What does power-beyond really mean?

[quicksandfarmer]

I bought a new Prince valve recently, which can be configured as open-center, closed-center, or power-beyond. I was surprised to read the following description of power beyond in the instructions:

This option provides both an outlet and a power beyond port (also referred to as a high pressure carry over port). This allows another valve to be connected downstream. When all the spools of a RD-5000 series valve are in neutral high pressure oil can go through the open center core and out the power beyond port to the inlet of downstream valve. The downstream valve only receives oil when all spools of the first valve are in neutral. This option must be used with open center spools and the outlet of valve must be connected to tank.

A couple of things about this description don't jibe with what I thought I knew about power-beyond.

This allows another valve to be connected downstream.

This sentence is ambiguous, but one way of reading it is that without power-beyond you can't connect another valve downstream. My understanding is that the whole point of open-center devices is they can be used in series.

The downstream valve only receives oil when all spools of the first valve are in neutral.

This doesn't sound quite right either. My understanding is that the point of power-beyond is to overcome the shortcomings of open-center when using more than one valve at a time.

In the valve specifications, it says maximum tank pressure 150 psi, but it doesn't specify in which configuration. In an open-center circuit, a valve has to take full circuit pressure on its outlet, unless it is the final one in the circuit.

The instructions are not terribly helpful, but I get the feeling that they're recommending that power-beyond be used if more than one valve is used in a circuit, and that the valve is not capable of being used in a true open-center fashion.

Any thoughts?

Thanks.

 

[Bird]

I'm a long ways from being a hydraulics expert, but that sure sounds right to me.

The downstream valve only receives oil when all spools of the first valve are in neutral.

I didn't actually use a power byond port on a valve, but on my B2710, when I used the auxilliary hydraulic outlet to go to and from an open centered two spool valve for my hydraulic top 'n tilt, that was the case. Since the 3-point valve was downstream from that auxilliary outlet and the new valve, the 3-point hitch would only work if both top 'n tilt valve spools were in neutral. Of course that was no problem because I didn't have enough hands to work any more valves all at once anyway.

 

[3RRL]

I have posted this link from Baum Hydraulics a couple times to help understand power beyond.
Hope it helps.

Hydraulic Terms and Definitions

To address this
This sentence is ambiguous, but one way of reading it is that without power-beyond you can't connect another valve downstream. My understanding is that the whole point of open-center devices is they can be used in series.

Yes, you can hook up valves in series without power beyond, but for the reasons given in the link, it is better to use a power beyond sleeve and return to sump for each valve.

 

[bx24]

Thanks for sharing 3RRL, that is great info

 

[RonMar]

That description from prince is Perfectly accurate. You have to make the distinction between outlet and PB. The term outlet as they are refering to it is the outlet from the valve returning to the resovoir. It needs to be a low pressure, basically unrestricted path to the resovoir. The most restriction it should have is a return line filter. The PB outlet is an extension of the high pressure inlet on an open center valve when the valve is in neutral(which is the only time a "open-center" valves center is "open". When it is not in neutral, The fluid is diverted to a working port and on to a cylinder or motor. The fluid returning from the cyl/motor returns thru the companion work port for that spool, and goes to the outlet/return port and not the PB port. Any fluid that goes thru the pressure relief valve also goes to the return/outlet port.

Without PB, what is normally the PB port is tied together with the outlet/return and has a pressure limit(150PSI tank pressure?). I have two 5000 series valves, and the last 5000 series pamphlet I looked at had that max return pressure at 500PSI. At any rate, you can't connect another valve downstream, at least succesfully, because without a PB sleeve, putting another valve downstream on the first valves combined return/PB port will apply full system pressure where it dosn't belong, when the downstream valve is operated.

If you look at the diagram of the PB adapter sleeve(should be in the 5000 pamplet or is available on princes website), it is a threaded tube that looks like a pipe bushing with an "O" ring out near the end. This sleeve screws in and separates the outlet/return port from the high pressure open center port and gives you that high pressure capable outlet from the center of the PB sleeve.

The reason for the pressure restriction on the return port is that the return galeries in this type mono-block valve are at the ends of the spools and the final seal for this pressure is the "O" rings at the ends of he spools which have to be free enough to allow spool movement.

My first 5000 series valve was installed incorrectly. It had the 3PH and steering downstream of that combined PB/return port. This is only a problem if you are lifting something heavy on the 3PH, such as a HD 5' brushhog, and exceed the 500 PSI return port pressure rateing. After working this new Brushhog a few hours I was introduced to a warming sensation on might right boot caused by the blown spool "O" rings on the valve pissing warm hydraulic fluid every time I lifted the 3PH and brushhog.

