HST and Relief Valves

[paccorti]

Like any good tractor lover, I've been pouring over my service manual of late /w3tcompact/icons/smile.gif. In particular I've been reading about the Boomer HST. What a fascinating piece of engineering. Whoever developed the original HST concept did not lack imagination! In every sense of the word, an HST is an CVT (constantly variable transmission).

In any event. I was reading how my HST has a high pressure relief valve (2 actually: forward and reverse). It seems that for all the big boomers (TC 35, 40 and 45) the same relief valve setting is used: 5000 psi. This tells me that from a pushing (or pulling) standpoint, all three tractors are capable of exerting the same maximum force.

For example, say that you have three big Boomers all in low gear (TC 35, 40 and 45). Assume that all three had plenty of traction (no wheel spin). They would all have the exact same ability to dig with the loader, push a pile of dirt, whatever. In all three cases the relief valve is the limiting factor in the tractor's ability to push. I know that I've buried my loader bucket and while in low gear with LOTS of traction (ballasted wheels, heavy rotary cutter off the back, etc.) the tractor just kinda stops pushing at some point. The wheels don't spin, the engine may slow down but it dosen't stall. I assume that this is the HST relief valve doing it's job. If a larger Boomer came along (a TC 40) they (with similar traction) could not do any better. Their relief valve would pop at the exact same point.

Mind you, this is not a criticism of my big engine brothers. If you take any engine and gear it down enough (for torque) you'll find that the engine is not the limiting factor. It may be the relief value or traction or something else (the pile of dirt) that gives first. Obviously in higher gears or other situations (PTO) I may run out of power first before my big engine brothers. This is simply the observation that higher horsepower does not necessarily mean better digging ability.

Comments anyone?

Peter

 

[MChalkley]

Peter - As you say, the hydrostatic transmission is an incredible piece of engineering. Rarely do you find something with so many advantages, and so few compromises.

Regarding your assessment of available pushing force between the three different models: Assuming the transmissions are otherwise identical too (and I have no doubt they are), I'd suspect that if the transmission is properly matched to the tractors (and I have no doubt that it is) and you manage to increase the traction on all three to the point that wheel slip isn't a factor, you'll probably find that horsepower becomes the limiting factor in at least the smallest of the three and, most likely, the two smallest. In other words, the tractor with the largest engine would have enough hp to allow depressing the HST pedal further, where it's operating at a more efficient swashplate angle, so more motive force would be generated. With the smallest engine, attempting to do the same would just stall the engine.

MarkC

 

[TomG]

I think there is an unanswered question of how much a HST pump is capable of generating. For example, there is a relief valve in my 3ph in addition to the main system relief. The 3ph valve is set over 3,000# while the system relief is about 1000# less. The purpose of the 3ph relief is to provide safety relief against suddeen load shocks. On the other hand, the purpose of the system relief valve is to set the maximum operating pressure. I don't know if the hydraulic pump actually is capable of generating enough pressure to open the 3ph relief valve.

Anyway, I don't know the exact purpose of HST relief valves. If the purpose is similar to a 3ph relief valve, then the 5000# setting may have little to do with operating pressures generated by the HST pumps on different tractors. Power delivered to the wheels, of course, is limited by the maximum operating pressure of the HST pump, and different pumps may deliver different power irrespective of the relief valves. Maybe somebody knows the answer.

 

[paccorti]

I thought this would be a good discussion! Let me add some more comments.

An HST pump and motor are both positive displacement. A piston slides in a cylinder, fluid has got to move. Any given engine can have enough power to generate extremely high pressures in the right circumstances. To take an extreme example, if all of the available power of an engine drove a pump that has an extremely small piston diameter (say a pin head) then you could generate very high PSI. Obviously this is not practical... You would "pump" very little fluid. So it's not inconceivable that a pump could be "small enough" to generate sufficient pressure to pop the HST relief valve.

The question, of course, is does this actually happen? Well I think it does. We can all try this test. Put your tractor in it's highest gear range and set the engine RPM for max rated power. Now go find some situation that will stress the transmission (climb a steep hill, dig aggressively with the loader, whatever). If you have the situation where: the engine is running, you are pressing the hydro pedal, and the tractor is stationary something is bleeding off the pressure from that positive displacement pump. I contend that it is the relief valve. I mean what else could it be. This also implies that the tractor cannot be stalled (by HST alone) under most steady state circumstances.

