Right size PTO shear bolts

[cornish wood burner]

Not something I've bought for a long time. You might find a few shear bolts in 4.8 but will be rare now. I would try a fastening supplier for what is basically a mild steel bolt. Just had a search and K engineering list 4.8 bolts.

A plain shank coach bolt must be about 4.8 but you need a means of holding the head while you tighten. Tackweld a nut under the head of one if you fancy doing a trial first perhaps?

As I mentioned previously you will need a reasonable well engineered shearing mechanism to cut the bolt. If the plates can part you will find the broken bolt will bend too much before it breaks. It will then give you grief to remove. Might be too weak anyway but you might have an idea of the power required from listening to your tractor.

I have seen grooved shear pins around so I think we might have missed the boat on that one. Also I've sold my lathe so I couldn't make you one anyway.

As an option if the bolt is long enough to hold you could take some of your 12.9 or 8.8s to your local machine shop and get them to machine grooves to reduce their strength. Get the groove in the right place and it would be better than 4.8s if that is what suits. Run an angle grinder cutting disc around a bolt is another thought. Not accurate but might work.

A few options for you, good luck with it.

[ArthurBottlesworth]

Simpler way may be to simply put bolt in drill and use hacksaw to cut slot

 

Strength will be proportional to x sectional area, so on 10mm bolt 1mm cut would give big reduction in strength .

 

Cross drilling bolt would give similar effect .......with bit more accuracy

 

Surprised there is nothing more on the net about this, perhaps people prefer to buy gearboxes ?

[CHC]

Why not just use a fully threaded bolt with no shank?

[ArthurBottlesworth]

Might do just that

 

I'm just being pedantic (stupid) but I really like to understand what I'm doing and why, I feel better then.....

 

Really p***es me off that I used wrong bolts, and could have caused damage......

 

Better to ask and be ignorant for 5 minutes than never to ask and be ignorant for ever

[cornish wood burner]

Why not just use a fully threaded bolt with no shank?

 

Set screws ( fully threaded bolt )are not ideal because they do not bear fully on the shear plate holes. If the sets are highly stressed or not tightened enough, you will get wear over time because of the poorer fit in the holes

Best option is to use weakened high tensile bolts.

[ArthurBottlesworth]

Nuff said - will do

Thanks everyone, feeling happier now....