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Jerry
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Discussion Starter · #1 ·
Can someone help me understand where the horsepower is lost going from the engine through the transaxle and then to the rear PTO.

My X748 diesel has 24 HP at 3425 RPM. The PTO is driven at 540 RPM. The speed reduction and thus the theoretical power multiplication factor is 6.34 (3425 divided by 540). Theoretically, the 24 HP engine should drive the PTO with a 152.16 HP force (24 X 6.34). This theoretical power gain is reduced by power loss in transmission friction of course.

I read in one thread where someone had found the rated PTO HP rating for a JD CUT with the 24 HP diesel to be 18 Horsepower. Can someone tell me where all the power is lost if there is indeed only 18 HP available at the rear PTO. I realize hydraulic transmissions rob power but I would think the power link from the transaxle input shaft on through to the PTO shaft to be gear driven with clutch control. I do not expect 150 HP but I would think it would be much more than 18 HP!

The reason I ask is I have been trying to figure out if my X748 will drive a 54” or 60” tiller. The tiller manufacturers list power requirements either in tractor HP or PTO HP.

Any help understanding this issue would be appreciated. Also, my apology if this has been discussed previously. :thanku:
 

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You don't get any horsepower change with gear reduction.

1 horsepower is moving x pounds up 1 foot per second
With gear reduction you could increase to 5x pounds but the speed would be lower by 5x so the HP works out the same.

Don't have the exact formula in front of me but I believe thats how it works.

Torque is probably what you are thinking of in terms of multiplication.
 

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PTO HP for all John Deere X700 series is 18PTO HP Your other Limiting Factor for the X700 series is weight which is 450Lbs for the Limited Cat 1 3point Yes it will Lift More But You damage Your Tractor Because You have exceeded the weight Limit During the warranty Period Deere Can Void Your warranty . So Keep that in Mind as well for a after Market Tiller for Your X748. For PTO driven equipment for Your tractor always Go Buy the PTO HP not engine HP:thThumbsU
 

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On cars with AWD, the drive train power loss is estmated at 20 - 25 percent. 24 hp to 18 hp is within that range. Of course the only accurate way to calculate drive train loss would be to first check the HP on an engine dyno, then dyno the vehicle on a chassis dyno.
 

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Tractor Nut
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I've got a 52" tiller and mine handles it with ease... HP has never been an issue with the tiller... With your diesel, I'd bet you could do it. Of course soil conditions also matter and breaking new ground is a lot different than tilling an existing garden.
 

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HP does not get multiplied from gearing it down. Torque gets multiplies but HP does not. Gearing it down takes more time for each revolution an the the pounds/per ft/sec formula for computing HP would reduce the effect of the torque multiplier because the it would take more turns to accomplish the distance measurement so the HP stays unchanged. In fact, it drops due to the sum of all the friction losses along the drive path from the engine to the PTO shaft.
 

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I faced the same dillemma last year when I purchased my tiller. I decided to go with the JD 647 , and don't regret it. It turns it pretty good, but on the second pass, when the soil is broken up. It will actually start pulling the rpm down. So for this reason, I think, if I went wider, I would have been disappointed, because it would take more horsepower to turn the unit, itself. Our soil here in Ct is dark,moist, and pretty heavy.With a wider tiller, I think it would pull it down to an un acceptable level. A 4' tiller is well matched to the 700 series tractors for both weight and power. As for the horsepower drop, that is just normal parasitic loss through a hydraulic drive system. The only way to see close to a true 24 hp to the pto,it would have to be a solid mechanical connection. The proof of hp loss would be the heat generated by the hydraulic system.
 

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As others said, there us no multiPlier effect by reducing the rear pto to 540 rpm. The reduction in HP is due to the inefficiencies of hydrostatic transmissions. The rear pto is not being driven mechanically by the engine's output shaft. Instead, the engine shaft drives hydraulic pumps in the transmission which in turn drive everything else including the rear pto. But this arrangement is only about 70-75% efficient, so the transmission output (whether it is to the drive shafts that turn the wheels or ptos) will be a lot less hp than the engine's rating.

The x748 with the earlier 3tnv76 yanmar probably had about 18 hp at the pto -- because that was the published spec for the 2305 with the same engine (different tranny, though). But I have never seen a published spec for pto HP on any X700 series model. There are many different engines that have been used in the x700 series (and predecessors) each with different engine HP ratings. So the pto HP would presumably vary with the engine.

The x748's current 3tnm72 Minimax engine is also used in the new 1023e. In that model, the pto HP is 15.3. I would assume for your purposes that you have not much more than that available, even if you are spinning it a little faster.
 

