HST, not an automatic according to some

[Doc_Bob]

HST, not an automatic according to some, but sure sounds like it to others. If I understand how an HST works, the harder you press on the “pedal”, the faster it goes because it is in a higher gear. The less pedal, the lower the gear. This means, that when pulling a heavy load, to move it in a lower gear, you don’t press on the pedal, but in fact, let up so that the HST can “select” a lower gear??? Is this right?

Bob

 

[dubba]

The HST on my JD (110 TLB) is more like a standard transmission with an infinitely adjustable torque converter. I have 3 ranges (gears). In any gear, by varying the pressure on the pedal, I can go from inches per hour to the full speed of the gear selected. The HST would rather I selected the proper gear for the job (low gear for heavy work). Though I tend to run my engine at ~2000 RPM for general loader work, the manual says to use a gear appropriate for the job, hold the pedal all the way down, then set the throttle for the ground speed desired.

 

[jinman]

</font><font color="blue" class="small">( The HST on my JD (110 TLB) is more like a standard transmission with an infinitely adjustable torque converter. I have 3 ranges (gears). )</font>

Dubba, that's as good an analogy as I've ever seen. Now, I'll let you answer all the follow-up questions on how/what makes a torque converter work. /forums/images/graemlins/laugh.gif /forums/images/graemlins/wink.gif

 

[RalphVa]

The BIG difference you'll notice in operating an HST is when going down hill. How many automatics will actually really hold you back and stop on a hill? The HST will. Let the pedal up all the way; it stops, at least in L range.

An HST is basically a hydraulic pump going through a variable valve regulated by the pedal. An automatic has vanes spinning in fluid that transfers the fluid action to another set of vanes maybe a millimeter away. Lots of fluid slippage and heat generation in that slippage space. A HST generates heat, mainly because of some inefficiency in the pump. Don't think there's hardly any slippage. That's why it'll actually stop you on a hill.

Ralph

 

[rtimgray]

I don't think that the HST is going to "change gears" in a a given range/gear. Most of the HSTs that I have seen and worked with only have 2 or 3 sets of gears. Beyond that, there is only a hydraulic pump that is connected to your final drive. The harder you push the pedal, the faster the pump goes, the faster the final drive moves - it doesn't change gears like an automatic transmission. At least, this is my understanding of the concept, but I was wrong one time before when I was a little kid and didn't know what I was doing.

 

[Bob_Skurka]

</font><font color="blue" class="small">( I don't think that the HST is going to "change gears" )</font>

I think you misunderstood DocBob's intent. He was making a simple statement that it is "like" changing gears. The hydraulic pump acts like a series of gears.

Think of a Jeep, a Touareg or Land Rover that has a true HIGH and a LOW range selector.

So the operator selects HIGH range, the HST pedal then acts like an additional gear selector within that range where barely pressing the pedal provides the most torque within the HIGH range and pressing the pedal to the ground provides the most speed but the least torqe. . . hence, while in HIGH range it is acting like a gear selector and pressing down all the way would be akin to being in 5th gear in a car, but barely pressing the pedal would be akin to being in 1st HIGH gear.

But if you shift the range selector to LOW range, then barely pressing on the pedal is like being in 1st LOW in a Jeep. And 1st LOW in a Jeep is nothing like 1st HIGH. Ditto a tractor. However in a Jeep there are actual gears. On a tractor the HST works wiht a pump to "simulate" gears.

 

[Doc_Bob]

</font><font color="blue" class="small">( </font><font color="blueclass=small">( I don't think that the HST is going to "change gears" )</font>

I think you misunderstood DocBob's intent. He was making a simple statement that it is "like" changing gears. The hydraulic pump acts like a series of gears.

Think of a Jeep, a Touareg or Land Rover that has a true HIGH and a LOW range selector.

