pto horsepower question

[unclehan]

Hey guys, I have some very basic (I think) questions on tractors.

1) Most all tractors have a pto shaft to run auxiliary equipments. When someone talk "pto power", does that mean the power available to do the auxiliary job alone (not counting the power needed to move the tractor), or does that mean the net power from the engine, minus the loss to fan, pumps... and transmission? So in the latter case, "pto power" also includes the power needed to move the tractor. Which one is right?

2) Speaking of transmission, do tractors usually have 2 independent transmissions? One for the auxiliary equipments such as mower or tiller, and one for the tractor wheels. These 2 functions have to be independent from each other so I think tractors must have 2 transmissions. Am I right?

3) Hydrostatic transmissions are not as efficient as mechanical gearboxes, yet they are popular. There must be a compelling reason for this. Is it mainly because of the many tasks a tractor must do and each requires a different gear ratio? A hydro offers infinite ratio.

4) Lastly, the engine power is generally split into running the tractor (moving it), and powering the equipments (such as spraying, tilling, harvesting...). Do you guys know which one takes the bulk of the engine power? Is it the actual moving of the tractor itself, or running the equipments? I realize the answer to this question will depend on what type of task is being performed. For simplicity, just assume tilling.

Thanks for your help.
Appreciate it.

 

[beenthere]

Some information on PTO horsepower can be found here.

http://ianrpubs.unl.edu/FarmPower/g579.htm#ptp

2 transmissions? One with output to the wheels and the PTO

Hydro is convenient.

Power goes from the engine to operate the hyd. pumps (usually two) and the transmission. As long as the power requirements do not exceed the engine limits, all is fine. Something has to give if the limits are exceeded, or the engine will stall. Deere has an electronic sensing system that will maintain engine rpm and decrease demand for power to the wheels, as an example of how to combat exceeding the limits. Otherwise, the operator will have to make the decision and any limitations.

 

[have_blue]

Hi Han,

PTO power is horsepower available at the PTO shaft for running implements. It's that simple. Think of engine power as gross power, and PTO power as net power for driving implements. Both tell you a lot, and both are fairly important to know when choosing implements.

A tractors power is routed through the transmission, differentials, and some is bled off to the PTO drive at some point. But no, I don't think tractors really have 2 separate transmissions in the usual sense of the term.


Hydros are popular mainly because some people don't like to shift. Aside from that, HST's are popular on tractors that do work requiring them to change directions in a small space, such as using a front loader.


As for power, tractors are geared low, and contain a lot more heavy gearing than a car for example. All the tractors power goes to drive both the wheels and the hydraulics and PTO. I'm sure a lot of HP gets lost in the geat train before it reaches the wheels and PTO, but we really don't need to get involved in the minute details. A 40 HP brand "A" tractor weighing 3000 pounds will have very close to the same available power as brand "B" of equal HP and weight.

Hope this helps a little, and welcome aboard.

 

[rambler]

1. They hook up a dynometer to the pto, & see how much power the tractor puts out as equiped from the factory. Fans & pumps & such count but none of these options are operated beyond normal non-use of them, since tranny is in neutral it really doesn't pull anything.

2. A pto can be transmittion driven (uses same clutch as the tractor tranny) or live (uses the same pedal as the tranny, but a second clutch) or be independent (uses a seperate clutch & clutch lever). None of these pto controls would be considered a transmision, but rather just a clutch. Old tractors like Ford N's use the same clutch as the rear end tranny, so no there does not have to be 2 seperate clutches, and it would not be a seperate transmission in any case? A pto control, if live or better, is just an on/off selection, not a real transmission with multiple gearing as I would understand it?

3. Hydro is a conveinience factor. For some uses or some operators it is handy. For real hard pulling, the power loss/ innefficiency is too much to use a hydro. This topic is like a color war, people love their (hydro; gear) tranny and think the other style is terrible - so you better not make too much of this point. /forums/images/graemlins/smile.gif

4. Most ag tractors are designed to supply full engine power to either powertrain - pto or tranny. It is your choice which one consumes how much power. For snow blowing, most power goes to the pto. For pulling the plow, pto doesn't run, all power goes to the rear wheels. You can split your power however you want/ need.

--->Paul

 

[unclehan]

why is the hydro even less efficent at high load applications?

 

[unclehan]

By the way, if anyone has had experience dealing with the hydraulic piston pumps (such as those in the hydro tractors), can you tell me how much friction is between the piston and the cylinder? How "freely to move" are the pistons in a hydraulic pump.