shear bolt question(s)...place your bet

[alltherage]

I have a 721CTS
http://www.loftness.com/Literature/filezone/index.php?path=./SnowLogix/Obsolete Snowblowers

The pto shaft calls for (2) 5/16"x1" grade 5 bolts. The small sprocket on the blower's chain drive calls for (2) 3/8"x1" grade 2 bolts. The large sprocket calls for (2) 7/16"x1.5" grade 2 bolts.

What should break first given this scenario on a tractor that's 23 pto hp? What do I want to break first? Seems to me that the best scenario is if the pto shaft bolts break first, because the pto shaft is heavier duty than the sprockets. I broke my first shear bolts the other day, and they were the pto shaft bolts. I didn't realize the shaft had bolts, so needless to say I FREAKED OUT at first. They were really rusty on the outside though and wondered if this same scenario would play out with fresh bolts.

What say you? Thanks!

 

[PHPaul]

The idea is for the PTO bolts to shear first. That way the entire drive train is protected.

My little Pronovost has one 1/4" Grade 2 in the PTO connector and one in the chain drive for the auger. I've only broken the auger bolt when I've gotten something jammed in the auger before it gets to the fan.

 

[woodchipper]

There is no sheer bolt that will break first. and there is no reason for one to break before the other. the purpose of a sheer bolt, it to protect the drive components that are in direct relation to the per say sheer bolt.
The chain drive sheer bolt will break if any intrusion in its own components gets in the way. For exemple: a rock too big to get exhausted by the impeller fan will want to jam the fan against the housing, most likely breaking the chain drive shear bolt first, but not allways. If the PTO shaft rotates at an excessive angle relative to the snow blowers input shaft, it will put a lot of strain on its own sheer bolt, and basically none on the chain drive. Therefore the PTO bolt is most likely to let go before any damage to the universal joints, bearings, and tractor PTO internals could occur.
It is of good practice to respect the suggested bolt strength. The higher the number, the higher the tensile strength of steel. A grade 2 is very soft, while a grade 7 is semi-hard. Grade five is the most commonly used bolt in the machinery sector. They are all used for specific purposes. The same goes with the number of thread per inch. The more threads per inch the tighter the bolt can be torqued without stripping the threads.