Re: A proper hydraulic connection????

Posted by Rod (NH) on August 24, 2004 at 18:55:01 from (12.148.160.138):

In Reply to: A proper hydraulic connection???? posted by dave-o on August 24, 2004 at 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.

Rod

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