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?

 

[Fallon]

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

A HST is just a hydraulic pump directly connected to a hydraulic motor. Variable displacement pump so it can almost go instantly from 0 flow to full flow or anywhere in between. At 0 flow, the motors don't move, so they hold you at a stop. In practice they leak down a bit so depending on the gear the machine will creep a little bit.

Tractors have a clunky 3 speed transmission on them. Not actually sure if it's before or after the HST. Need to come to a complete stop to shift that tranny. No keep to stop to change speed within those 3 ranges. Some cheap machines out there only have a 2 speed tranny. Know a few machines like some mini TBLs that only have a 1 speed HST. I guess excavators & many skid steeres fall in that bucket as .

 

[ericm979]

The range transmission is after the HST on my Branson. The newer models have a four speed range trans with an H pattern shifter instead of the clunky in line shifter on mine. And it's got synchromesh so you can shift while moving. I think that applies only to the gear models though as the HSTs do not have a clutch. Being able to shift while moving would be really handy.

 

[Vigo327]

This is an old thread but going back to the beginning, if someone were to be curious about how an HST 'drives', the thing that probably most people can relate to is a riding mower. Most all modern ones are HSTs and essentially they function the exact same as an HST tractor, other than the aforementioned multi-speed trans that can be between the HST pump and the axle.

It's sort of ironic that newer tractors can have HST followed by a synchromesh gear trans, because if all the old gear tractors had synchros I'd barely see a need for HST in the first place. I know they have their strengths but im a car guy first and a manual transmission enthusiast and I for one don't mind shifting like mad even while doing FEL work with my little b6100. I just dislike not being able to change gears while underway except with extreme caution. Ive been moving ~120yds of dirt about 300-400 feet with my little tractors (gear b6100 & hst b8200) and I've been doing a lot of unsynchronized 2-3 upshifts while moving and getting that down. It's sort of fun as a challenge but if i could snap my fingers and put synchros in there i'd rather do that!

When im running the b6100 on this job it's L1+diff lock while pushing into dirt, then HR (high reverse), H2 to roll out, H3 when underway. So both gear shifting + range shifting + diff lock pedal going on. In the heavier HST b8200 it's just Low to push into dirt, High to travel and dump, and it doens't need the diff lock because its so much heavier (backhoe, filled tires etc). But the low-medium-high range shifter on the 8200 is so much more finicky and irritating to operate than both shifters on the 6100 that i don't enjoy using the HST tractor any more than i enjoy all the shifting of the 6100. So HST is not the only factor there.

 

[5030]

The downfall of all HST's (versus gear drive) is the parasitic power loss associated with the 'fluid drive' itself and why most require a frontal mounted heat exchanger, to get rid of the heat produced by the fluid coupling. Het produced in operation is wasted power.

Why I won't have one and prefer a hydraulic shuttle gear trans. Very little parasitic loss and residual heat produced.

 

[finn1]

The downfall of all HST's (versus gear drive) is the parasitic power loss associated with the 'fluid drive' itself and why most require a frontal mounted heat exchanger, to get rid of the heat produced by the fluid coupling. Het produced in operation is wasted power.

Why I won't have one and prefer a hydraulic shuttle gear trans. Very little parasitic loss and residual heat produced.

Not sure why people dwell on the power loss with a hst Transmission, especially on a light weight compact tractor that is traction limited. As far as I know, most manufacturers offer multiple power ratings within a frame size, but most customers, I think, based on what I see, chose the lower or intermediate power ratings, indicating that power isn’t a real issue.
For example, Kioti offers a CK2610, 3510, and 4010. Most of the ck series tractors seem to be, again, based on my observations, the lower powered 2610 or 3510. That tells me that power isn’t that big a deal to most. They are predominantly hst, too.

I personally have the 4010, and find that weight and traction are limiters, more than power is, with the hst.

The other thing is that most compact and even the larger utility tractors aren’t used much for pto work by the typical owner, especially since I don’t think the vast majority of owners are farmers whose primary income from working the land.

Even things like construction equipment are, for the most part, hydro.

 

[finn1]

The other thing to consider is the extinction of a manual transmission in light duty trucks and the trend towards automatics in medium duty trucks.

Manual transmissions were dropped from the Super Duty line, and eventually both competitors. It seems that the manuals cost more to build than an automatic, but commanded a lower market price, and has higher warranty costs, as well as lower torque capability, and lower resale value.

I suspect that the same is true with a non-hst tractor.

Medium duty trucks were at something like 60% market share automatic thirteen years Aho when I left the industry, and rapidly increasing.

 

[mikester]

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.

Some HST tractors DO shift. My M59/M62 has an electronic HI-LOW range lever that works for each gear.

So if I'm operating in 3-HI the HST auto shifts between LOW-HI depending on machine speed and throttle position. To answer the OP yes there is a slight but noticeable delay during shifting.

Skidsteers, CTL's, TLB's, wheel loaders all have some sort of HST shifting.

 

[finn1]

Some HST tractors DO shift. My M59/M62 has an electronic HI-LOW range lever that works for each gear.

So if I'm operating in 3-HI the HST auto shifts between LOW-HI depending on machine speed and throttle position. To answer the OP yes there is a slight but noticeable delay during shifting.

Skidsteers, CTL's, TLB's, wheel loaders all have some sort of HST shifting.

I don’t recall any delays when changing range on my CTL.

Does snap your head back, though.

 

[5030]

The other thing to consider is the extinction of a manual transmission in light duty trucks and the trend towards automatics in medium duty trucks.

Manual transmissions were dropped from the Super Duty line, and eventually both competitors. It seems that the manuals cost more to build than an automatic, but commanded a lower market price, and has higher warranty costs, as well as lower torque capability, and lower resale value.

I suspect that the same is true with a non-hst tractor.

Medium duty trucks were at something like 60% market share automatic thirteen years Aho when I left the industry, and rapidly increasing.

The main reason is today, the younger set don't have a clue how to operate a standard transmission. Had a devil of a time finding a puddle jumper with a standard transmission but I found one...finally.

I do know that now, if you get a CDL, it has to have a standard transmission cert on it, least here in Michigan.

I learned on an 18 speed twin stick and I'm good with a standard.

 

[finn1]

The main reason is today, the younger set don't have a clue how to operate a standard transmission. Had a devil of a time finding a puddle jumper with a standard transmission but I found one...finally.

I do know that now, if you get a CDL, it has to have a standard transmission cert on it, least here in Michigan.

I learned on an 18 speed twin stick and I'm good with a standard.

No, you don’t need to take your test with a manual to get a class A CTL in Michigan. You can take a test with an automatic equipped truck, but you will only be permitted for an automatic.

I doubt the same applies to a B license, since so many are now automatics.

Very few new cars are even offered with a manual anymore. They just languish on the lots because nobody wants them.

Same for light trucks. In fact, light trucks arguably led the way in eliminating automatics. The manual transmission is functionally obsolete in the marketplace.

And don’t blame it on the “younger folks”. My Bil is 68, my brother is 63, my meighbor is 72, the other neighbor is 79, none have ever driven a manual, at least not in the last forty years. I don’t think they’re atypical.