PT AL-629 ( can it be done ? )

[BobRip]

KentT, we need to know the grade of the slope. I have similar problems to yours when I am on a 25 Degree slope. Other than that I have no problem getting around, except when stuck in the mud. You may need a new tram pump, or something. Let's fix the wear problem first (if ther is one) and then redesign if needed.

 

[KentT]

Bob,

Personally the grade of the slope isn't the issue with me -- I keep the piece of property whether the PT works on it or not...

I find it hard to believe that my tram pump is worn out in 250 hours -- when all indications are my machine was serviced quite well by PTRich, the previous owner. I say 250 hours, because it has performed the same way as long as I've had it...

Again, I could perhaps understand that I may need to adjust a relief valve or something to get optimum performance. But, where is that valve? On the pump itself? According to the part number we have, it isn't in the wheel motors, so it must be on the pump... Further, according to others here their performance on slopes is also "marginal" when the oil is hot, so I'm not sure that tweaking a relief valve setting would really solve the performance issue. If it would, why did Tazewell go to different wheel motors to increase torque? Seems to me that the designed performance for the machine, as is, is marginal for the conditions I'm trying to use it in.

Though, like Bob999 stated, I don't think I'm exceeding 25 degree slopes -- which is the performance limitation on the engine's oiling system. I think I could safely operate on these slopes if I had enough low-end torque to do so...

 

[BobRip]

Bob,

Personally the grade of the slope isn't the issue with me -- I keep the piece of property whether the PT works on it or not...

I find it hard to believe that my tram pump is worn out in 250 hours -- when all indications are my machine was serviced quite well by PTRich, the previous owner. I say 250 hours, because it has performed the same way as long as I've had it...

Again, I could perhaps understand that I may need to adjust a relief valve or something to get optimum performance. But, where is that valve? On the pump itself? According to the part number we have, it isn't in the wheel motors, so it must be on the pump... Further, according to others here their performance on slopes is also "marginal" when the oil is hot, so I'm not sure that tweaking a relief valve setting would really solve the performance issue. If it would, why did Tazewell go to different wheel motors to increase torque? Seems to me that the designed performance for the machine, as is, is marginal for the conditions I'm trying to use it in.

Though, like Bob999 stated, I don't think I'm exceeding 25 degree slopes -- which is the performance limitation on the engine's oiling system. I think I could safely operate on these slopes if I had enough low-end torque to do so...

I agree that it is marginal and that is exactly why they went to bigger wheel motors. I was just hoping that a minor fix would help. The pump can wear very quickly if dirt gets in it. I have gotten stuck in ruts on my property and was very disappointed until I learned to wiggle my way out and I do understand the need for more torque. Many times around here and other places I have seen the urge to redesign (we must all be engineers at heart) when fixing would have worked. I have seen many engineers do this also.

I looked on the hydraulic diagram for the tram ciruit, and could not see a relief valve in it. If its there it may be on the pump. Call PT and ask.

 

[pah]

Kent
I have a 422 with 412 hrs. I have 20 acres of rocks trees and hills here, many of the hills are similiar and some are a little more dramatic than yours. When cool and for the first hour or so my machines performance is great. I can mow up my steepest hills no worries. When hot performance is still adequate. Looking at the pictures of your hills I don't think I'd have any trouble moving materials up same with my machine hot or cold. Your description that you can bog down and kill the engine while the oil is cool is troubling and completely contrary to my experience. I'm with JJ Check out your drive system pressures. If it was me I'd take it to a good hydraulic shop and make sure the existing components are ok before modifying anything. good luck

 

[toy]

I just realized there were questions here that I had not answered:

JJ -- I am not sure that the tram pump is pumping to spec, for two reasons (1) I don't know what those specs are, and (2) this machine only had 250 hours use on it when I got it, and is up to about 370 hours or so now, and I've never considered/had the need to hook up gauges to it...

