Actual HP Loss w/Hydro

[Adirondacker]

In comparing Hydrostatic transmissions vs. gear or modified gear (glideshift), the PTO HP is rated lower with the Hydros. Is this reduction only when the using the tranny with PTO engaged (i.e. mowing, tilling, etc.) or does it also apply when running the PTO to run an Aux. hydrolic system like with a backhoe? Just trying to figure out if the power reduction is only when you use them together or for any PTO use. Hope this makes sense. Thanks for any insights.

 

[Darren]

It's my understanding that in any fluid type drive there's a greater hp loss than in a gear arrangement so it would apply whether you're measuring power at the wheel or at the pto. No matter what you do, the engine output has to be transmitted through the fluid system in a tractor with a hydrostatic drive.

 

[JackIL]

PTO horsepower is what is measured at the PTO shaft with the tractor stationary and engine delivering power at rated speed. On engines used in virtually all sizes of tractors, PTO horsepower is approximately 85% of the Net engine horsepower measured at the flywheel (which is hard to do). Now in hydrostatic tractors, the engine has to run the hydro pump at all times whether the tractor is moving or not. This consumes some power. Therefore, the PTO HP of a hydro tractor is less than a comparble gear tractor and is typically about 82-83% of Net flywheel horsepower.

The SAE standards for measuring engine horsepower define 2 basic types. Gross Horsepower does not consider the power losses from exhaust and intake systems, cooling fans, and other "accessories." Net Horsepower does take these "parasitic" effects into account. In smaller engines such as on lawnmowers, most manufacturers quote Gross Horsepower because this is the highest number--makes it sound bigger than it really is. In tractors from about 20 to 50 HP, most manufacturers first quote Net (flywheel) horsepower and then secondarily sometimes provide PTO horsepower. In farm tractors starting at about 50 horsepower the usual advertised horsepower is PTO horsepower. However, for large articulated 4WD tractors the advertised hp is usually Net Engine HP since most of these large tractors are sold without a PTO that could even be used to measure PTO hp.

Probably more than you wanted to know, but at least I think this answers your basic question.

JackIL

 

[frank_f15]

very interesting and ENLIGHTENING post! thanks

 

[Adirondacker]

JackIL and others,
Just the kind of information I was looking for.
a big thanks!

-Adirondacker-

 

[markie61]

My NH TC40D (HST) was purchased used with 31hours on it. Just to be sure it was in tip-top shape, I had the dealer agree to do a pressure test on the hydro and dyno the PTO. The results were very enlightening:

The specs say that the engine puts out 40hp and the PTO puts out 33.2 at 2600rpm (the gear model would have put out 35 - difference of 1.8hp). On the dynomometer (a static test at rated rpm), the dealer found out it was putting out 38hp - much higher than advertised. I think manufacturers may be a bit conservative in their ratings....

I would think that multiple demands on the same engine with a finite amount of horsepower reduces the amount of hp to each task (for instance, when mowing up a hill and raising the loader above an obstacle, the tractor slows because of the demands of the drivetrain, loader, and mower.)

Yesterday, I was pulling up trees with the loader. If it tried to curl my bucket up while moving forward, I stopped and the bucket wouldnt break out. So, I planted the bucket under the roots (maximum single application of hp/hydraulic fluid), stopped, then curled the bucket up (maximum single application of hp/hydraulic fluid). It worked.

With a backhoes, everything is hydraulic - whether it is plumbed to the tractor or used a separate pump. The PTO hp only comes into play with the pump and the backhoe mfr should have minimum hp specs needed to maintain the gpm flow of the hydraulic fluid. If you plumb to the tractor, make sure the backhoe's gpm requirements are met by the tractor's system.

I don't know if this answers your question, which I think is "How much horsepower does the drive system take from the maximum available to the PTO on an HST?" The answer would have to be, "It depends...." It would have to vary depending on gear selected, how far you are pushing the pedals, what the terrain is like, what implements are operating, how much extra weight is added, etc. The ratings from the mfr and static test from the dealer give the best-case scenario and allow you to judge which tractor and implements are right for a your particular application.

Mark

 

[chrisjbell]

<font color="blue"> does it also apply when running the PTO to run an Aux. hydrolic system like with a backhoe? </font>

When using a BH you'll either run off the tractor hydraulics or use a PTO driven pump. If you use tractor hydraulics, it is a moot point (the pump in the tractor will produce the same flow regardless of tranny type). If you use a PTO pump, I wouldn't worry about the output - my little B7300 HST turns that pump plenty fast enough to produce enough pressure that I usually run it at about half speed (to keep the BH from moving too quick). Once in a while I rev it up to get more power, but most of the time it is pretty much loafing along.

 

[OkieG]

1.) <font color="blue"> I usually run it at about half speed (to keep the BH from moving too quick). </font>

2.) <font color="blue"> I rev it up to get more power </font>

Chris, this is a minor point that I will attempt to make. I believe a tractor's advertised hydraulic flow in gallons/minute is measured with the engine running at rated rpm. Reduced engine rpm = reduced hydraulic pump rpm = reduced hydraulic flow, which makes your first comment a valid piece of advice. Flow is what determines the speed that the BH moves.

However, for the most part, hydraulic pressure is not a function of engine speed. Instead, it is determined almost exclusively by the setting of the pressure relief valve. Actual measured pressure should hover around the rated pressure, regardless of engine speed. Revving your engine up may make a big difference in responsiveness, but will make little difference regarding power...or am I missing something. Wouldn't be the first time!

OkieG

 

[chrisjbell]

Heck, I don't know. But my BH is driven by a PTO pump and it moves a lot slower (where I can control it) when I run the engine at lower RPMs. That sucker is *fast* when the thing is turning at high revs - too much for my amature abilities.