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How Do You Run Your Hydro?


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#31 HowardsMF155 OFFLINE  

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Posted March 27, 2012 - 08:48 AM

WRbourget, great job on the diagnosis.

I'm trying to wrap my head around the operation of a hydro, and how it differs from a standard transmission. Somebody check me here. A standard transmission changes the gear ratio between the engine and the ground speed, and also changes available torque. So, with perfect traction, the tractor can drag a load in low gear that it can't move in high gear. Total horsepower is presevered because the max load changes depending on ground speed.
With the hydro, my impression is that TORQUE remains constant. Therefore, with perfect traction, if the tractor can move a load at slow speed across the ground, it will be able to move the same load across the ground at full speed. Actual horsepower output by the unit changes because the same load is moved at different speeds. Available torque is limited by the drive ratio between the hydraulic motor and the wheels and the torque of the hydraulic motor.
Does that sound right? I suppose the key difference is in the "drag factor" of the total moveable load by the tractor.

#32 TUDOR OFFLINE  

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Posted March 28, 2012 - 04:23 AM

Ummm, close, but not quite.

Given full input rpm, the hydro pump can vary the available horsepower from 0 -100% by the position of the swash plate which creates the flow and the pressure developed up to the relief pressure. The pressure is created by the work being done and max torque is at max pressure, no matter how much flow over 0% is available. Gear reduction in the rear end multipies the available torque from the motor.

Given a specific load to pull, a low angle on the swash plate will result in low flow and the pressure developed from trying to move the load will build until it is high enough to move the load or the relief valve pops. Once the load begins to move, pressure drops off and output torque follows suit. As the swashplate angle increases, so does flow, and, to a lesser or greater extent, pressure increases to accomodate accelleration from the increased flow and the resultant speed.

Bottom line is, 100% torque is available at any flow rate at maximum pressure before the relief valve pops. Full horsepower is only available at full deflection of the swash plate and maximum pressure before the relief valve pops. In 2300 hours of use and abuse, my Sundstrand has never popped the relief at full throttle, and only rarely at 1/2 throttle. It breaks traction first. The tractor with operator weighs over 2400 lb. (MF1655, FEL, 5' backblade plus weights, loaded tires and chains)

Note: The maximum horsepower and torque referred to are not what the engine will produce, but what the hydro will handle. That is dependant on the maximum displacement per revolution of the pump and the relief pressure. For entry level hydros, that's about 7 horsepower at XXXX psi and for the big, top of the line hydros (eg. Sundstrand Series 15 in the older GTs), about 12.5 horsepower at 1500 psi.

Gear drive GTs have full engine output horsepower and torque available (minus line losses) right off the bat, but have to slip the clutch or drive belts to keep from digging holes, or throttle back the engine with a resultant loss of available torque. Hydros have full torque available even when the engine is idling. They just don't have very much flow to go with it.

Hydros are power transmitting and control units. Manual transmissions are power transmitting units only. Both have the same final drive reduction with hydros having an infinite number of intermediate reductions and manuals having only 3 or 4 selectable intermediate reductions. (I'm referring to only one range on each.) Some manual transmissions also incorporate a variable ratio input system which allows much better control within each gear. But that's the catch. It only works one gear at a time whereas the hydro covers the entire range in both directions.

In your scenario, at some point the hydro will run out of available horsepower due to insufficient forward speed caused by a lower than optimal swash plate angle and peaking pressure. A manual transmission does not have that restriction. On the other hand, first, the manual has to get the load moving, second, it has to do it without killing the engine or smoking the clutch or belts. If you want to match the hydros speed when it runs out of horses, you have to start in the gear that will match that speed. Perfect traction bites!