Bob999 -- It exhibits some symptoms of both descriptions you gave. It will bog the engine down to the point that it will actually kill the engine when the oil is cooler. The only way to avoid this is to back off on the treadle, and ease back down on the treadle, trying to strike a balance that doesn't choke the engine. When the oil gets hotter, it doesn't bog the engine down to the point of killing it quite so easily, but it just loses torque. As I've said, I've had to combine "feathering the treadle up and down" with waggling the PT to climb up that back driveway more than once when loaded pretty heavily. I have gotten it into situations in the woods, when hot, where all it would do is whine, and I'd have to waggle my way out of the predicament... So, I think I'm seeing some symptoms of both. I'm hoping that by replumbing it to parallel, that I shift the torque curve enough that it will not be as prone to bogging the engine down, and I can then maintain enough RPM, speed and momentum to climb these hills and not get into situations where the wheel motors are bypassing -- if that is what the whine is coming from....

Note also that if the part number that SnowRidge provided for these White wheel motors (400230W31222AAA) is correct, then it is 14.2 ci motor WITHOUT relief valves. So, if it is bypassing, where is it bypassing?

http://www.whitehydraulics.com/pdf/catalog/UScat04_ce.pdf

If the machine is stalling out the engine on the hill could your problem be with the throttle cable linkage not opening up the throttle all the way because your hydraulics is demanding more than the engine is putting out. The releive valves are factory set but I have wondered why don't the relief valve open up before the engine stalls out it seeems like it should.

 

[KentT]

I want to close the loop on this topic also, since it may mislead someone in the future. I've abandoned the plan to replumb my PT-425s system from 2 series circuits to 4 parallel circuits... not because of any theoretical issue in doing so -- I still think that replumbing would double the torque and halve the speed.

I've had to abandon it because I do NOT have White CE Model 230 14.2ci wheel motors that are rated for up to 3000 PSI. Instead I have White RS Model 141 12.5ci wheel motors that are rated to 1500 PSI (continuous). My old wheel motors would not handle doubling the pressure in the system. I am now looking at installing larger displacement wheel motors (18ci or larger) that can increase the torque without replumbing the system.

 

[Bob999]

I want to close the loop on this topic also, since it may mislead someone in the future. I've abandoned the plan to replumb my PT-425s system from 2 series circuits to 4 parallel circuits... not because of any theoretical issue in doing so -- I still think that replumbing would double the torque and halve the speed.

I've had to abandon it because I do NOT have White CE Model 230 14.2ci wheel motors that are rated for up to 3000 PSI. Instead I have White RS Model 141 12.5ci wheel motors that are rated to 1500 PSI (continuous). My old wheel motors would not handle doubling the pressure in the system. I am now looking at installing larger displacement wheel motors (18ci or larger) that can increase the torque without replumbing the system.

It seems to me that the peak pressure in the hydrostatic drive system in the PT would occur when no wheel motor is turning--with the result that there would be very little flow in either circuit. This further suggests that each circuit would see the same peak pressure and that pressure would be approximately equal to the peak pressure developed in the variable displacement pump circuit.

To put it differently--the fact that PT uses wheel motors rated for a maximum pressure of 1500 PSI suggests that is the maximum pressure developed by the variable displacement pump. The limit may be achieved by a pressure limiting valve or it may reflect the maximum output of the variable displacement pump.

 

[KentT]

It seems to me that the peak pressure in the hydrostatic drive system in the PT would occur when no wheel motor is turning--with the result that there would be very little flow in either circuit. This further suggests that each circuit would see the same peak pressure and that pressure would be approximately equal to the peak pressure developed in the variable displacement pump circuit.

To put it differently--the fact that PT uses wheel motors rated for a maximum pressure of 1500 PSI suggests that is the maximum pressure developed by the variable displacement pump. The limit may be achieved by a pressure limiting valve or it may reflect the maximum output of the variable displacement pump.

If that were the case, as I understand it, then each motor in the two series circuits would be producing only the power equivalent of 750 PSI -- 750 PSI to the front wheel and 750 PSI to the rear wheel. That means these 12.5ci motors would only be producing a maximum of 1386 in lbs of torque, done at a 2gpm flow, yielding 34 RPM. That's only 115.5 ft lbs of torque -- surely that is not the case.

