Go BottomEngine gross power vs Engine net power vs PTO power
 | DenisS
Join Date: May 2006 Posts: 367 NJ 
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2006-09-13 135157
I understand PTO power is what is actually transmitted to the PTO shaft. But what are gross and net figures measuring?
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 | Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York 
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2006-09-13 135165
Gross normally is with out water pump alternator or any other apendages to the power where net includes them. Pto is at the rear of the tractor accounting for all power loses of hydraulic pumps as well as transmission and drive train. ....
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| DenisS
Join Date: May 2006 Posts: 367 NJ
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2006-09-13 135167
Art,
So when an attachment is rated for a certain HP range of a TRACTOR (where PTO is not specifically mentioned), it would be more correct to look at the net, gross or at the PTO? ....
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 | greg_g
Join Date: Jan 2004 Posts: 1816 Western Kentucky 
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2006-09-13 135170
Not sure what value the gross number would be, other than advertising maybe. From the aspect of work, net engine horsepower is what you want to know. PTO horsepower is how much of that net engine horsepower that is available at the PTO spline. When the owner's manual doesn't specify, you can ballpark the PTO hp by subtracting 14% from the net engine hp.
One you didn't mention is drawbar horsepower. That's of interest to owners of ground engagement equipment. How much can it pull. It's a somewhat archaic rating nowadays, but drawbar hp is ballparked by subtracting 14% from PTO horsepower. Example: my KAMA has 45 (net) engine horsepower, ~39 PTO hp, and ~33 drawbar hp.
//greg// ....
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 | earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2006-09-13 135171
And you have to take into account the different types of drive trains: pure hydrostatic pump-to-motor, pump-to-motor-to-gear transmission, etc. My hydrostatic NH has less wheel horsepower to the wheels than the same non-hydro CUT. With my machine, engine power goes through a hydraulic pump then to a hydraulic motor which then powers the 3-range gear transmission.
If you a google search for Nebraska tractor tests, you should find all kinds of into dating back to the early 1900's as they ahd to compare all machines with the same criteria. ....
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2006-09-13 135174
With the new tractor ratings of net engine being more advertised then PTO, which is all that counted till the 80's when the compact tractors started showing up in force,I'd wonder that many short line companies might not be figuring on that for the operation of their equipment. For us here many tractors will do just fine on the flats with a mower but when they hit a hill that tells what the engine do! Drawbar horsepower is a number that has gone by the way side as how do you rate it? Dirt is not concrete and there are many ways to set tractors up for different performance. ....
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2006-09-14 135184
As EW mentioned, and Art alluded to, the efficiency of the drivetrain can be very important in certain situations.
The Gross -vs- Net hp figure is the only measure of the 'rolling' efficiency of a tractor, big or small.
In applications such as loader work, or pulling hay wagons or trailers, the Net hp is the only real yardstick you have to judge available power since most small tractors, as Greg mentioned, are not drawbar hp rated.
When you see a PTO implement, like a tiller, rated for hp, they mean PTO power, usually they state not only the minimum hp required, but the size of the gear box capacity. This is not just to limit damage, but as a guide of how sturdy it is built. For things like a a rear blade though, it is Net hp they are talking about. I can put a BX sized blade behind my 'Bota, but if it catches on something the tractor will definitely do some damage to it, likewise, my 1,000 pound box blade would likely do some damage to a BX too.
Best of luck. ....
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| DenisS
Join Date: May 2006 Posts: 367 NJ
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2006-09-14 135187
If a rear mower is operated at standard 540rpm, what kind of damage can a tractor do to the mower if the mower is rated say between 25 and 50hp and the tractor Net HP is at or above 50hp? The rpms stay the same, there's a sheer bolt / slip cluth to protect against obsticles, why have a maximum HP rating in this case? ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2006-09-14 135188
Denis: Typically when a mfg. rates their product they (hopefully) have done some extensive testing. That said, some one determined that after a certian HP things start breaking or wearing rapidly. The shear pin for a 4' cutter won't be the same size as one for a 6' for reasons such as there's heavier/thicker blades or rotating mass that can cut more at one time, which means the drive train has withstand greater shock loads more often. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2006-09-14 135189
Not only that, but when the mower, as a complete unit, is designed, it is assembled out of a bunch of individual components, most made by another company, like the 90° gear box.
They know, that in the case of your example, that an average 40 hp tractor will load up and lose rev's under a heavy load of wet grass. The operator will then do something to reduce that load, slow down or raise the deck or both.
Put that same mower on the back of a 100 hp tractor, and you likely wouldn't even know it was overloaded, that is until the gearbox grenaded from the heat and wear of the overload. I've seen exactly that happen. An orchard near me has a little tiny 90 hp Carraro tractor, they use a 4' mower on the back to go between the rows. Someone thought they could save money by buying a 4' mower intended for a small CUT. It only lasted a few months before they showed up at my place asking if I could weld the gearbox back together, it had snapped clean through in the neck.
Best of luck. ....
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| DenisS
Join Date: May 2006 Posts: 367 NJ
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2006-09-14 135193
EW and Murf,
Ok, now that makes sense. ....
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 | Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2006-09-14 135194
Murf; Could you get away with it if you added a slip clutch? I guess you would have trouble calibrating if the clutch is large and you could smoke the clutch moving through wet grass before you noticed the slippage?
It would be nice to have an adjustable magnetic clutch, probably too expensive for AG though. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2006-09-14 135195
Denis, glad to help.
Eric, as you know a slip clutch is a PITA to start with, on a big noisey machine it's worse. Who wants to sit turned around backwards to see if the clutch is slipping or not? We had this problem on a commercial turf harvester I helped design a few years back, in the end we settled on the only viable way to do it, and drove it hydraulically. By electronically monitoring and regulating flow & pressure we could do it fairly easily. It was spendy, but so are warranty claims.
Best of luck. ....
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2006-09-14 135200
Today anything that I compare for a pto is from the ag industry and the standard has been the independent hydraulic wet clutch. They have a slip built in to the engagement and are a soft engagement. There are other type systems that require a more experienced operator but are still very effecive like the two stage clutch or just a hydro. ....
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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2006-09-14 135201
Art and Murf some expensive lobe and gear pumps in the chemical industry and automation have magnetic couplings. The lobe pumps alone are $40-100 K +. I have seen the variable magnetic resistance couplings. Some using fluid that changes resistance with viscosity with small electromagnets. It would be interesting to be able to dial the max PTO torque before slip to manage any size equipment with a large tractor. Not sure the utility, I guess it would be good for an grain auger or PHD but a shear pin also serves that function. ....
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2006-09-15 135202
A few years ago that was addressed with our larger Magnum series tractors and they do vary the pto engagement as to the load. The system is engaged with a toggle switch and so far we have had good effortless results and it has not been a service issue at all. We have seen many hydraulic pto systems some even with single disc engagements that might cause shear pin problems as they are to quick of an engagement. ....
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