To get a really heavy load moving, you want a hydro. Once it's moving, you want a geared transmission to get the most power to the ground.
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#33 KennyP OFFLINE  

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Posted March 28, 2012 - 05:19 AM

Intersting read! Opens the eyes on how these work. Thanks!
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#34 wrbourget OFFLINE  

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Posted March 29, 2012 - 08:03 PM

So in the case i was talking about It wouldn't back up the trailer ramps idling it would lug the engine a little but it would just hang there because the flow was to low, so why would the governor affect it that much. once i changed the spring location it would back up the ramps at idle.
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#35 HowardsMF155 OFFLINE  

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Posted March 29, 2012 - 10:08 PM

Bob, excellent explanation and I thank you. While I did not express it as eloquently as you, I think I was close in my concept of how the hydro works and how the gear transmission works. I did fail to take clutch and belt issues into account. It still appears to me that ultimate torque is limited in a hydro transmission, that no adjustment of the control handle will increase the torque available. I agree that the torque is still considerable and that the control method makes it easier to use the available torque.
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#36 TUDOR OFFLINE  

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Posted March 31, 2012 - 05:00 AM

So in the case i was talking about It wouldn't back up the trailer ramps idling it would lug the engine a little but it would just hang there because the flow was to low, so why would the governor affect it that much. once i changed the spring location it would back up the ramps at idle.


Since the governor was malfunctioning, it couldn't maintain a normal idle rpm thus reducing flow to the point that even at relief pressure, insufficient horsepower was available to back up the ramp. When you corrected the malfunction, the engine had a few more revs, the pump moved a little more oil, and enough extra horsepower was generated to deal with the ramp without hittting relief pressure. Although it was probably very close to popping the relief.

Horsepower = (Pressure/1714) X Flow (gallons/minute)

Edited by TUDOR, March 31, 2012 - 07:31 AM.

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#37 TUDOR OFFLINE  

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Posted March 31, 2012 - 07:16 AM

Bob, excellent explanation and I thank you. While I did not express it as eloquently as you, I think I was close in my concept of how the hydro works and how the gear transmission works. I did fail to take clutch and belt issues into account. It still appears to me that ultimate torque is limited in a hydro transmission, that no adjustment of the control handle will increase the torque available. I agree that the torque is still considerable and that the control method makes it easier to use the available torque.


Your concept was very close. It just needed a little tidying up for clarity.

The phrase "constant torque" I felt needed to be better explained and qualified, and then I developed diarhea of the keyboard from that point. (It's a curse. No excuse.)

"Ultimate torque" is a slippery phrase. I prefer maximum torque, which refers more towards the "as is manufactured and allowable" context of the discussion. Ultimate torque is more along the lines of a "what mods can be made to increase torque until the hydro fails" kind of discussion.

Yeah, I know. It's just semantics. But on a Sundstrand, the "normal" 1500 psi will generate about 160 in-lb of torque. Modifying the relief valves to the max that Sundstrand will allow, will result in almost 500 in-lb of torque. Since they will allow it (under specific circumstances), then the "ultimate torque" for that hydro will be something a bit higher, and soak up a lot more horsepower than we have in our tractors in the process.

I don't quite understand how you feel that a hydro's torque is limited. Compared to what? Certainly not the engine, since some hydros will actually output more torque than the engine can produce, simply by having a bigger motor than pump, and pretty much all hydros will output more torque than the engine when it's idling if they are reasonably matched.

Since max torque is available across the entire range, I don't see a need to exert control over it. The pressure required to move the load will make the required adjustments to the torque automatically, within the available pressure range as set by the relief valve. We control the flow, and therefore the horsepower and speed available, with the throttle and the drive control.

Edited by TUDOR, March 31, 2012 - 06:49 PM.

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#38 wrbourget OFFLINE  

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Posted March 31, 2012 - 07:42 PM

Since the governor was malfunctioning, it couldn't maintain a normal idle rpm thus reducing flow to the point that even at relief pressure, insufficient horsepower was available to back up the ramp. When you corrected the malfunction, the engine had a few more revs, the pump moved a little more oil, and enough extra horsepower was generated to deal with the ramp without hittting relief pressure. Although it was probably very close to popping the relief.

