Go BottomJD 790 PTO Question
 | John Weaver
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2000-05-13 16240
Does anyone know if the PTO on the JD 790 is designed such that a bushhog could "push" the tractor? I don't really understand exactly how this occures, but if so, it seems dangerous. I have heard the term "overriding PTO", but don't know if it applies to the JD 790 (or even what "overriding PTO" means). Any help?Thanks, John (newbie).
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| Halsey Green
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2000-05-13 16242
What overrunning is, is when the flywheel action of the mower blade drives the power train of the tractor. I am not sure how the internals of the JD790 is connected, but you should be able to check by jacking up the tractor and see if turning the pto truns the wheels. I just purchased one today but won't be picking it up until Tuesday. I can't find a reference in the manual. Overrunning is a problem with balers and there large flywheels.
Halsey Green
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| bud in oh.
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2000-05-13 16243
John, an over riding pto is when an implement driven by the pto is turning faster than the pto it self. Most balers have a flywheel that is set up on a type of ratchet so if you slow the rpms on the tractor or disengauge the pto it allows the fly wheel to override (travel faster) the speed of pto on tractor. Balers are set up with a fly wheel for momentum to drive the bales through the chamber and to handle the changing loads.(windrow sizes). A three point mower does not have the capabilites to push a tractor like say a rototiller would. Hope this helps, budinohio. ....
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| Rob Munach
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2000-05-13 16245
The 2WD 790 has an overunning clutch on the PTO so the implements will "freewheel" when you push in the clutch. The 4WD has a 2 stage clutch (also can be called live PTO). Halfway down clutches the tractor, all the way down clutches the PTO. It is very handy when you need to halt foward progress of the tractor using the clutch but still want the implement to be powered (such as when you are crunching up huge amounts of brush in a rotary cutter) ....
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| John Weaver
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2000-05-14 16278
I have test driven the JD 790 with 4WD. The clutch all the way down disengages the PTO. I guess the danger would be in an emergency when you could not operate the clutch if the PTO continues to "push" the tractor. I understand that there is an operator presence switch in the seat so probally not a problem. Am I looking at this correctly? I have never owned a tractor and interested in safety. ....
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| Jim White
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2000-05-16 16327
The only time a tractor can be pushed by the fly wheel effect of an emplement is if the PTO drive comes off of the drive train of the tractor instead of an independent drive. The 790 has an independent drive for the PTO and will not push your tractor no matter what. ....
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| Danny Yerks
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2000-05-16 16336
I came real close to buying a JD790, so I researched them well. This is a question your JD dealer should be able to answer, if he can't, then my advice is to find another dealer. I'm seeing a variety of answers here. First, let me explain what is meant by the implement pushing the tractor. This will only happen with implements that have large flywheels or blades. For example, a finish mower has lightweight blades and they do not provide enough momentum to push a tractor. However, a bush hog (rotary cutter) has large heavy blades and when they get spinning at full speed, they create a lot of momentum! On some tractors (especially some of the older ones like Ford 8N's and JD B's), the PTO is engaged by sliding a gear so it's connected directly to the transmission drive, so when the tractor is moving so is the PTO. Unfortunately, the reverse is also true, when the PTO is moving so is the tractor. What this means is, when you are going along cutting with a bush hog and you need to stop quick (because you see a rock or something), if you disengage the clutch the engine will no longer be driving the tractor, but the momentum of the bush hog will! And believe me, it has a lot of momentum! I've done a lot of bush hog mowing with a JD B and when I needed to stop quick I had to stand on both brakes and hope for the best! An overrunning clutch is a device that allows the tractor to drive the PTO, but not the reverse. It's basically like a ratchet wrench, so when the mower tries to drive the PTO, it just slips and makes a ratcheting sound. Some tractors have these built in, but if they don't you can easily add one on externally (I think they're about $65). If the tractor has an independent or live PTO, you don't need and overrunning clutch. I was told by the JD dealer I was dealing with that the JD790 2WD without power steering has a transmission driven PTO and would require an overrunning clutch for a bush hog. He said the 2WD with power steering comes with a 2 stage clutch live PTO and would not require an overrunning clutch. He said the 4WD comes standard with power steering and a 2 stage clutch live PTO and would not require an overrunning clutch. With the 2 stage clutch, you can push the clutch in half way and the tractor will stop, even though the PTO is still running. This allows you to stop quickly if need be, and you can change gears, switch to reverse, etc, while the PTO is still running. If you push the clutch in all the way the PTO will stop (once the momentum of the implement dies down). I hope this clears up the confusion!
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