Traction Power Trac Traction

[duane]

One of the previous posts regarding the comparison of Kubota vs. Power Trac traction makes for interesting discussion.

The Kubota provides power to the wheels through front and rear differentials.

The Power Trac provides power to each individual wheel through a individual wheel hydraulic motor with common
hydraulic pump. Torque is achieved by hydraulic pressure. If one wheel starts to spin, the hydraulic pressure at this individual wheel motor drops (flow increases) since it is spinning faster than the other wheels. Is the torque to the other wheels limited to the amount of pressure to the spinning tire?

Some of you guys that have seat time in your Power Tracs, please give us your experience. If you lift the two front tires off of the ground through front attachment down pressure, the front tires will spin in the air, but is hydraulic pressure still sufficient to move the tractor. Or has the free spinning front tires lowered system hydraulic pressure enough to prevent torque from being delivered to the wheels on the ground.

You guys that have PTs or Earthforce hydraulic driven tractors, try the above and report your experience. Maybe the Power Trac engineers have designed the hydraulic supply so that hydraulic pressure to each wheel motor is un-affected by the traction at the other wheel motors.

Chime in
Duane

 

[Charlie_Iliff]

Duane:
There are a number of threads on the board about the theory and function of hydraulic/hydrostatic transmissions & motors. I must confess I haven't studied enough to be able to explain what the Power Trac is really doing. I can say that I can put the edge of the bucket on the ground, lift the front wheels clear of the ground, and dig with the real wheels driving forward. When I visited Power Trac, that's how Mike demonstrated a 425 and a 1430, and how he recommended digging. (For the 1845, Terry put me on it and merely asked that I not dig up any of the asphalt road.)
When one wheel starts to spin, I have the subjective impression that the others have reduced torque, but that may be completely wrong. The feeling may just be because I now only have three pushing. I have never just observed, but always changed pedal pressure, steering, lift or downforce, or all of those.
I believe that even with three spinning, one with good traction will move the machine. I don't know whether the pump has enough capacity to keep the full 3000 psi to the non-spinning wheel, but certainly there is enough to drive it with some power.
I think all of us would be interested in comparing traction and pulling power among the various machines, in varying conditions. There are a lot of variables to consider, however, and a full-blown test program would be a formidable undertaking. Of course, if it were a TBN project, we'd never be able to agree on the protocol for the test. We'd all be afraid that some "other color" machine might win.

 

[MossRoad]

I was backgrading granulated limestone(moon dust) at the ball park with my wide bucket by tipping the leading edge down enough to lift the front tires off of the ground about an inch, then going in reverse. The two rear wheels pulled the machine along very well on hard dirt, while the front wheels just spun in the air. I have turf tires, so on grass, if I try this(and I have) the rears will tend to spin instead of pulling, but they are turning. And the neat thing was, because it is articulated, I could still steer!

 

[marrt]

This indeed is a very interesting question. I had the same thought when testing the PT. To scoop dirt, I would lower the bucket to the point (due to inexperience) that I would release enough pressure off the front wheels that they would spin and I couldn’t push the bucket into the pile of dirt more than 3 or 4 inches. Unfortunately, I did not think to look at the back wheels to see if they were also spinning.

If you really loose torque to the rear tires if the front tires are spinning, then I imagine one could rig up a valve to temporarily reduce flow to the front tires. What I would call “a poor man’s differential lock.”

 

[bubenberg]

posthole digger and sufficient strength in the hydrostatic distribution of power.
our soil composition is topsoil=clay, subsoil=heavier clay and a foot or so down sheal. since we have our 9 inch posthole digger for the pt we never have been stock, never shortened a post because of insufficient depth due to obstruction and have not restored to blasting yet. since we purchased our pt posthole digger we installed about 350 sections of fence.
in order to get through the above mentioned layers we have to apply pressure to the auger by lifting the front wheels. we have not seen any degradation in power of either the auger or the kohler engine.

 

[PTRich]

The best way to think of it is torque times resistance. If a tire has a lot of resistance 20 lbs of torque will turn the tire slower than if a tire has 2 lbs of resistance. The flow rate is guided by the pedal and the pressure rate is governed by the engine speed. That is one of the reasons letting off the pedal provides more pulling power. With a little seat time traction is not a problem, there is 2 variables controlled by the driver (engine speed, treadle speed) and 1 variable controlled by mother nature (actual traction surface). After you get used to the driver variables you can quickly change them to cope with mother nature. Keep in mind one of the good things about a tractor that articulates is with a turn of the steering wheel you can get a bit of fresh material under your tires.

