Function of Range and RPM on an HST

   / Function of Range and RPM on an HST #1  
Joined
Oct 26, 2021
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44
Location
Southern Vermont
Tractor
Kubota MX6000HST
I've dug through the forums here and even read some scholarly articles on the subject but I still don't understand how revs affect the performance of the HST when using hydraulic implements like a loader. I can feel the difference it makes in pulling, cutting and ground engaging implements but the only difference I've noticed for hydraulics is speed of lift, whether it's off my three point or my loader. Is there a hidden significance or another benefit here I'm missing? It seems like due to the DPF I should just run wide open all the time and keep diesel on hand.

And then as a separate question for HST operators: are you guys running ranges other than L strictly when you want to do what you're doing faster or does it confer a change in the gear ratio that makes for more meaningful differences? I run L almost all the time because my impression is that you get the most torque to the wheels when you're in it, your revs are high and you're gently edging the pedal towards that peak. That and I thought I'm supposed to keep my revs up so if I'm traveling the property shouldn't I be avoiding cruising along distances in M at 1300rpm?

I guess I'm making yet another "please explain hydrostats to me" thread. There just seem to be so many bad analogies and explanations out there that I'm not sure what to believe. Moreover, I'm wondering what the practical applications are in order to get the best use out of my machine.

Thanks in advance!
 
   / Function of Range and RPM on an HST #2  

Michael In Tennessee

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Mar 14, 2018
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402
Location
Niota, TN
Tractor
Kubota MX4800HST
Gear ranges. I'm impatient. I use M for most traversing my 24 acres. I use H on the road in front of the house. I've gotten to where I use M for light loader work as well. Just a bit quicker to move around. Heavy work is always in L. If you don't need the torque, may as well get some speed when you want it.

Yes, RPMs matter to the hydraulic circuits. As you noted, mostly its speed, but it can be performance when you are talking about using a lot of flow like when using a motor. If you can't get flow rate, you can't get power. Cylinders don't care much.

Yes, on a modern DPF equipped tractor, you want to run fairly decent RPMs anyway. I run my MX4800 about 1800 RPM unless mowing, which is over 2k RPMs (don't know the exact number, just put the dial on the PTO indicator).
 
   / Function of Range and RPM on an HST #3  

Midniteoyl

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May 20, 2013
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4,634
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N. W. Indiana
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Kioti CK3510SE HST, Ford 3400, Gizmow ZTR, Simplicity 7016H
I run "idle" @ ~1500rpm and use Linked Pedal when just putting around or doing loader/grapple work. 1500 is more than enough for a quick loader and keeps the DPF happy on my tractor.

I run in M most of the time, even for most loader work, unless the situation calls for a different range, like traveling longer distances in High, or pulling roots in Low.

From what I understand, once you have enough HP to support your hydraulic's max PSI setting, which could be at idle depending on engine, the only thing more RPM does is increase flow, which in turn increases speed.
 
   / Function of Range and RPM on an HST #4  

ericm979

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Nov 25, 2016
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Santa Cruz Mountains, Ca
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Branson 3725H
Engine rpms determine hydraulic flow. If your loader moves too slow, run the engine faster.

The HST and hydraulics are separate circuits that use the same tank. The HST is basically a fixed displacement pump and a variable displacement motor connected together. The pump speed is determined by engine rpm. The motor speed by the pedals. Slower motor speed (less pedal) gives less speed and more torque to the wheels, like choosing a lower gear in a manual transmission.

On my tractor, low range is really slow. I use it only for very steep slopes or pulling something heavy. High range is so high that it's only good for flat ground and I don't have much of that. Nearly everything is done in medium range. Newer Bransons have four ranges which would be really nice.

If there is a choice between lots of HST pedal in low range and barely any pedal in medium, like say going up a steep hill, low range and lots of pedal will heat the oil and the HST less. That's probably not a consideration for 100' of steep road but might be if it's 1/2 mile.
 
   / Function of Range and RPM on an HST #5  

3gunr

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Apr 17, 2018
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wentzville mo
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mahindra 2538 mahindra 2810 cub cadet 7260 massey 255
AS ericm979 says . It is better ti use full pedal in low gear than little pedal in med . I do all finish mowing , most brush hogging, and disking in med range , tilling and digging in low , always 2000 rpm or above.
 
   / Function of Range and RPM on an HST
  • Thread Starter
#6  
OP
PlasterProspector
Joined
Oct 26, 2021
Messages
44
Location
Southern Vermont
Tractor
Kubota MX6000HST
Engine rpms determine hydraulic flow. If your loader moves too slow, run the engine faster.

The HST and hydraulics are separate circuits that use the same tank. The HST is basically a fixed displacement pump and a variable displacement motor connected together. The pump speed is determined by engine rpm. The motor speed by the pedals. Slower motor speed (less pedal) gives less speed and more torque to the wheels, like choosing a lower gear in a manual transmission.

