Help me understand the HST

[skidoo]

I'm an electrical engineer and have a good grasp of mechanics. But, at present, I'm lacking sufficient detail in understanding of the HST. So, I am asking for the HST gurus to help me understand the following:

1) My 2520 works well for most scooping and engaging the rear blade while at 1500 to 2K rpms. It will either spin the wheels or bog the engine. So, when the rear blade digs too deep resulting in the engine to bog rather than spin the tires, what happens when the rpms are increased and then neither the tires spin nor the engine bogs, but the HST just whines louder?

2) In a condition like 1 above at max rpms, plenty of traction, with a deep ground engaging load resulting in no movement, how far can one press the go pedal (how much HST whine) and still be safe to proceed?

3) As above, if the engine is at max rpms, tires not moving, and the go pedal pressed hard (loud HST whine), where is the energy now being concentrated?

 

[DieselPower]

It's a Kubota web page but it gives a good description of the workings of different types of tractor transmission systems.

http://www.tractorsmart.com/main/Tractor Transmission Types.htm

Hydrostatic Drive. There is nothing new-fangled about this transmission. It has been around for a long time. Nevertheless, it is among the least understood of all transmission types. It is common for it to be compared with the automatic transmission in an automobile. They are different animals altogether. The only similarity is that they both use oil to transmit power. Tech types describe a hydrostatic transmission as being a variable-displacement hydraulic pump, driving a fixed-displacement hydraulic motor. Now, to the un-initiated, that's a bunch of gobblety-gook. So let's s t r e t c h that explanation out a little bit. Any hydraulic pump's sole purpose in life is to deliver some amount (volume) of fluid to some other device, which in turn moves whatever is attached to that device, causing work to be performed. This can be either through linear motion (as in a hydraulic cylinder), or rotary motion (as in a hydraulic motor). If we want to control how quickly our device moves (or rotates), one way we can do that is to vary the amount of oil that the pump sends to it. If our pump is delivering four gallons a minute, things will happen four times faster than if we only pump one gallon a minute. (Are you with me, so far?) Since we can control our pump's output, we can control the motor's speed. Now, unless we just want to go around in a circle all day long, we've got to figure out how to change directions. No problem. (You knew that, didn't you?) Most hydraulic motors don't care whether they are turned clock-wise or counter-clockwise. (They probably don't even know the difference!) Since a hydrostatic transmission operates in what is called a closed loop system, consisting of the pump and motor units, we simply reverse the direction of flow from the pump, causing the motor to operate in the opposite direction. Oh, by the way, closed loop means that a fixed amount of system oil is trapped, or contained, within the pump/motor circuit. Assume that a certain hydro transmission is of a size that it contains 500 ml of oil within the closed loop. That volume remains constant, never changing regardless of tractor speed or direction. So, for any change that we make in the output volume and/or direction of flow of oil from the pump section, a reaction must occur in the motor section, inducing a corresponding change in motor speed and/or direction. In reality, there is a certain amount of oil that is constantly escaping from and being replenished back into the closed loop section of the transmission. This is because a small amount of oil is allowed to flow all around the various components for lubrication and cooling purposes. The greatest advantage of a hydrostatic transmission is the ability to infinitely vary the ground speed and quickly change directions. It's like having a million speed transmission. If you need a travel speed of 1.200589 MPH, it is available. Another advantage is reliability. This transmission is, by way of design, pretty much self-protecting from operator abuse. Also, on foot pedal controlled transmissions, there is a built in safety factor in that you need only lift your foot from the pedal, to bring the tractor to a controlled stop. The only disadvantage of note is a slight loss of power at the PTO shaft. You must also remember to apply the parking brake should you park the tractor on a slope. Hydrostatic is, by far, the best choice for turf mowing applications or for any tasks that require constant speed and direction changes within a small area.

 

[VABlue]

Good explanation, thanks for posting it. I've never really thought about how it works, just accepted that it did. PFM...

In the case of the wheels not moving, I suspect that it won't hurt the HST. The metal parts should be designed to handle any amount of pressure the pumps can throw at it. It may get a little warm, though.

 

[Toolguy]

Maybe there is a pressure regulator adjustment that can be made. In Low range, with good traction, the engine should stall or at least bog down considerably before the wheels stop turning.

 

[Woody_NH]

Skidoo,

I'm another EE that had to do a little research to better understand HST operation. About a year ago there were some threads here that were very useful, suggest a search here and on the net in general, there are several good explanations out there. Swash plate pump 101 stuff.