Horsepower = (Pressure/1714) X Flow (gallons/minute)


I was young then and those tractors are long gone, but to validate my governor fetish setting them to factory setting after repairs and such has proven to me to be valuable as far as the performance of the equipment i am working on.
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#39 TUDOR OFFLINE  

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Posted March 31, 2012 - 08:07 PM

Oh, yeah! If the governor is right, everything comes alive!
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#40 IHCubGuy OFFLINE  

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Posted March 31, 2012 - 08:39 PM

To get a really heavy load moving, you want a hydro. Once it's moving, you want a geared transmission to get the most power to the ground.


Many big farm tractors have a variant of this idea today where they use a Hydromechanical transmission. It's called different names by different manufacturers but they all work alonq that principle. John Deere calls it the IVT, Fendt calls it the Vario and CaseIH and Agco have a version as well in their lineups.
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#41 HowardsMF155 OFFLINE  

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Posted April 01, 2012 - 10:56 AM

"I don't quite understand how you feel that a hydro's torque is limited. Compared to what? Certainly not the engine, since some hydros will actually output more torque than the engine can produce, simply by having a bigger motor than pump, and pretty much all hydros will output more torque than the engine when it's idling if they are reasonably matched."

OK, Bob, you have challenged me to dig deep and find out what I am REALLY trying to articulate LOL. Horsepower is a function of a load or drag moved a certain distance during a certain amount of time. A hydro has a fixed final drive ratio, with full throw on the swash plate producing a maximum speed on the hydro motor. (at this point I feel like a half wit engaging in a battle of wits). Since I don't think you can create something fom nothing, if the engine is producing 12 horsepower, and the tractor is USING 12 horsepower to move a load, (dragging Ducky's triple bottom plow) then it seems to me that axle torque can be calculated by using this formula:
Torque=(hpx5252)/rpm
I would assume that RPM is full speed on the hydro, around 8mph in the case of a MF12. Using original tires, circumference =23x3.14=72.22 inches. To reach 8 mph, the axle must turn about 120 RPM. (120x(72.22/12)x60)/5280=8.2 mph
So: Torque=(12x5252)/120=525.2 Ft. Lbs of torque.
I assume (probably incorrectly) that this is the maximum torque that can be acheived by the hydro, and that this is the torque available when moving slowly as well.

I feel quite certain that a geared transmission with the same ratio of engine speed to ground speed will produce the same torque. However, the gear that would be selected to move at that speed would be 4th, again for a MF12 gear variant. If the gear transmission were shifted into a speed which was HALF the ground speed, the axle would rotate half as fast as well. Then the torque equation would be:
Torque=(12x5252)/60=1050.4 Ft Lbs. of torque.

This is what I am referring to when I say I think that torque on a hydro is limited.

Now, during this exercise I havn't allowed for efficiency, nor have I tried to figure out what sort of clutch is needed to keep from smoking the remainder of the driveline on a geared transmission. I've just tried to explain why I think total available torque from a hydro is limited compared to a gear transmission.

Why does it matter to me? I have both a 12G and a 12H. I'd like to make sure I am matching them to the jobs they are best suited for. If dragging a plow through the garden is too much for the hydro, should I switch to the gear? After all, I won't be moving through the garden at 8 mph. The hydro certainly is convenient when mowing. However, my garden is bounded by fences. If I'm not popping the relief valve on the hydro, the easy reverse certainly helps turn around at the end of a row. I'm tempted to use the hydro for both, but don't really care for removing and reinstalling the deck every week. Maybe I just need to buy another hydro :laughingteeth:
hpx5252/RPM=torque

#42 TUDOR OFFLINE  

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Posted April 01, 2012 - 06:19 PM

"I don't quite understand how you feel that a hydro's torque is limited. Compared to what? Certainly not the engine, since some hydros will actually output more torque than the engine can produce, simply by having a bigger motor than pump, and pretty much all hydros will output more torque than the engine when it's idling if they are reasonably matched."

I see that I used the wrong word in the above statement. With a 1:1 hydro, it should read ".... hydros have the potential to output more torque....".

