Shear pin misery... only thing left is adding a slip clutch?

Dadnatron · Jul 9, 2017

I just purchased a new flail mower. Straight from China. I ordered it, shipped it in, and put it together.

Everything seems to work normally. Everything turns by hand, but the flail is 84" long with hammers.

The gearbox isn't the biggest one I've seen, but I suspect it should be adequate, at least for some time. However, I am breaking shear bolts upon startup each time I try to get it going.

I can get it started, with the PTO and the machine turning, but long before it spools up to regular power, the shear bolt breaks. The original bolt was a Grade 5 bolt. I went through 3-4 of them again, without success. I decided WTH, and put in a TSC Grade 8 and same thing happened. They are shearing upon startup. Everything moves, and it doesn't appear that anything is bound up. It just seems as though the inertia of getting this thing moving is higher than the strength of even a G8 5/16" bolt.

Overall, the size of the bolt seems pretty small and I wonder if the PTO shaft is undersized for the size of the machine. I measure the U-joint to be 70mm so this would be a Series 3 PTO shaft according to U-joint measurements. Given the PTO requirements rated on the machine and the HP of my tractor, this should be between 70-100hp and a Series 8 PTO shaft. But I see no torque on the shaft (I've removed the cover and have just been trying to make sure everything is up and running before I put it back on.) It moves... the flail drum freely moves and spins for quite a while after the bolt shears. I suspect it gets up to about 40% speed, lasting about 2 sec before the shear pin breaks.

I'm thinking I need to add a slip clutch to the shaft in order to be able to tighten it adequately to make the system work. And while I've used PTO shafts with the clutch already installed, I've never installed one myself.

Any thoughts on this issue? I know the typical is 'something is too tight and it is binding' but I don't think this is it... I think it just takes 'too much power' to get everything up and moving, and by the time that has occurred, the shear bolt has broken.

PS: I am contacting the manufacturer and telling them about the issue. I think, looking at the situation, that they simply included the wrong size PTO shaft for this machine. A Series 3 shaft would be typically with a 20-25hp machine.

If this is the case, and I want to get another 'domestic' PTO shaft, what is the best source for one? I'd like to get one with a CV joint and a slip clutch if possible. Any options on US source? I need 540rpm collapses to 72" between shafts with expansion of about 12-16" it appears.

3Lfarms · Jul 9, 2017

I may be mistaken, but I think most flails are belt driven, and the belts provide the slip if the flail binds up. I don't think I've seen a shear bolt or clutch on a flail. My Berti doesn't anyway.

k0ua · Jul 9, 2017

And I have never seen anything smaller than a 1/2 inch shear bolt on a mower. I can't imagine a 5/16 shear bolt. That just doesn't seem like much on a mower of that size.

CalG · Jul 9, 2017

You might take a "survey" of comparable equipment and consider up sizing the shear bolt to match the need. Drilling out holes is cheap and easy. ;-)

The PTO shear bolt on a 5 foot snow blower (Bueler) is 3/8" grade 8, just as a point of reference. The auger drive is 1/4" grade 2.

stuckmotor · Jul 9, 2017

Here's my standard comment on shear bolt problems. I was breaking them like crazy until I realized the ones I was using were too small. I didn't think one with a greater diameter would fit but it did and I saw a great improvement. May your problem be as easy to solve.

ericm979 · Jul 9, 2017

Dadnatron said:
I suspect it gets up to about 40% speed, lasting about 2 sec before the shear pin breaks.

Are you engaging the PTO with the tractor at idle?

AgriSupply has a lot of PTO shaft models if you can't get this one to work.

pmsmechanic · Jul 9, 2017

How loose is the sheer bolt bracket assembly on the shaft? Is the U-joint assembled properly? Is the shaft out of balance?

oosik · Jul 9, 2017

The 5/16" grade 8 shear bolt is too small. My Wallenstein BX62S uses a series 5 pto shaft with a 3/8" grade 8 shear bolt.