As an operator, the fundamental concept to understand is that pressing down on the peddle increases the gear ratio. When encountering resistance that starts to bog the engine down our initial instinct is to push that peddle to the floor just like in a car or truck. Wrong choice as this will call for the high end of the gear ratio! You should ease up on the peddle to lower the gear ratio which is counter intuitive.

If your 2520 has “loadmatch” you should normally have it engaged. This is a closed loop control system that attempts to maintain the no load engine rpm that you have set. As you encounter resistance loadmatch will reduce the HST gear ratio to maintain rpm even if you have pushed that peddle to the floor.

 

[skidoo]

Thanks so far.

These answers make sense. Except that it does not match with my experience with my 2520. No, it does not have a load match.

If I engage the rear blade too far, and with the rpms near 2500+, no mater how little or how far I press the pedal, the wheels do not turn. The engine does not bog down. But, the HST whines louder as I press the pedal more. I am afraid to press the pedal too far because my HST was just replaced.

If you have a 2520, does yours function like this?

In theory, I should be able to just barely touch the pedal (very low gear ratio) and it should turn the wheels. Maybe it does, but too slow to notice. Eventually, with the higher gear ratio (more pedal depression), it should bog the engine. It doesn't, or I am not pressing far enough. The HST just whines louder and there is more resistance to pressing on the pedal.

Under this condition, can you press the pedal to the floor without fear of damaging the HST? If it does not bog the engine before reaching fully depressed, what does that mean for the HST?

 

[Mickey_Fx]

Gear ratio is an inproper term in discussion of HST. As in the article, HST use a varable displacement pump. Displacement can vary infinately between zero and max capacity and NO gears are invloved. Since fluids are not compressable, the design has to include a pressure relief valve in the design to prevent exceeding the max pressure limits the engineers have deemed appropreate.

Any time there is no turning of the wheels and engine is running and the pump is displacing fluid and nothing is broke, the pressure relief valve has come into play. Fluid will be dumped internally as long as the pressure exceeds the set limit. Tis possible something is wrong if pressure relief valve if opening sooner than specified set point.

If all is working correctly, no damage should happen when max pressure is exceeded.

 

[skidoo]

That sounds reasonable.

Except that if I press just a little, no wheel spin, no engine bog. Is the fluid being dumped at this point but just dumping a very low volume?

A little further, same, but more volume?

If I press all the way with max fluid dump, that would be a lot of energy to dissipate concentrated in the valve I assume. How long can this be done before damage occurs. i.e. 1/2 sec, 1 sec, 5 sec, etc...?

 

[dieselmotorhead]

I have a 4600 with HST. John deere may have given your machine more protection than mine. If I have good/excellent traction and high load I can stall the engine. I have even had the engine run backwards at one point. Very bad as the oil pump was sucking oil not pumping it. I am hoping it was not hurt too bad. I have started to learn to ease up and not mash the pedal when the load is high. The odd thing is there is a hydralic actuator that sets pump stroke. The engine needs to run to lower the pump stroke, not a problem except when stalled out like above it will still try to move on restarting for a fraction of a second.

 

[Chris2520]

I have a BB2060 on my 2520 and if i sink the scarfers in the lowest position in the ground in hard clay. yes it will stop you cold. in low range. if i lay down on it. yes it will whine. but it will spin all 4 tires. i suppose i must of never hit the relief valve. i have been in situations where i almost killed the engine. but never layed all the way down on the peddle at 2500 RPM's and not spin the tires. i guess some relief valves are set a little different. if i stick it. it always spins the tires before it bogs down. at that point i know i am to deep and lift the blade a bit. and keep on digging....

 

[SFish]

Skidoo:

Something is wrong if you can't spin the wheels with the pedal depressed a small amount under the conditions you describe. I would talk to the service rep at the dealer.

Steve

 

[texasjohn]

Skdoo, having read all the above, including your careful description of your concern, I, too, agree... something isn't working right...you should be seeing some sort of effort to turn the wheels and hear engine effort increase/bog down when just slightly hitting the HST pedal when using its lowest manual gear. Time for a call or taking it to your repair guy...

you said it has a new HST... I'm hoping that you have a warranty....

actually, yours is the first time I've heard of anybody on TBN saying that their HST has been replaced.

 

[skidoo]

As I mentioned, the HST was replaced just last week under warranty. The Dealer was excellent and checked it out thoughly by working it and getting it warmed up. The reason it was replaced is because after it warmed up, it lost a lot of power to the wheels. I am trying to understand what was going on or how to prevent any potential damage.

