PTOs have been a staple on the back of tractors for decades. Before the three-point hitch was even on the rear of older machines, you had that stub shaft coming out the back in order to put power, the power takeoff, out to some kind of implement. There are four main drive mechanisms that get power out to this shaft. The oldest and most basic of them is called a transmission-driven PTO, where this shaft is literally tied into the same gearing that's turning the wheels on the back of the tractor. As those wheels are turning, that shaft is also turning as well. That's found on the oldest machines that are out there. I'm not aware of any modern equipment that's going to have a transmission-driven PTO. One main reason for that is that those straight transmission-driven PTOs can actually drive the tractor forward. If you have an implement behind the machine, it has a large moving mass on it, and that moving mass is turning this shaft. Even when say the tractor's power has come down and you're trying to slow down, that moving mass can actually drive the tractor forward.
It wasn't too long after that PTO was out that an overrunning clutch was added onto it. When that moving mass is spinning, if the shaft starts to go faster than what the tractor is, it allows that shaft to overrun the machine and not drive it forward. You will occasionally still find that on the most basic tractors sold today, the most economy class machines. Sometimes we'll still have that transmission-driven PTO, but it is extremely uncommon.
The newer version of that transmission-driven PTO is called a live PTO or a two-stage clutch. Essentially what happens there is rather than having that PTO driven off the transmission itself, it's driven off of a separate shaft that has a clutch pack attached to it. Now, that pedal is going to work a little bit differently than you may expect a clutch pedal to work in say a car. When you push this pedal to a whole way to the floor, you can go and shift the gears on your tractor and let your pedal back out again and off you're going to go. Drives like a manual transmission does. However, that clutch pedal actually has two phases to it. The first half of the clutch pedal's tractor is going to clutch the transmission, allowing you to shift gears, but if you depress the pedal the entire way, you clutch both the transmission and your PTO.
The important thing to know there is that as you're driving along, you can keep your PTO under power while you're shifting gears. With that transmission driven PTO without the two stage clutch, every time you clutch to change gears, you're also clutching your PTO, so any implement that you have behind you, say a rotary cutter in the grass or whatever, is losing its power when you depress that clutch pedal. However, having that second stage in there gives you that mechanism to be able to drive a manual transmission, and also to be able to independently clutch your implement on the rear of the tractor.
The most modern version of the PTO is referred to as the independent PTO. Now all the gearing for the power going out the back is completely independent of the transmission. Your independent PTO can be done in one of two different ways. You can have a mechanically independent PTO, where again, you're using a clutching mechanism in order to de-clutch that PTO from the tractor's transmission, or more commonly hydraulic independent PTOs, where the tractor's using some kind of electronic actuator in order to engage and disengage that shaft. That's going to be found on all modern ag equipment and any compact tractor from about 30 and 40 horsepower on up.
Now, we've talked a lot to this point about how the PTO and transmission interact with one another. The selection of your transmission plays into this a little bit as well. If you're choosing a hydrostatic transmission, really none of this matters, because you're not using the clutch pedal, you're not de-clutching that transmission as you're varying your speed as you would be in a gear driven transmission. Any kind of function of is this a live PTO or a two-stage or an independent almost doesn't matter if you're buying a tractor with a hydrostatic transmission. If you're buying a gear driven machine or a power shift or a shuttle shift, you're up into the bigger ag equipment, you're going to want to be aware of these kinds of things, but many of you making compact tractor purchases, 95% of which are hydrostatics, very little of this really is going to have an operational impact on your day today.
When you start moving up into bigger utility tractors, say above 90 PTO horsepower or so, you'll start to find some machines that are going to have different shafts out the back. Now in our smaller compact tractors, 540 RPMs and a relatively small shaft has been the standard for decades. However, in your bigger machines, when you need to move more power through that shaft, you're going to have some higher needs available, and that's when you'll start seeing a thousand RPM PTOs, where that shaft is spinning faster in order to transfer more power. Those thousand RPM PTOs are sold in two different sizes, and this can get a little confusing sometimes. They're generally referred to as big and small. The big PTO stub is going to be one and three quarter inch and have 20 splines on it. The smaller version is going to have 21 splines and be one and three eights of an inch.
Now those different shafts are generally going to be interchangeable. You're usually going to have multiple options on your tractor to be able to either flip the shaft or stick a different stub in or unbolt this and bolt the other version on, but you sometimes can have some compatibility issues depending on what your implement expects and what your tractor is providing. If you're looking at used implements in particular, you want to make sure you're matching them up or that your tractor's going to be able to handle all of those configurations. Those bigger machines also occasionally will have a function audit called E-PTO or economy PTO. That puts a gear selector in line with that PTO mechanism that allows you to run your tractor at lower RPMs on your engine but still get required thousand or 540 for your implement.
Those are good for applications where say you have a 200 horsepower tractor, but only might need 100 horsepower to go out to your bat wing mower, and that allows you to run that engine at a little bit lower RPMs and potentially save some fuel. To me today, that feature is almost a little bit silly, unnecessary. Our modern tier four engines with their electronic fuel injection really are burning fuel based upon a torque load and not just based upon your engine RPMs. That feature today almost, in my opinion, should be removed from tractors because it's not really the best scenario for your modern engine. You'll learn a little bit more about that in some of the engine videos that we've done.
There are actually several types of PTO shafts down here. The end that hooks onto your tractor can be engineered in a couple of different ways, and there's some differences when it comes to service and maintenance, and also the angle that that shaft can run at in relation to your implement. So today, we're going to go and talk about PTO shafts. What I have here is the most common type of PTO shaft. This is called a yolk joint. A yolk joint is going to have two U-shaped yolks that go 90 degrees to one another with a cross that goes across the middle that supports them. Now those are going to move very freely when they're in line with one another, and they can rotate around their respective axes. But when they start to be off center from one another, you can get some chattering motion that you're going to notice. And you'll notice that in your implements, when you take your three-point hitch, say the whole way down, or you're making a headland turn, and your tractor is running at a hard angle from your implement, you'll start to hear a chatter from your yoke joints. They'll start jumping around a little bit. That noise that you're hearing when it gets to those more extreme angles is that shaft fighting as it goes around, and over time, it'll start to wear out the cross piece that sits in the middle of that joint.