According to the HP formulas at Surplus Center, at 750 PSI and 8 gpm, each of these wheel motors would only be putting out 2.845 HP. Multilply that times 4 and the 25-HP PT would only be putting 11.84 HP to the ground -- running full throttle. I don't think so...

 

[duane]

When tramming forward, hydraulic oil flows first to the front wheel motors and then to the rear wheel motors. In reverse, the hydraulic oil flows first to the rear wheel motors, and then to the front wheel motors. Full tram pressure is available to the first hydraulic motor in the series (front or rear depending on tramming direction), and remaining hydraulic pressure depending on pressure/torque consumed by first motor in series goes to the second motor.

 

[KentT]

When tramming forward, hydraulic oil flows first to the front wheel motors and then to the rear wheel motors. In reverse, the hydraulic oil flows first to the rear wheel motors, and then to the front wheel motors. Full tram pressure is available to the first hydraulic motor in the series (front or rear depending on tramming direction), and remaining hydraulic pressure depending on pressure/torque consumed by first motor in series goes to the second motor.

Duane, the following is how I understand it. If I'm wrong please help me understand.

The first wheel in the series is exposed to the full output pressure of the pump-- but it doesn't produce either torque or RPM based upon the full prssure of the pump, because it also has the back-pressure of the remaining wheel in the series against its return line. The first wheel motor produces the torque and RPM based upon the pressure of it's supply line less the PSI at its return line caused by the second wheel motor. The second wheel motor produces torque and RPM based up the pressure at its supply line less the pressure at its return line, i.e. the remaining resistance in the circuit as the oil flows back to the pump.

It helps if you think of the pump as the source of flow (gpm) and the wheel motors as the source of resistance to flow (the pressure). The pressure in the system is caused by the wheels resistance to movement, in addition to the small amount due to friction as the oil is confined by hoses, valves, connectors etc.

Note that I mention RPM in the above discussion simply because of inefficiency in the system. RPM is theoretically a function of flow (gpm), but RPM is also effected by PSI because at higher pressures more fluid will "leak" by the rotors, vanes and/or pistons of the motor. The system isn't 100% efficient. For example 6 gpm flow on the 12.5ci wheel motors produces 72 RPM at 750 PSI, yet only 68 RPM at 1000 PSI and 60 RPM at 1500 PSI. The difference is the "leakage" which increases at higher pressures, in addition to the small amount of increased resistance to flow in the lines and fittings themselves. BTW, leakage is inluenced by how thick the oil is -- its viscosity -- the thicker the oil the less leakage.

So, the first wheel motor in the series must be able to withstand internal pressure (at least on its inlet side) that is equal to the full pressure of the pump. However it also has the outlet pressure of the second wheel motor in the series. It operates at the "Delta PSI" in producing torque and RPM, because of the outlet pressure.

That's how I understand the system, and the impact of a series circuit. In a parallel circuit, each wheel motor would operate at the full PSI of the pump, but would only get 1/4 the pump's gpm output instead of 1/2... that was the "theory" behind the Stray mod.

BTW, Char-Lynns performance charts for their wheel motors has this definition in the preceding specs:

Delta Pressure: The true delta BAR (delta PSI) between the inlet port and outlet port.

They also the define Maximum Inlet Pressure for the S series motors as:

172 Bar (2500 PSI) without regard to the delta Bar (delta PSI) and/or back pressure ratings or combination thereof.

So, it appears the Char-Lynn S series housings are rated to 2500 PSI while the specific motors I looked at are rated for either 1500 PSI continuous (18.2ci) or 1300 PSI continuous (22.7ci).

Their S-series seals are rated to 1500 PSI, which is why I spec'd the High Pressure Seals option, which is rated at 2500 PSI for the S series. If these wheel motors were not in a series, I would not need to be concerned about this max inlet pressure nor the seals to withstand it.

Bottom line is that the 25HP gas engine isn't large enough to produce 16gpm and 3000 PSI at the pump. That would require 27.8HP, assuming 100% efficiency. I think it likely produces 16gpm (or something close to that) but only something in the 2500 (or a bit more PSI) range, due to inefficiency. (Of course, pressures can intermittently spike higher.) That means the wheel motors are actually operating at less than 1500 PSI continous, likely something laround 1250-1300 PSI, yet the first wheel motor is exposed to twice that amount on its inlet side.

