Right size PTO shear bolts

[cornish wood burner]

????

 

If a machine has a pto shaft setup for the machine the shear pin is designed to break before the machine does. Simples.

 

The drive train on the tractor is designed to be strong enough to handle the engines horsepower.. hence the bigger tractors have stronger drivetrains.

 

The only time you have problems is when shafts get mixed up...

 

When a tractor stalls the engine rpm drops from around 1500 rpm to zero in the blink of an eye. The inertia of the engine will either cause the pto clutch to slip or twist the shaft. Either way it will put more strain on the tractor drive train than the engine power alone can produce.

[woody paul]

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.

[CHC]

It would not matter on size of bolt as if the sheer part of the pto yoke can take a m8 bolt you would not put a m6 in as it would potently damage the hole on the pto where you put the bolt through and if it stalls the tractor it would not damage the tractor as its no different to stalling it normally. if you look for the maker on the pto then do a search of the internet that might come up with the grade. Just to give you a idea though I run a big baler on a 200hp tractor and that has a m8 10.9 tensile strength bolt for the shear bolt but. A 8.8 bolt might also stretch rather then brake as the higher the tensile the more brittle they are. Sounds odd but but you can bend a 8.8 bolt but a 10.9 will brake before it bends as its a harder metal. But might be worth asking a ag dealer which Walterscheid or similar make if not sure

[ArthurBottlesworth]

Implement came without driveshaft so just bought one from Malpas tractors

I think it's rated at 60hp ( tractor is 40)

 

What's confusing me is that if the PTO drive is transmitted through the shear bolt, then a bigger engine must need stronger pins .......

 

Am I missing something fundamental ?

[CHC]

Also might be worth taking out bolt and running it to see if it seized

[cornish wood burner]

Implement came without driveshaft so just bought one from Malpas tractors

I think it's rated at 60hp ( tractor is 40)

 

What's confusing me is that if the PTO drive is transmitted through the shear bolt, then a bigger engine must need stronger pins .......

 

Am I missing something fundamental ?

Depends on what power you want to put through it. If you had a 200 hp tractor with a shear pin to suit then the pin probably wouldn't shear but the machine would be damaged first. Shear pin should be the weak link and break before your machine or tractor drive train should.

[biggarlogs]

The shear bolt protects the implement not the tractor.it is designed to protect from sudden impact loads, not torque, which is better Co trolled by a slip clutch.hitting a tree stump will give you an impact load and a shear bolt will break.long rushes will give an l creased power consumption,and hence a slip clutch will protect machine better.it is important to use a shaft which is designed for you implement.most machines have belts which both take the impact and slip when over torqued.it's all down to machine design,see your handbook!

[Justme]

Implement came without driveshaft so just bought one from Malpas tractors

I think it's rated at 60hp ( tractor is 40)

 

What's confusing me is that if the PTO drive is transmitted through the shear bolt, then a bigger engine must need stronger pins .......

 

Am I missing something fundamental ?

 

 

The engine wont be using all its power.

 

If a tool needs 40hp at 540rpm that is all the tractor will supply. If the tool starts to bog down the tractor will up the power to try to keep it at 540rpm. If the tool bogs down too much the shear bolt will shear.

[CHC]

The engine wont be using all its power.

 

 

 

If a tool needs 40hp at 540rpm that is all the tractor will supply. If the tool starts to bog down the tractor will up the power to try to keep it at 540rpm. If the tool bogs down too much the shear bolt will shear.

 

 

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

Edited July 11, 2016 by CHC

[Goaty]

Implement came without driveshaft so just bought one from Malpas tractors

 

I think it's rated at 60hp ( tractor is 40)

 

 

 

What's confusing me is that if the PTO drive is transmitted through the shear bolt, then a bigger engine must need stronger pins .......

 

 

 

Am I missing something fundamental ?

 

 

The bolt itself only keeps the two halves aligned. Not all the power is transmitted through the bolt just as a vehicle only takes part of the trailer weight.

Still on that analogy a 200hp will pull the same trailer as a much lower power and not break the coupling as it's rated for the trailers maximum potential. Not the vehicles.

 

The shear breaking point is the same.

 

 

Does the tractor really struggle and the pto have to be feathered excessively when engaging. If not I would carry on as you are, if it's an art engaging then a bigger power is required.