Gross HP vs PTO HP - HST vs Gear

Wildman1 · Nov 1, 2003

Ok, now after reading all of the postings about this subject, let me interject one other variable. As I understand it, the PTO hp is measured in neutral or when neither pedal is engaged. But, when the swashplate begins to be tilted when a pedal is being depressed, the displacement of the pistons in the hydraulic pump becomes larger and more fluid is being transferred to the hydraulic motor that couples to the gearbox. I take that to mean that more hp is being lost by this process.

Would this then mean that the real advantage, PTO hp-wise, of a gear would be under way at speed? IE; the faster you go with a HST the more hp loss through the hydraulics.

Sorry to add any confusion.

Old_Hickory · Nov 1, 2003

Excellent post Art...excellent post !!! /forums/images/graemlins/cool.gif

Soundguy · Nov 3, 2003

( The pto drives of tractors I'm familier with are not run thru the hydro unit so there is no loss with that aspect of it as suggested by sound guy, they are a direct shaft )

Likewise.. all the equipment I'm familiar with uses total hydrostatic / hydraulic pto setups.. no direct clutch to flywheel connection for the pto. .. Mostly this is heavy equipment, but at least one of them is a compact tractor.. etc.

Soundguy

art · Nov 3, 2003

Sound Guy what make and model compact is that that runs a PTO thru a hydro? I don't see how they could do it. /forums/images/graemlins/ooo.gif.

jrlichina · Nov 3, 2003

The engine is directly connected to the HST input shaft. All Kubota B/BX series HST tractors power the PTO through the HST unit. The PTO clutch shaft is driven by the input shaft of the HST unit. The clutch shaft has a hydraulic clutch that inturn drives the PTO output shaft. This is percisely why the PTO hp rating is less for a HST tractor. The engine is always driving the HST so it will always put some parasitic load on the engine therefore reducing the hp available at the PTO output shaft. This is obviously the least when the at rest(pedal in neutral position). I would assume that the maximum PTO rating is done at some speced load limit on the HST unit(possibly low range full swash plate deflection). This would then account for the lower HP rating at the PTO because of the load imposed by the HST unit. Actually all HST tractors I have ever seen(and worked on) are designed this way, but that's not to say that some could use a split drive system(but highly unlikely due to cost and performance).

Docsknotinn · Nov 3, 2003

Art, Its great to see you here ! I missed your posts. For those who dont know Art he has I think, no, Actually I know he has forgotton more about tractors than I should ever hope to know. Its tough to argue with a guy thats been turning wrenches longer than I've been alive /forums/images/graemlins/grin.gif This seems like one of thjose Mysterious topics that is never quite settled. I think we can thank manufacturer specs for that. It also easy to miss place a single word and get a whole different meaning. I'm sure Both Art and Richards points have some degree of validity. I have to believe its at least a bit of a sore spot with Cowboy after the typo or innaccurate literature on his JD that he had to have bought back earlier this year. That would make any bodys Rice crispies soggy. /forums/images/graemlins/mad.gif Either way I Think we are all Lucky to have two guys as knowledgeable about the idio synchrasies of Tractors on the forum as our Friends Art and CowboyDoc. Now about that Swash plate, Does that come with a swash buckle /forums/images/graemlins/grin.gif Dave

jrlichina · Nov 3, 2003

There is no mystery here, modern HST tractors will have fewer PTO hp than its gear tranmission version. This is inherent to their design, so unless that changes it will always be true.

p.s. - Swash buckle not included /forums/images/graemlins/smile.gif

art · Nov 3, 2003

Take a closer look, that shaft goes straight thru and the PTO picks up it's power from that input shaft. That means then it is only using the same input shaft not running it thru the whole hydro but actually taking the power thru idler gears to the PTO clutch assembly which inturn takes hydraulic fluid from the power steering to engage the PTO. The loss of the horsepower is from the additional pump to drive the hydro pump for the transmission of the tractor not from driving the pto thru the hydro's variable speed system. Just look at the service manual and you can pick it up on the drives from the idler shafts used behind. There is not a great picture for you to work from put I do trust you can find it in the service manual of your tractor. Or you could go to your local dealer and look at the parts on the computer link. Tractor dynos are a stationary unit attached to the PTO shaft. The old Cadet hydro pumps and many more Deere included had a stub shaft out the back for a direct coupling, maybe you've seen that before, it's non-hydro use that spun with a direct link to the engine for auxillary power drive at engine speed for future drives. On those units it is visable but on these newer units with space constraints they are enclosed. I'm not denying the power loss but it is not as you have stated.

art · Nov 3, 2003

We do agree on that part. Next time I have one apart for you I'll have to take some pics for you so you can see how it does get the power to the pto.

jrlichina · Nov 3, 2003

I've worked on many of these and that's exactly what I stated, " The PTO clutch shaft is driven by the input shaft of the HST unit. "

The PTO input shaft is connected to the HST input shaft and since the input shaft turns the HST pump(parasitic loss) it is using HP to do it which therfore reduces the PTO available for the PTO.