 

[Bob999]

If that were the case, as I understand it, then each motor in the two series circuits would be producing only the power equivalent of 750 PSI -- 750 PSI to the front wheel and 750 PSI to the rear wheel. That means these 12.5ci motors would only be producing a maximum of 1386 in lbs of torque, done at a 2gpm flow, yielding 34 RPM. That's only 115.5 ft lbs of torque -- surely that is not the case.

According to the HP formulas at Surplus Center, at 750 PSI and 8 gpm, each of these wheel motors would only be putting out 2.845 HP. Multilply that times 4 and the 25-HP PT would only be putting 11.84 HP to the ground -- running full throttle. I don't think so...

12 HP to the ground doesn't sound too far off the mark to me. When the machine is used as a mower a lot of power goes to the PTO circuit for the mower. When all the losses are factored in I wouldn't expect a lot more power to the ground.

However, I take it that we are both guessing--I know I am--I still think it would be to your benefit to actually measure the pressures before investing big money in the project.

 

[KentT]

12 HP to the ground doesn't sound too far off the mark to me. When the machine is used as a mower a lot of power goes to the PTO circuit for the mower. When all the losses are factored in I wouldn't expect a lot more power to the ground.

However, I take it that we are both guessing--I know I am--I still think it would be to your benefit to actually measure the pressures before investing big money in the project.

That 12HP would be WITHOUT the PTO or steering/lift circuits robbing any power from the engine. That would be the max available HP at the wheels, regardless of the other constraints.

Pressure is not really generated by the pump, per se -- pressure is generated by the resistance to the wheel motors turning as a volume of fluid goes through them, in addition to the small percentage created by hoses, connections, etc. The pump is pumping a volume of oil, which happens to be correlated to pressure because it is to easier to pump against lower pressures. The more resistance to movement of the wheel motors the more the pressure -- to the point that the pump can no longer push that volume of oil through the system.

The only way I can see to measure the pressure would be to put the PT up against an immovable object such as a tree, and running wide open, measure the pressure at different treadle settings. That would be pretty difficult to do...

 

[Bob999]

The only way I can see to measure the pressure would be to put the PT up against an immovable object such as a tree, and running wide open, measure the pressure at different treadle settings.

I think about it differently. There is an outlet port on the variable displacement pump (maybe two--one for forward and one for reverse) and there is a hose connected to the port(s) that carries the fluid. To measure the maximum pressure developed by the pump you need to terminate that hose in a pressure guage--because we both agree that maximum pressure is generated at zero flow.

 

[KentT]

I think about it differently. There is an outlet port on the variable displacement pump (maybe two--one for forward and one for reverse) and there is a hose connected to the port(s) that carries the fluid. To measure the maximum pressure developed by the pump you need to terminate that hose in a pressure guage--because we both agree that maximum pressure is generated at zero flow.

There are two ports on the tram pump. Which is the inlet and which is the outlet simply depends on which way you're pushing the treadle, unless I'm mistaken.

But, I don't know if you can get meaningful info with your test. In the real world the flow is never quite limited to zero when the swash plate of the pump is open, because of leakage in the wheel motors. If you terminate the line completely into a gauge you should either (a) stall the engine when you push down on the treadle since the oil has no place to go, and all of it cannot "leak" through the pump, or (b) it should kick in any internal bypass/pressure relief valve in the pump. Based on my experience with the PT and cooler oil in the system, the first will likely occur -- the engine will die when you push down on the treadle -- after the pressure gauge momentarily spikes...

Perhaps I'm wrong, but I don't think that would be a good measurement. It would confirm that the pump is pumping -- yes, but I already know that. It won't tell me how much pressure that pump will produce under a constant load nor at maximum flow rate because I can't go "full treadle" to see. Right now the wheel motors can "leak" and relieve some of the pressure build-up. With the line terminated in a gauge, they can't, so I don't think you'd ever get to a meaningful treadle position (i.e. flow) to take a reading.

