Shear pin keeps braking

[whynot162]

I have a howes 5 foot brush hog mower. I have used it for years with no issues. Last year I broke the shear bolt when hitting a rock. I have since replaced the bolt almost every time I use it. What do I need to check? mower turns fine by hand, and gear box is full of oil.
Could the bolt have been a grade 8 instead of a grade 5? I have only been mowing grass, so I do not know why this is happening, and need some help...

Thanks
Doug

 

[George2615]

I doubt the shear bolt would be a grade 8 unless you have a slip clutch. If you have a clutch then the bolt wouldn't be a shear bolt and may be a grade 8.
Does it break while mowing or only when engaging the mower.
Is the shear bolt hole elongated or not perfectly round. If the bolt is really loose or sloppy in the hole that will allow it to chuck back and forth when engaging or disengaging the mower leading to premature failure.
If the hole is larger than it is supposed to be you can drill it out to the next size say going from a 1/4" to a 5/16" or if 5/16" drill up to 3/8". It's also possible the shear bolt may be a metric size instead of standard.

 

[OZisKTB]

My JD 503 Rotary Cutter Manual says: "If shear bolt fails, replace with a M8 x 50, 8.8 grade cap screw and lock nut."

I can't get the grade from the Howse site, do you have a manual?

 

[whynot162]

I do not have a manual,
the hole seems fine, I have been using a 1/2 X 3 1/2 bolt. and it seems to fit? I do not know if it could be metric? I do not notice that it is broken until the mower stops working, so I can not tell if it is when using it, or when it starts. I do not have a slip clutch.
The tractor is 32Hp.
Thanks

 

[Gary Fowler]

IF it is shearing that easily, I suspect that you have replaced the original with a smaller bolt OR a lower grade or maybe both of these. Go up one grade in strength and see if that helps. It could have been a metric bolt that could be a few thousands larger than an SAE bolt AND a higher grade bolt. By combining a slightly smaller bolt And a lower grade, this would cause the problems that you are having.
The shear bolt should be a snug fit and no slack between the two sides. If you have a micrometer, measure the inside dimensions of the hole and find a bolt that exactly fits that hole within 5 thousandths.

 

[stuckmotor]

I was breaking shear bolts like crazy until I found out I was using bolts one size too small. I didn't even know larger bolts would fit in the hole but, they do and are a vast improvement.

 

[whynot162]

That may be it, The bolt does go in easy, I have a caliper, But do not remember if it will measure the inside.
Off to look.

Thanks

doug