PTRich

 

[king_black_shoes]

Greetings P.T.ers, I agree with Charlie and PT RICH. At first it seemed like one tire spinning made the others cut out or loose power. I had to stop thinking of the pedal as a throttle and ease up on it. Like Charlie I find that adjusting your steering and lift arms and backing off the pedal really helps. Sometimes on an incline I also increase the throttle lever a bit so that when my tires do grab I won't stall. My PT422 is the lightest of the bunch as I couldn't afford ROPS but I've been amazed with the traction. During mud season I had a large dump truck get stuck in my yard. After getting it out the area looked like a mud pit with 1 foot deep ruts from the truck. I was able to drive through the mess several times (with full bucket) without losing traction .One side of my septic mound has the maximum slope allowed by law (I won't get into how much slope or what percent grade......} . I'm able to mow straight up and over it without spinning or feeling like I'm about to roll over. It did take a lot of practice to dig without spinning all the time. When I can just barely see the tips of the bucket teeth I'm able to move forward and fill the bucket without much spinning. I press my right elbow against my side so I have a lighter touch on the stick. For me it seems like it's best to let the bucket get 2/3 full before I start to curl it back. Hey PT Rich I'd like to her your opinion on hydraulic fluid- vs.- 5w30 motor oil. Thanks, Tom

 

[PTRich]

A friend of mine runs a company that sells rock drilling equipment that uses a lot of hydralics. Using motor oil is a lot more common than I thought. Some pumps are designed for motor oil. A lot of variable speed pumps use motor oil. I would only put in what they reccomend, there is a big difference in viscosity.
PTRich

 

[kcook]

Some real Hydrostatic tractors us motor oil as well!

KCook

 

[MChalkley]

Duane, et. al. -

I don't own a PowerTrac yet, but know a little about them. I do own an EarthForce, of course, so I can dissertate in some detail on them. The two machines use very different means to transfer power to the wheels. So, with those facts in mind, here's my take on the issues involved. First, with respect to PowerTrac's:

PowerTrac machines use a piston pump to drive individual wheel motors, as everyone here knows. The key to understanding how they will function if one loses power is to know the type of circuit the PowerTrac uses and the properties of those circuits. As I understand it, from talking with several of the folks at PowerTrac, they use two series circuits, each driving two motors. I'm pretty sure the PowerTrac circuit drives the front and rear on the same side, and I'm certain that each circuit drives one front wheel and one rear wheel.

So, given that, a few thoughts on series circuit properties. Remembering that, in a hydraulic circuit involving a motor, flow in the circuit determines its speed, and pressure determines the torque, we can now generalize a bit. Since, in a series circuit with two motors in the circuit, all the fluid must flow through both motors in the circuit, you can guarantee that both motors will always turn the same speed because the flow through them has to be the same. However, the pressure generated in a motor, and therefore the torque it's producing, will always depend on the amount of resistance to rotation there is. So, you can quickly see that if one wheel in a circuit is completely off the ground it will continue to turn at the speed determined by the rate of flow, but will produce almost zero pressure because there's no resistance. Therefore, all the pressure produced by the pump is available to produce torque in the other motor in the circuit. By necessity, the wheel with maximum traction produces the most torque, which is a major advantage of using a series circuit.

As for how to produce maximum torque at the wheels, it boils down to how to generate the most pressure. You've got x amount of horsepower being produced by the engine. That amount of horsepower can product a certain amount of flow at a given pressure - the higher the flow, the lower the pressure that can be produced, and vice versa. If you want maximum torque, you need to produce the minium amount of flow to produce the minimum wheel speed you want, which will allow the pump to use the available engine horsepower to produce maximum pressure. It's possible, of course, to have enough horsepower at your disposal to produce maximum pressure at maximum flow, but this is not very practical in a tractor, because of cost, fuel consumption, size and space consideration issues, etc. So, yes, using the least amount of treadle pressure that will produce the wheel speed needed will produce the maximum possible torque, assuming the same engine speed, of course. Many pumps will not produce rated pressure except near maximum rpm, so the engine speed must be near maximum to produce maximum pump pressure.

Now, as to EarthForce machines: they use a hydrostatic pump and motor combination that is much more similar to CUTs (actually hydrostatic TLB's, but that's another story) than to PowerTrac's. There are some important differences as well. The EarthForce uses planetary drives in each wheel, for example, to greatly increase available torque, at the expense of speed, of course. It uses a variable displacement pump and motor, insead of just the pump (though some CUTs now use variable displacement motors now, too). Automotive type differentials transfer power from the motor to the planetary hubs. The differentials have a limited-slip mechanism in them to transfer power to the wheel that's getting the most traction, just like four-wheel-drive trucks use. Unlike the PowerTrac, or CUT-type hydrostatic transmissions, though, generating maximum torque at the wheel with the EarthForce is just a matter of mashing harder on the accelerator, because there's an automatic internally self-adjusting control system that balances pump flow and pressure based on motor pressure and speed (which in turn is determined by how hard the wheels are to turn, i.e. how much motive work you're trying to do). This is called an "automotive" speed control, using the literal meaning, and having nothing whatsoever to do with what we call "automobiles".

