Function of Range and RPM on an HST

[jyoutz]

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!

This is how I operate:
L= loader
M= mowing
H= highway

 

[PlasterProspector]

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,

This is wonderfully helpful, thank you.

So then, as I understand, when I press down on the pedal I'm increasing oil pressure which changes the angle of the swashplate, thereby causing the pistons to begin to pump, which increases the displacement as the engine continues to turn the whole thing at a speed relative to RPM. How then, since gears aren't in play, do we see the torque peak and then drop off as the vehicle continues to increase in speed as we push the pedal farther down? Is this to do with the varying effects of swashplate angle? Is there a particular reason we see the pedals rockered for forward and reverse as opposed to being separate?

You've been wonderful at helping me understand this and I really appreciate it.

 

[ljjhouser]

I was also told whatever gear I use with HST, better to keep rpms high to insure cooling of the fluid and transmission. Great video on the swash plate pump.

 

[ericm979]

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 think that's how most CUTS do it. Absolute efficiency is not that high a priority. Yanmar does something different to make the HST most efficient in the range people use most often but it's still a fixed displacement pump.

Keep in mind that when the HST is in neutral the pump is pumping but since the motor displacement is zero, the fluid isn't doing any work, so there's no real resistance on the pump.

I do believe that both pump and motor are variable in some systems.

Kubota's HST+ uses a two speed pump kind of like a log splitter to fake an additional two speed range transmission. It's a neat idea.

I would expect there are more advanced HSTs like maybe those in big ag tractors that have both sides being variable.

 

[Vigo327]

when I press down on the pedal I'm increasing oil pressure which changes the angle of the swashplate, thereby causing the pistons to begin to pump, which increases the displacement as the engine continues to turn the whole thing

I think in most cases your pedals are actually mechanically connected to the swashplate angling shaft (dont know a better term for that) through a linkage, and there is not a fluid connection between your foot and the swashplate. What IS in between is a fluid-filled damper, like a shock absorber, that resists you moving your pedal too quickly (which would cause a pressure spike in the system and possibly break things). So in terms of the oil pressure you mentioned, the pressure you're changing is fluid pressure between the HST pump and the motor it's trying to spin.

How then, since gears aren't in play, do we see the torque peak and then drop off as the vehicle continues to increase in speed as we push the pedal farther down?

So the torque of a hydraulic motor is based on the pressure you're feeding it. It's rpm is based on the volume of fluid flow you feed it. When you first begin rolling, the pump can make flow faster than the driven motor can receive it because while the pump is only hooked to your foot, the driven motor is hooked to the entire weight of the tractor, several thousand lbs. So, inertia is not limiting your swashplate movement but it's limiting the drive motor's movement. Since the pump is trying to send more flow than the pump can initially receive, pressure (and thus torque) shoots up. Ultimately torque output will be limited by a pressure relief valve which will open and let fluid bypass the motor to prevent fluid pressure rising to dangerous levels. As the driven motor starts to spin and there is less difference between what the pump is trying to send and what the motor is able to receive, the pressure between them drops. Ultimately POWER (such as horsepower) is derived from torque AND rpm, so if you had a 20hp tractor you could transmit 20hp with very high pressure/torque and very low rpm, or very low pressure/torque and high rpm. So even though the torque drops off as speed increases, the total power delivered is about the same. The actual sensation of acceleration is more tied to torque than horsepower, though, so you 'feel' more 'power' when accelerating from a stop.

 

[Vigo327]

The HST is basically a fixed displacement pump and a variable displacement motor connected together.

Is that true in all compact tractors?

I think that statement was a simple mistake: it's backwards. It's a variable displacement pump hooked to a fixed displacement motor.

I've made some notations on pictures of a Kubota HST unit i found in an Ebay ad.

 

[MHarryE]

Vigo said it before I got to. Hydros are variable displacement pumps, fixed displacement motors. Think of it if your pump was always putting out 50 liters per minute. At max displacement on a motor, pump and motor would turn the same speed. Reduce displacement of the motor and you still have 50 liters per minute coming from the pump so the output speed would increase. As the motor displacement approaches zero, output speed being the displacement ratio, it would be 50/0 or infinity. Hydro motors are always variable. Motors are usually dual displacement except on lower end. Skid steers, compact track loaders, excavators, Kubota Grand L, all I have operated have had dual displacement motors. My GR2120 and RTV900XT have single displacement motors. Dual displacement - large displacement equals lower speed, higher torque. Lower displacement higher speed lower torque.

 

[npalen]

Vigo said it before I got to. Hydros are variable displacement pumps, fixed displacement motors. Think of it if your pump was always putting out 50 liters per minute. At max displacement on a motor, pump and motor would turn the same speed. Reduce displacement of the motor and you still have 50 liters per minute coming from the pump so the output speed would increase. As the motor displacement approaches zero, output speed being the displacement ratio, it would be 50/0 or infinity. Hydro motors are always variable. Motors are usually dual displacement except on lower end. Skid steers, compact track loaders, excavators, Kubota Grand L, all I have operated have had dual displacement motors. My GR2120 and RTV900XT have single displacement motors. Dual displacement - large displacement equals lower speed, higher torque. Lower displacement higher speed lower torque.

Can you explain a bit how the dual displacement works? Not familiar with how shifting between the two ranges takes place.

 

[rScotty]

Vigo said it before I got to. Hydros are variable displacement pumps, fixed displacement motors. Think of it if your pump was always putting out 50 liters per minute. At max displacement on a motor, pump and motor would turn the same speed. Reduce displacement of the motor and you still have 50 liters per minute coming from the pump so the output speed would increase. As the motor displacement approaches zero, output speed being the displacement ratio, it would be 50/0 or infinity. Hydro motors are always variable. Motors are usually dual displacement except on lower end. Skid steers, compact track loaders, excavators, Kubota Grand L, all I have operated have had dual displacement motors. My GR2120 and RTV900XT have single displacement motors. Dual displacement - large displacement equals lower speed, higher torque. Lower displacement higher speed lower torque.

I don't understand. You say that hydros (hydrostatic transmissions) are variable displacement pumps hooked to fixed displacement motors...and then you go on to explain in terms of motors that vary in displacement form max to zero. How is it that a fixed displacement motor varies from max to zero displacement?

What does Kubota use? Does anyone really know if Kubota uses a variable displacement pump or variable displacement motor? That would be a nice place to start.

What is it that the typical L/M/H three speed range shifting is actually shifting ?

What does HST plus do in addition to the standard range shifting ?
rScotty

 

[RalphVa]

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!

Depends on whether you're running a JD or Kubota. JDs do not have enough flow for strong hydraulics at lower speeds. Kubotas do, almost too much such that their hydraulics are more jerky unless a very soft touch is employed on the controls.