Hydraulic valve question

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Didn't quite know where to post this, (yea I know a hydraulics forum lol) but I got some good info here before on the subject. I'm putting hydraulics on an Economy Power King tractor and would like to use the hardware I have to keep cost down. What I want to do is use two separate cylinders, one for the belly mower and snow blade and another for the 3 point hitch. I don't have a duel spool valve but I do have two singles. Can I run the pressure line from the pump into one valve and take the out line from the valve (which would normally go to the reservior tank) to the pressure in fitting on the second valve? These are both used on double acting cylinders. Any help appreciated. Please keep wording simple, for the novice. Thanks RON
Reply:Depends on your specific valves and hydraulic circuit.  WARNING!  I'm not familiar with tractor hydraulics, but did study and repair helicopter hydraulics in the army about 50 years ago.Very simplistically stated, hydraulic circuits come in two flavors.  One uses open center valves that simply allow the fluid to flow freely from the pump through the open center of the valve and back to the reservoir whenever the valve is centered.  In the centered, neutral position the system is under very little pressure.  When hydraulic actuation is needed, the flow is diverted to the side branch to do the work and the fluid returning from the actuator passes through a portion of the spool valve and back to the reservoir.The second basic system type uses closed center valves that block flow from the pump when in the neutral position.  After system pressure builds up to operating pressure a separate pressure unloading valve diverts fluid flowing from the pump back to the reservoir at low pressure while holding operating pressure in the line to the control valve.  When actuation is called for the unloading valve senses the drop in pressure in the line to the control valve and diverts fluid from the pump to the actuating circuit.You should know what type of valves you have (open or closed center) in order to decide if you can use your valves and how to plumb them.  (Perhaps the entire tractor world uses only one type of circuit and valve - I don't know but would speculate that they use open center systems for  minimum cost and complexity.) Again speaking very simplistically, open center valves would be plumbed in series as you proposed.  Closed center valves would be plumbed in parallel, with the pressure line split to feed the input port of each valve.  But then you have to provide a pressure unloading valve, which is not cheap.  If you can blow air freely through your valves' inlet ports in the centered position, they are open center valves.  Using closed center valves in a system lacking a pressure unloading valve (or a sophisticated and expensive pressure regulating pump) would lead to instantaneous damage to your pump and/or bursting of your hoses.Since hydraulic system design is actually a fairly sophisticated endeavor, I would suggest getting as much info as you can from a study of existing tractor hydraulics, books, manufacturers, and repairmen.  Proper sizing of pumps, cylinders, valves, and hoses is important to getting the operating speed you want and can manage without wasting power in undersized components.  Filtering, including system purging on initial fillup is very critical to system component life.  Valves plumbed in series must be rated for system operating pressure on the outlet port.  You should provide a pressure relief valve to prevent excessive system pressure in the event of a blocked or overloaded actuator.Have fun.awrightLast edited by awright; 09-27-2008 at 03:31 PM.
Reply:Thanks for that explanation. I do believe the valves I have are open center as you explained. They are all from ag equipment and golf course machines.I will test the valve as you suggested. I actually have the system set up with the one valve and cylinder and it works fine.(maybe I just got lucky) I will be adding the second one after testing. I now have a resevoir that I fabricated and like I said It works fine.I could see the possibility of needing more reserve oil for two cylinders. Is there a formula for cylinder volume and tank size? Is it as simple as one to one plus a certain percentage extra? RON
Reply:Most AG uses open center system. As far as tank size, it depends on the pump. You want a tank at least the size in gallons that you pump puts out. For example, if your pump is a 10 gpm (gallons per minute) you want at least a 10 gallon tank.Ed
Reply:Now you're getting into design areas that are beyond my knowledge and require some reading.  I'm sure design handbooks from the manufacturers will give you rules of thumb for reservoir design.  The basic requirement for reservoir sizing is (1) enough surface area to dissipate heat generated by the pump, and (2) long enough fluid dwell time in the reservoir for bubbles in the return fluid to float up to the surface before they get sucked into the pump inlet.  Bubbles are very damaging to pumps.  If you are drawing bubbles into the pump, it will sound rough.  Looks like Ed has a good rule of thumb for starters.  Actually, that sounds like a familiar rule of thumb.If you already have a reservoir fabricated, just monitor fluid or reservoir surface temperature.  Look up recommended max fluid temp in the literature.  Cooler is ALWAYS better in hydraulics.  If temp seems too high you can either increase reservoir surface area or put a radiator type oil cooler in the return line.  You could also weld cooling air passages through the middle of the tank, but that would be a hassle.  Never put any restriction (like an oil cooler) in the pump intake line and be sure the pump intake line is generously sized and as short and direct as feasible.If you are not getting rid of bubbles before they reach the pump intake, you can weld baffles into the tank to force a long path from return to pump intake lines.  However, you also want low fluid velocity through the reservoir to promote bubbles floating up to the surface.  This is where engineering comes in.Required reservoir capacity is not necessarily increased because you have two control valves and actuators.  It is more determined by pump capacity and the actual amount of work the actuators are doing.  The system doesn't really know or care how many valves and actuators you have but rather, how much hydraulic work you are doing.  If the cylinders are just used occasionally to adjust mower blade height and such, the system is probably just idling at free flow most of the time and not generating much heat beyond that of the idling pump.No need to test the valves if they are already working in your test system.  You would have known pretty quickly if they were closed center.awrightLast edited by awright; 09-27-2008 at 05:10 PM.
