HST - How Does It Work?

[Harv]

Seems kinda late in the game for me to be asking this question, but it's mostly intellectual curiosity at this point. /w3tcompact/icons/clever.gif

In the <A target="_blank" HREF=http://www.tractorbynet.com/cgi-bin/compact/showflat.pl?Cat=&Board=buyjd&Number=99527&page=&view=&sb=&o=&vc=1>Trencher Attachment</A> thread, it's been stated that it's better to have HST for trenching operations 'cuz you need to go real slow. That got me to wondering a little more about the specifics of how HST does what it does.

From what I've read on this board in the last year or so, I've gathered that pushing down on the HST pedal is roughly equivalent to increasing the gear level that is driving the tractor. The farther you push it, the faster you go with less torque to the wheels. You let off and you slow down but increase your pulling power. Just like shifting to high gear and then back down to first on a manual transmission.

So my question is, what exactly is inside an HST box? If it indeed allows you to go slower (lower) than a manual low gear, does that imply that it is analog? Meaning there are no increments, but rather a true continuous range of operation? And if that is so, what is the actual mechanism?

Feel free to post pictures and diagrams. And maybe talk real slow...

 

[MossRoad]

My PT425 uses a variable volume pump for forward and reverse motion. Here's a quote from the owner's manual;

"The treadle is a gear shifter and not an accelerator; therefore, only partially depress when going up a hill because the full stroke may cause the engine to bog down."

What does that mean? I don't know either /w3tcompact/icons/tongue.gif

 

[MossRoad]

Here are two interesting links that I just found. I haven't read them yet, so I'll be busy for a while.

<A target="_blank" HREF=http://www.bru-hyd.com/getriebe.e.htm>The Hydrostatic Transmission</A>

<A target="_blank" HREF=http://www.fpef.org>The Fluid Power Educational Foundation</A>

 

[DFB]

Harv, from my manual the HST is described as providing " infinitely variable speed control" thru (in my case) two ground speed ranges, High and Low. There are four major components. The charge pump, valve block, piston pump, and piston motor. Power transfer is accomplished by displacement of a "swashplate" which is controlled by the operator's foot pedal. Moving the swashplate off the center vertical position starts oil flowing to a variable displacement piston pump which in turn rotates a fixed displacement piston driven motor which is gear coupled to the final drive. The greater the displacement of the swashplate the faster the vehicle will travel in the selected range.

Clear as mud right? /w3tcompact/icons/tongue.gif

As for decreasing the pedal travel to increase torque the manual describing my tractor's operation doesn't seem to specifically address that.

DFB

 

[MChalkley]

Harv - To add to what's already been said (and I haven't checked out MossRoad's links yet, but plan to), the term HST, which is really a misnomer that stuck, technically only applies to "closed-loop" systems, which means the hydraulic motor is connected directly to the hydraulic pump on both the input and the output sides. The only way oil can get out is via internal leakage into the cases of either, or through relief valves. The reason this is important is that, since "for every action there is an equal and opposite reaction", if more fluid comes out of the pump, as when accelerating, the motor has to turn faster. Conversely, if the motor is trying turn faster than the pump, as when decelerating, the motor has to "drive" the pump, which in turn has to "drive" the engine.

In HST transmissions, both the pump and the motor are piston designs, which are the most efficient, and highest pressure, types generally available. So why aren't they always used everywhere? They need to be kept very clean because they have a very low tolerance for dirt.

There are two ways to increase the output of a piston pump: increase the speed you're turning it, which is pretty self-explanatory, or increase the stroke of the pistons. The second factor is where the variable angle swashplate comes in. Picture a round steel block with holes drilled in it and pistons set in the holes. The back side of the pistons are against a plate (we'll call it a swashplate), that is set to the same plane as the end of the big steel block. If you now spin the block, the pistons don't do anything - they're just resting on the swashplate. But if you tilt the swashplate, the pistons have to move into the block where the swashplate is closest to to it, and they move out of the block where it's farthest away. The greater the tilt of the swashplate, the greater the stroke of the pistons and the larger the flow from the pump - also, the more hp it takes to turn it, of course.