I would suggest that unless you hear otherwise from the manufacturer - go to at least series 5 pto shaft with appropriate slip clutch.

ruffdog · Jul 9, 2017

Always check the pto shaft to make sure the u-joints are in phase. If it is out-of-phase, it is more apt to shear pins.

Here is some good reading on u-joint phasing:
http://www.tractorbynet.com/forums/kubota-owning-operating/372559-going-through-second-stage-bolts-4.html

RPW · Jul 10, 2017

Are you starting the PTO at idle and slowly bringing it up to speed? Any strange noises?

Dadnatron · Jul 10, 2017

I do not bring it up from idle. That is not a feasible situation in my real world. I cannot idle down every time I want to begin mowing.

I have no strange noises. Everything begins and appears fine. It begins spinning, and everything is smooth, but I think the stress of getting all that metal moving simply overcomes the shear force of the pin.

I am extremely limited on what I can add as far as size of a bigger bolt. There simply isn't very much 'meat' to drill out and maintain integrity.

If I try to weld it (and I'm not sure whether it is cast steel or iron) and add an adaptable slip clutch at the machine, will that 'work' as far as protection of the tractor? I was able to find a slip clutch at TSC which slips over the shaft at the gear box, and onto which the PTO shaft can be attached. At least that is what it appears to be set up for. I thought, perhaps if I weld the 'shear pin pieces' together, then I could still protect things with the clutch. Any thoughts about this?

The only other thing I can do is to purchase a completely new PTO shaft with a slip clutch integrated already. This is probably what I should do. Get a new PTO shaft Series 6 or 8 with a CV joint and a slip clutch. I am sure this would cure 'all' the problems which have arisen.

IF anyone has a recommendation on where I can find such a shaft collapsed length of 72" with at least 18" working expansion, please link me or give me a name. The length that is required is making it difficult to find.

Gator6x4 · Jul 10, 2017

Dadnatron said:
I do not bring it up from idle. That is not a feasible situation in my real world. I cannot idle down every time I want to begin mowing.

I have no strange noises. Everything begins and appears fine. It begins spinning, and everything is smooth, but I think the stress of getting all that metal moving simply overcomes the shear force of the pin.

I am extremely limited on what I can add as far as size of a bigger bolt. There simply isn't very much 'meat' to drill out and maintain integrity.

If I try to weld it (and I'm not sure whether it is cast steel or iron) and add an adaptable slip clutch at the machine, will that 'work' as far as protection of the tractor? I was able to find a slip clutch at TSC which slips over the shaft at the gear box, and onto which the PTO shaft can be attached. At least that is what it appears to be set up for. I thought, perhaps if I weld the 'shear pin pieces' together, then I could still protect things with the clutch. Any thoughts about this?

The only other thing I can do is to purchase a completely new PTO shaft with a slip clutch integrated already. This is probably what I should do. Get a new PTO shaft Series 6 or 8 with a CV joint and a slip clutch. I am sure this would cure 'all' the problems which have arisen.

IF anyone has a recommendation on where I can find such a shaft collapsed length of 72" with at least 18" working expansion, please link me or give me a name. The length that is required is making it difficult to find.

View attachment 514946
View attachment 514948 View attachment 514949

Call these people by phone, Agri Supply has farm supplies, tractor implements & mower parts.

Everything Attachment is a real good source. Tractor PTO Shafts | The best quality shafts at the best price available and as always free shipping.

You may also want to review Everything Attachments video on how to measure and cut a PTO shaft. (The first one on this page) everything attachments how to cut a pto shaft - Google Search

RandyT · Jul 10, 2017

I have seen that exact shear bolt setup that uses a 5/16 bolt on a bushhog squealer SQ72 that belongs to my neighbor. I have to start all pto attachments at idle with my Kioti Otherwise you will get the chock load of your life. And a 1/2 grade 5 won't survive at anything above idle on my cheap rural king bush hog. Went to slip clutch years ago because of it.

vvanders · Jul 10, 2017

What's the side of the flail look like, does it have a belt? If so then there's you're slip protection. FWIW Caroni flails don't have a shear pin or clutch, just 2-3 belts.

