Right size PTO shear bolts

[ArthurBottlesworth]

Running Chunker last week and put in log too big for the machine

 

Instead of snapping the shear bolt it just stopped the tractor dead - scared me to death - I seem to have got away with it but have now resolved to find the right strength of shear bolt

 

I assumed there would simply be a chart where you looked up the tractors hp and selected the right shear bolt but it doesn't seem to work like that ...

 

It can't be right that a 20hp tractor uses the same bolt as a 200hp tractor

 

Can somebody give me some technical advice on how to select a bolt that will actually shear ?

[Peasgood]

Don't know what machine you have but 8.8 M8 are usual shear bolts.

Technically they should all break at the same point (and it makes no difference on what power the tractor is) but in practice they do vary quite a lot.

 

A good explanation of the reason the tractor power makes no difference is the same as a matchstick snapping when a 5kg weight is hung off it. How strong the person holding the matchstick is has no influence on it.

[cornish wood burner]

First of all you will need the tractor pto torque. If you don't know this the engine spec might tell you. Multiply engine speed divided by pto speed x engine torque at rated pto speed, this will give pto torque.

Find the radius of the shear bolt from the centre of the rotating shaft.

Calculate the force on the bolt using torque divided by radius

Diameter of shear pin will give you the cross sectional area of the

bolt.

You then need force/ area in whatever units you are working in to give the required shear strength.

Find a bolt with that shear strength. However I suspect you will need to go down several sizes of bolt if your machine covers 200 to 20 hp

[cornish wood burner]

First of all you will need the tractor pto torque. If you don't know this the engine spec might tell you. Multiply engine speed divided by pto speed x engine torque at rated pto speed, this will give pto torque.

Find the radius of the shear bolt from the centre of the rotating shaft.

Calculate the force on the bolt using torque divided by radius

Diameter of shear pin will give you the cross sectional area of the

bolt.

You then need force/ area in whatever units you are working in to give the required shear strength.

Find a bolt with that shear strength. However I suspect you will need to go down several sizes of bolt if your machine covers 200 to 20 hp

Actually I doubt whether the machine would take 200 hp before the shear bolt sheared. You could try as soft a bolt as you can find. Even aluminium and work up to a 4.8

[Goaty]

Don't know what machine you have but 8.8 M8 are usual shear bolts.

Technically they should all break at the same point (and it makes no difference on what power the tractor is) but in practice they do vary quite a lot.

 

A good explanation of the reason the tractor power makes no difference is the same as a matchstick snapping when a 5kg weight is hung off it. How strong the person holding the matchstick is has no influence on it.

 

I'm with this:thumbup1:

 

It is a shear point to protect the implement and tractor from shock load. Pto drive shafts can fail internally. Which is not good. Expensive pto shafts have clutches built into them. Rotavators often have that type.

It's always better to have a tractor that is at least slightly gaffer of its task.

 

You could try softer bolts. I reckon they will snap often.

I've have stalled many times with pto driven equipment and never had a tractor failure. I think it is engineered this way, that the weak point is the engine power. It's similar to using or mistakenly selecting the wrong gear and stalling

Edited July 11, 2016 by Goaty

[Tom D]

Why bother? Changing a shear bolt is a pita, turning the key on the tractor is much easier. The shear bolt is setup for the strength of the shaft / machine that it is attached to. If your tractor hasn't got the power to shear the bolt then thats fine, it hasn't got the power to bend anything either....

[cornish wood burner]

Why bother? Changing a shear bolt is a pita, turning the key on the tractor is much easier. The shear bolt is setup for the strength of the shaft / machine that it is attached to. If your tractor hasn't got the power to shear the bolt then thats fine, it hasn't got the power to bend anything either....

 

Not quite true. You haven't considered the tractor pto drive. Constant shock load coupled with the engine inertia will exceed its design strength. Probably enough built in safety margin but who knows.

[Goaty]

I have a John Deere 710. A 1967 tractor. When I got it the internal shaft that runs from the clutch to the 1000rpm speed shaft which was one solid 40mm diameter shaft about 120 long it was sleeved as a repair with grub screws👎 the pto didn't turn when engaged. I did get a scrap tractor to get the donor part. I imagine yield of steel is improved now.

[shavey]

If it helps in any way i do acres of rough topping every year and if ground is uneven or lots of tussock grass and bracken I use three different types of tractor toppers all 3m one is straight forward two blade one is double blade mulching and the other is big flail

now if iam in the above rough territory to save destroying the rubber doughnuts or right angled gearboxes instead of using the m10 x 50mm shear bolts

I use cheap m10 x 50mm bolts with thread from bolt head to nut that way you have a weak point on the threads when you catch high ground or heavy lumps its strong enough to do normal cutting but if you hit something it's gone before any damage is done

[Tom D]

Not quite true. You haven't considered the tractor pto drive. Constant shock load coupled with the engine inertia will exceed its design strength. Probably enough built in safety margin but who knows.

 

????

 

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...