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....