So the operator selects HIGH range, the HST pedal then acts like an additional gear selector within that range where barely pressing the pedal provides the most torque within the HIGH range and pressing the pedal to the ground provides the most speed but the least torqe. . . hence, while in HIGH range it is acting like a gear selector and pressing down all the way would be akin to being in 5th gear in a car, but barely pressing the pedal would be akin to being in 1st HIGH gear.

But if you shift the range selector to LOW range, then barely pressing on the pedal is like being in 1st LOW in a Jeep. And 1st LOW in a Jeep is nothing like 1st HIGH. Ditto a tractor. However in a Jeep there are actual gears. On a tractor the HST works wiht a pump to "simulate" gears.



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Yes, my question was not as clear as it could be, but you got it it right. One thing about an HST that seems counter intuitive is the harder you push on the pedal (assume low range, FEL work, heavy load) the less actual effective work you are getting done.
Bob

 

[_RaT_]

"Yes, my question was not as clear as it could be, but you got it it right. One thing about an HST that seems counter intuitive is the harder you push on the pedal (assume low range, FEL work, heavy load) the less actual effective work you are getting done.
Bob "


I don't understand this statement. If you press harder on the pedal, you will turn the wheels faster providing you still have the power to do it. That would mean to me that you are or at least have the potential to effectively get the work done faster. If you do not have enough power from the engine to continue to drive the wheels at a given pedal position, by backing off you again are enhancing the effectiveness by now continuing to move the tractor. What am I missing? /forums/images/graemlins/confused.gif

 

[daTeacha]

Perhaps the best automotive equivalent of an hst is the new to the market CVT, or Continuously Variable Transmission. These have two pulleys of variable diameter connected by a belt. The computer of the car varies the size of the pulleys in response to input from the throttle position, engine speed, ground speed, and load or strain on the drivetrain. Depending on how you look at it, there are either no gears at all or an infinite number of them. The ratio between engine rpm and wheel rpm can be anything within the limits of the sizes of the pulleys, but there are no meshing teeth making things go around.

With the hst, the variation in ground speed is controlled by two basic things -- the position of the pedal and the speed of the engine driven the primary pump. The pedal controls how much fluid goes to a second "pump" wherein the fluid is making the hard parts move around as opposed to the primary pump where the hard parts move the fluid. There are bypass valves that keep the thing from blowing its seals (most of the time). Maximum efficiency is achieved when the minimum amount of fuel use results in the maximum amount of motion. Higher rates of flow from a fully depresed pedal generally do not produce maximum torque on the rear axle, just more speed, thus equating to the gears of a mechanical system.

Think in terms of a garden hose. Open the end of the hose completely and you will get lots of water at relatively low pressure. Close it down to a small stream and you will get more pressure.

I know this was far from a perfect explanation and analogy, but maybe it'll help someone.

 

[john_bud]

Hi,

There have been some interesting explanations of HST's, mind if I add mine?

Ok, there are 2 parts to an HST tractor transmission. One part is the range selector, it is usually, Low, Medium and High. Then there is the peddle (or peddles).

I think of it like a 10 speed bike. Really, a 15 speed --> one with 3 gears on the front where you peddle. The left gear selector, moves the chain on the front gears from the little one for going slowly up hills to the biggest one for going down fast. That is the same thing the range selector does for the HST - low for power, high for speed.

On the bike, the right side gear selector moves the chain on the back gears. Up is for the biggest gear, down is for the smallest gear. When peddling up a hill, you move the lever up for more power (easier to push the peddles) at a slower speed. After you reach the top of the hill, you move the lever down for higher speed (but you have to peddle harder). That is exactly what the HST foot control does. As you push farther down, the HST tries to make the tractor go faster, but the engine has to work harder.

In my experience, the HST peddle gets confused with a car's gas peddle. As the engine reaches it's max output, they instinctively push the peddle down thinking it will give more power to the engine. Going back to the bike example - imagine you are ridding up a steep hill and as you begin to reach your max output, you push the gear selector down for a higher speed gear. What happens? You stall out, fall over red faced and panting. What you should have done was to pull up on the gear selector to go to a high power, low speed gear.

jb