Reading my JD-790 hydraulic schematic...

[Wayne County Hose]

Hydraulic pumps - part 1

Wayne County Hose. I think you need to start back at the basics. Please click on link.

And what did they say that I didn't or was incorrect?

I think that's a cheap shot. If you think I posted inaccurate information, quote what you thought was incorrect, and explain why it is incorrect. Anybody can cut and paste a link.

 

[AKKAMAAN]

I am sorry Andy @ Wayne County Hose, if I did not explain clear enough. My intention was not to make anyone upset, about this. I am just seeking the JD motivation about this design. Trying to learn!! Interesting to hear from J_J, that equipment with this suction filter bypass, are now getting rebuilt.....




I try to use the qoutation system , on my reply so there wont be any misunderstanding about my replies.

Also please read these "facts" from Wikipedia....

AKKAMAAN, I believe you are waaaay overthinking this. And actually, there are completely sealed systems out there (Like on a Moffett piggyback forklift found frequently on Lowes and Home Depot delivery trucks) which have absolutely zero atmospheric pressure in the hydraulic tank or system. There goes the "no atmospheric pressure= no oil in the pump" theory.

Here is my own quote:
"When a pump "sucks" in the oil, it is technicly the atmospheric pressure that pushes the oil into the empty space that the pump creates on the inlet side when turning.
NO atmospheric pressure= NO oil into the pump."
My statement of course concider the standard VENTED tank setup, we had on the drawing. It also concider perfect vacuum as referens point for pressure.

I am sure you don't mean it is vacuum in that
Moffett tank system, or pressure below atmosheric pressure. Completely sealed systems has been around for a long time. The ones I have come across, have a breather cap that sets up a 1-2 psi over-pressure in the tank, to secure oil "feeding" the pump. Atmospheric pressure PLUS 1-2 psi!

In most systems, initially, yes it is atmospheric pressure pushing oil into the pump. However, once the pump starts going, there is no longer atmospheric pressure pushing oil into the suction line, there is instead a vacuum created by the pump. It is also kinda misleading to state that a suction filter bypass valve opens at ANY psi.

Not misleading at all, if the referens point for pressure is perfect vacuum, zero psi in absolute pressure!

It opens from vacuum. It is sucked open, not pushed open. If you look at a gauge on a suction filter, it is not a pressure gauge, it is a vacuum gauge. The needle rotates counter-clockwise for vacuum, pressure gauges rotate clockwise.

If we, daily in hydraulic life, call it that pump "sucks" the oil in, is not a big deal, we all use that way of discribing it, but when it comes to understanding any pressure science that occurs below atmospheric pressure, we need to use the absolute pressure referens, perfect vacuum is ZERO. everything above is "plus" on the pressure scale.

There is oil under pressure there but it's pressure is caused by gravity more than atmospheric pressure. Just because we make a box and seal it doesn't mean it has 14 psi in it

I never said that, thats your interpretation....

Just wondering, where did you come up with that pumps cavitate at 15 psi? It would be rated at inches of mercury anyway, and I have seen pumps pull way more vacuum than 15" without cavitating.

I never said that either, thats your interpretation....
Inches of mercury is not a common unit in hydraulics...

I do appreciate you sharing your knowledge and I am not being a "wisenheimer" so please don't take it that way. I too have a ton of hydraulic schooling, most of it application specific. Maybe I'm not understanding something in this but I'm referring back to my tech manuals and 25+ years of field experience.

Thanks, Andy

Again...no harm! I probobly sound more like a "wisenheimer" than you did.... (what ever that means, I suppose it is not something good...LOL).
I have been in tougher discussions than this. I have allways backed off when I have been proofed wrong. But never when I can proof I am right. All my students over my 20+ year teaching career, have always walked out of the classroom happy with this scientifcally accepted "theory".

I am sure all of you other guys here at TBN have more experience, training and skills in YOUR part of the hydraulic world than I have!
My hydraulic world have been teaching and training logging equipment to operators and technicians. I have two different engineering degrees, forestry and construction. Hydraulics by self learning and and a lot of 1-3 week classes at JD (former Timberjack) and Komatsu (former Valmet)
Thats why I choose to be here at TBN, trying to learn more about, to me, new applications, but also give back some of my knowledge and experience.

Cheers
Per A, "aw come on"....

 

[Wayne County Hose]

Ahh, don't worry. Not a tough discussion unless somebody makes it one. I felt the need to explain that I'm not being a weisenheimer or questioning your knowledge cuz sometimes, on the 'net, things can get taken the wrong way.

I do see where you are coming from. I guess we went to two different schools. I got taught from a more practical side while you got taught more indepth theory. We did discuss atmospheric pressure and it's effects, however, we were told that for all intents it is almost irrelevant.

We did discuss vacuum on the suction side and expressed it in inches of mercury. We never bothered measuring it or was told to use a measurement of it in troubleshooting but they wanted us to be sure of it's existence and how different levels of it affects different systems.

