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 | runslikeadeere
Join Date: May 2004 Posts: 22 Mississippi 
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2006-08-30 133810
I own a JD 790 and have a question that I have not seen addressed in the owner's manual. I have noticed when using the lift to raise a heavy implement the engine lugs down some when running at idle. Should one routinely increase the throttle above idle RPM when operating the lift? Thanks for the help!
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 | DRankin
Join Date: Jan 2000 Posts: 5116 Northern Nevada 
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2006-08-30 133812
"Should one routinely increase the throttle above idle RPM when operating the lift?"
Absolutely. Your tractor is telling you what it needs and you are listening.....
Low idle should be used for just that.... idle, as in doing nothing. ....
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 | earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2006-08-30 133819
Both my CUT and skid steer recommend running the engine at or near full throttle all the time. There is no need to run anything in these categories at idle unless they are sitting still and are cooilng down the turbo (but who does that? LOL)--it lugs the engine and increases load on the hydraulic pump. And in some cases with hydrostatic drives can damage the pump(s) since they require pressure inside the pump against the swash plate. ....
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 | beagle
Join Date: Jan 2004 Posts: 1333 Michigan 
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2006-08-30 133833
We run at about 80% full rpm for everthing but mowing work. At 75-80% rated rpm, you are running at the optimum torque rating of the engine. RPM's over that point increase power, but reduce torque because the efficiency of the engine drops off.
Never run the hydraulics at idle, or move the tractor if it's a hydrostat. The back pressure on the hydrostat sytem can cause damage similar to trying to push the tractor (or tow) against the system. ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2006-08-30 133835
Beagle I take it all your equipment is gear-drive versus hydrostatic?
From my exp. and my particular applications of a CUT and SSL the engine torque doesn't come into play as much as HP i.e., power curves vs. wheel torque (this excludes the 3 speed ranges of the CUT, and the 2 speed ranges of the SSL). For grins-and-giggles my SSL has a 78 HP turbo, 3 cyl. engine putting out something like 120 lb-ft of torque. But at the wheels it is capable of an outstanding and mind-blowing 6,300 lb-ft of torque! Also, I have a 40' Grove telescoping, self-propelled (hydrostatic), manlift/work platform that weighs 8,200 lb. empty. But it's powered by a 24HP Kohler gas engine! ....
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| wingwiper
Join Date: Jun 2004 Posts: 676 
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2006-08-31 133839
EW
Am I wrong to say that Diesel Engines put out their HP and Torque at LOW RPMs and the curve remains pretty flat for several hundred RPMS, but to get the Hydralic pump to work and move more fluid you need to increase RPMs. ....
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 | greg_g
Join Date: Jan 2004 Posts: 1816 Western Kentucky
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2006-08-31 133842
Diesel engines develop torque early on the power curve, but HP continues to develop. My KAMA for example develops max torque at ~1700 rpm, but doesn't develop the full 45hp till ~2300. It will continue to rev until the governed ~2650, but that's basically to gain ground speed only. HP actually trails off after 2300. That max HP point on the power curve is generally coincident with the engine revs required to produce working PTO rpms.
I suppose some credence could be given to the theory that you could operate the hydraulics down at the rated max torque RPM. But since the design engineers consider PTO speed to be where the engine is working most efficiently, I consider that to be a practical speed at which to run loaded hydraulics as well.
//greg// ....
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| beagle
Join Date: Jan 2004 Posts: 1333 Michigan
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2006-08-31 133843
No, even the CUT is a hydro. 2100 rpm supplies plenty of fluid and pressure in the hydrostat. The tracor has no problem telling you if you aren't giving it enough. I use the hydro whine to let me know if I need a lower gear range or more rpms.
For the engine, I like to run at the peak of the performance curve, which is around 80% when doing heavy work, with the dozer or the CUT. The dozer is also hydrostat (D4B), as are most of them anymore.
When mowing with the CUT, I run the rpm's up to get the performance out of the mower decks.