OK, Bob, you have challenged me to dig deep and find out what I am REALLY trying to articulate LOL. Horsepower is a function of a load or drag moved a certain distance during a certain amount of time. A hydro has a fixed final drive ratio, with full throw on the swash plate producing a maximum speed on the hydro motor. (at this point I feel like a half wit engaging in a battle of wits). Since I don't think you can create something fom nothing, if the engine is producing 12 horsepower, and the tractor is USING 12 horsepower to move a load, (dragging Ducky's triple bottom plow) then it seems to me that axle torque can be calculated by using this formula:
Torque=(hpx5252)/rpm
I would assume that RPM is full speed on the hydro, around 8mph in the case of a MF12. Using original tires, circumference =23x3.14=72.22 inches. To reach 8 mph, the axle must turn about 120 RPM. (120x(72.22/12)x60)/5280=8.2 mph
So: Torque=(12x5252)/120=525.2 Ft. Lbs of torque.
I assume (probably incorrectly) that this is the maximum torque that can be acheived by the hydro, and that this is the torque available when moving slowly as well.

I feel quite certain that a geared transmission with the same ratio of engine speed to ground speed will produce the same torque. However, the gear that would be selected to move at that speed would be 4th, again for a MF12 gear variant. If the gear transmission were shifted into a speed which was HALF the ground speed, the axle would rotate half as fast as well. Then the torque equation would be:
Torque=(12x5252)/60=1050.4 Ft Lbs. of torque.

This is what I am referring to when I say I think that torque on a hydro is limited.

Now, during this exercise I havn't allowed for efficiency, nor have I tried to figure out what sort of clutch is needed to keep from smoking the remainder of the driveline on a geared transmission. I've just tried to explain why I think total available torque from a hydro is limited compared to a gear transmission.

Why does it matter to me? I have both a 12G and a 12H. I'd like to make sure I am matching them to the jobs they are best suited for. If dragging a plow through the garden is too much for the hydro, should I switch to the gear? After all, I won't be moving through the garden at 8 mph. The hydro certainly is convenient when mowing. However, my garden is bounded by fences. If I'm not popping the relief valve on the hydro, the easy reverse certainly helps turn around at the end of a row. I'm tempted to use the hydro for both, but don't really care for removing and reinstalling the deck every week. Maybe I just need to buy another hydro :laughingteeth:
hpx5252/RPM=torque


Nothing wrong with your math, you're just forgetting that the hydro has an infinite number of gears compared to a manual transmission's 3 or 4. You're also forgetting that there are 2 parts to the transaxles, the transmission (gear or hydro) and the rear end reduction/differential. The rear end is common to both transmissions. It just happens to be incorporated into a single case with the geared transmission, rather than being a separate unit as with the hydro.

I've been basing this discussion on the output of the hydro where the power goes to the final drive reduction. You're looking at the torque at the rear wheels. Torque at the rear wheels will be the same for both transmissions, given the same load and speed. That is where the work is being done.

If the geared tractor is in 2nd gear and the engine is at full rpm, that is all the torque that it can develop. Engine rpm is the limiter. A hydro, with its infinite speed ranges, and given a load that is within it's capability, can accellerate from the geared tractors speed to a higher speed, and that takes more torque. The additional torque comes from more flow due to advancing the drive control, which increases the available horsepower, of which torque is a component, and the increased load due to accelleration builds more pressure to create that torque.

A thought just occured to me. Many people are under the misunderstanding that max pressure is always in the system. It is not! A hydro only generates enough pressure to do the work. If you're tootling up the street at full throttle and speed to have some refreshment with your neighbour, the hydro may be developing, let's say, 375 psi. When you come back with his garden trailer and a load of dirt in tow at the same speed and throttle, the pressure may be up to 475 psi due to the extra load. The extra load requires more pressure, and therefore more torque, as well as more horsepower to move at the same speed.

Here's one that I pulled off.

