Shear bolts

[almost13ranch]

I have a JD 513 mower that I bought used although it was in very good shape. I don't have the manual for it and I'm wondering why I'm breaking shear bolts. I've broken two so far and I'm using M10x aprox. 3" long, lock nuts. The top of the bolt is marked 8.8 also. I do have a lot of rocks and dirt piles where i have to mow and it doesn't seem to be after hitting these thing that the shaft slips. I'm wondering if I'm setting the mower up on the tractor right as well.

Now, and I have re-read this 3 times in the owners manual, I'm starting the PTO at 1500 rpm and shutting it down at idle. I'm beginning to wonder if the tractors owners manual might be wrong and causing the bolts to break?

 

[CharlieS]

engaging the pto at 1500 adds alot of shock load to not only the pto, but attachment as well. i always engage the pto at idle.

 

[beenthere]

"almost"

My JD 513 manual says "To avoid premature failure of shear bolt on cutter driveline, engage PTO slowly at low engine rpm".

I think the ehydro tractors have a more abrupt startup to the PTO shaft, so the rpm being low may be more important.

 

[DMW]

If the shear pin is on the tractor side of the shaft, you should also check the angle of the PTO shaft when raised/lowered. The PTO shaft might be binding enough to break the bolt. Has the shaft/joints ever been greased? You said it was picked up used.

If all seems ok then read this thread: http://www.tractorbynet.com/forums/owning-operating/17269-jd-513-eating-sheer-bolts.html#post189543

 

[almost13ranch]

If the shear pin is on the tractor side of the shaft, you should also check the angle of the PTO shaft when raised/lowered. The PTO shaft might be binding enough to break the bolt. Has the shaft/joints ever been greased? You said it was picked up used.

The bolt it on the mower side of the shaft.

The shaft was very easy to slide and the joints were in good shape. Also the angle of the shaft in an up or down position is very slight. The shaft has markings on the plastic cover indicating that it was manufactured in Italy. I greased everything to make sure. One other thing I thought was odd was that there is a grease fitting on the shear bolt side of the yoke.

I may have to find a manual as i still don't know if I set it up correctly. It's level when it's down, and the lever is about in the middle, but it 's only about 8" when it's all the way up

 

[Ford850]

The way you have it set up sounds fine to me. I had problems shearing bolts the first few times I used my cutter on my DX55. Now I engage the pto when slightly above idle. I s-l-o-w-l-y engage the lever until the shaft starts to spin and then fully engage it. Then bring the RPM up to speed. I think I've only broke one since then, and that was on a dirt mound.
According to what I could find on your bolt marking, that 8.8 seems equivelent to a grade 5 bolt. While stronger, it could be more brittle than a grade 2 shear bolt. Any equipment I've used with shear bolts called for grade 2. I buy them in bulk at TSC. Your JD mower might require the 8.8, but I would verify that and use what they suggest.

 

[LoneCowboy]

You WANT the bolt to break.
it's protecting the rest of the driveline.

Use cheap nasty made in china junk bolts, you want them to go (despite it being a pain), at any attempt at jamming the system.

and yes, you should engage at idle and very slowly.

 

[almost13ranch]

Here's an update. As per the bolt, JD spec's the 8.8 bolt for that mower. I also talked to a mechanic at the dealer ship regarding the reason that the engage speed is so high. It is to keep the engine from stalling in the event of needing full horsepower to start an implement. It is an electric engage, and as such there is no adjustment on how quickly it engages. He recommended that I engage it in the lowest speed that will not cause stalling. I can start the PTO on this mower at an idle without stalling, and will now continue the practice.

Thanks to all for your help.

 

[Carykong]

I,also, engage the PTO on my brush mower fully lowered into tall grass. The grass tends to help absord the shock load. As an aside, I do not use sheer bolts but like to use carriage bolts. Carriage bolts are fully threaded and will break easily with an impact. Like everyone has said, its a pain to replace the bolt but its a whole lot better than shaft,gear, or PTO damage.

 

[beenthere]

Carykong
Seems like less load on startup would be better for the shear pins, rather than more load. :confused3: