Anyone who owns a proper farm tractor will tell you two things: they’re indispensable and they’re expensive to fix. I recently pursued a quote for repairs on our John Deere 5105, and I was shocked to find that, unlike a car or truck, tractor shops bill you by the day, not the hour, or at least my local shop does! Not to mention the three-day project came with a heavy price tag to boot. My problem was the brakes on my tractor, but many other things can break as a result of abuse, especially with PTO (Power Take Off) driven implements. That’s why a PTO slip clutch should make it onto your farm tools list.
What is a PTO Slip Clutch?
You’ve probably seen one before; they’re a simple round disc and spring device found on most PTO-driven rotary mowers. A PTO slip clutch is a torque limiting device that, when overcome with too much torque, limits the amount of torque being transferred from the tractor to the implement. This is accomplished by, you guessed it, slipping or spinning free which allows the two sides of the PTO shaft to spin at different speeds. Slip clutches are commonly provided with bush hogs and rotary tilling devices, but can also be used with most PTO-driven devices such as post hole augers, hay tedders, balers, cord wood saws and other common farm equipment.
What is a Shear Pin?
A shear pin is the alternative to a slip clutch. Many implements that the manufacturer suspects will see limited instances of locking or jamming will be sold with a far cheaper shear pin torque limiting device. Effectively a shear pin is exactly that; a pin, or bolt that is intended to shear or otherwise break under a specific amount of torque. When you blow a shear pin because you hit rocks with your post hole digger, the pin shatters, causing your PTO shaft to spin free without transferring power to the implements, basically creating a fail-safe disconnect. You’ll need to pull the auger out, unscrew the auger from its hole or otherwise free it from the stoppage. Once free of obstruction you need to turn off your tractor, realign the holes that pin lived in and replace that pin with a new one. Replace 10 pins in two hours and you’ll see the wisdom of investing in a slip clutch assembly.
Unfortunately, sometimes an operator has a stroke of genius and realizes that, if they replace the shear pin with a hardened bolt, it won’t snap. This is true, but when your trusty grade 8 bolt holds together when a shear pin would have broken, you destroy gears in the implement or your tractor, neither of which are a cheap fix.
Why You Want a Slip Clutch
A PTO slip clutch provides a safety measure, an intended point of failure, for when your PTO driven implement jams such as catching a stump with your bush hog, or binding your post hole digger up in roots or rocks. Even the best small farm tractor, when running at 3800 RPM or higher and the implement you’re using suddenly stops, something will give; be it a torque limiting device, your tractor’s clutch, your transmission, the implement’s gear box or worst case scenario; your PTO shaft fails catastrophically. Having a slip clutch device avoids all this expensive damage, and gives you the convenience of not having to change pins several times a day. Once your PTO implement is free of the obstruction, your slip clutch will resume transferring 100% of the torque between your tractor and implement. Handy, isn’t it?
How to Repair a PTO Slip Clutch
Slip clutch assemblies are relatively simple pieces of equipment. The major parts include the input side or tractor side clutch flange, the output or implement side clutch flange, a baffle or pressure plate, friction disks, sometimes an intermediate steel plate, bolts to hold it together and springs to provide the correct level of friction. If you use and abuse your slip clutch, it is possible to wear out or otherwise break your friction disks, like I did the last time I used my brush hog. In my situation, my clutch had seen many hours of abuse and the disks were worn super thin. It only took a little sapling, a little slippage and BAM!, my clutch disks disintegrated and my brush hog ceased its helpful spinning motion. By the way, if you’re mowing and have a hard time keeping traction in the field, consider using tractor tire fluid to add traction.
Repairing the clutch assembly was a breeze, but finding the parts was a challenge. Eventually, I found the appropriate clutch plate for my assembly on Amazon, new springs (for good measure) on another website as well as new bolts and nuts from the hardware store to do a complete rebuild. If you’re faced with replacing your clutch disks and you can’t find part numbers or original owners manuals, check and record the outer diameter (OD) and inner diameter (ID) in both inches and millimeters. Thicknesses are relatively universal across brands and your old disks are worn, so don’t bother measuring that. Many clutch lining manufacturers offer universal friction disks, so find the one that most closely resembles your dimensions, or if you can find a competent equipment dealer, have them order the rebuild kit for you.
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Originally published in 2016 and regularly vetted for accuracy.