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PTO Horsepower and Generator Question
Historically I have only really run gear units, for technical reasons, but have recently added a HST unit to my 'personal use' department.
Based on what I know of them the way it works is remarkably like the way your vehicles power steering system, the pump is creating full pressure constantly, whether or not it is required. However the pressure relief valve opens and shuts constantly maintaining that pressure level.
This though is nothing to do with the amount of WORK the pump is required to do, which is a LOAD on the system. If the PTO is running and the tractor is stationary then the only thing the pump is doing is is maintaining the pressure set by the relief valve. There is some intrinsic power loss by even just this amount of work being done, but not nearly as much as pushing the tractor along.
My overall caution is that tractors are not overly efficient at rated output, nor do they like to produce at 100% for very long. Generally when we are spec.'ing equipment we like to use 75% as a rule of thumb. In other words if your machine puts out 28.5hp at the PTO, consider it to be about 21 hp and it will perform well without beating up the equipment.
Best of luck.
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PTO Horsepower and Generator Question
Tom, you were heading down the right track until you got to "My impression is that the speed pedal controls tilt of the swash plate.". This is where you made a wrong turn friend. As I mentioned earlier (maybe not clearly enough), and Mark H. re-iterated, the foot pedal on a hydrostatically driven unit is merely operating a valve which directs the flow of hydraulic fluid. It has nothing to do with swash plate dynamics, that is self-controlled based on input from sensors & valves, etc.
The variable rate pump, BTW, is only required so that you can have a wide range of functions served by one pump instead of several. For low motor speed applications, like loader work, you need maximum PSI, but not as much in the way of flow, as you do for high motor speed applications, like road speed travel, where you need an immense amount of flow, but not at as a high a pressure.
The best example I can think of to help visualize the need for a variable pump is in a log-splitter, you want the cylinder to travel as fast as possible 'unloaded' (on the return stroke and the down stroke before it contacts the wood) to mimimize the wait times, but then you want it to develop maximum power and reduce speed in order to actually split the wood.
Clear as mud, huh?
Best of luck.
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PTO Horsepower and Generator Question
Congrats to both (all 4 actually) of you. I'm just a baby, and on a second 'other half' so I have a long way to go before I get to that benchmark of life. I recently hosted a 50th anniversary 'garden reception' for my parents last month. I was beginning to think I wouldn't see 50 years, never mind being married that long but the Doctor now says I'm "gonna suffer along with the rest of us for a long time yet." 
Back to splitters though, Doc, don't buy one unless it has a two-stage pump on it. Also, don't waste your time (and money) on one with 'two direction' splitting, it isn't nearly as powerful on the 'back stroke' and it's a waste. If you are going to be tackling the 'big' stuff you may want to look at the 'block lifter option' if it's available, in fact it might be worth it to eliminate any manufacturer that doesn't offer it. While your at it, check out the ones that flip to a vertical position too, it can be a real back-saver.
Mine is a two part splitter, the 'power unit' is outside in a 'dog house' and the 'splitter' portion is mounted mine into a long, very sturdy steel bench in between bay doors in drive shed at the farm. If you back in with the wood in the back of a truck, or trailer, you move the wood straight sideways onto the bench, then slide it through the splitter and out the other side into another truck or whatever you want, boxes, etc. This way you only have to pick up the wood once instead of many times. Besides if your splitting in bad weather (including hot sunny days) you are inside doing it, not to mention STANDING UP...
Best of luck.
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PTO Horsepower and Generator Question
Mark, that was not very PUN-ny at all ...
Infact I think it was only 2/3 of a PUN ... the PU part.
Ohhh-oh, here we go again....
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