A hydro will take off with a heavy load at just over idle and then ramp up to full throttle and full drive speed. I've done this with an 18' travel trailer, and I was not gentle with either the drive control or the thottle. A visitor next door saw me hook up and said that the little tractor wouldn't pull the trailer, so I did a little showing off. The engine stumbled slightly for a second when I bashed the throttle full up after doing the same with the drive control. That trailer weighed 3600 lb and it was at full speed within 15'. Never dug a hole, didn't smoke the belt, and didn't pop the relief, even with the ham-fisted way that I abused the controls. That MF12H also had calcium loaded tires, wheel weights, a hydraulic 3PH, and a FEL. Traction was not an issue with almost 1400 lb of tractor and operator.

That was over 30 years ago. I've learned a lot of the why's since. Most of them the hard (expensive) way.

Does that answer your questions about torque development and plowing with the 12H?

Perfect traction doesn't exist in normal operations with these little beasts. They don't weigh enough and will break traction if the load is too much for them. The above tractor broke traction at 2/3 throttle on a regular basis while excavating for my pool lines with the FEL, even with a 125 lb weight box on the back in addition to all the rest. The possibility of using all of the available torque at full throttle borders on the "it ain't happening" side of "not bloody likely" with either machine.

Don't sweat it, man. I still run into things that I can't seem to get my head around. I've learned to accept the "what it will do's" and wait for the "why it won't's" to rear their ugly heads. LOL

Edited by TUDOR, April 01, 2012 - 06:56 PM.

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#43 HowardsMF155 OFFLINE  

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Posted April 01, 2012 - 06:35 PM

"not bloody likely"

:worshippy2:
Notice I have been saying "perfect traction" while being aware that that would be a VERY hard objective to achieve.

"I've been basing this discussion on the output of the hydro where the power goes to the final drive reduction. You're looking at the torque at the rear wheels. Torque at the rear wheels will be the same for both transmissions, given the same load and speed. That is where the work is being done."

I caught something in your previous explanation that told me we weren't QUITE discussing the same thing. We are certainly on the same page now.

"Does that answer your questions about torque development and plowing with the 12H?"

It tells me I can plow all I want with a hydro and not to worry. I see two hydros in my future, one with a mower deck and one with a sleeve hitch. I'll use the 12G for throwing out fertilizer and giving trailer rides to my kids. Thanks for your posts, as someone else has said, I feel like I'm in a technical school for Garden Tractors, and I enjoy that!

#44 TUDOR OFFLINE  

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Posted April 01, 2012 - 07:32 PM

Howard, if you're going to plow 3 or 4 acres, at least take an IR temperature gun to monitor the hydro. It's one thing to pull a huge load on wheels a hundred feet on a level hard-packed surface, It's something else to pull a constant heavy load for a couple of hours. A quick temp check every 15 minutes or so until it stabilizes will do your confidence in the machine wonders. As long as it stabilizes at 180* or less, you're good. Normal is 140* - 160*, over 195*, get the plow out of the ground and drive around at full throttle for a bit to cool it off. The only cooler for that tranny is the fan and the fins.

I had a gear drive tractor.........once. After the 12H, never again. I now have 7 hydros, 2 operational, 3 needing minor repairs, 1 needing a little more than minor, and 1 basket case. All MF except for a Husqvarna 20 hp grass cutter. Most of my hours are on a 1655 with a FEL.
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#45 coldone OFFLINE  

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Posted April 01, 2012 - 10:02 PM

Let me try to get my head around this.

So with the Hydro we have two variables, pressure and flow. The pressure only builds when there is resistance to flow. The amount of flow is controlled by the swash plate, which would be the only operator control. So the pressure climbs slower if the flow rate is small. The pressure climbs fast if the flow rate is large. If those statements are true, then max operational pressure is dependent on load and how fast your reach the max operational load is dependent on the amount of flow you have.

Will the pressure be the same to move 500LBS at 5mph as it would be to move 500lbs at 2? Flow rate should be less but it should still take close to the same pressure once the load is rolling. Assuming all load variables are equal.

Close?
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