Right size PTO shear bolts

[ArthurBottlesworth]

There's no problem

Except when I put too big a log in ........

The bolts are marked 12.9

Does that sounds like its too high ?

[cornish wood burner]

Shear bolts were often used in a softer form years ago. The old grade 2 imperial which was around 30 tons per sq inch. Metric equivalent about 4.6 to 4.8. Shearing action was often quite plastic especially if the plates and bushes were a bit worn. Bolts are more commonly 8.8 or 10.9 now and tend to snap cleanly in comparison. I've no doubt weaker bolts are still available but the shearing action works best if it was designed to use them.

[cornish wood burner]

There's no problem

Except when I put too big a log in ........

The bolts are marked 12.9

Does that sounds like its too high ?

 

Drop down a grade or two 8.8 or 10.9 to shear quicker without stalling the tractor

[gdh]

I would be careful putting bigger shear bolts in. We did it with our chipper after snapping a few and the pto stub on the tractor snapped instead. We fixed the problem by adjusting the stress control in the end.

[Justme]

If the tractor is say 70hp and is running at 540rpm which would be about 1900-2000 rpm It will be putting out about 60hp roughly. So if a mower need 20hp the tractor will still put out 60hp other wise you would be able to put a 150hp tractor on a implement that only been 50hp to run it you can do it but if you push it to hard you will ruin the implement

 

 

If you have an engine running with no load it is only producing minimal power to keep the speed up. As you add load the governor will add fuel to allow the engine to produce more power at the same speed.

 

Power out is not fixed re rpm. The outputs are quoted maximums. IE 60hp at 3000rpm. You can do 3000rpm at less power out.

 

The danger is as you say if you push it the engine can supply too much power. So thats a good reason to have a slip clutch or shear bolt.

[ArthurBottlesworth]

I have know clever people to weld the to plates on pto shaft just because bolt kept breaking. Also check your bolt is not 12.8 instead of 8.8 this makes a lot of different's I remember a old winch truck i drive all ways snapped 8.8 then we were told it should run a 12.8 never broke any more.

 

I bought box of shear bolts with the driveshaft

They are 12.8's will get them changed for lower strength

 

Is there anything weaker than 8.8's ?

 

IMHO Better to start weak and work your way up ........

[ArthurBottlesworth]

It strikes me nice chart showing breaking points and "numbers" 8.8 12.8 etc available would be useful for us pragmatic types ....

 

We could start weak and work up to what doesn't break very often in a sensible controlled way

[cornish wood burner]

I bought box of shear bolts with the driveshaft

They are 12.8's will get them changed for lower strength

 

Is there anything weaker than 8.8's ?

 

IMHO Better to start weak and work your way up ........

 

You could try a 4.8 bolt. However the weaker and more ductile your shear bolt is the more important the shear plate design is. Ideally the plates should be held tightly together with a hard replaceable insert to cut cleanly. If this is not the case you might have a few problems getting the broken pieces out as it may not be a clean shear.

[cornish wood burner]

If your shaft transmits 60 hp with a 12.9 bolt

A8.8 gives you 41 hp

A 4.8 gives you 22 hp

This is a very rough calculation with unknowns of a good shearing mechanism which could make a difference.

[ArthurBottlesworth]

If your shaft transmits 60 hp with a 12.9 bolt

A8.8 gives you 41 hp

A 4.8 gives you 22 hp

This is a very rough calculation with unknowns of a good shearing mechanism which could make a difference.

 

That's really useful

Do you know supplier for 4.8 shear pins ?

 

Been thinking a lot about this today

There is (small) business opportunity for someone doing shear pins in various strengths .....

 

Maching a slot around the pin would reduce cross sectional diameter and could be used to produce various strength shear pins from a standard sized blank

 

If you knew torque requirement of implement and tractor pto torque output you could match them properly instead of just guessing - possibly supplying them to implement manufacturer

 

Would be at specific rpm though ........

 

Interesting stuff

 

Picked up 8.8 bolts today as step in "right direction"