Mark and Tom how does this fit with your experience? Have you ever had the situation where the tires are not moving and the hydro pedal is depressed? Where else would that pressure go or does your engine stall from the effort?

Peter

 

[MChalkley]

Peter - I would say you are absolutely correct in your assessment of the purpose and function of the high-pressure relief valve.

As for experiences, if I try to pull something that I can't move in a high-traction situation (e.g. on good solid ground with 2,000 pounds of backhoe on the back for extra ballast) in high range, the engine will stall. If I try the same thing in low range, the relief valve pops. I know that's what's happening because the temperature of the fluid going to the HST cooler goes up faster under this scenario than any other I've seen (I've got a temperature gauge with sensors in several places, one of them being the line going to the HST cooler). Also, if you have the shop manual and you look at the hydraulic schematic for the HST transmission, you'll see that this is the whole purpose of the high-pressure relief valve. There is a low-pressure relief valve, too, but it has a different function.

MarkC

 

[TomG]

Well, I'm a gear guy myself. Had to go used and gears to get the size and implements I needed with the budget I had. I'm slowly picking up HST concepts, but this will be mostly learning on my part. I'm raising theoretical possibilities here rather than making statements of fact. My purpose is to come up with ways of understanding how this stuff works.

First thing is that I didn't know that HST pumps are piston/cylinder types. I believe my hydraulic pump is a gear type, which I thought might also be a type used as HST pumps. I don't imagine that gear pumps are capable of achieving the same pressures as piston pumps, and a gear pump would reach a pressure where the oil would simply stall. However, I guess a gear pump still would need a safety relief since oil in the pump chamber would heat up in a hurry. So, one possibility for the question of where does the pressure go is that HST pumps are gear types, and the oil just stalls.

Somewhere I heard that vane type HST's do, but they would be very uncommon. But, it's another possibility. In a vane design, the oil would just stall but also probably would become hot.

There are two other theoretical possibilities for the question where does the pressure go that I can think of. The first idea was mentioned. If the pump is big relative to the engine, then the engine itself could be the pressure relief. A design is conceivable where the engine would stall in any gear and traction within pressure limits of the design. Since you both observe that the engine stalls in high, but not low, range, such a design doesn't seem likely.

Another possibility is that HST pumps are variable displacement pumps. I think they are, and the HST speed pedal operates by changing pump displacement. Conceivably, a high-pressure condition could revert a pump to a zero displacement condition. However, I don't think that's how things actually work, and the idea that the relief valve sets max operating pressures seems the best bet.

 

[paccorti]

Tom,

You have some good points there. Let me try to fill in the blanks. A gear type pump (as I understand it) is also a positive displacement pump. If the pump spins it's got to move oil (there is no orifice in the pump by which the oil may escape). It very much reminds me of a "roots type" supercharger. The kind of superchargers that sit on dragsters.

Oil can definitely stall in a vane type design.

An HST pump (at least for my tractor) is a positive variable displacement pump. Pistons do move (but there is no crankshaft or connecting rods). I'll explain in more detail if your interested.

I haven't had a chance to test all this thinking since I started the post but one interesting thought occurs to me. On my TC 35D the HST is connected directly to the engine. The range selection (hi/low) is next and then the rear differential. Engine->HST->Range Selection->Rear Diff. If my tractor is in high or low range and if it can't move the engine will not stall (cause the HST relief pops). Think of it this way, if the tractor is not moving (and the HST pedal is depressed) then hi or low the HST motor is not spinning. My relief valve must pop. In either case the the HST "feels" the same to the engine (cause the range is downstream of the HST).

Mark, I wonder if your HST is downstream of the range selection (engine->range selection->HST->rear differential). In that manner a high range would speed up your HST pump (as it should) while also making the HST "feel" like a taller gear (to the engine). That would explain why the relief valve could pop in low but not high (instead the engine stalls because of the taller "feel" of the HST to the engine).

Is this true?