On my tractor, low range is really slow. I use it only for very steep slopes or pulling something heavy. High range is so high that it's only good for flat ground and I don't have much of that. Nearly everything is done in medium range. Newer Bransons have four ranges which would be really nice.

If there is a choice between lots of HST pedal in low range and barely any pedal in medium, like say going up a steep hill, low range and lots of pedal will heat the oil and the HST less. That's probably not a consideration for 100' of steep road but might be if it's 1/2 mile.
So on an HST tractor are cylinders actually being turned off and based on the amount of transmission oil flow? What then does the swash plate actually function as? Does it slow the transmission oil flow? How does this variable displacement result in the torque curve we see that peaks early and then declines?
 
   / Function of Range and RPM on an HST #7  

Vigo327

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Jun 25, 2021
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259
Tractor
Kubota B6100

This should give you some idea what's meant by swashplate. A piston pump is like an engine, and the swashplate changes the 'stroke' of the pistons. It basically does the job of a crankshaft in an engine as far as moving the pistons up and down, although it is shaped completely differently.

No tractor engines disable engine cylinders as far as i'm aware. It is common on cars but really only in response to regulatory hurdles the manufacturers are trying to clear, not because they want to or customers are asking for it.

You could say the something like disabling cylinders happens in an HST pump, and it's actually far better than what happens in car engines. When the swashplate is in a 'neutral' position, basically flat, the cylinders have zero stroke and the pistons don't move up and down in the cylinders at all! This is great because unlike a car engine, you aren't still wasting energy slamming those pistons up and down even though the cylinder isn't doing any work.

The engine rpm affects the hydrostatic pump in a couple of ways. The total flow you can get out of the pump is basically (pump displacement based on current swashplate position) X (engine rpm) = Total flow out of the pump. So speeding up the engine makes the pump pump more in any given swashplate position. So if you find low range still slightly too sensitive you can lower engine rpm to make it less sensitive, say if you are trying to tuck pallet forks into a tricky little pocket on something, lining up a tow ball, etc.

The other thing is that all pumps leak internally (piston seals, 'valve sealing' etc) and the leakage is a function of pressure and time. So at lower rpms the pumps will leak more internally because the cylinders are under pressure for a longer time (each stroke takes longer to complete) which means more wasted input power and more waste heat generated for the same result vs running the pump at a higher rpm. This is the same reason a worn out engine with low compression (ie a lot of internal leakage in the cylinders) will be hard to start and weak at low rpm but will make almost full power at high rpm because the time during which the cylinder can leak pressure out gets shorter and shorter at higher rpm.

Basically just do whatever feels good. There isn't really an easy way to screw the thing up other than being in too high of a gear range for the work you're trying to do, and you've got that covered already. Also, if the HST isn't making loud noises, you're probably not mistreating it. If you do something like put it in high range and try to push it into a dirt pile, it'll SOUND unhappy, and as long as you're not making it sound like that, you're pretty much ok.
 
   / Function of Range and RPM on an HST #8  

nybirdman

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Feb 1, 2009
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5,042
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north of upstate ny
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Kubota L4240 HSTC,L3000DT
After 16 years with two HST tractors(32+42 HP) I have use medium about 95% of the time other than roading(toggle up/down with Kubotas) and run 1800 rpms or so.Rarely use stall guard.
 
   / Function of Range and RPM on an HST #9  

npalen

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Joined
Nov 17, 2009
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3,122
Location
Beloit, KS
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Kubota B9200 HSTD and Mahindra 3015
Engine rpms determine hydraulic flow. If your loader moves too slow, run the engine faster.

The HST and hydraulics are separate circuits that use the same tank. The HST is basically a fixed displacement pump and a variable displacement motor connected together. The pump speed is determined by engine rpm. The motor speed by the pedals. Slower motor speed (less pedal) gives less speed and more torque to the wheels, like choosing a lower gear in a manual transmission.

On my tractor, low range is really slow. I use it only for very steep slopes or pulling something heavy. High range is so high that it's only good for flat ground and I don't have much of that. Nearly everything is done in medium range. Newer Bransons have four ranges which would be really nice.

If there is a choice between lots of HST pedal in low range and barely any pedal in medium, like say going up a steep hill, low range and lots of pedal will heat the oil and the HST less. That's probably not a consideration for 100' of steep road but might be if it's 1/2 mile.
The HST is basically a fixed displacement pump and a variable displacement motor connected together.

Is that true in all compact tractors? I would think it more energy efficient for the pump to be variable displacement so that full volume isn't being pumped at all times relative to engine RPM?

I do believe that both pump and motor are variable in some systems.
 
   / Function of Range and RPM on an HST #10  

BackRoad

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Apr 30, 2020
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693
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Rural PA
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Kubota MX6000, MF 35
Vigo...very helpful video! Thanks...
 
 
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