So, I am just afraid to press the pedal all the way down at high rpms and heavy load until I understand that this will not damage something.

It is definitely pulling much better than before and I don't think there is anything wrong with it now. But, I need to know what is normal and what is not.

So, let me ask the questions a bit differently.


For discussion purposes only, if one was to somehow lock the wheels in place so they could not move, then if at 2500 rpms, if the pedal was pressed just slightly, what happens? Would the engine bog? I believe not. So, one applies more pedal, the HST whines some more, but the engine does not bog. So, one applies even more pedal. The HST whines even more and then maybe the engine may exhibit signs of slowing. At lower rpms, the engine would bog at less of a pedal depression.




Chris, when you say you lay on it, how far do you press the pedal? Do you feel confident that you can press the pedal to the max if you are engaged with a heavy load? No fear of damage?

 

[Chris2520]

As I mentioned, the HST was replaced just last week under warranty. The Dealer was excellent and checked it out thoughly by working it and getting it warmed up. The reason it was replaced is because after it warmed up, it lost a lot of power to the wheels. I am trying to understand what was going on or how to prevent any potential damage.

So, I am just afraid to press the pedal all the way down at high rpms and heavy load until I understand that this will not damage something.

It is definitely pulling much better than before and I don't think there is anything wrong with it now. But, I need to know what is normal and what is not.

So, let me ask the questions a bit differently.


For discussion purposes only, if one was to somehow lock the wheels in place so they could not move, then if at 2500 rpms, if the pedal was pressed just slightly, what happens? Would the engine bog? I believe not. So, one applies more pedal, the HST whines some more, but the engine does not bog. So, one applies even more pedal. The HST whines even more and then maybe the engine may exhibit signs of slowing. At lower rpms, the engine would bog at less of a pedal depression.




Chris, when you say you lay on it, how far do you press the pedal? Do you feel confident that you can press the pedal to the max if you are engaged with a heavy load? No fear of damage?

I have done it many times.... and if has to big a bite i cant dig. if you put it on the mat. it will spine all 4 tires. yes it will whine. your loading it. dont worry about the whine. All HST's whine even large skid steers. lets put it this way . it has a releaf valve in the system. if for some reson you did have enough load on the machine to put the peddle to the floor and it not spin. the relief valve would kick in. thats what protects the system. I have never hit this. as i dig in dirt. and not pull something that big. i have the same box blade you do. and i have stopped the machine cold. but at 2500 if i put it on the mat. it will spin.

Now let me tell you what you are experenceing...
It was a defective HST from the factory. we know that. Right?
So now you are gun shy. and dont want to mash on it. for fear of it breaking. it ant going to break. take it from someone who has been building 700 inch pro stock motors for years. and thay break. thats part of raceing. after it breaks. you fix it. and you got to get back on the horse. and poor the coal to it.

Here is my take on the deal. people gave me crap for lifting a 2000 pipe bender. I want to see what it can do now. Yes i take care of it. clean and grease it. but i run it like a house on fire. because its under warrenty. if its going to let me down , i want it to do it now. Now i dont mean abuse it. you do haft to use common since. but run the machine the way it was made to be used. i asure you. if JD made a HST that broke everytime it was over loaded. thay would be out of bissiness.....

So to finnaly answer your question. how far would i push the peddle down ?
at operating RPM i would push it all the way to the floor. it ant going to do but 2 things. bog down and hit the relief valve. witch means choke motor or no tire spine. or 2 just break tires loose and spine. eather one ant going to hurt it. then you back up raise blade or whatever your doing , and start again . with blade not so low this time....
Sorry for the long explaination. i just wanted to explain my own experences of what i have found out by running the machine.
and i dont abuse it. and i dont baby it eather.
keep in mind it was built to do what your doing with it...

Hope this helps

Chris.....

 

[VABlue]

After pulling over 1000# with an 18HP Craftsman HST lawn tractor, I have to agree with Chris - you're not going to break the HST. And if you do, JD will fix it again...

 

[SFish]

I agree that hitting the bypass shouldn't hurt the transmission, that is what the bypass is for. However, I would not do this for more time than required to determine that it is not going to go.

Skidoo, I am a little concerned about the notion of putting the pedal to the floor to make the tractor go from stop with a load. The hydrostat pedals are not llike a throttle and produce high torque/low speed at a small amount of depression, and low torque/high speed at full depression. Pushing the pedal to all the way when starting with a heavy load, without loadmatch (automatically adjusts wheel torque to the set engine RPMs), is like trying to start out in your car or truck in 4th or 5th gear.

