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Tractor Talk Discussion Forum

A proper hydraulic connection????

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08-24-2004 15:48:01

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I've been trying to plan my conversion of my trip loader bucket to full hydraulic. I have done a through search of the archives of this forum and have become more and more confused. Does anyone really know the proper use of hydraulics and correct set-ups?

So far I have learned:
1) you absolutly CAN NOT rig open center valves in series
2) you absolutly CAN rig open center valves in series
3) you absolutly can not use a double acting open center valve with a single acting cylinder without doing damage to the pump, valve or tractor
4) you can use a double acting valve on a single acting cylinder with no problem whatsoever

I'm lost. My library has no books on this subject and the forums have proved to have conflicting information. Those who have posted on the subject always seem to think they are right then someone comes along and says the 100% opposate. Can aynone offer help to a proper source of information or can anyone clarify the above contradictions. Thank you.

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08-25-2004 06:46:28

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
Why not just add hoses to the boom cylinders and make them doubling acting. Saves having to have extra oil capacity in a reseviour. Instead of adding valves to valves. Just purchase a set of two,three,four,etc valves together in one body unit?

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08-25-2004 07:16:02

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 Re: A proper hydraulic connection???? in reply to buickanddeere, 08-25-2004 06:46:28  
1st: The boom cylinders don't have that option (only 1 port). 2nd: expense.

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Rod (NH)

08-24-2004 18:55:01

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
Hi Dave,

Well, I'll add to your confusion by saying the following:

Yes, you can indeed install open center control valves in series without the power beyond option...BUT it is generally not recommended and you have to be careful about the pressures involved. What happens when you do that is the exhaust (return) cavity of the upstream valve can be pressurized up to your system relief valve setting. With many modern systems, this pressure will exceed the exhaust cavity pressure rating and the valve can be damaged catastrophically with possible collateral damage -- a personal safety matter. The pressure beyond option is intended to overcome this problem. A control valve has to be machined to accommodate such a feature however. It cannot simply be retrofit with an "adapter" unless the feature is provided for beforehand. With older tractors than run a significantly lower pressure than modern equipment, a series installation of non-power beyond valves is possible.

Here's how I have done it for the last 20 years or so. My Oliver 77 front loader is controlled by a two spool double acting control valve. My rear 3 point hitch and rear accessory function is controlled by another identical two spool double acting control valve. The control valve serving the rear functions is installed in series downstream of the control valve for the front loader. My system runs at a maximum of 1200 psig -- that's where my system relief valve is set. The relief valve is always located upstream of any control valves. The system is open center. Both of my control valves have nominal 2500 psig pressure ratings with exhaust cavity ratings of 1000 psig. Therefore, when I use the rear control valve for something that causes the system relief to function, the exhaust cavity of the upstream valve is actually overpressurized by about 200 psig. I have never had a problem but I would not recommend a deliberate overpressure of any amount to anyone else. However, doing so up to the rated exhaust cavity pressure (1000 psig in my case) would not be a problem.

So. Independently evaluate all the maximum pressures involved and the detail ratings of the equipment while operating under the worst case. You may be able to run two valves w/o PB in series afterall -- if you pay careful attention to detail.

While I have not done so, I would think that a double acting, open center control valve could, indeed be used with a single acting cylinder. One way would be to plug one of the two work ports. This would mean you'd run the system up to the relief setting whenever the cylinder is being retracted. Not a particularly good idea. I am assuming that the valve is a typical 3 position, spring loaded to neutral center spool. Alternately I would think you could just plumb the unused work port direct to return and it would work fine, unless I am missing something. You would lose operating pressure to any other function that you might want to use simultaneously with the retract operation so that might be a consideration. Since I haven't actually done this, I'am speculating about a positive result. If anyone knows of a technical reason why such an arrangement would not work, please advise.


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Gerald J.

08-24-2004 18:26:57

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 Re: Hydrauics 101 (long) in reply to dave-o, 08-24-2004 15:48:01  
Generally an open center valve comes with one return port open. There can be another port but it has a plug in it. That plug combines the oil passed through the valve when not actuated and the return oil from the return side of a cylinder and sends the oil to the hydraulic reservoir.

When there are the two ports at the valve outlet side, that valve is usually capable of being converted to power beyond so you can use a series of valves.

A power beyond conversion plug separates those two oil flows and has a thread in it that can supply oil to another valve. Each valve needs a line to the reservoir for the cylinder return oil, but they can be connected together with pipe Ts.

For the open center hydraulic system, the oil flows through the valve(s) and returns all the time. The pump moves oil all the time, mostly at low pressure so there's not much heating. When you switch that flow to a cylinder pressure builds as the cylinder is loaded. That pressure rise requires there be a pressure relief valve in the system to vent excess oil to the return line or you can burst hoses or shear off the pump shaft.

When you connect a single acting cylinder to a 4 way valve, you have only one hose. Works fine going out because the absent return hose is connected by the valve to the return pipe and there's no oil, no pressure and no problem. The trouble arises when you move the valve to return that single acting cylinder. Then the back side of the valve tries to put oil to the back side of the cylinder but the port is blocked and pressure hits the pressure relief valve. So long as the pressure relief valve is big enough for the full flow of the pump and you don't do that often you won't heat the oil too much, but you will heat the oil. You can plumb that cylinder return port on the double acting valve to the return line that goes back to the reservoir, or you can find a valve for open center that is MADE for a single acting cylinder. I happen to have one that I'm not anticipating using. I don't think it has the capability of being converted for power beyond, but the last valve in the line doesn't need power beyond. It came from Energy.