I know you'd definitely kill the engine on a fixed displacement pump -- I've dealt with "hydraulic locks" before. I'm just not sure how this variable displacement pump would respond, nor how meaningful the test result might be....

 

[Bob999]

If that were the case, as I understand it, then each motor in the two series circuits would be producing only the power equivalent of 750 PSI -- 750 PSI to the front wheel and 750 PSI to the rear wheel. That means these 12.5ci motors would only be producing a maximum of 1386 in lbs of torque, done at a 2gpm flow, yielding 34 RPM. That's only 115.5 ft lbs of torque -- surely that is not the case.

According to the HP formulas at Surplus Center, at 750 PSI and 8 gpm, each of these wheel motors would only be putting out 2.845 HP. Multilply that times 4 and the 25-HP PT would only be putting 11.84 HP to the ground -- running full throttle. I don't think so...

I need to go back to your power calculation--it seems to me that it may be low by a factor of 2.

Wouldn't the flow through each motor be 4 gallons rather than two (this assumes the total pump output is 8 gallons/minute and that the flow is divided into two circuits)? If the flow is double in each circuit then the total HP would also double.

 

[J_J]

There are two ports on the tram pump. Which is the inlet and which is the outlet simply depends on which way you're pushing the treadle, unless I'm mistaken.

But, I don't know if you can get meaningful info with your test. In the real world the flow is never quite limited to zero when the swash plate of the pump is open, because of leakage in the wheel motors. If you terminate the line completely into a gauge you should either (a) stall the engine when you push down on the treadle since the oil has no place to go, and all of it cannot "leak" through the pump, or (b) it should kick in any internal bypass/pressure relief valve in the pump. Based on my experience with the PT and cooler oil in the system, the first will likely occur -- the engine will die when you push down on the treadle -- after the pressure gauge momentarily spikes...

Perhaps I'm wrong, but I don't think that would be a good measurement. It would confirm that the pump is pumping -- yes, but I already know that. It won't tell me how much pressure that pump will produce under a constant load nor at maximum flow rate because I can't go "full treadle" to see. Right now the wheel motors can "leak" and relieve some of the pressure build-up. With the line terminated in a gauge, they can't, so I don't think you'd ever get to a meaningful treadle position (i.e. flow) to take a reading.

I know you'd definitely kill the engine on a fixed displacement pump -- I've dealt with "hydraulic locks" before. I'm just not sure how this variable displacement pump would respond, nor how meaningful the test result might be....

Just tee in a pressure gage at the pump outlet, then you can read the pressure at any time. You could also extend the pressure gage line up to the front console.

On my 1445, I have installed a tee with a quick attach fitting. I put one in the PTO circuit, and the lift circuit. On the test gage, I put the other half of the quick attach. I just plug in the gage into the circuit I want to test, or just leave it plugged in.

 

[KentT]

I need to go back to your power calculation--it seems to me that it may be low by a factor of 2.

Wouldn't the flow through each motor be 4 gallons rather than two (this assumes the total pump output is 8 gallons/minute and that the flow is divided into two circuits)? If the flow is double in each circuit then the total HP would also double.

I mentioned 2 gpm in my example because the performance charts for the White 12.5ci RS motor shows that its torque at 750 PSI is highest at 2 gpm -- i.e. that would be the "best case" scenario of the amount of torque it could produce at 750 PSI. GPM has nothing to do with PSI or torque necessarily -- GPM determines RPM, not torque. Pressure (PSI) determines torque. The combination of torque (pressure) and flow (gpm) determines horsepower. For the HP calculation in my earlier post I used 8 gpm flow at 750 PSI -- the maximum power that the wheel motor could provide at that PSI if the 16 GPM output of the pump is correct (16 gpm/2 circuits = 8 gpm at each wheel motor).

