GST vs HST during a shift change

[skyhook]

I know my 8 speed gst trans has a hesitation when shifting on the fly.
There's that "pause" before it enters the next shift.
Do the hst tranny's do the same?, or is it a smooth transition without the pause?

 

[diesel lover]

I know my 8 speed gst trans has a hesitation when shifting on the fly.
There's that "pause" before it enters the next shift.
Do the hst tranny's do the same?, or is it a smooth transition without the pause?

Hst transmissions do not shift. They are a complete fluid drive. They do have 3 or 4 ranges at which to be operated but I personally only change ranges at a dead stop.

Pushing lightly on a hst pedal provides the highest torque value. Pushing the hst pedal to the floor creates the highest speed but lowest torque.

All variable speeds are created by different amounts of fluid flow combined with pressure etc. Hst works with a hydraulic pump creating flow and the flow is received down the line by a hydraulic motor. The pump is normally variable and the motor is normally fixed.

 

[skyhook]

Thanks for that info, DL,
So as you press on the peddle to shift , there is no loss of momentum like the gst, correct.
I know, if going uphill, on a gst, if you were to shift, the tractor would start to reverse slightly,
This would be eliminated on a hst.

 

[jeff9366]

Hst transmissions do not shift. They are a complete fluid drive. They do have 3 or 4 ranges at which to be operated but I personally only change ranges at a dead stop.

Pushing lightly on a hst pedal provides the highest torque value. Pushing the hst pedal to the floor creates the highest speed but lowest torque.

All variable speeds are created by different amounts of fluid flow combined with pressure etc. Hst works with a hydraulic pump creating flow and the flow is received down the line by a hydraulic motor. The pump is normally variable and the motor is normally fixed.

Great explanation. I read about HST transmissions via Wikipedia but its explanation was not as clear as yours. Thanks.

 

[diesel lover]

Thanks for that info, DL,
So as you press on the peddle to shift , there is no loss of momentum like the gst, correct.
I know, if going uphill, on a gst, if you were to shift, the tractor would start to reverse slightly,
This would be eliminated on a hst.

Please think of a tractor with a front end loader or simply a log splitter with a single hydraulic piston. Now how does the loader work? A loader recieves fluid from a rotary pump and coverts it into a linerar motion using the hydraulic cylinder. So rotary motion from the pump is simply turned into back and forth motion at the cylinder as it moves in and extends out.

Back to the hst consept, we take rotary motion at the pump and push the fluid to a motor that also is creating rotary motion!! I hope that helps!

In this way torque is created. On a truck the driveshaft spins much faster than the rear wheels correct? How much faster? Multiply the speed of the wheels by the rear end ratio and this is equal to the speed of the driveshaft!
The rear end creates torque by lowering the speed of the driveshaft and transmission. In the same way a hst transmission can create a ton of torque from 0 movement at all! If engauging a clutch, it will slip from no movement at all. This causes a loss of torque. If engaging a hst drive torque is multiplied!!

 

[Gary Fowler]

Thanks for that info, DL,
So as you press on the peddle to shift , there is no loss of momentum like the gst, correct.
I know, if going uphill, on a gst, if you were to shift, the tractor would start to reverse slightly,
This would be eliminated on a hst.

The easiest way to think of an HST is think of it like an automatic transmission on a car. HSTS has 2-3 ranges and usually you have to stop to change these gears (unlike an automatic car transmission) however I think there is some new tractors coming out that allows on the fly range changes. The HST does work backwards a bit from a car where the more pedal you push the more power you have. HST (most of them) are not related to engine speed in a strict sense. You set the RPM at a constant rate and then control the speed and power with the pedal. Push it just a bit and you start to move slowly, push it down more and you move faster. Just like a car transmission has more power in low than high, the same applies to the HST. Slow has more power to turn the wheels so if you are say going up a hill (engine speed is Wide open throttle(WOT) so you have no more engine HP to use) and the engine begins to stall, you let up a bit on the HST pedal which slows the speed down but increases torque to the wheels because you have theoretically shifted to a lower gear.
It does take some getting used to how to drive an HST compared to a gear. Most HST transmissions need the engine speed to be 1500 RPM to WOT to operate smoothly so the speed is determined by first which range you have it in ( L< M< H ) AND by how far down you push the pedal. It is unlimited speed forward and reverse in each range up to the max speed in that range. The transmission is turned by a hydraulic motor feed from the engine driven HST charge pump so it requires a bit of RPM to supply enough oil to operate smoothly but it does supply a constant supply of oil to the hydraulic motor so there is never any hesitation in forward or backward motion.
It is rather difficult to explain the operation, but rather simple to operate once you know that by slowing the speed you get more power

 

[skyhook]

Got it, thanks guys.:thumbsup:

 

[k0ua]

There is quite a bit to HST engineering, and how they work, but operation is the simplest of any transmission. There is no shifting in a range as pointed out before, unless you are considering a Kubota Grand L with HST+. HST+ is a next generation hydrostat, without going into how it works, with a discussion of swashplates etc. lets say it allows you to select a high and low setting within each of the 3 ranges by flicking a column mounted lever. As an example in any 3 range hydro the M range is a good compromise between the maximum torque of Low range and the Maximum speed of High range. Most people especially when doing loader work will choose M, except perhaps when pushing into the pile of material where more torque may be wanted, but the travel speed of M is usually sufficient when loaded. The throttle (fuel delivery) is usually set at a fixed setting and your ground speed is controlled by the hydrostat pedal or pedals infinitely within the range. The exception is HST+ where the throttle can be also controlled by the hydro pedal. Most of us get along great without the advanced features of HST+ with our hydrostat equipped tractors. I don't have one and I love my hydro equipped Kioti, but I can appreciate some of these advanced features (there are several more I have not mentioned) and lust after them. This video will give you some more info about HST+

https://www.youtube.com/watch?v=hzO_-LCd838

 

[k0ua]

Keep in mind that the HSDS automatic shifts in the HST+ is not actually shifting gear ranges it is changing the two speed motor. Here is an article with more discussion of this.

Operating Kubota HST-Plus

So as you can see there is a lot to this, and you asked a simple question, does hydro transmissions pause when they shift? The answer of course was they don't shift. Except in HST+ and they sort of shift. Happy reading.

 

[marcwert]

Please think of a tractor with a front end loader or simply a log splitter with a single hydraulic piston. Now how does the loader work? A loader recieves fluid from a rotary pump and coverts it into a linerar motion using the hydraulic cylinder. So rotary motion from the pump is simply turned into back and forth motion at the cylinder as it moves in and extends out.

Back to the hst consept, we take rotary motion at the pump and push the fluid to a motor that also is creating rotary motion!! I hope that helps!

In this way torque is created. On a truck the driveshaft spins much faster than the rear wheels correct? How much faster? Multiply the speed of the wheels by the rear end ratio and this is equal to the speed of the driveshaft!
The rear end creates torque by lowering the speed of the driveshaft and transmission. In the same way a hst transmission can create a ton of torque from 0 movement at all! If engauging a clutch, it will slip from no movement at all. This causes a loss of torque. If engaging a hst drive torque is multiplied!!

where is torque multiplied in an hst. is there torque converter?