Some tractor (8N and MF) conversion valves have a screw that opens the return line to prevent the pressure build up. So they "convert" for a single acting cylinder.

Baum Hydraulics catalog has a few pages on hydraulics and how to hook them up. They do have a web page. I don't remember the URL.

The sectional valves from Prince that are sold by Northern Tool are very handy when making up a small but complex valve and do include both open and closed center capability and single and double acting cylinders.

On a closed center system, the power beyond port is plugged so oil doesn't pass back to the reservoir except oil returned from a cylinder. There the pump holds the oil at pressure. There's generally no pressure relief valve because the pump destrokes or contains the pressure relief valve. More recent tractors have a mix that runs like a closed center system but at only a couple hundred PSI until there's a load, then the pump pressure is let to rise to its limit. They claim the fast response of the closed center system with less energy wasted moving the oil.

When you add valves to a closed center system you need a return line from each pump and a power beyond plug on all valves but the last one, there you need a closed center plug.

Since the oil pressure is maintained high by the pump in a closed center system, letting a valve open to a blocked port is no problem and so you can use a single or double acting cylinder on a double acting valve with no changes. I do that all the time.

Gerald J.

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08-24-2004 18:21:52

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
John Deere publishing has a 'Fundamentals of Service' text with the title 'Hydraulics'. It and the Army manual is what I use with students teachign hydraulics.

You can connect open center valves on an open center machine in series.

You can connect closed center valves in series on a closed center machine

I have no idea about how to rig a double acting valve (if you mean joystick valve). If you mean a valve that reverses oil flow on a single set of circuits sure you can (with a 1 way cylinder), Ford, David Brown and others have factory valves this way.

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08-24-2004 18:17:19

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
Dave, Power Beyond means the open center valve has a port that the pump flow returns to tank through when you are not operating any circuit. There is a plug you can put in there and route the oil flow through another open center valve and then to tank. If you use a valve for a double acting cyl on a Ram/single acting cyl just run the unused port back to tank so it will not load up the pump. I do this for the snow plow on the W6.
Later Bob

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08-24-2004 17:48:33

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
Dave: Below is a link to the Army"s hydraulic training manual--everything you ever wanted to know and then some. It"s a sure insomnia cure, but there"s some good info there. As far as not being able to hook open center valves in series, all I can say is that there"s gonna be about a million backhoe, loader and excavator owners who are going to be AWFUL mad when they find out their machines were plumbed up wrong from the factory....

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08-24-2004 17:37:54

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
To solve your problem you first need to find out if your tractor has a "closed loop" or "open loop" hydraulics.

Once thats established the proper componets can be selected to accomplish your task.

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08-24-2004 18:25:32

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 Re: A proper hydraulic connection???? in reply to mme, 08-24-2004 17:37:54  
Its open

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Mike M

08-24-2004 17:04:58

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
I agree with the go to town part,but once there find a loader tractor with a full hyd. set up and copy the way it is set up.

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08-24-2004 16:04:47

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 15:48:01  
All 4 of your points are 100% correct.

You need to use power-beyond port, or you will damage things.

You need the _right_ valving to allow constant use of a double valve with a single cylinder. Some tractors have a little twist valve, some valve assemblies have it. The oil flows the wrong way if you don't adjust this little valve, and starves the pump.

Marry the right parts together & all will be well. Put the wrong ones together, & you will have expensive scrap metal.


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08-24-2004 17:07:58

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 Re: A proper hydraulic connection???? in reply to paul, 08-24-2004 16:04:47  
To clarify, the use of valves in series was WITHOUT the use of any kind of "power beyond."

Is there a power beyond PORT? What I have read is "power beyond CONVERSION PLUG." Whats the differance.

Your satment "double valve with single cylinder" is confusing. I was refering to the use of a one-spool 4-way valve (a valve designed for use with a double-acting cylinder) used with a single acting cylinder.

My confusion grows!

Do you have a text or source of informtion about designing hydraulic circuts that you can recommend? Thanks

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08-25-2004 08:19:29

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 Re: A proper hydraulic connection???? in reply to dave-o, 08-24-2004 17:07:58  
Reading these,it looks like alot of confusion is in terminology.Single/double valve or single/double acting-two diff things.Same with cyl's.Joystick? 1 lever for two valves,2double acting,2 single acting,1 of each,vs's 1 lever for each spool.Power beyond,if the valve is convertible the return becomes the port.As other post,the problem w/valves in series,some can't handle the pressure at the return port a second valve can create.Not that there isn't alot put together that way.It all depends (Doesn't everything?),on the components used,not all valves/manufacturer's are the same.None of this takes into account the 'more than one way'& diff parts available.The ref mentioned are good,most stuff I've seen aim to industrial/factory.Just did'nt look in the right place maybe.The sa cyl w/a da valve,hook unused port to return.Maybe better to scout around for a 'standard' loader valve,1 body,2 spools.Used on lots of stuff.Hope this wasn't long & confusing(er).

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Allan in NE

08-24-2004 16:35:39

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 Re: A proper hydraulic connection???? in reply to paul, 08-24-2004 16:04:47  
So Paul,

What you are saying then, is that it would be a heck of a lot easier just to run into town and trade loaders? :>)


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