Help me understand the HST

[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