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Jerry
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Discussion Starter · #9 ·
Thanks to all who have posted information about power transmission and with their experiences with different tillers.

I tend to agree that a 48” tiller is perhaps the safest way to go, if for no other reason, to not overload the system. Also, if there was a mechanical problem with the tractor, JD could use the use of a larger tiller to perhaps void any warranty work.

In my mind I keep thinking of having shifted my car down to a lower gear under a load going up a hill. This shifting to a lower speed gear thus increasing the speed differential between the engine and axle speeds gave me (the car) the ability (power) to get up the hill. To me, this is similar to running a PTO shaft at a reduced speed thus giving the PTO more power…not less. Sometimes what seems logical is not necessarily true in reality.

Again, thanks to all for the information.
 

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Downshifting your car gives you more torque to the wheels, not more power.

Power is how much work gets done in an amount of time.

When you downshift, you can pull bigger loads, but it takes more time. The power is the same.

In a higher gear, you can go faster, but can't pull as much weight. Power is the same.

No matter what speed the PTO turns, you will always have less power than the power at the shaft on the engine.

If it worked any other way, a Mack truck would have a 3HP Briggs and Stratton under the hood and a huge transmission to make up the difference!

--Justin
 

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Engine horsepower - maximum horsepower to move the tractor = PTO horsepower.
 

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Downshifting your car gives you more torque to the wheels, not more power.

Power is how much work gets done in an amount of time.

When you downshift, you can pull bigger loads, but it takes more time. The power is the same.

In a higher gear, you can go faster, but can't pull as much weight. Power is the same.

No matter what speed the PTO turns, you will always have less power than the power at the shaft on the engine.

If it worked any other way, a Mack truck would have a 3HP Briggs and Stratton under the hood and a huge transmission to make up the difference!

--Justin

That's a great way to visualize/explain it. Thanks Justin.

One thing I said in my previous post:

The reduction in HP is due to the inefficiencies of hydrostatic transmissions.
This is part of the reason why you see much higher hp engines in tractors with hydrostats, as compared to older geared transmissions (like my little 8 hp 110 with a 4 speed). With a 70-75% power transfer efficiency, a tractor with a hydrostat needs about 1/3 again more engine hp to get the same hp at the pto as a geared tractor. (For example, if JD wants +/-18 hp at the pto, they will need to spec a roughly 24 hp engine.)

But the hydrostatic transmission also introduces efficiencies, so in a lot of respects it more than makes up for the power loss. Probably the best thing about the hydrostat transmissions, is they allow variable ground speed all while maintaining the engine at peak power and ptos at constant rpm. No shifting, no rpm adjustments. No worries about losing hydraulic pump pressure. Just press a pedal (or push a lever) to adjust forward speed, or another one to adjust reverse speed. And the engine can be sized just right knowing that it will be continuously operated at peak power.
 

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Downshifting your car gives you more torque to the wheels, not more power.

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--Justin
When you downshift your car, you also increase the rpms of the engine for the same road speed, so you are most likely moving to a point in the power band where the engine is producing more hp, as most gasoline automotive engines produce their peak hp at higher rpm's.
 

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This is part of the reason why you see much higher hp engines in tractors with hydrostats, as compared to older geared transmissions (like my little 8 hp 110 with a 4 speed). With a 70-75% power transfer efficiency, a tractor with a hydrostat needs about 1/3 again more engine hp to get the same hp at the pto as a geared tractor. (For example, if JD wants +/-18 hp at the pto, they will need to spec a roughly 24 hp engine.)
This statement is somewhat correct. With a fluid connection between the engine and the rear end, there is bound to be some power loss through internal leakage. The piston pumps and motors used in most hydros are each 90% efficient at transmitting power for a combined 81% efficiency, not 70-75%, although JD may use a lower percentage as a fudge factor to take some wear into consideration.

JD has set aside a reasonable amount of horsepower (6) for driving the tractor when operating a PTO powered implement. The factors governing this horsepower calculation are;

a) The maximum speed that the tractor might be required to attain.

b) The maximum tractive effort to be needed prior to breaking traction while a PTO powered implement is engaged in it's task.

c) The power handling efficiency of the transmission.

Since the 2 most power hungry PTO driven attachments, that require forward or reverse motion to be used, are snowblowers and tillers, and neither can be used at high speed if they are working hard, 4.8 horsepower (in this particular case) developed at the rear wheels is adequate. In the case of snowblowers, available traction is very much a limiting factor (hydraulic pressure in the transmission) and with tillers, speed is the limiter (flow rate of the transmission pump). The combination of pressure, flow and efficiency rating dictate the horsepower needed at the rear wheels to drive the tractor.

