a genuine replacement parts.

There is also a safety issue concerning parts that are not designed to shear at a set loading.

Cheers
Roscoe

 

[Soundguy]

For safety.. I'd check the mower manual. My ford 9XX series manual states to use a grade 2 bolt as the shear pin.

MY KK manual gives the part number to a 4-pack of shear bolts.. OR specifies the use of a grade 2 bolt of the correct dimensions.. etc.

Soundguy

 

[flakrat]

Thanks for the expanded info.

To answer the question regarding how the shaft connects to the rotary cutters drive shaft, it requires the bolt to turn the drive (i.e. no splines on the Rotary cutter side).

The RC is a Clipper 400. I've not been able to find anything via google regarding this cutter (a manual would be great ) so I don't know if the PTO shaft that cracked was the original or a replacement, and for that matter if the bolt was the original.

Based on what y'all have said, a grade 5 bolt would be ulikely.

The 1/2" shank fits in the hole nice and snug.

As for why it broke, I noticed that before it broke, the end of the big part of the PTO shaft (the side that the smaller diameter side slides into) was wallowed out, so my best guess is that this shaft was too short for the job and didn't have enough small diameter shaft inside the larger shaft.

 

[jinman]

Flakrat, when you install the shear bolt into the PTO shaft and cutter gearbox, make sure your grade 2 bolt is long enough that all the threads are out of the hole. run your nut all the way down and then use pliers or some other tool like a punch to flatten the threads just behind the nut. That will keep the nut from backing off better than using a lock washer. I've never had any luck using a lock washer because the bolt bends and stretches and soon the nut becomes loose on the lockwasher, allowing it to back off on the threads.

If you use a short bolt with threads that stay in the hole, you actually have a situation where the diameter of the bolt is less than 1/2" because of the cut of the threads. This allows the bolt to break and shear easier than the plain 1/2" shank. This is just something I've learned through experience.

Finally, I have one PTO drive shaft that is bowed two inches in the middle. I had a brain-dead moment and raised my 3PH too high with the mower attached and bent the PTO drive shaft. It was comical watching it flop around after I bent it. /forums/images/graemlins/blush.gif It was not comical paying $125 for a new one. /forums/images/graemlins/frown.gif /forums/images/graemlins/smirk.gif

 

[BB_TX]

<font color="blue"> I have one PTO drive shaft that is bowed two inches in the middle. I had a brain-dead moment and raised my 3PH too high with the mower attached and bent the PTO drive shaft. </font>
Can't quite match that, but I did manage to cut thru the plastic safety shield before I could get it back down. /forums/images/graemlins/blush.gif

I use a nylon insert lock nut on my shear bolts. Works well

 

[flakrat]

Even more good advice, thanks y'all

 

[jinman]

</font><font color="blue" class="small">( I use a nylon insert lock nut on my shear bolts. )</font>

That sure does work good, Bill. I still have a couple of those in my bolt box, but I quit using them because I'm cheap. /forums/images/graemlins/wink.gif You've seen my place and its "interesting" topography. I've gone thru so many shear bolts it's embarrassing. I almost never find the nut end of the shear bolt when I have an "event." I think the aircraft locking nuts are about $0.40 each. I decided to use regular nuts I could buy by the pound at tractor supply for pennies apiece since I go thru so many. /forums/images/graemlins/crazy.gif

 

[BB_TX]

Surprisingly I have not lost any. They are always laying inside the plastic shield or at least on the deck. The angle iron bracing keeps anything from rolling off.
As far as cost. I sheared my last one several months ago. Went to a farm store in Bonham (closest place to my land) and picked up several bolts and some extra nuts just in case. Expected about $6 or $7 based on the bin price. Lady threw them all on a scale and charged me less than $2. /forums/images/graemlins/grin.gif I asked if that was correct and she said they charge by the weight. If it wasn't so far out of the way, they would get all my business.