 

[quicksandfarmer]

Here's a link that describes my understanding of how an open center circuit works:
Fluid Power Systems

(this is a Navy training manual). The description of open center begins under the heading "Fluid Power Systems," second paragraph.

The key part of this description is "the system pressure goes through each directional control valve."

This is at odds with the description in the link that Rob gave, where it says:

This sleeve outlet prevents back pressure on the exhaust or return passage of a control valve which normally is not designed to take high pressure.

(emphasis is mine)

In the diagram of open center in the Navy manual, the exhaust passages clearly are meant -- in fact, required -- to take full system pressure.

Looking at the list of purported advantages to open-center in the article that Rob linked to, the advantages assume that either the exhaust passage of the valve can't take full pressure, or there is no system relief valve. So yeah, if you have equipment that's not meant for open center, or not plumbed properly, power beyond is going to work better.

I guess what I'm realizing is that open center and power beyond are different beasts. While you can mix them to a certain extent, they have different philosophies and put different requirements upon the equipment.

What has me a little mystified is why a valve manufacturer would label a valve as open center capable, and then say the exhaust gallery can't take more than nominal pressure. That makes no sense at all.

 

[RonMar]

Here's a link that describes my understanding of how an open center circuit works:
Fluid Power Systems

(this is a Navy training manual). The description of open center begins under the heading "Fluid Power Systems," second paragraph.

The key part of this description is "the system pressure goes through each directional control valve."

This is at odds with the description in the link that Rob gave, where it says:
(emphasis is mine).

System pressure does go thru each control valve. Each spool valve in a valve body is a directional control valve. The return ports on these monoblock valves are not designed for system pressure for the reasons I mentioned in my last post. They are just designed to provide a path to the resovoir.

In the diagram of open center in the Navy manual, the exhaust passages clearly are meant -- in fact, required -- to take full system pressure.

In that diagram, yes they are, but that is a VERY basic drawing meant to show the principal of open center flow, and does not reflect the actual flow in a Prince 5000 series monoblock valve(or most any other for that matter).

I guess what I'm realizing is that open center and power beyond are different beasts. While you can mix them to a certain extent, they have different philosophies and put different requirements upon the equipment.
What has me a little mystified is why a valve manufacturer would label a valve as open center capable, and then say the exhaust gallery can't take more than nominal pressure. That makes no sense at all.

Open Center and PB are different features, but both can easilly be on the same valve. The term open center just refers to the flow thru each spool when it is in the center position. With the valve spools in the center position, there is no restriction to the flow and the fliud flows thru all the spools like the valve body is a piece of pipe. With open center, you only build system pressure when you move a lever and perform work with a hyd cylinder. When the levers/spools are in the center, there is no pressure, just high flow and no load on the engine which saves fuel.

An open center valve can be PB capable, or not depending on the manufacture. Without power beyond capability, it must however be the last valve in the system and it's outlet, a combination of the flow from the open center gallery and the returns from the work ports and the safety relief, must have a unrestricted low pressure path to the resovoir.

The valve is open center(with the correct plug in it's side). If you want to put another valve downstream of it however, you need the $13 PB sleeve from surplus center to separate the high pressure capable flow of the open center, from the low pressure return/relief gallery.

 

[MJPetersen]

My first 5000 series valve was installed incorrectly.

RonMar is exactly right. I have frequently seen photos and examples of loaders, which are hooked up incorrectly, even on dealer installs. If you have a FEL installed on your tractor, have an open center system and you have a 3 pt hitch, then you almost always should be using the PB port of the loader valve for the the 3 pt. (The exception could be if you are using the PB port of the 3 pt to operate the loader valve.

If your tractor is OC, with FEL and 3pt, AND the FEL valve has 6 lines to it then it is NOT using the PB port to power the 3pt (or any other valve down stream). If it has 7 lines then it is using a PB to power the 3pt or other valve. [4 lines for cylinder actuation, 1 line in from pump, 1 line out to sump, and 1 line out to power additional valves.

If your FEL does not have 7 lines and your OC system powers the 3pt after the FEL then you too are risking the warm oil on the right toe.

Mike

 

[Soundguy]

Another feature that wasn't directly mentioned.. but was touched on.. about having a hyd circuit provide fluid pressure while another circuit was also getting pressure.

this could be done with a priority valve.. like.. having power steeirng on a priority setup when fluid flow is limited and shared..

soundguy

 

[shaley]

PB is also called series/parallel connections. Here is a PDF that explains it rather well.
View attachment Series_open_center.pdf