Right/wrong/or indifferent, that's the way we've done it for years, and seems to work pretty well for us with the diesel equipment. With a gas engine, let it fly!!!! ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2006-08-31 133844
wing: "Am I wrong to say that Diesel Engines put out their HP and Torque at LOW RPMs"
>>>>>That's a loaded question, no pun intended. Doesn't it depend on the application? My dad had a Miller portable welder that sounded like an old hit-and-miss that it ran so slowly. The older diesels of yore I recall even the Dodge Cummins weren't designed for high RPM otherwise they'd blow apart. My '03 Cummins spins much faster it's early kin. Back to applications: on a skid steer the high RPM is where the engine has been designed to be married to the hydro and offer the best performance, particularly for low-vehicle-speed wheel torque.
"... and the curve remains pretty flat for several hundred RPMS, but to get the Hydralic pump to work and move more fluid you need to increase RPMs."
>>>>>>>>>>>Again this a loaded question. Application is all-important. If I take my either my CUT os SSL and run the engines at say half-RPM I can get locomotion out of them, but not necessarily the speed I'm needing. Now try pushing or pulling something I can do it but it takes longer (I like getting things done quickly, get paid, move on to something else). The SSL when run at half-RPM will pick up just a little less than when run at full RPM. However, drop the RPM to just above idle the action is noisey (hydraulically speaking) and a bit jerky, and I can't lift as much, but that's probably due to the pump's inefficentcy.
....
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| wingwiper
Join Date: Jun 2004 Posts: 676
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2006-08-31 133846
EW
Good answers, I was a bit vague in my post.
I meant that Diesel Engines have a Low Troque point and their HP point is also way lower than a Gas. Cummins have peak HP at about 28 or 2900 RPM and their Torque is at 1600. Many Gas Engines will hit their peak HP in the 5000 RPM range. I really didn't mean the Torque and the HP were peaked at the same point. You made good examples.
I run my 4115 with the BH at about 2000 to 2200 and have noticed that past a certain point I don't seem to gain a thing in BH performance, that BIG rock was lifted to X numbr of feet and when I add more RPMs, it stays at the same level.
I am NEW to Hydralics and am trying to understand what I know about Diesels and don't know about Hydralics. I do not want to over work my CUT but I want to make it do what it can with as less abuse as possible.
....
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 | DennisCTB
Join Date: Nov 1998 Posts: 2707 NorthWest NJ
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2006-08-31 133847
I operate my tractor just slightly below PTO speed which is for all intents full throttle. PTO speed generally = Full Throttle.
Over the years on TP one of the most common misconceptions of new owners is the complaint that the Tractor is "lugging". And we usually find out that they are running it just above idle.
On the subs like the BX they don't even come with a Tachometer because you really don't need one, it is either idle when parked, or full throttle which is PTO speed when it is being worked.
Art or Murph are the experts on this and I suppose they will chime in with their insight. ....
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2006-08-31 133850
I get called lots of things but rarely "expert", thank you Dennis.
First of all there are several factors here, each one is unique, both in it's effect, and in it's need.
Torque, this is one measure of the "work ability" of the engine or a drive motor, it is the grunt in the equation. High speed does not require as much torque, as getting something moving in the first place. In a HST application, it together with HP, govern how much work can be done in a given period of time.
HP, very similar to torque above for this conversation.
The two critical ones are PSI or pressure & GPM or flow rate.
In a HST the big one is flow rates. It is pretty much a given that any modern hydraulic system, especially in a HST, will always give enough pressure.
Flow rates are what EW was talking about with regards to mobility, and at what speed.
In most HST's the pump is of a variable displacement, or swash-plate type. This type of pump uses a two basic moving parts, a shaped housing which spins with the impeller, and a piston which not only spins, but tilts on an axis. This tilt (or swash) is what makes the difference. By tilting the plate the pump can be infinitely varied between a higher volume, but (relatively) low pressure unit, and a high pressure, but lower volume unit.
This means one pump can produce the barely moving but all the grunt necessary to move a mountain, as well as the high speeds but it slows down for a good head wind travel capabilities.
So, after all of that, yes, you need more engine RPM's than idle to make useable hydraulic power. How much more? Well, the rule of thumb we use (for a CUT) is, 1,300rpm or better for anything, 1,750 rpm or better for unladen slow speed travel, and full PTO speed for PTO aplications, high speed travel, or any heavy work.
