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PTO Horsepower and Generator Question
Now that's a real interesting question and maybe Chief's dealer will have a simple answer.
I'm with Harvey and think the ratings are taken from dyno readings and the tractor wouldn't be moving. At least my manual states 'observed pto hp, and I imagine 'observed' means read off a dyno. Besides, I know that tires, pto and hydraulics do share power from the same engine. My PHA will lug the engine if it's bitting good and I try to pull up the auger with a lot of dirt on top of it. Sometimes going into a pile of gravel with the loader I have to clutch the tractor before the hydraulics will break out the bucket. If the tractor were moving, high traction should reduce power available to the pto, so what traction load would be chosen for the tests? It's got to be static I think.
On the other hand PTO specs for HST tractors are usually less than for gear tractors. I can't figure why a pto HP for a HST should be less than for a gear pto. I think most HST's in neutral just spin the swash plate on the pump around. There aren't many moving parts and no oil is moved. I don't know why that would take much power or at least more power than a gear TX. I think there's something here I've missed and maybe an answer will pop up in the next several days.
I forgot to mention in comments about oversized generators in another thread that smaller breakers can be used to better match a tractor to a large generator. It's probably not desirable to have to reset main breakers frequently if the load is subject to high surges but it's still probably better than having to restart the tractor or have motors overheat.
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PTO Horsepower and Generator Question
Good response and it does point to an assumption I made. We might get to a more general understanding of HST's here if that's interesting. I think most HST's use variable displacement axial flow pumps or motors. They can be of swash-plate or other design.
My assumption is that most HST's use a variable displacement pump rather and a fixed displacement motor but it is an assumption. A swash-plate type axial flow pump doesn't move oil or develop pressure in neutral because the pistons don't move. Of course, that wouldn't be true if it's the motor that's variable. There are other types of variable displacement pumps often used in closed center hydraulic systems that do maintain pressure.
I think I've heard of some HST's that use variable displacement designs for both the pump and motor. In that case one of them becomes the range selector.
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PTO Horsepower and Generator Question
The link below is the best pic I have of an axial flow pump, which is a type I that think is commonly used for HST's. The swash plate is rotated by the input shaft and pulls pistons in the cylinder block in and out. The pump pictured has a fixed displacement. Variable displacement ones have a control that controls tilt of the swash plate. The plate spins all the time, but if the plate is perpendicular to the cylinder block then the pistons don't move and there will be no flow or pressure; that is unless there's something I haven't grasped yet. My impression is that the speed pedal controls tilt of the swash plate.
Here's where I am perplexed. If the pump works the way I've thought it does for quite awhile now I can't see any reason why it would develop circulation or pressure when in neutral. On the other hand, I also can't understand why a spinning swash plate would sop up much HP. The power loss from gears to HST would make more sense to me if the pump were doing something. Don't know! Another impression of mine is that the pto input shafts are pretty much separate from the pump.
Full of impressions today but then I'm just a gear head myself.
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PTO Horsepower and Generator Question
There doesn't seem to be much continuing interest in the HST PTO rating question. I'll persist with one more comment. I can't say I'm right because I'm not claiming expertise, bit I don't think I've exactly gone wrong either, nor do I think Murf and I are saying radically different things.
Control of ground speed/torque is achieved by varying the volume pumped in variable displacement engines or motors or both. High volume = fast ground speed; low volume = slow for a pump and reversed for a motor. Common variable axial flow pumps usually are variable swash plate or bent axis designs.
Basically, the swash plate or axis angle has to change to vary the volume pumped or the motor speed. One way or another a swash plate or axis angle must change to vary ground speed. It can be on the pump, motor or both and the speed pedal as far as I know has to affect one or the other. As Murf said, the speed pedal doesn't operate directly on the motor or pump in most CUT's as far as I know. To my thinking if the speed pedal controls ground speed it has to affect displacement on the pump or motor somehow.
Here is a link that describes a HST more or less the way I think I did.
http://www.tigercat.com/hystdem.pdf
The link below gets into HST designs that use variable displacement pump and fixed motors, fixed pumps and variable motors or again both and the advantages of the various designs and throws in gear section in series as well. A set up for Baja Cars is described. However, all of this sort of begs the question. I don't know which design is common on CUT's and that's sort of what I was interested in.
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PTO Horsepower and Generator Question
Art's comment is well taken. At least in this part of the thread I'm guiltily of mixing and matching principles and specifics and without a grasp of the specifics. All of these designs seem to be out there and Murf pointed indirectly to different properties of the designs, which is something I hadn't thought about.
So, from Murf's comment I did some digging and found the link above that summarizes the designs. The link also says the variable pump only type is likely to be found only on lawn and garden tractors due to torque limitation of the design. The design would save much of the HP that seems to be sopped up by a HST when a tractor isn't moving.
Since power is sopped up, I'd guess that most CUT's use a design where both pump and motor displacements are variable. A variable motor only design is inefficient at extreme ranges of operation and on it's own can't achieve zero speed or reverse. The pump likely has a load sensing design so pressure is maintained and oil bypassed as Bill and others mentioned. A geared range section reduces improves efficiency by reducing the ratio of displacements from the pump and motor required to achieve extreme ranges of motor speed
Anyway, that seems to be where the HP goes. I'm not sure why since I'd guess zero speed as well as reversing could be achieved by the pump swash plate. The answer likely has to do with coordinating operations of the pump and motor. But, that's enough of an answer for me although a bunch of blanks are left to fill in sometime. It's also probably the answer for Doc's question now a long way above. I'd guess most CUT pumps work against pressure all the time, and that's where the HP goes.
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PTO Horsepower and Generator Question
The link below goes to a site I ran across awhile back. The site likely supports a commercial idea of the owner but I haven't figured out exactly what is being sold or if it's interesting. The particular page linked to is about the sorts of leakage Art mentioned. There are a couple of other relevant pages on the site.
This mixing and matching of principles and specifics does get messy but I don't read any angst or even dismay here. When it works content should become more specific and less speculative. I think we've got to be messy sometimes to keep up our content quality and everybody eventually benefits. I know my grasp of both principle and specific have ratcheted up through this discussion and I hope others benefited as well.
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PTO Horsepower and Generator Question
The links I thought I put in an above comment aren't there. I remember now that you can't put site addresses in text but I thought I included another one in link url box but maybe not.
Maybe interest in HST mechanics is exhausted for the moment but I included the links below the pump picture in the link below. The principles link describes constant torque, constant speed and other designs. The description is for a variables pump and motor design.
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PTO Horsepower and Generator Question
Hey, a pretty good way to ease out of a messy topic. At least I don't have to collect any thoughts and I'd have to have a second to make further contributions. Well, HST's don't have seconds so I'm out of luck. Don't have a HST either.
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