Drive Train Protection for a Sawmill

Pros debate the ins and outs of a slip clutch versus a shear pin to protect the power train of a sawmill driven by a tractor's power takeoff. January 27, 2007

What is the right kind and level of shear pin protection for a PTO drive from a 40 hp tractor to a sawmill? Overriding clutches seem to protect in the other direction. My application is for a Belsaw circular mill. The PTO has a clutch, but I think I want separate protection. How large a pin? What kind of coupler would you recommend? What is the original Belsaw connector like?

Forum Responses
(Sawing and Drying Forum)
From contributor G:
Use a 1/2" grade 2 bolt. If you shear this more than once every 6 months, go to a grade 5. The best way would be to put in a jack shaft and use belts. This would also move the tractor off to one side and you can then run the dust chain straight out the back. Note on a shear pin, you will need to cut groove in the end of the shaft for a C clip to keep the PTO shaft on when the pin breaks.

From contributor R:
"The PTO has a clutch, but I think I want separate protection."

Assuming you're talking about a drive line slip clutch, why? A properly adjusted PTO drive line slip clutch is a world better than a PTO drive line shear bolt.

Why not just properly adjust for your tractor what you already have and be happy that you have the better of two options already? Anyway, if you don't have a slip clutch or shear bolt protection in the drive line, you can buy a PTO shaft either way from tractor supply stores like TSC, or most any new tractor dealer.

From the original questioner:

It seems to me that a shear bolt is a more fail safe system than a clutch. Clutches can stick if not used or adjusted for a long time, but a shear bolt is going to break reliably at around the same load or less.

From contributor J:
You may want to rethink what you want to do. Shear pins are for shock loads, to protect the machinery, and sawing is not really a shock load event. But I can see a lot of drawbacks to a shear pin failure while sawing because you are imposing a great load on the shafts, and common sawing could break a 1/2 inch bolt easily, in my opinion. I would be more concerned with adequate guards or a stand behind to keep away from the blade arc than I would in having shear protection that saw manufacturers have never provided... must be a reason.

From contributor R:
"It seems to me that a shear bolt is a more fail safe system than a clutch. Clutches can stick if not used or adjusted for a long time, but a shear bolt is going to break reliably at around the same load or less."

Well, each to their own, but I have quite a few around here, and I much prefer the clutch every time. It has to set a year or more before I bother to loosen the springs, slip the clutch a little and readjust it. I haven't found one of mine stuck yet. If there's an overload, a clutch is a lot easier on things, too.

From the original questioner:
I appreciate the comments and am interested in learning more. The reason I started to focus on shear pin protection is that the Belsaw M-14 mill was advertised with a shear pin protected PTO shaft for direct drive PTO applications. I still have the flier describing it. No clutch, just shear pin. I'd like to see one of those shafts or have someone describe it in detail. But now, if I use a slip clutch as several recommend, how do I know how tight to adjust the clutch?

From contributor J:
Bear in mind that the load from a sawmill on a typical tractor is going to take all the available power... and slippage of any kind will result in problems. My mill is belt driven direct from the motor to the mandrel, and I must keep the belts tight or slippage occurs. And if it slips, the saw slows, heating results, and things go south from there. If you were sawing and sheared a pin while in a cut, I imagine having the blade hung up in the log would be a thrill also. I have seen several old Belsaw M14's and none with shear protection, but a few modified with belt drives to get a higher head saw rpm, since a PTO is 540 on older tractors and some blades run true at 600 rpm or better. I run my 52 inch blade at 625, which is the speed I had it hammered for. If you have the blade sent in to a saw doctor, they can hammer it for as low as 250 or 300 rpm.

From contributor D:
I have a M-14 TK that came with a factory shear pin pto shaft. It does work. Things do come to an immediate stop when they (2) snap and then it is a bit of a job to run the carriage back by hand. I do think mine fails because of metal fatigue and not necessarily because the saw is in trouble. I do think there should be something in the driveline though, to be honest - I'd been keeping my eyes open for a slip clutch. The old school mills around here use the three tire setup for driveline protection a lot, adjusting the tire pressure till they stop rolling smoke.

From the original questioner:
Contributor D, that's really useful. Can you describe that PTO shaft a bit more? What size and grade are the shear bolts? How far are they OC from the center axis of the shaft? What's the mechanism for holding things together when they snap? How long is the shaft telescoped in and out? I like your idea of combining this with a slip clutch. I'm running now with a 6 inch flat belt off a Mustang 2 2.8L V6. If I tighten the belt too much, it throws the lead off, and the engine is starting to give trouble too, so I'm lining up the next stage. I have a winch in place for pulling back the carriage on the rare occasions when I have to kill the engine in the middle of a cut to keep the saw from heating, so I don't mind that possibility. Thanks.

From contributor S:
Slip clutch and shear pin? You must be a belt and suspenders kind of guy. I am not a mechanical engineer, but I would suspect that the clutch would slip long before the pin would shear. And the clutch, as I know them, can be adjusted to slip at different amounts of torque. If anything, go with two clutches or add a belt somewhere in the drive line and a few remote kill switches for the panic attacks. There is no such thing as being too safe!

From contributor D:
This is a picture of the tractor end of my shaft. The bolts are 5/16 diameter. I just use hardware store ones (grade 3?). Although support for the mill has stopped, Timberking might be worth contacting. I have bought some spares from their NOS.

Actually, instead of belt and suspenders, I had been considering just replacing mine with a slip clutch. Contributor J's comments started me thinking about unknown slip. That might be why it is a make or break connection, don't know. I guess I did forget to add, there is a kill switch beside me back over to the tractor, so I guess there is some belt and suspenders going on.

Click here for higher quality, full size image

From the original questioner:
That picture really helps me out. It looks a little more complicated than one flange up against another. When the shear pins break, is there something that still keeps it together so the PTO shaft doesn't flail all over? Also, roughly how far are the two shear bolts from the axis of the shaft, since this is critical to the torque they protect against?

From contributor R:
Those shear bolt shafts are common and can be bought at a farm store or tractor dealer. I have several of them. You have to buy the size for the HP that it will be used with. You do *not* just buy one and shove any bolt in it and use it on any HP tractor.

As far as the slip clutch, many implements don't use them because they want to hold the cost down, and then they offer them as an option. As long as the slip clutch is adjusted properly, they last a long time (years if only slipped occasionally). When adjusted properly, if they are overloaded, they do not just slip and then destroy themselves! Mine will start slipping, then pull the engine down. If you don't take the load off, they will choke the engine down while slipping and kill the engine.