Unless the power transmission to the implement is via a hydraulic motor on the implement, I believe that all PTO's are pure, hard metal mechanical connections to the engine except for the mechanical clutching arrangement to connect and disconnect the PTO. To do otherwise would restrict available PTO horsepower even more.

For those who think that a tractor needs more than 5 hp to operate at speed, consider that it only takes 12 hp to move a 4000 lb car down a flat road at 60 mph if wind resistance is taken out of the equation. This is from a dyno demonstration when I was in high school, 50 years ago.

Wind resistance does not have an appreciable effect at a tractor's top speed.
 

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Tudor, that's all interestign stuff there. The 75% efficient figure I used above is pretty much consistent with what you see as pto hp relative to the engine hp, e.g. 24 hp engine, but only 18 hp at the pto. (Some of that "loss" may also be from the difference between gross and net hp.)

Now, are you sure about this statement below?

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Unless the power transmission to the implement is via a hydraulic motor on the implement, I believe that all PTO's are pure, hard metal mechanical connections to the engine except for the mechanical clutching arrangement to connect and disconnect the PTO. To do otherwise would restrict available PTO horsepower even more....
That is very different from my understanding of how a hydrostatic transmission works. This is from the Wikipedia entry on different transmission types:

Hydrostatic

Hydrostatic transmissions transmit all power hydraulically, using the components of hydraulic machinery. Hydrostatic transmissions do not make use of the hydrodynamic forces of the fluid flow. There is no solid coupling of the input and output. The transmission input drive is a central hydraulic pump and final drive unit(s) is/are a hydraulic motor, or hydraulic cylinder (see:swashplate). Both components can be placed physically far apart on the machine, being connected only by flexible hoses. Hydrostatic drive systems are used on excavators, lawn tractors, forklifts, winch drive systems, heavy lift equipment, agricultural machinery, earth-moving equipment, etc.
I'm not saying Wiki is the end all authority. Maybe they have it wrong too. But we've had quite a few discussions on MTF about these hydros. One of the things I've learned in those discussions, is that the "pto hp" of the transmission is the total available HP to drive everything. So a unit with 18 pto hp has to divide that hp between the drive system, the mower deck, and the rear pto implement if all are being used simultaneously. In other words, there is not a separate source of 18 hp available to drive a rear pto implement.

This issue recently came up when I was considering a 3PH Trac-Vac powered by the rear pto for a Kubota BX1860. In consulting with Trac-Vac and Kubota, they concluded the BX1860 would have insufficient pto hp (13.7) to power the tractor, the mower deck, and the Trac-Vac simultaneously.
 

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Quote:
Hydrostatic

Hydrostatic transmissions transmit all power hydraulically, using the components of hydraulic machinery. Hydrostatic transmissions do not make use of the hydrodynamic forces of the fluid flow. There is no solid coupling of the input and output. The transmission input drive is a central hydraulic pump and final drive unit(s) is/are a hydraulic motor, or hydraulic cylinder (see:swashplate). Both components can be placed physically far apart on the machine, being connected only by flexible hoses. Hydrostatic drive systems are used on excavators, lawn tractors, forklifts, winch drive systems, heavy lift equipment, agricultural machinery, earth-moving equipment, etc.


I'm wondering about the accuracy of the bold print above. Missprint, missed on proof reading, misunderstanding, or am I wrong in my understanding? I do note that they are talking about transmissions and are otherwise correct. PTOs are not mentioned.

For a hydro transmission to also supply power to a PTO, it would have to perform at maximum output while still giving speed control of the tractor to the operator. Ain't happenin' without a few extra, and expensive, components in the system. Can we say Case hydraulic drive, where the pump supplies full flow 100% of the time? Hydros use a swash plate to control flow from the primary pump, and that is hooked to the drive control pedals, not the PTO control.

While the shaft that supplies power to the PTO may pass through the transmission, and power the transmission at the same time, that does not mean that the transmission is powering the PTO. See the link to the Sundstrand Series 15 "U" drive below, page 6. Note that the input and PTO shafts are actually one shaft and that John Deere has used this transmission, I believe, in the 318, and drives it from the PTO end with a cooling fan on what is labelled as the input shaft.

http://www.sauer-danfoss.com/stellent/groups/publications/documents/product_literature/bln-9646.pdf

While this arrangement places all the power in the one shaft for both uses, generally, the transmission has a much lower requirement for that power and the majority goes to the PTO. If the swashplate is in the neutral position, a dyno hooked to the PTO would show a hp output closer to the engine rating than the PTO rating. The only loses would be from friction in the systems, and that will definitely not be 6 horsepower with the hydro not doing any work. I certainly don't want to deal with 6 horsepower converted to heat due to friction (about 15,000 BTU per hour).

