Bob, at the moment I'm really just looking to gain a bit of information rather than copy the PTs system specification.
I might be wrong, but I believe that the PTs only have one PTO circuit that is used for all the attachments, so I would suspect that the flow from that PTO is a compromise to fit as many attachments as possible. I'll be doing the sums as you suggest, probably even working back from the HP requirements for different sized mowers, but knowing the specs of the motors used on the PT attachments gives me a good grounding to work from.
JJ, the machine has 31HP, and it is set up so that the drive is capable of using nearly all of that (I believe about 25HP at full flow and pressure).
However, while using a mower I wont be going at full speed (~10mph) and it wont be running at full pressure so I'll have more HP available for attachments.
I'm thinking about the possibility of getting a larger motor that runs at 540rpm and attahcing that to the back of the tractor and using standard 3ph attachments with a PTO shaft, I can always add a diverter if I want to use that flow for a boom mower or something.
There is alot of possibilities and I havent really decided what I want to do yet.
There is an argument to adding a high flow pump that is capable of using the full 31HP, and sizing the attachments so that there is enough HP left over for drive. If the attachments need more HP I can just slow down on the drive.
That would also allow me to use the full engine HP through the PTO when the machine is stationary.
Thanks guys, its interesting to hear your thoughts on this. I dont have much experience with hydraulic driven mowers so its good to hear from those that have.
Mith,
FYI, the PT-425 (25HP gas) has an 8gpm at 2500 PSI (max) dedicated PTO pump, an 8 GPM at 2500 PSI (max) pump that powers the steering, lift, curl, and Aux PTO/QA circuits (with priority given to those multiple demands in that sequence), and a 16gpm at 1500 PSI variable displacement pump -- so the sum total output of those 3 pumps can potentially exceed the power available from the engine. All three are direct-connected to the crankshaft -- i.e. spinning at 3600 RPM max.
For example, if you're trying to simultaneously drive, steer, lift, and curl, with the PTO engaged -- everything potentially slows down (most noticeably the steering and lift/curl response) -- it can't produce the max pressures (only the gpm flow) on all three pumps at the same time. I can't tell by the sound if the mower on the dedicated PTO pump slows down, because there's no significant working load (you're not mowing with the deck up in the air only spinning the blades and chopping air)...
Theoretically, both the dedicated PTO pump and the steering/lift/curl/QA pump can produce about 8 HP each at the point of work, given the hydraulic calculators I've seen that factor in efficiency. I haven't done the calculations on the variable displacement drive pump...
Bottom line is that the 25HP machine spins a 60" finish mower (or 48" brush cutter) with about 8 hydraulic HP (max) available at the attachment... that may sound like low HP to those used to ag/CUT/SCUT tractors, but I'd remind them that my 12 HP Simplicity drives a 48" belly mower, giving arguably the best cut in the business (for a tractor), while hauling my butt around...
The design of the deck itself is also important -- the PT finish mower deck would handle tall thick (or wet) grass much better if it were a deeper deck with more room for the clippings to churn around underneath the deck, without having to chop them over and over...