Shear Bolts

[Other Daniel]

Hi all,
The question is, are all shear bolts the same for any given diameter ?
Ie: An M6 shear bolt will have a standard breaking force, as will an M8
or M10 ....
Hoping my question makes sense

 

[Kram]

Nope I believe there are grades to them., looks like they use standard bolt markings for the grade.

What is it for?

 

[Other Daniel]

Hi @Kram , thanks for the quick reply. I suspected as much.
It's from the pto shaft of my nieghbour's newly aquired topper.
It was delivered with 3 spares, but within 10 minutes he had
broken one; hence his visit over to me....
I was just thinking, in the spirit of good neighbourliness, to
obtain a few myself, which I would secrete within my workshop,
only to be pulled out at an appropriate moment in the future,
when he had never gotten around to ordering some for himself,
as per my advice to him.

Pic is stolen from t'web and is only for illustration.

...... and, we always like pictures

 

[addjunkie]

Hi @Kram , thanks for the quick reply. I suspected as much.
It's from the pto shaft of my nieghbour's newly aquired topper.
It was delivered with 3 spares, but within 10 minutes he had
broken one; hence his visit over to me....
I was just thinking, in the spirit of good neighbourliness, to
obtain a few myself, which I would secrete within my workshop,
only to be pulled out at an appropriate moment in the future,
when he had never gotten around to ordering some for himself,
as per my advice to him.

Pic is stolen from t'web and is only for illustration.

...... and, we always like pictures

View attachment 537199

More important question is why is he breaking the bolt in the first place. Rough ground, not running it fast enough, trying to start it up on the ground? I ran a topper like that for 3 years, really rough ground and never broke a shear bolt

 

[Agroshield]

What is said above about different grades is true, but I think you might achieve a level of consistency if you looked for 'PTO shear bolt' in the appropriate diameter and perhaps bought from a supplier selling them specifically as PTO spares.

 

[Erie Fred]

Might want to switch to a overload clutch for that application.

Snowblowers still run shear bolts as you only change them maybe 1-2 times a year at most.

 

[Other Daniel]

More important question is why is he breaking the bolt in the first place. Rough ground, not running it fast enough, trying to start it up on the ground? I ran a topper like that for 3 years, really rough ground and never broke a shear bolt

@addjunkie ,
Essentially all of the above
Whilst being an adorable couple (truly), he and his good
lady have left Paris to take up the good life here...
Bought themselves a very tired Fordson Dexta with a linkbox,
and, now, this new topper.
...and so their adventure begins.
I'll get a photo of the ensemble next time they get it out of
the barn.
Hence my question re the shear bolt. I'm happy to help out,
and am just seeking to pre-empt the rather predictable future
scenarios. Which, as we already know, will no doubt happen on
a Sunday afternoon, or a Bank Holiday
Their mechanical knowledge, and feeling, is as close to zero as
one could possibly get. I'm sure they have other qualities.
One example; whilst I was hooking the topper up to the Dexta for
them, I asked him if he had any grease for the pins. He replied yes,
went off and came back with a tin of WD40....
Explaing the gearbox of the Dexta to him did take a while.....

 

[addjunkie]

@addjunkie ,
Essentially all of the above
Whilst being an adorable couple (truly), he and his good
lady have left Paris to take up the good life here...
Bought themselves a very tired Fordson Dexta with a linkbox,
and, now, this new topper.
...and so their adventure begins.
I'll get a photo of the ensemble next time they get it out of
the barn.
Hence my question re the shear bolt. I'm happy to help out,
and am just seeking to pre-empt the rather predictable future
scenarios. Which, as we already know, will no doubt happen on
a Sunday afternoon, or a Bank Holiday
Their mechanical knowledge, and feeling, is as close to zero as
one could possibly get. I'm sure they have other qualities.
One example; whilst I was hooking the topper up to the Dexta for
them, I asked him if he had any grease for the pins. He replied yes,
went off and came back with a tin of WD40....
Explaing the gearbox of the Dexta to him did take a while.....

Ahhhhhh sounds like they'd be better with a mower deck on a little kubota or something, basically a biggish ride-on mower….if not suggest two loops around the outside / field heads and the mower up to speed before the arms are dropped, then stripes.

 

[m_c]

I thought this thread was going to be about security shear bolts.

PTO shear bolts are typically just 8.8 tensile bolts (not set screws!), and you just need to nip them up (there's often a bit play in the coupling, so if you tighten them too much, you end up with a bolt subject to tensile and shear forces which shears easier).
Sounds like your neighbour needs a lesson in being more gentle with mechanical equipment, as I'm guessing they're either dumping the PTO clutch in, and/or trying to cut things the topper isn't meant to cut.

However, just thinking, aren't Dextas a single clutch?
In which case being gentle with engaging the clutch again could be problematic, as any sudden engagement while manoeuvring could shear the bolt, so @Erie Fred 's suggestion of fitting an overload clutch instead of the shear coupling would be a better long term solution.

 

[Other Daniel]

Ahhhhhh sounds like they'd be better with a mower deck on a little kubota or something, basically a biggish ride-on mower….if not suggest two loops around the outside / field heads and the mower up to speed before the arms are dropped, then stripes.