Peter

 

[TomG]

I wish I had the language and felt 'educated' about these things. I'm not sure what a positive displacement pump is compared to other types. My grasp of gear pumps is impressionistic and based on seeing gears when priming electric fuel transfer pumps. I thought of them as forcing fuel along tapered gear faces into increasingly an increasingly smaller space to create pressure. Or, maybe trapping fuel in recesses on drive gear faces when they meshed with the driven gear. At any rate, my impression is that the barrier against back-flow would be less positive in a gear pump than in a piston/valve pump. I'm always happy to have my impressions cleaned up.

I've got to think this stuff through as well, and right now, I'm at a very basic level. So I'm thinking: How does this stuff actually work? The engine drives a variable displacement pump, which drives a HST motor. The motor output drives a variable ratio gear device and differential. The volume of oil pumped determines the HST motor speed, and the volume is determined by both engine RPM and pump displacement.

If I've got it right, I'm wondering if a comparison to hydraulic jacks holds up. Sort of like high school physics. The force applied by an output cylinder is related to the force applied to an input cylinder and the ratio of the input and output cylinder areas. Within this comparison, the hydraulic pump at low displacement would be equivalent to a jack with a small input cylinder. At a given engine RPM, the HST motor would turn slower than at higher displacements. Like a jack with a small input cylinder, each engine rotation would require less power, but more rotations are needed to move the tractor the same distance as at higher pump displacements. Boy, can I get basic, but I believe I'm starting to see the idea. The HST pedal is neither an accelerator nor a transmission, it is both.

Guess I'll keep the size of this down by just concluding that I'll digest this stuff and take a stab at relating it to HST relief valves. I suppose life would be easier if I was content to just use the equipment and never wonder how it works.

 

[MChalkley]

I think all tractor hydrostatic transmissions use piston pumps and motors. As far as I know, they are all connected directly to the engine, with any range selection gears being on the output side of the transmission, i.e. on the motor.

The pump and motor is considered to be a "closed loop" system in that any oil pushed by the pistons in the pump causes a direct and corresponding motion in the pistons in the motor. The displacement of the pistons themselves is fixed, but their output is varied by changing the stroke of the piston in the cylinder. There is a swashplate attached to the input shaft of the transmission (the shaft that transfers power to the pump). The swashplate rides on the pisons as it turns and its angle relative to the plane of the pump housing and the faces of the pistons is controlled by the HST pedal. When the swashplate angle is flat relative to the pump housing, the pistons do not move at all so no power is transmitted to the motor. When the HST pedal is depressed, the swashplate tilts, causing the pistons in the pump to be pushed in and out as it rotates, which transfers the power from the engine to the motor via the hydraulic fluid. The greater the angle of the swashplate the more the pistons move, the greater the amount of fluid sent to the motor, and the greater the amount of motion in the pistons in the motor and the faster the motor's output shaft turns. The motor's pistons also ride on a swashplate similar to the the one driving the pump except that it is almost always set to a fixed angle that does not change (though some low-speed torque advantage can be achieved via a variable-angle swashplate on the output shaft, but this increases the complexity and cost of the transmission). The motor generates rotational force using the in and out motion of the pistons in exactly the same way the pump does, but in a reversal of the process. Pretty ingenious.

So, Peter, to answer your question, the HST transmission in the Kubotas is also connected directly to the engine (through the clutch, of course). The reason the relief valves pop in high range but not in low in a given load situation is that the torque put out by the transmission is multiplied enough by the low range that it's sufficient to overcome the traction available, so the tires spin, whereas in high range, the transmission output torque is multiplied to a lesser extent, making it insufficient to spin the tires, so something else has to relieve the pressure - in this case it's the pressure relief valve.

MarkC

 

[TomG]

Thanks Mark. I read your note in detail. I might even save it and re-read it. I realize that, among participants of this board, I'm unusually driven to figure out how these things work. Becoming prisoner to a black box really does drive me wild. It's nice to have content here that sheds some light into my black box.

Years ago I encountered a black box in a government operation. A data system produced reports, and the reports determined the distribution of quite a few bucks. Since I used the reports, I asked questions, and found the incoming data went through a program that did some calculations and formatted the reports. However, the program designer and documentation had disappeared years before. Nobody knew what the program did. 'You're kidding,' I said. Didn't make myself too popular I guess. Some people just don't fit in.