I'm glad that you are here with a vast knowledge of theory. I too will state my point if I believe I am correct and will also readily admit when I'm wrong. Hey, they crucified the only perfect man.

Thanks again for sharing your knowledge.

Andy

 

[J_J]

AKKAMAAN,

Disabling the bypass on the hydraulic filter, is something new to me. Apparently they have been doing this for a good while. Terry at Power-Trac verified they were. doing this 20 years ago, and even then I am not sure that someone has not replaced the housing for the filter. So, I really don't know what I have, except it is a filter assembly, with a 10 micron filter. They do not have any manual for the model I have, which was built in the mid 80's. early 90.

I can see the logic, and that is to prevent any trash getting in the closed loop system, but if you don't keep up with the hours, you might jeopardize the VSP unit, and if it fails, it ain't cheap. I wished someone would tell us about these modifications. What is unique about the closed loop system in the PT's, is that the pump sucks fluid from the wheel motor output, and the charge pump replenishes whatever is necessary to maintain the correct volume.

On the filter housing gage, I believe, that it has to be a vacuum reading/indicator, as there is no pressure , except atmospheric. A vacuum gage in the pump input line will read zero with the pump not running.

 

[AKKAMAAN]

I was taught back in the day that it is not a good idea to pressureize the return core, or the outlet in the valve body.
I would not take for granted all valves can stand the outlet pressureized! They are not meant to, unless they are designed for it!

Power beyond is not possible on every valve setup. If there is no extra outlet, it cant be done with out pressureizing the return core on the flanks of the valve housing. Not many valves are designed to withstand pressure on the flank return core.
To make safe power beyond, I would use it on valves with extra outlet AND designed for a power beyond plug.


I took a look at Prince RD5300 valve and found out their system. See this Prince pdf-file!

I have set up a few pic's to show the Prince system, which should be about the same for most brands.

They also have an option to convert to Closed Center!

Also to note that power beyond don't allow operation on PB valve if mainvalve is operated (Open Center fully closed). UNLESS it is possible to connect the pressurized parallell core from Main valve into the PB valve.

 

[Cocre]

So the best solution for me and my JD 3320, in a perfect world, is to connect A to the Prince valve inlet, B to the Prince valve power beyond, C to the Prince valve outlet (return)?

 

[AKKAMAAN]

So the best solution for me and my JD 3320, in a perfect world, is to connect A to the Prince valve inlet, B to the Prince valve power beyond, C to the Prince valve outlet (return)?

I like your pictures!!

Q's!

• I assume you are connecting only one, 3-spool Prince valve unit??
• Which one of A and B is pump supply line??It looks like A to me.
• Do you have a drawing with symbols too?? That would help me to understand your system better!

If you are not connecting another valve beyond the prince valve, your B and C on the Prince valve is the same thing. One of them shall just be plugged. See my picture...

Just make sure you get the right type of plug, like the one on the picture.
NOT a power beyond plug!!
How all this connect to your tractor, I can't tell until I know more about that tractor system....if A and B would be just a regular AUX, it should work fine to connect there, but it looks like B connects to the rockshaft?? then it seems like C should be the return line, to the sump.
So if that is how it works, your rockshaft will be disabled when you hook up the Prince valve.

 

[Cocre]

I actually have a two spool Prince SV series valve. I am installing a TnT using the normal method as described by KennyD and others elsewhere.

A is pressure coming thru the standard JD SCV. With B connected to A the 3 pt hitch works. Using the backhoe, B is left loose (no 3 pt hitch function), A supplies oil to the hoe and the hoe returns oil to the tank via C.

Many home built TnT valve mounts connect A to the Prince valve inlet and B to the Prince valve oulet, leaving C unused. But all the talk about no high pressure on the outlet side of valves made me rethink. I thought I could replace the open center plug with a power beyond plug and use the flow thru it to feed the 3pt hitch. The outlet would just dump "used" oil back into the tank.

I may have all this mixed up, but if $20 for a PB plug and a piece of hose will save me dollars down the road, that is OK with me. Just looking for knowledge.

 

[AKKAMAAN]

I actually have a two spool Prince SV series valve. I am installing a TnT using the normal method as described by KennyD and others elsewhere.

A is pressure coming thru the standard JD SCV. With B connected to A the 3 pt hitch works. Using the backhoe, B is left loose (no 3 pt hitch function), A supplies oil to the hoe and the hoe returns oil to the tank via C.

Many home built TnT valve mounts connect A to the Prince valve inlet and B to the Prince valve oulet, leaving C unused. But all the talk about no high pressure on the outlet side of valves made me rethink. I thought I could replace the open center plug with a power beyond plug and use the flow thru it to feed the 3pt hitch. The outlet would just dump "used" oil back into the tank.

I may have all this mixed up, but if $20 for a PB plug and a piece of hose will save me dollars down the road, that is OK with me. Just looking for knowledge.

I got the full picture now!!
I can't see other than that would work just fine!
Get a PB plug and execute!