How many SCV s: John Deere Older  -- Antique Tractors Discussion Forum and Review How many SCV s: John Deere Older -- Antique Tractors Discussion Forum

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Discussion Boards > Active Subjects > Messages as Posted > John Deere Older Forum

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 08-26-2004, 13:24 Post: 94611
AnnBrush



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 How many SCV s

I dont really understand hydraulic / tractor terminology. I fully understand how hydraulics work (physics principals)but am uncertain as how to classify the tractor I have with respect to its hydraulic capability. Here is what it does, how would y'all classify this, BTW it's a JD 4300:
I have a hydraulic lever on the right fender that operates the MMM up and down (push lever forward MMM goes up, push it back and MMM goes down, I am not sure if it powers down or is just gravity fed, that lever actually operates two rear facing hydraulic couplers to which the MMM single cylinder hydraulic unit is connected. Q. is this also known as a power beyond? Is this a SCV?
I also have a joy stick that operates 4 hydraulic couplers situated beneath the right foot plate where a FEL used to be connected (I don't have one), the lever moves right and left and up and down and on one position there could be a detent although I am not sure. Q. how many "SCV's" do I have here and how would you describe this setup. Q. would I have power up and down on 2 cylinders or just power up on 4 cylinders with a return hose running back to storage resevior.

Reading other threads I think I know what is being spoken about but am never sure. Perhaps some of you with more knowledge in this area could fill me (and perhaps others) in. Much appreciated. Other comments are of course welcome.
Ann Brush






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 08-26-2004, 13:38 Post: 94613
kwschumm



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 How many SCV s

I'll take a stab. You probably know that SCV means selective control valve. I take that to mean that the operator can select and operate it on demand. It sounds like you have three SCVs, the first two operate the loader circuits and the third operates the mower. The third is plumbed to the rear outlets. There is one supply and one return circuit for each SCV. A Power Beyond circuit supplies full hydraulic pressure without a separate operator control.






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 08-26-2004, 20:07 Post: 94639
beagle

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 How many SCV s

Just to expand on kw a little, a Power Beyond Circuit is a circuit in series with another hydraulic circuit. It is created at the PB port on the SCV, at the exit end of the valve. The valve body needs to be manufactured with the Power Beyond port, allowing "full" pressure fluid to flow to the circuit in series. It really isn't "full" pressure, since the volume of fluid and the total pressure used in the power beyond circuit is added to the first SCV load. In other words, if there is any load from actuation of the first SCV, it is deducted from the flow available on the PB circuit.

Another way to put it is that a SCV with Power Beyond Capability allows a seperate circuit to be "tapped into" an existing hydraulic circuit.

Maybe I expanded a little more than a little.






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 08-27-2004, 05:47 Post: 94679
TomG

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 How many SCV s

You likely have four control valves (two for the loader and one for the MMM and one for the 3ph (there are other valves in a 3pd as well). You also may hear the terms spooling and direction control in this context.

If the MMM 'couplers' mentioned are hoses and one is at each end of the MMM cylinder then the MMM is likely powered both up and down (the cylinder is called a double-acting type). If one hose attached to the MMM valve runs to the loader valve then it is a power-beyond application.

That is almost certainly the case since there are few alternatives to power-beyond given the open centered hydraulics used on these tractors. Open centered means that the pump delivers full-flow at all times in a loop from the reservoir. Power beyond means that the outlet from one valve runs to the inlet of another, or they are in series as mentioned.

You should be aware that in a power-beyond application, operating an upstream valve blocks the flow from downstream valve. In many setups neither the MMM nor 3ph would operate while a loader valve was operated. Since nothing is connected to the loader work ports, pressure would build and the relief valve would open. It makes a sort of screaming sound and the engine would throttle up. A tractor shouldn't be operated for long periods while a relief is open.






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 08-27-2004, 07:07 Post: 94685
beagle

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 How many SCV s

Just to add a few words to clarify, you cannot run a circuit from the exhaust or outlet port of one valve to the inlet port of another to create a series. You must have a power beyond port on the valve that allows the continuation of the power loop. In series hydraulic circuits, the exhaust of all SCV's in the series return to the reservoir. The power beyond of the first valve routes the the inlet of the second, and so on.

Many SCV's allow for flow through the power beyond port even when the first valve in the series is actuated. The net flow is available for the PB circuit.






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 08-28-2004, 07:29 Post: 94784
TomG

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 How many SCV s

Good morning. I guess this is a thread about terminology and there is some here. Hope it helps Ann.

I used 'outlet' in a general sense. Most PB rated valves have two outlets or return ports. The second one usually is called the low-pressure or TANK return and it carries exhaust oil directly to the reservoir or tank. True enough that the PB line from one valve goes to the inlet or pressure port of a second valve but it is an outlet line. I tend to think of PB more as an application than as a physical unit. It's just anyplace where one valve assembly downstream of another in an open centered system.

Exhaust oil results when a double acting cylinder moves. It is the oil on the low-pressure side of the piston that is displaced as a cylinder moves. If a valve doesn't have a TANK line it goes into a single return line that carries all return flow. I believe there are PB rated valve assemblies that use a single return line and my hoe may be an example. In such a valve the exhaust oil will move a downstream cylinder if its valve is operated.

The passage Beagle mentioned that allows several valves in the same assembly to be powered is sometimes called a power passage. It provides tandem connections when several valves are operated Tandem connections produce 'the lightest load moves first' operation rather than true simultaneous operation. Most newer valves have the feature. My hoe has it but my loader doesn't.






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 08-28-2004, 08:48 Post: 94787
Captain B



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 How many SCV s

This is a related question. I have power beyond off on my JD990 off which I run a backhoe. I have been thinking of adding a hydrualic top link for my box blade. I see a variety of control valves listed in catalogs like Northern Tool's. I don't feel that I know enough to pick the appropriate one. I want a valve that, roughly put, "locks open" so that I can still get unrestricted flow to the backhoe when I attach it. What valve supports this application?






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 08-28-2004, 09:48 Post: 94791
DRankin



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 How many SCV s

I have the Deere power beyond kit too, and top and tilt.

I run the top and tilt from the power beyond system. In other words, the PB circulates fluid though a pair of open center SCV's that operate the T n'T.

When I need the backhoe or the log splitter, it is a simple matter to unplug the PB from one and attach it to the other.






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 08-28-2004, 10:12 Post: 94797
Captain B



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 How many SCV s

Thanks DR. My question, however, is: What specific type of control valve do you recommend? For example, I see listings for a "two-way detent valve", a "one-spool control valve with float",and a "two-spool control valve." I can't determine which one I should buy to run a hydraulic top link off of my two-port(out and return), rear-mounted power beyond hook up.






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 08-28-2004, 10:15 Post: 94798
DRankin



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 How many SCV s

Where is Tom G. when we need him????






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