I hope this made a little sense. And that it's right. /w3tcompact/icons/wink.gif

 

[duane]

Mark, a lot of food for thought

When I get my PT tomorrow, I have a lot of studying to do.

thanks
Duane

 

[PTRich]

Sometimes you can make one wheel spin faster than the three other wheels under certain conditions when applying excess pedal power.
PTRich

 

[Sedgewood]

MChalkley<font color=red> As I understand it, from talking with several of the folks at PowerTrac, they use two series circuits, each driving two motors. I'm pretty sure the PowerTrac circuit drives the front and rear on the same side, and I'm certain that each circuit drives one front wheel and one rear wheel.</font color=red>

It's not really clear from the drawing in the manual but I get the idea that the front motors are in parallel with each other but are in series with the rear motors, which also are in parallel with each other. I should go look at the tractor to verify this some time.

It also feels in a seat-of-the-pants way like torque is reduced progressively with the number of wheels spinning and that increasing pedal increases torque (which can give one a surprising bit of acceleration if the tires "hook up" at full pedal). I'll have to give this some closer attention in future to see if these seat of the pants observations are indeed right.

 

[MChalkley]

Sedgewood - <font color=blue>It's not really clear from the drawing in the manual but I get the idea that the front motors are in parallel with each other but are in series with the rear motors, which also are in parallel with each other.</font color=blue> Is this the sort of manual that has removable pages you could fax? I'd sure like to see a copy of the diagram, especially if they've got a hydraulic schematic. I doubt it's a simple combination of parallel and series circuits. If it includes any sort of parallel circuit at all, most of the advantages of the series circuit are compromised. On the other hand, if there's priority valving or something like that involved, that would be different. But, then, it also wouldn't really be a parallel circuit anymore, either...

<font color=blue>It also feels in a seat-of-the-pants way like torque is reduced progressively with the number of wheels spinning</font color=blue> this could certainly be true, but I'm not sure how you could tell, unless it's from the wheels still getting traction not having enough power to turn <font color=blue>and that increasing pedal increases torque</font color=blue> this should definitely not be the case - it might give the appearance that torque is increasing, because of the centrifugal force of the spinning wheel, or something, but it should have less true torque. The only way I can think of that this wouldn't hold true is if there's also some sort of sensing circuit similar to the one in the EarthForce machines involved.

When I talked to the PowerTrac factory folks, one of them several times alluded to the "valves" that control the flow to the wheel motors. But, the "valves" I saw were simply distribution blocks. And when I asked the president of the company, who is also the founder and primary designer, if there was a valving mechanism in the "drivetrain" to control the flow to the motors, he said no, that it was series circuits. At least, that's my recollection of the conversation - not infallible, I'm afraid...

 

[Sedgewood]

MChalkley <font color=red>I'd sure like to see a copy of the diagram, especially if they've got a hydraulic schematic.</font color=red>See attachment. Sorry, no schematic, I wish there were one but this plumbing diagram is all they've put in the manual.

<font color=blue>It also feels in a seat-of-the-pants way like torque is reduced progressively with the number of wheels spinning</font color=blue><font color=red> this could certainly be true, but I'm not sure how you could tell, unless it's from the wheels still getting traction not having enough power to turn </font color=red>That is probably what my pants sense.<font color=blue>and that increasing pedal increases torque</font color=blue><font color=red> this should definitely not be the case - it might give the appearance that torque is increasing, because of the centrifugal force of the spinning wheel, or something, but it should have less true torque.</font color=red>This part I'm unsure of and need to observe more closely but it seems that the tractor finally starts to move after I've added pedal.

 

[Sedgewood]

MChalkley, I think now that I look at the diagram again that I have misread it. #19 is a return line and connects to #17, rather than the supply line I thought it was going from front to rear. So it looks like all wheels may be in parallel with some modifier at the #3 Splitter Block Manifold. Or perhaps front wheels are in parallel with each other and with rear wheels, which are in series. There's a fuzzy bit in the memory bank from the flurry of my visit to PT, something about series/parallel wheel motors that is innovative and gives traction to all four wheels.