Reading my JD-790 hydraulic schematic...

[AKKAMAAN]

Wayne County Hose,

Thanks, I learn something everyday. Did not know that.

AKKAMAAN ,

Are you saying that if that filter has a 25 psi bypass that the pump will cavitate before it reaches the bypass rate? If that is true, I don't think very many people know that.

Of course!
Pressure is a relative factor. In hydraulics we usually only talk about the pressure ABOVE atmospheric pressure. We can call it over-pressure!
So when measure the system pressure with a pressure gauge,

we let the oil pressure inside the Bourdon tube, and on the outside the tube, we have the atmospheric pressure.

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.
(This a challenge for for aviation and space vessels, when they get on such high elevation so they loose the atmospheric pressure.)

And we all know that the atmospheric pressure varies between 14-15 psi, depending on weather, altitude etc?

When the pump turn around and creates this empty space on the inlet side, if this space dont gett filled it will become vacuum. 0 (zero) psi.

This filter on the inlet side will cause a pressure drop depending on size of flow. If filter is clogged, and the bypass is set to crack open at 15 psi or more, there will not be any oil going in to the pump. There will be vacuum only on the inlet side of pump. Cavitation???? Guess!!

The only reason to have a bypass set above atmospheric pressure, is to make it disabled.

We are used to return filter and they have made us accept 10-20 psi back pressure. If same size and gauge of filter is used on suction side it will cause the same pressure drop, 10-20 psi....Cavitation???Guess!!

This is the reason I am so curious about this desig with both strainer and suction filter on the pump inlet side. And of course even more comfused about the design when being told bypass open at 25 psi......

 

[AKKAMAAN]

Any kind of gauge on the suction side is a vacuum gauge. The bypass on a suction filter works on the same principal as on the return side, just in reverse. The suction side bypass will pull the bypass valve open whereas the return side bypass pushes it open.

Actually, to be technically correct, it is the atmospheric pressure the pushes the valve open. Atmospheric pressure can only push 15psi. we will need a charge pump to crack that "25"psi by pass valve open.......

 

[Wayne County Hose]

Actually, to be technically correct, it is the atmospheric pressure the pushes the valve open. Atmospheric pressure can only push 15psi. we will need a charge pump to crack that "25"psi by pass valve open.......

Actually, to be technically correct, it's the vacuum created by the pump that pulls the valve open, in inches of mercury, not pounds of pressure.

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.

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. 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.

Using your atmospheric pressure logic, you make it sem like there's 14 psi of oil sitting waiting for the pump to turn on and we all know that's not the case. 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, same with an oil reservoir/pump.

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 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

 

[soulasphil]

IF you can find a "tank" port on your machine, then you can use a PB equipped valve with the PB port feeding the backhoe and/or the 3PH.

If no "tank" port can be found, one can be created by drilling a hole like the one on the pictures below of my JD 3520.

 

[J_J]

Well, I stand corrected. I just found out that PT modifies the base of the filter so it does not bypass so it will not get to the point of sucking any crud into the charge pump, and then through the VSP pump. So the gospel is to change out the hyd filters every 50 hours. I knew about the 50 hr, but did not know why. I think some of the larger machines hyd filters are changed out every 100 or more.

You think you know something until someone tells you differently.Terry at PT. Why do I believe him, because he is the head mechanic and builder at Power-Trac, and he has been there forever. If they would only write a good manual.

Did any of you PT owners know about the modified filters. So a heads up is in order to anyone that has changed out the original filter to a base unit with gage, or indicator. That is the first instance that I know , of anybody doing that.

All this has convinced me to install a timer on the engine.

 

[George G]

Hydraulic pumps - part 1

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

 

[Cocre]

OK, now I am confused. All the talk about in series or not in series got me jumpy. Is something like what is show in the attached picture OK? This is not mine, but I am doing something similar. Going from the JD installed Power Beyond supply into a Prince valve inlet and then from that valve outlet (not its optional PB outlet) to the JD PB return that supplies the hitch. With this setup the 3 pt hitch can be used via the open center Prince valve as well as operating the TnT. From what I can deduce, the Prince valve would be in series with the 3 pt hitch valve, right?

On my JD 3320 I have installed the valve, plumbed it in, and the 3 pt hitch works fine, just want to make certain I do not mess something up.

Thanks for any insight.

 

[kennyd]

OK, now I am confused. All the talk about in series or not in series got me jumpy. Is something like what is show in the attached picture OK? This is not mine, but I am doing something similar. Going from the JD installed Power Beyond supply into a Prince valve inlet and then from that valve outlet (not its optional PB outlet) to the JD PB return that supplies the hitch. With this setup the 3 pt hitch can be used via the open center Prince valve as well as operating the TnT. From what I can deduce, the Prince valve would be in series with the 3 pt hitch valve, right?

On my JD 3320 I have installed the valve, plumbed it in, and the 3 pt hitch works fine, just want to make certain I do not mess something up.

Thanks for any insight.

If you ask me, (and quite a few others who have done it this way) it is fine. If you ask JJ, your valve will probably blow up.

 

[J_J]

If you ask me, (and quite a few others who have done it this way) it is fine. If you ask JJ, your valve will probably blow up.

Don't be such a wise^^^ around Christmas. It doesn't gain you much.

I hope we are talking about the same thing, for I think you are mistaken if you think it is alright to connect the return port from one valve to the next valves IN port.

 

[kennyd]

Don't be such a wise^^^ around Christmas. It doesn't gain you much.

I hope we are talking about the same thing, for I think you are mistaken if you think it is alright to connect the return port from one valve to the next valves IN port.

You keep reading the manuals, I will keep relating what works in real life-not only by me (My projects are well documented) but by plenty of others both here and elsewhere.

I do understand that using a valve with PB is always the best choice...but you have to understand it is not always possible either. Like I said before, How may backhoe valves Edit: and LogSplitter valves are exploding?

See what you started Will