Did I mention it's a very tired Dexta ?
It's certainly not revving as high as it should, that's sure.
I would guess, flat out, the engine's getting to about 1200
'ish rpm. Naturally, the revcounter isn't working.
But, you've given me an idea....
I can poke my workshop tachometer at it...
More to the point, I could see what speed the pto is
achieving. It's supposed to be 540 rpm.
Sure, they would be far better off with better adapted
machinery/tooling ; but, that's far too sensible.
They're living the dream....

 

[Other Daniel]

I thought this thread was going to be about security shear bolts.

PTO shear bolts are typically just 8.8 tensile bolts (not set screws!), and you just need to nip them up (there's often a bit play in the coupling, so if you tighten them too much, you end up with a bolt subject to tensile and shear forces which shears easier).
Sounds like your neighbour needs a lesson in being more gentle with mechanical equipment, as I'm guessing they're either dumping the PTO clutch in, and/or trying to cut things the topper isn't meant to cut.

However, just thinking, aren't Dextas a single clutch?
In which case being gentle with engaging the clutch again could be problematic, as any sudden engagement while manoeuvring could shear the bolt, so @Erie Fred 's suggestion of fitting an overload clutch instead of the shear coupling would be a better long term solution.

The neighbour's learning curve is at around 89° atm.
The (this) Dexta has a two plate clutch, btw.
But, yes, he needs to learn how to live with his machine.
This may take longer than 3 shear bolts

 

[Bouledogue]

I thought this thread was going to be about security shear bolts.

PTO shear bolts are typically just 8.8 tensile bolts (not set screws!), and you just need to nip them up (there's often a bit play in the coupling, so if you tighten them too much, you end up with a bolt subject to tensile and shear forces which shears easier).
Sounds like your neighbour needs a lesson in being more gentle with mechanical equipment, as I'm guessing they're either dumping the PTO clutch in, and/or trying to cut things the topper isn't meant to cut.

However, just thinking, aren't Dextas a single clutch?
In which case being gentle with engaging the clutch again could be problematic, as any sudden engagement while manoeuvring could shear the bolt, so @Erie Fred 's suggestion of fitting an overload clutch instead of the shear coupling would be a better long term solution.

As @m_c says, they are typically 8.8, and - even if they're not - no harm would come of using that grade. Better too weak than too strong.

I seem to remember that Dextas changed at some point from single to dual clutch. Perhaps moving to live drive? Can't remember the specifics. I also have a shadow of a thought that even the dual clutch needed some kind of pin moving to make it work.

Don't be too hard on the guys. Amateurs break shear pins at first. Eventually they get a feel for the machine and stop doing it (or do it less).

Incidentally, I once knew a bloke who welded up the coupling on a topper when he got tired of breaking shear pins. I think he did about £6000 of damage to both machines that afternoon...

 

[Bornfree]

Make sure he gets bolts long enough so the plain shank of the bolt is through both halves of the coupling.

 

[Erie Fred]

Does the PTO coupling having an overrunning clutch on it ?

 

[Other Daniel]

Does the PTO coupling having an overrunning clutch on it ?

Sorry @Erie Fred , can you explain what an over running clutch is ?

 

[Erie Fred]

Sorry @Erie Fred , can you explain what an over running clutch is ?

On the old fordson & others, if you are brush hogging, and need to stop forward motion (like to keep from going out into a road) if you just pushed the clutch
down, the PTO (still engaged) will flywheel and continue to move the tractor.
That big disc back there makes a very good flywheel.

https://www.teamtractor.com/blog/understanding-over-running-clutches--28040

This does not detract from my suggestion to ditch the shear pins for a PTO overload clutch (at the gearbox input shaft)
As changing shear pins get's old real fast.

 

[Other Daniel]

As @m_c says, they are typically 8.8, and - even if they're not - no harm would come of using that grade. Better too weak than too strong.

I seem to remember that Dextas changed at some point from single to dual clutch. Perhaps moving to live drive? Can't remember the specifics. I also have a shadow of a thought that even the dual clutch needed some kind of pin moving to make it work.

Don't be too hard on the guys. Amateurs break shear pins at first. Eventually they get a feel for the machine and stop doing it (or do it less).

Incidentally, I once knew a bloke who welded up the coupling on a topper when he got tired of breaking shear pins. I think he did about £6000 of damage to both machines that afternoon...

This one is definitely a dual clutch. I've tried it myself.
It's exactly the same as my Super Dexta.
The second (ie: pto ) part of the clutch is really at the extreme of the
pedal travel. You really have to have it down on the footplate. But, as I say,
mine's the same.

 

[Other Daniel]

On the old fordson & others, if you are brush hogging, and need to stop forward motion (like to keep from going out into a road) if you just pushed the clutch
down, the PTO (still engaged) will flywheel and continue to move the tractor.
That big disc back there makes a very good flywheel.

https://www.teamtractor.com/blog/understanding-over-running-clutches--28040

Aaah... ok.
I can confirm that when you depress the clutch pedal enough to stop forward motion,
the tractor will stop, whilst the pto continues to drive the implement.

 

[Erie Fred]

Aaah... ok.
I can confirm that when you depress the clutch pedal enough to stop forward motion,
the tractor will stop, whilst the pto continues to drive the implement.

Good to check, as it sounds like the new owners are very in-experienced.