Here's the specs at 750 PSI and their conversion to HP:
.5 gpm = 1250 in lbs torque, 6 RPM (.12 HP)
1 gpm = 1360 in lbs torque, 15 RPM (.32 HP)
2 gpm = 1386 in lbs torque, 34 RPM (.75 HP)
4 gpm = 1349 in lbs torque, 72 RPM (1.54 HP)
6 gpm = 1322 in lbs torque, 110 RPM (2.31 HP)
8 gpm = 1228 in lbs torque, 146 RPM (2.84 HP)


1228 in lbs X 146 RPM divided by 63025 = 2.84 HP at each wheel motor

 

[KentT]

Just tee in a pressure gage at the pump outlet, then you can read the pressure at any time. You could also extend the pressure gage line up to the front console.

On my 1445, I have installed a tee with a quick attach fitting. I put one in the PTO circuit, and the lift circuit. On the test gage, I put the other half of the quick attach. I just plug in the gage into the circuit I want to test, or just leave it plugged in.

Have you tested the variable displacement tram pump with your test gauge? If so, what kind of readings did you get? I'm guessing that the PSI moves all over the place, unless you're doing something like tramming down a level road at a steady treadle. In normal use working, I'd presume as the load on the wheel motors goes up (climbing even a slight slope, pushing or pulling something, making a turn, etc.) the PSI increases, then as the load goes down the PSI decreases...

IMO, the only way to test the max PSI output would be to essentially stall the engine when at full throttle -- and see the spike reading just before the engine died -- or the relief valve opens up.

 

[Bob999]

I mentioned 2 gpm in my example because the performance charts for the White 12.5ci RS motor shows that its torque at 750 PSI is highest at 2 gpm -- i.e. that would be the "best case" scenario of the amount of torque it could produce at 750 PSI. GPM has nothing to do with PSI or torque necessarily -- GPM determines RPM, not torque. Pressure (PSI) determines torque. The combination of torque (pressure) and flow (gpm) determines horsepower. For the HP calculation in my earlier post I used 8 gpm flow at 750 PSI -- the maximum power that the wheel motor could provide.

Here's the specs at 750 PSI and their conversion to HP:
.5 gpm = 1250 in lbs torque, 6 RPM (.12 HP)
1 gpm = 1360 in lbs torque, 15 RPM (.32 HP)
2 gpm = 1386 in lbs torque, 34 RPM (.75 HP)
4 gpm = 1349 in lbs torque, 72 RPM (1.54 HP)
6 gpm = 1322 in lbs torque, 110 RPM (2.31 HP)
8 gpm = 1228 in lbs torque, 146 RPM (2.84 HP)


1228 in lbs X 146 RPM divided by 63025 = 2.84 HP at each wheel motor

I went to two different calculators on the web and used 1500 PSI and 8 gallons per minute. That equated to 7 HP--assuming 100% efficiency of the motor which is obviously unrealistic. If you reduce the pressure to 750 PSI the HP drops to 3.5 --again assuming 100% effeciency. Alternatively doubling the flow will double the HP. What is missing in all of this is any anchoring in data--what is the output of the variable displacement pump.

I really don't think it is realistic to try to back into this by making assumptions--either you need specs for the variable displacement pump or you need good real world measurements to make good decisions about replacing wheel motors.

 

[KentT]

I went to two different calculators on the web and used 1500 PSI and 8 gallons per minute. That equated to 7 HP--assuming 100% efficiency of the motor which is obviously unrealistic.

I really don't think it is realistic to try to back into this by making assumptions--either you need specs for the variable displacement pump or you need good real world measurements to make good decisions about replacing wheel motors.

But, the two wheel motors in a series circuit where the pump only puts out 1500 PSI (your propositition) would only be using 750 PSI each -- 750 PSI at the front motor, and 750 PSI at the rear motor... that's where you're misunderstanding lies. For a wheelmotor in a series circuit to produce the power that corresponds to 1500 PSI, the pump would have to output 3000 PSI.

The output of a wheel motor is determined by the delta PSI ... the PSI at the inlet less the PSI at the outlet. The front wheel motor has 3000-1500=1500 while the rear wheel motor has 1500-0=1500. (Those are "100% efficiency numbers" assuming no resistance from hoses, fittings, valves, etc.)

To be semantically correct, the pump does not output PSI, it outputs GPM. The PSI is the resistance to flow at the wheelmotors, 1/2 at the rear and 1/2 at the front.