If the engine developes 24 horsepower, with 18 hp at the PTO, then the balance is for friction and drive.

Moving to your Kubota discussion, if drive requirements have already been included in the PTO calculations, why did the people you were discusing your possible purchase with include the drive requirements a second time. PTO power is for attachments and implements, not transmissions. You can always slow down when the engine loads up.

"One of the things I've learned in those discussions, is that the "pto hp" of the transmission is the total available HP to drive everything."

The PTO of the engine is the total available horsepower. The PTO output of the transmission goes to the rear wheels along with a small component from the charge pump for the implement lift cylinders. Most hydro applications will pop the relief valve well before they hit max engine horsepower. A hydrostatic drive transmission consists of a charge pump, a variable displacement piston pump and a fixed displacement piston motor. There is no hydraulic provision there for a PTO except the charge pump, and it doesn't have the flow capability.

If the hydraulic capability was there, there would be no need for slip clutches and a limited need for shear pins. That's what relief valves are for.

I'll have to remember your comment about 75% of output for PTOs with a hydro transmission. The shuttle shift Massey Ferguson 2600 series uses 85%+/- for PTO output, as a comparison with the relative loses of hydros. They both make a handy and easy to remember rule of thumb if the actual numbers aren't available.
 

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I'm wondering about the accuracy of the bold print above. Missprint, missed on proof reading, misunderstanding, or am I wrong in my understanding?....
Yeah, I see what you mean. I didn't notice that on first read (think I read it the opposite way). I don't understand hydros well enough to know one way or the other.

Here is one of those discussions I was talking about:

Questions About PTO HP

As the Kubota dealer explained it, and the Trac-Vac dealer confirmed, the tractor propulsion would use about 4-6 hp, deck about 4-5 hp, and Trac Vac about 4-5 HP (these were rough estimates and would vary under different loads). They felt a minumum of 15 PTO hp was required to be operating both ptos simultaneously while mowing. They felt the BX1860 could struggle at times with only 13.7 pto hp.

So as I understand it, the full pto hp is available at the rear only if nothing else is using any. For instance, if the tractor is parked and is being used to power a 3PH PTO generator.
 

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Yup. I remember that thread. It caused me to do some thinking. (Always a dangerous thing!)

What it should have done was to cause me to do some research, as I did last night after reading your post.

From the information that I found, you are right and I was wrong!!!

It seems that there are many ways to list the horsepower for tractors. Gross, net, wheel, drawbar,and taxable are the ones that I can immediately remember, and the State of Nebraska is the only place that makes the tractor manufacturers produce proveable real world horsepower ratings with independant confirmation. Unfortunately, they limit that requirement to 40 hp and up.

I still had a hard time swallowing a 15% power loss between the engine and the PTO of a gear drive tractor considering how few opportunities there are for that loss untill I remembered that even universals have a loss factor attached and each loss is compounded by the previous. Once that got got hammered through my granite skull, it became easy to accept even the additional 10% loss for a hydro.

I can be more than a little stubborn at times (I should live in Missouri!) and made a couple of calls to try for a dyno to confirm these losses on my MF GC2310. Unfortunately, the local college covered their's with concrete about 30 years ago, and while I'm still waiting for a call from the new high school, I doubt that a dyno was included in their purchases for school equipment due to budgetary restrictions. Fifty years ago, we had one in the new high school that I attended.

If anyone can gain free access to a dyno for their tractor and post the results, I'm sure everyone would be appreciative.

Please disregard my posts in this thread. I'm sorry to have muddied the waters and I apologize.
 

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Please disregard my posts in this thread. I'm sorry to have muddied the waters and I apologize.
Now hold on a second. I thought your posts were interesting and useful. We are having a discussion here, exchanging information and knowledge (limited, in my case). Sometimes we all miss our mark (often, in my case), but that doesn't make the contribution insignificant.

As for right and wrong, we were also discussing the source of power to the PTOs. Are they directly gear driven from the engine drive shaft, or somehow powered by the hydro tranny? I thought they were somehow powered by the hydro tranny, whereas you felt they were directly gear driven. I haven't been able to confirm my understanding, and the more I think about it, it would make sense for them to be directly gear driven. So you may well have me there. :)

Hopefully someone else will chime in with a good resource (EngineTech -- you out there?) I hope the OP will forgive us for the hijack!
 
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