PTO clutches

What are pto overrunning clutches and slip clutches and when should they be used?
Thanks

ORC need to be used on any tractor that does not have live PTO the reason is if your using a machine like say a brush hog it will push you forward till the blades on the hog stop. That happening can cause you to run into thing you do not want to run into. A slip clutch is a safety devise that helps keep you from braking things of some how you hit something or get something in a machine that stops the machine from turning like it should
Hobby farm

An ORC is basically a one-way gear. It's engineered so that you can transmit power from the PTO to the implement, but the the inertia of the implement (like the momentum of the disc and blades spinning on a bush hog) cannot transmit power back to the PTO shaft. A critical thing to have if you don't have a live PTO.

The basic, non-live PTO takes its power directly though gears off of a shaft in the tranny. If you are running an implement like a bush hog that has a lot of momentum, you won't be able to stop the way you would otherwise with the implement running because the momentum of the moving parts in the implement, without an ORC, would keep turning the input shaft, transmitting their momentum back to the gears in your tranny. The net effect is that instead of the motor powering the tranny from the front end, the power is coming into the tranny from the PTO on the back end, so depressing your clutch pedal makes no difference, and you can find yourself unable to stop and possibly in a mess.

A slip clutch is designed to relieve the stress of overloading or can be a comparatively cheap alternative to a differential, depending on the application. In the former, you can choke a sickle mower so that the knife will stop running without killing the tractor motor, or breaking things in between the two. In the latter, you'll find them on ground drive implements like hay rakes and the old beater-style manure spreaders. In a straight line, both wheels provide power to turn the rake or the beaters on the spreader. On a turn, where one wheel will necessarily turn faster than the other, the slip clutch will slip and let the two wheels turn at different speeds. neof the most basic, and longest lasting designs is two discs with ridges cast in a radius on their faces so that they mesh/nest into each other (picture the ridges in the middle of an orange-juice squeezer, only flat). They're held together under a spring, whose tension can be varied to control the point at which the ridges can pop past each other.

ORCs are mostly to to protect the tractor and operator from power backfeeding from the implement. Slip-clutches are to protect the implement from the power coming from the tractor, whether that power is coming from the PTO, hydraulics, or from a ground drive.

You also need the ORC on many tractors WITH live PTO. That because many have a PTO brake that will get torn to pieces with the momemtum of a PTO shaft trying to turn with the PTO off and brake automatically engaged.