When starting out under load you want the equivelent of automotive low gear, which is a small amount of pedal. If the tractor is working properly and hooked to an immovable object the tires should spin. I could recreate what you describe with my rig by lifting the maximum with the loader and the maximum with the 3 point to maximize traction, and then trying to pull the immovable object. This would cause the transmission to bypass at even low pedal depression. The only way I have made my tractor bypass and stop without wheel spin under "normal" conditions is going up a steep hill in high range (C range on my 3320). Get your tractor checked out by a pro.

Steve

 

[dynasim]

What is happening is the relief valve on the pump outlet(motor inlet) is relieving. At lower rpm's, the engine does not have enough power to increase the pump pressure to its the maximum, and the engine quits. This assumes that, at lower rpms, you push the pedal in significantly to engage the pump.

At higher rpms, the engine can provide the power to turn the pump and increase the pressure to the relief capacity at the normal initial pedal position.

I would bet that, if you just barely pushed the pedal in at lower rpms, you would get the same symptoms as you get at the higher rpms with normal pedal pushing.

I can lock my brakes, and barely push the pedal in on by NH TC45D, and the tractor will not move, but will whine. At full engine speed, the transmission does not have the capability(on mine) to stall the engine.

At low rpms, the tractor will stall, because I am used to pushing he pedal in enough to move the tractor along.

Chris

 

[skidoo]

Great feedback!

Thanks Chris. It is good to know from your experience with the exact model too.

SFish, I am not quite convinced yet that the simple analogy with low gear ratios are accurately representative. It does not quite fit with actual operation as described by Chris, nor quite like mine...

Does an increase in rpms increase the pressure available from the pump?

Does an increase in pedal travel increase the pressure/flow to the load?

How does the pedal (control valve) control pressure or flow through the system? Does it drop pressure across it, i.e. say the pump has a pressure of 500 PSIG. Then with no pedal depression, there should be 0 PSIG across the load. Then, say with half pedal depression, may there be approximately 250 PSIG across the load? And, if so, then is half the energy (heat) being dissipated in the control valve? Or does it use a more efficient method of energy control?

What is the source of the HST whine? Is it fluid flow through the control valve, or through the bypass, or is it due to the total energy going through i.e. pressure/flow?

Thanks for the previous responses. I think I will now go out and try a few experiments...

 

[dieselmotorhead]

If the RPMs are higher then more flow is available. But the HP available is what controls. Flow is the key. The resistance of the hydraulic motor is what makes the pressure. The variable output pump varies the flow. Flow times pressure is power. So when resistance ( pressure) times flow exceeds what the engine produces it will bog down. If the flow is reduced the pressure does not change but the speed slows down, lowering power needs. The pump is just a series of piston pumps rotating with one end riding on a staionary circular plate. At 0 pedal the plate is flat. As the pedal is depressed an actuator tilts the plate. Now all the pistons move back and forth pumping oil. The more tilt the more oil flow. This flow goes to a motor with a similar setup of pistons with a fixed stroke. Therefore it is infintely variable and the only waste of power is the headloss and friction of the pumps and motors.I don't know what the whine is.

 

[SFish]

What Dieselpower posted and Dieselmotorhead stated (two diesel guys) explains how the transmission works. With a small amount of pedal the pump pistons (or equivelent) move a shorter distance and thererby produce less flow at a greater pressure and more torque to the wheels. At full pedal it is pumping the maximum amount of fluid and the trade off is less pressure at the same engine speed and torque. If you have Motion Match the pedal equivelent is adjusted to keep the engine RPMs up. Without Motion Match (or turned off), flooring the pedal under a heavy load will stall the engine down because it is unable to produce the pressure to move the wheels.

A hydrostatic transmission is a variable ratio transmission with the ratio between engine RPM and wheel speed being minutely adjustable. A gear drive transmission has variable ratios, but only by way of a series of steps in gear combinations. One major difference between the two is that the pumps and motors of a hydrostatic setup are not necessarily based upon straighforward one-to-one fluid displacement and there are inefficiencies that result in fluid leakthrough. This is one reason why a tractor with a hydrostatic transmission is rated at slightly less torque to the wheels than the same one with a gear transmission.

I would really like to hear from someone here who has worked on these pumps and motors to explain the principles they work on. It has been a long time since I looked inside one of these things.

Steve