PT AL-629 ( can it be done ? )

[J_J]

JJ, that's interesting because I've also heard water systems used as an analogy to explain DC electrical circuits...

What you're not considering in the "house analogy" and why it breaks down is that there is a fixed volume of fluid coming out of a hydraulic pump at any given RPM (less any leakage or inefficiency in the pump). It is the volume that is a constant, not the pressure... pressure is variable. In your analogy, you assumed (correctly) pressure inside the house as the constant, not volume. Somewhere in that water system supplying your house is a relief valve that is already bypassing (or shutting the pump off) to hold the pressure constant -- otherwise the pressure would continue to build up as long as the pump turned, when the outlets were closed...

So, the analogy doesn't quite fit.

Unless the hydraulic pressure builds to the point where the relief valves (in either the wheel motors or pump) bypass to relieve excess pressure, changing the circuits from series to parallel would double the pressure....

The house analogy breaks down like this, Every outlet is the house has the same pressure, which is pump pressure, or city water pressure. .The only way to change that is to put a booster pump in the circuit. Volume is controlled by the opening on any valve in the house. Now open all the valves, what happens? Pressure drops, volume decreases.

My background also includes Electronics technician U.S.Navy. Irrigation installation, and a fair to good knowledge in hydraulics, and some welding.

 

[KentT]

Volume is controlled by the opening on any valve in the house. Now open all the valves, what happens? Pressure drops, volume decreases.

BTW, if you have four open valves on four independent lines you'll lose more water than if you have two lines, with two open valves in each one...

Parallel versus serial -- try it sometime!

I appreciate your experience -- it brings a lot to this forum. But, it doesn't mean you're always right, though...

 

[BobRip]

You only have pressure when you have resistance to the fluid being pumped, such as a cylinder working, or a motor running, or steering, etc.things being in parallel do not amplify fluid pressure.

I agree with part of your statement. "you only have pressure when there is resistance to fluid being pumped". However, a cylinder working without mechanical load or a motor running without load will have very little back pressure. Once there is a load then the pressure goes up. If you have three motors in series and two are in the air with one on the ground, almost all of the back pressure comes from the one on the ground with torque. Of course when a cylinder hits the end of travel back pressure will build up. Again I am ignoring friction losses. Do you agree or do I misunderstand the technology.

 

[J_J]

I agree with part of your statement. "you only have pressure when there is resistance to fluid being pumped". However, a cylinder working without mechanical load or a motor running without load will have very little back pressure. Once there is a load then the pressure goes up. If you have three motors in series and two are in the air with one on the ground, almost all of the back pressure comes from the one on the ground with torque. Of course when a cylinder hits the end of travel back pressure will build up. Again I am ignoring friction losses. Do you agree or do I misunderstand the technology.

Yes, that is correct, except that just extending the cylinder with no load will show very little pressure, until it reaches the end of it's travel, or it is acting upon, say a load of stone. The cylinder with max load will show something close to pump pressure.

 

[J_J]

Parallel connection (right)
Pump pressure can be lower in a parallel circuit because in a series circuit the pressure at the pump must be the sum of the pressure drops across the motors. However, where motor pressures vary widely, there is a loss of efficiency in supplying the motors requiring lower pressures. This circuit is most efficient where the load on each motor is the same. Raising the pressure on one motor renders the others less efficient and may disrupt the speed relationship. In parallel circuits, the only way to increase torque of the highest-pressure motor is to increase system pressure

 

[KentT]

Parallel connection (right)
In parallel circuits, the only way to increase torque of the highest-pressure motor is to increase system pressure

Yep, which it does... the point that started this whole issue....

BTW:

Yes there are similiarities between "current flow" and viscous flow in a pipe.

Ohm's Law for current : V = IR or I = V/R ,where I : current, V : Voltage difference between ends of conductor, R : resistance of the conductor

Fluid Flow : Q = P/Z , where Q : flow rate, P : pressure difference across ends of pipe, Z : impedance of the pipe

Nature of R : R = const*L/A , where L : conductor length, A : cross section area

Nature of Z : Z = contant*L/A^n , where L : length of straight pipe, A : cross section area , n : typically between 2 and 4 depending on whether the flow is laminar or turbulent.

Series and Parallel Conductors : R = R1 + R2 + R3 +... (Series)
1/R = 1/R1 + 1/R2 + ... (Parallel)

Series and Parallel Pipes : Z = Z1 + Z2 + Z3 + ... (Series)
1/Z = 1/Z1 + 1/Z2 + ... (Parallel)

These similarities often make it useful to treat hydraulic systems like circuits and use "circuit analysis" to figure out different quantities at various points in the system.

Source: Some DC Circuit quick questions Text - Physics Forums Library

 

[J_J]

The hydraulic pressure can be adjusted slightly by adjusting the relief valves, but you run the risk of exceeding the design limitation of the pump, and the motors that use the hydraulic fluid.

If anyone can come up with a way to double pressure on a fixed displacement pump, I am sure that someone will make plenty of money. I hope you guys solve this hydraulic problem. In most cases, you can not get something for nothing.

 

[KentT]

JJ,

First of all, it is a variable displace tram pump, not a fixed displacement pump.

Second, it isn't "something for nothing" -- it is trading off speed for torque, a common calculation and "balancing act" done in hydraulic systems, whether it be motor speed or reaction time of a hydraulic cylinder.

Third, the White wheel motors are rated for 3000 PSI, twice what they appear to be running now...

I'm not sure what the tram pump is rated for... nor am I sure how well the wheel motors will stand up at that pressure AND the wheels reversed.

But, as long as I don't damage the tram pump, I have little to lose -- because the only alternative I see for making the PT-425 useful for me is to replace those wheel motors with higher torque ones anyway...

I also find it interesting that you weren't a naysayer when Stray was originally discussing his mod:

http://www.tractorbynet.com/forums/power-trac/69056-two-speed-modification-pt-422-a.html

 

[BobRip]

KentT, I think it's a good idea. I don't see why the hydraulic motor engineer did not like it. Maybe you should give him a call and ask for clarification. I suspect there may be some misunderstanding. With the diverters I don't see how it could not work. I may just be blind though.

 

[BobRip]

but he also said that they should not be hooked up independently via. a manifold because of the loss of speed and the demand on the pump to supply a constant flow to all motors equally.

Beechwood, I agree with the loss of speed and it has been discussed and accepted.

Supplying all motors with equal flow can be taken care of with the diverters (a standard hydraulic component), so I don't see a major problem. After all there are two parallel circuits (left front and right rear plus right front and left rear) that balance nicely. The diverters would probably be needed (IMHO), but it can be done.