I bring mine up from idle every time, helps with drivetrain shock and once it gets spinning it takes a good ~30s to spin down so I usually only need to do it once. You'll probably stop breaking shear pins if you bring it up from an idle.

SPYDERLK · Jul 10, 2017

Dadnatron said:
I do not bring it up from idle. That is not a feasible situation in my real world. I cannot idle down every time I want to begin mowing.

I have no strange noises. Everything begins and appears fine. It begins spinning, and everything is smooth, but I think the stress of getting all that metal moving simply overcomes the shear force of the pin.

I am extremely limited on what I can add as far as size of a bigger bolt. There simply isn't very much 'meat' to drill out and maintain integrity.

If I try to weld it (and I'm not sure whether it is cast steel or iron) and add an adaptable slip clutch at the machine, will that 'work' as far as protection of the tractor? I was able to find a slip clutch at TSC which slips over the shaft at the gear box, and onto which the PTO shaft can be attached. At least that is what it appears to be set up for. I thought, perhaps if I weld the 'shear pin pieces' together, then I could still protect things with the clutch. Any thoughts about this?

The only other thing I can do is to purchase a completely new PTO shaft with a slip clutch integrated already. This is probably what I should do. Get a new PTO shaft Series 6 or 8 with a CV joint and a slip clutch. I am sure this would cure 'all' the problems which have arisen.

IF anyone has a recommendation on where I can find such a shaft collapsed length of 72" with at least 18" working expansion, please link me or give me a name. The length that is required is making it difficult to find.

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View attachment 514948 View attachment 514949

I think you should go with the new shaft with slip clutch. That will solve the problem.

IF I were going to try to make what you have work, I would machine that hole out to half inch on a new center 3/32" inward from its present center. A very difficult operation on that extremely hard steel. -- But like you said, just drilling it bigger on center would critically thin the outer web.

ruffdog · Jul 10, 2017

Yeah, just plunge a 2 flute carbide end mill on the new location.

Caution...only go up one size at a time...a 3/8 has much more meat that the 5/16

Dadnatron · Jul 10, 2017

I tried bringing it up from idle and got it running full speed. It was pretty nice because it had absolutely no vibration, which really surprised me. I expected at least some vibration or shimmy but it worked well. I tried it out a bit, but must admit I didn't have a lot of confidence in that shear bolt. But from idle up, it held for at least 4-6 runs.

Cut well, recognize I need to work on the height adjustment. But I did a quick swath along the road and ran it vertically to clear off some things growing finto the road a bit. Worked great.

I'll do a full review of my experience at a later date. I saved about 75% from US based purchase, from what I'd found. There are likely others, but what I'd found was out of my willingness to pay.

I think I'll look to get a Series 6-8 shaft with a clutch. It has belts, but there are 4 of them and this is a pretty good sized machine so by the time the energy has it alll turning, I'm thinking those 4 belts might hold strong enough to still damage something. I don't know, but I know my peace of mind would benefit if nothing else.

I'll see what I can find this week.

airbiscuit · Jul 10, 2017

First/Best Fix - engage the PTO from Idle
Second/cheap fix - drill a second shear pin below the first one. (do 1/4" first to see if that is just enough extra and still maintain function).
Third fix - new PTO Shaft with slip clutch.

vvanders · Jul 11, 2017

I suspect unless you *really* hit something that shear bolt will hold. The flails should bounce off anything small they hit, the driveline shock of going 0 -> 540 is pretty substantial. I'd carry a couple extra with you and see how well it holds up but a clutch probably won't hurt.

ruffdog · Jul 11, 2017

The problem with clutches is they freeze up. If the unit sits for a while, it takes much more effort to break them loose and the tractor may be damaged.