PT AL-629 ( can it be done ? )

[Beechwood]

Beechwood, the nice thing about a hobby is you don't have to justify it.
I think the extra wheels and motors will add no torque as discussed above. It will help if wheels slip and give you more surface area. According to a conversation with PT tech support the relief valves on the motors are set very high and don't nomally trip, so adjusting them will not buy you much.

Bob, I'm going to think outside the box, so bear with me because I must be missing something. Lets say the 180 weighs 1000# and it has 4 wheel motors with each motor puting out 3,000 in/lbs. of torque ok?, then I built a 1000# wheel barrel and power the front wheel with 1 of the same size motors, same displacement (3,000 in/lbs. of torque. ), same tire size etc. Then I load 4 /80 lb. bags of ready mix in each. Which machine is going to move easier with less required force or power? ( I'm not talking about traction ). The pt with 4 -3,000 in/lb torque motors or the wheel barrel with 1- 3,000 in/lb motor?. In theory the wheel barrel is going to use the full 3,000 in/lbs. of torque and the pt with 4 motors will use 1/4 the torque on each motor. to acheive the same task. So 6 wheels will should provide more power than 4 wheels. This seems to make sense to me. Or my way of thinking is wrong.

 

[BobRip]

Bob, I'm going to think outside the box, so bear with me because I must be missing something. Lets say the 180 weighs 1000# and it has 4 wheel motors with each motor puting out 3,000 in/lbs. of torque ok?, then I built a 1000# wheel barrel and power the front wheel with 1 of the same size motors, same displacement (3,000 in/lbs. of torque. ), same tire size etc. Then I load 4 /80 lb. bags of ready mix in each. Which machine is going to travel up a slight grade better ( I'm not talking about traction ). The pt with 4 -3,000 in/lb torque motors or the wheel barrel with 1- 3,000 in/lb motor? The wheel barrel is going to use the full 3,000 in/lbs. of torque and the pt with 4 motors will use 1/4 the torque on each motor. to acheive the same task. This seems to make sense to me. Or my way of thinking is wrong.

You thinking is accurate, but your example is not totatly relevant. A PT with 4 wheel motors or 6 wheel motors (if plumbed in the series parallel configuration that PT now uses) will theoretically have the same pulling force. Since if one wheel motor puts out more torque, the other one in the series has less pressure applied to it and will put out less torque. In the actual situation you will probably get a little more torque with six wheel motors since even at stall some fluid flows through a motor and goes to another motor. How significant this would be I don't know. You real limitation here is the pump that drives it all. It only has so much pressure and flow and you have to manage this for your desired results.

My suggestion is to put in a valve to allow either parallel or series operation with diverters so that a slipping wheel motor does not hog all of the flow. This will double your torque and half the top speed. Do this when adding your extra wheels and you would have something or just do it without the extra wheels and you would really have something. I still prefer the PT 425 Big Wheel. Plenty of torque and lots of ground contact with low pressure. You can also get a two speed wheel motor that allows low torque/high speed or high torque/low speed. It is switched with a solenoid.
Disclaimer, I am an electrical engineer, so check all of my suggestion with your local hydraulic expert.

 

[Beechwood]

Bob, Thanks! now I'm beginning to understand. If I were to power each motor independant of each other instead of in series I would have more torque ( or I should say equal torque )available from each motor.

 

[BobRip]

Bob, Thanks! now I'm beginning to understand. If I were to power each motor independant of each other instead of in series I would have more torque available from each motor.

Yes, assuming that your power sources to these motors totaled to greater than the single power source.
It might be good to get a book on hydraulics and study it, although many people here know a lot more than I do. I just think in the electrical analogs and that is usually pretty close, if I do it correctly.

Hey, you could put PT425 motors on your machine. It would run slower, but have more torque. You would probably/might need an adapter plate and different hoses.

 

[Bob999]

Bob, I'm going to think outside the box, so bear with me because I must be missing something. Lets say the 180 weighs 1000# and it has 4 wheel motors with each motor puting out 3,000 in/lbs. of torque ok?, then I built a 1000# wheel barrel and power the front wheel with 1 of the same size motors, same displacement (3,000 in/lbs. of torque. ), same tire size etc. Then I load 4 /80 lb. bags of ready mix in each. Which machine is going to move easier with less required force or power? ( I'm not talking about traction ). The pt with 4 -3,000 in/lb torque motors or the wheel barrel with 1- 3,000 in/lb motor?. In theory the wheel barrel is going to use the full 3,000 in/lbs. of torque and the pt with 4 motors will use 1/4 the torque on each motor. to acheive the same task. So 6 wheels will should provide more power than 4 wheels. This seems to make sense to me. Or my way of thinking is wrong.

I think about this a little differently. The power available to drive the four (or one or six wheel motors) is determined by the engine and variable displacement pump that create flow/pressure in the hydraulic system. For any given combination of engine and pump the maximum power available (the product of flow and pressure) is going to be a constant. You can use the power to drive one motor or four motors or six motors but the amount of power available is a constant.

 

[Beechwood]

I just got off the phone with one of the hydraulic engineers at White Drive Products Inc. in Hopkinsville, KY. I told him what I was trying to accomplish. He knew about the PT180 and told me what model motor, displacement,etc is being used on the 180. He said if I were to add two more motors the same size etc. and hooked them up in series that I would definitely get an increase in pushing and pulling power, 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. He said that a 29 Hp motor would be overkill for the 180 wheel motors and suggested staying with my upgraded 18Hp motor. The costs for the new motors are less than I expected. So now I'll do some thinking and decide if I want to make this a winter project or can the idea.

 

[J_J]

The pressure will remain the same -- at the wheel motors -- only if there are bypass valves set to keep the pressure the same... otherwise, it will theoretically double as the volume is reduced in half. Again, this is at the wheel motor, not at the pump...

Kent, You can not compare electrical circuits with hydraulics. A better analogy would be water. Consider the water in your house. No matter what kind of circuit you have be it series or parallel, the water pressure will be the same.

In a six wheel configuration, you have increased the over all cubic inch displacement, and you would need a larger hydraulic pump to give the same speed. All the hyd motors have a max safe pressure, and if you exceed this, you run the risk of blowing the seals. A larger pump will also take more hp.

 

[BobRip]

Kent, You can not compare electrical circuits with hydraulics. A better analogy would be water. Consider the water in your house. No matter what kind of circuit you have be it series or parallel, the water pressure will be the same.

In a six wheel configuration, you have increased the over all cubic inch displacement, and you would need a larger hydraulic pump to give the same speed. All the hyd motors have a max safe pressure, and if you exceed this, you run the risk of blowing the seals. A larger pump will also take more hp.

Yes, you do increase the cubic inch displacement, but they are in series and all of the flow will go through all motors. How can it not? Is it leaking out somewhere? The speed is the same, but the torque is divided amongst the motors. Again, I am out of my field, but this is how I understand it. The pressure drop is divided across the three motors, but the flow goes through all three. I read what the expert says, so I am probably wrong, but I don't think so. I am ignoring additional losses caused by and extra motor.

 

[KentT]

Kent, You can not compare electrical circuits with hydraulics. A better analogy would be water. Consider the water in your house. No matter what kind of circuit you have be it series or parallel, the water pressure will be the same.

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....

 

[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....

I would like to know where and how you are getting the extra pressure from? Your hydraulic pump is rated for a certain gpm, at some pressure, which is the usable pressure. Let's assume that the pressure is 3000 psi. That is your max pressure you can use before it is relieved back to tank through the relief valves. As a matter of fact, if no work is being done, the pressure is min. 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.

 

[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.