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.