The other confusing factor with a hydraulic system is relief valves, it doesn't matter how many RPM's the engine or pump makes, when the fuild reaches a set point, the valve opens and pressure stops building, period. This is why as WW pointed out, the RPM's don't change the lift capabilites, when the rock in the bucket, and the weight of the B/H itself create a system pressure above that of the relief valve, it opens and the lifting stops.
CAUTION: This valve only has an effect on a "live" circuit though. I.E. A circuit to which a valve is open. If you were to raise, for instance, the FEL up to the height of a truck, and rolled something heavy off the truck and into the bucket, without touching the FEL controls you could quite easily create more pressure on the system than it was designed to take. If you were to then lower it, and drive off with it, any bump would create an extra load, called a shock load, which could easily be enough to cause a failure. You could blow a line, cylinder(s) seal, or even a valve seal.
Anything I didn't cover in that?
Best of luck. ....
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| beagle
Join Date: Jan 2004 Posts: 1333 Michigan
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2006-08-31 133852
Can we add that Horsepower is the speed at which the torque is used to accomplish work. The physics professor out there somewhere would probably like to see the time dependency of "Power" mentioned in this discussion. ....
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 | kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2006-08-31 133853
Here's some equations for you.
time = money
knowledge = power
power = work / time
so, after some substitutions
knowledge = work / money
Solving for money we get
money = work / knowledge
thus, as knowledge decreases money increases
This explains a lot about CEO salaries. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2006-08-31 133855
Beagle, you are of course correct, but I was trying to keep it fairly easy to understand.
As a Prof. of mine used to say "I didn't want anyone pulling a muscle trying to understand what I was saying."
Best of luck.
....
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| wingwiper
Join Date: Jun 2004 Posts: 676
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2006-08-31 133863
Murf
Very impressive response. It was of great interest when I read the section about Shock Load, just that paragragh alone gave me some Super Caution Info. I have used my CUT many times to dig up and move Boulders that would lift only half the distance of the capable travel of the FEL and then I would slowly move the Boulder to a dump area, which has been my property boundary and dumped, making a Rock Wall. I am glad I have chosen to go slow and steady and now I will be certain to do so.
Very Good Info.
My Tach has a Orange Mark at the 2350 RPM ( I believe) and have ONLY used that Pre RPM setting for PTO Tiller work.
I will need to read your post at least a few more times for it to really sink in.
Thanks
....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2006-08-31 133870
WW, thank you.
You brought up an interesting point I had skipped over. Not all PTO work requires full PTO speed, in your case, as with most CUT's, about 2,300 or 2,400 rpm.
Implements such as liquid or air pumps will run just fine below PTO speed, and some often work even better going slower, such as with a Post Hole Digger (PHD).
However, PTO-driven implements such as a tiller should never be run at less than PTO speed, the type of load they create, combined with the slower speeds, which can put excessive strain on internal bearings and such, can cause serious (read: expensive) damage to your PTO.
Also, if you want to be able to carry heavy loads on a regular basis with a little more margin of safety, you can easily install a device called an accumulator in the lift side of the FEL lift circuit.
It is basically just a sping-loaded piston in a small casing. When you hit the pressure point it is designed to, the spring gives a little, the piston moves back, and fluid has somewhere to go safely without ruining a seal, or blowing a line. When the shock load is over, the spring pushes everything back to where it belongs.
They are a cheap means to get a little more safety.
Best of luck. ....
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| beagle
Join Date: Jan 2004 Posts: 1333 Michigan
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2006-08-31 133880
And those shock or "impact" loads can be easily as high as 50% of the lifted load. A soft impact (cable slung) load is designed at 120% of the lifted load, and for AASHTO bridge design, rolling impacts for pnuematic tires on a smooth surface are designed at 125% of the axle load.
Maybe something MDOT needs to revisit, if you've ever driven a Michigan highway.
Real loads to keep in mind while carrying with the FEL for sure. ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2006-08-31 133881
beagle: Do we "drive" the roads or do we "dodge" them? ....
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| beagle
Join Date: Jan 2004 Posts: 1333 Michigan
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2006-09-01 133910
I'm about to dodge them in my Chevy for the Labor Day weekend. You would't know Michigan is struggling when you see the traffic on I-75, but it sure gets obvious when you see the number of vacation homes for sale when you start running the back roads.
Better times ahead...........probably. ....
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