The motor works the same as the pump, except that the fluid coming into it is spinning the block, instead of the other way around.

Also, in most HST transmissions, the swashplate in the pump can be pivoted over center in either direction. One way, the tractor goes forward - the other way, flow is reversed and it goes backward.

Usually, the motor's swashplate angle is fixed at an angle about half-way between its maximum and minimum angles. At least one CUT has a "high torque" mode that forces the motor to maximum displacement (maximum swashplate angle). On my EF-5, both the pump and the motor have completely variable swashplate angles - which gives it almost as great a speed range as a CUT without any mechanical gearbox (or "range selector"). Picture the pump turning with a very small swashplate angle - very little flow will be produced, and the motor turning with a very high swashplate angle - maximum flow is required to turn the motor, which results in a very low "gear" with very low speed and very high torque. On the other hand, if the pump is turning with maximum swashplate angle, producing the most flow it's capable of, and the motor is set to minimum swashplate angle, requiring very little flow to cause it to spin, you have the equivalent of a very high "gear", with high speed but with low torque. There is also an override on the EF-5 that forces the motor into highest-displacement high torque mode. So why don't all CUTs use variable displacement motors and do away with the range selectors? Cost is one factor. Also, motors aren't equally efficient at all swashplate angles so, while they can produce almost as great a speed variation as tractors with 3 ranges, they're not as efficient throughout the entire range. There's no free lunch. Personally, so far, I think I prefer the variable displacement motor to the gear range selector, because I never have to stop to shift, it's always in the right "range" depending on how fast the engine is turning and how much work I'm making the machine do, e.g. it will automatically select "low range" when I'm pushing into a pile of gravel by setting the motor to maximum displacement, then "shift" to high range as I carry the gravel away. One other disadvantage of the purely hydraulic approach is that you don't usually get quite as much range - my EF-5 has a top speed of 13 mph (measured), whereas the L4310 would go 16 mph.

Now that I've totally confused you, you can probably appreciate the links MossRoad posted a lot more. /w3tcompact/icons/smile.gif

 

[Harv]

<font color=blue>Now that I've totally confused you</font color=blue>

Wrong again, amsoil-breath! /w3tcompact/icons/grin.gif

Looking at a sketch in a Grand L brochure, DFB's manual, the animations in MossRoad's links and now your 2 cents (more like a buck fifty

), I think I'm finally getting the picture. /w3tcompact/icons/clever.gif

I had seen references to the "swashplate" in several places, but until I read your message just now, I couldn't quite figure out exactly what it was. I got the concept of the hydraulic pump and hydraulic motor, but it was your dissertation here that finally shed some light on how the fabled HST manipulates the speed and torque of the drive.

I'm happy enough with my ol' manual transmission for now, but I gotta admit, HST is a friggin' piece of genius. /w3tcompact/icons/cool.gif

 

[MChalkley]

Harv - Glad you got something out of it. Back when we used to have "HST vs. gear wars" /w3tcompact/icons/wink.gif, we discussed the principles of operation (among other things) a few times. Once, I think I remarked that I sure would like to be able to take credit for having designed the HST concept. I searched for that post and couldn't find it, but I did find <A target="_blank" HREF=http://www.tractorbynet.com/cgi-bin/compact/showflat.pl?Cat=&Board=nhown&Number=22838&page=&view=&sb=&o=&vc=1>this link</A>, in which we discussed some of the same things as here, and a few of us marvelled at the genius of the design. I guess one of the things that most amazes me about it is that I don't think it would ever have occurred to me to generate force in that way. A really radical concept. As you said, pure genius.

 

[TresCrows]

Here is a pic of a turbine engine hydro pump with variable displacement. Imagine two pumps, one fixed output and another variable driven as a motor from the fixed, then you have tractor HST.
J

 

[TresCrows]

Another pic of hydro pumps, one fixed and the other variable. The variable changes output by changing the angleof the piston body, a swashplate works in a similar fashion.
J

 

[TresCrows]

Another pic showing a variable hydro pump/motor with a swash plate.
J