Reply:Thanks Edking & Awright. These were the kind of explanations I was looking for. I find this pretty interesting. Going to do some research on the internet and library to learn more. As you said it is an occasional use situation. Heat does not seem to be a factor with the present setup. The pump intake has an unrestricted 5/8 ID line to it that works well. I'll have to get the numbers off the pump and check the GPM. I believe it is a Webster? I can run the motor at just above idle, about 900 RPM and the cylinder action is still strong and crisp.RON
Reply:Explanation ....Open center is constant flow, variable pressure ....  All the oil has to go thru all the valves, free-flow, Valves are plumbed in series.  Pressure is only created when one or several valves are actually operated.Closed center is constant pressure, variable flow.   All the valves are plumbed parallel, think of a main pressure line, with a branch tee for each valve.  NO oil flows, until a valve is operated.   Similar to your house plumbing ......   constant pressure, no flow until you turn the sink or shower on.   Your drain becomes your "return" line.Many valves today, can be converted to one or the other .....  just need to install the correct plug, open center allows thru-flow, closed center shuts it off.The vast majority of ag power supplies (tractors) being produced today are closed-center .....  So many advantages ...  less heat, less oil breakdown, smaller reserviors needed, etc. etc.Many single-purpose power supplies (log splitters is one example) are still open-centered ....   cheaper pump and plumbing ..... and also, is only usually running when needed (unlike a tractor, which can many times run hundreds of hours, for only one hour of actual hydraulic work).  Again, heat and oil breakdown is a factor, log splitters get to cool down during beer breaks Last edited by mark8310; 09-27-2008 at 06:01 PM.
Reply:If it's an open center valve you're going to need a power beyond plug for the first valve in the circuit if it's a constant displacement pump. I.E. gear or vane type.If you don't, when you pressure up the second valve in the circuit you're gonna blow the seals in the first due to you putting pressure on the return circuit, something it was not designed to do.Obviously this is providing the valve doesn't already have one ....Mike
Reply:Well, this is getting a bit more involved than I first thought. Now I'm thinking of using the existing valve to operate both lifts with the one cylinder. (not to difficult, basically run a lift rod from the cylinder connection to the rear 3 point hitch.) or look into finding a 2 spool valve, maybe on e-bay. I have seen some that were reasonable. I've seen a "float valve" referred to in some descriptions, is this something I should be concerned with. Thanks everyone for your help with a non-welding topic. RON
Reply:Float will allow the cylinder to act as if it had no lines to it. Its a 4th position in the valve.  Up , neutral (cylinder locked in place) down and float.  The valve has a detent that locks it in the float position, usually all the way forward.David Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Originally Posted by mark8310The vast majority of ag power supplies (tractors) being produced today are closed-center .....  So many advantages ...  less heat, less oil breakdown, smaller reserviors needed, etc. etc.
Reply:John Deere log skidders use closed center systems with a multiple piston pump and a squash plate.  This operates everything using a priority flow valve for the steering, then brakes, then the rest of the machine.  The pump drives off the front of the crankshaft.Closed center system has the advantage of pressure already there before you pull the valve.  Open center runs cooler and saves on HP when not in use.  Both systems work and both have their place.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:Not intending to pick nits, but hoping to nip what I believe to be an error in terminology before it propagates too far:I think David R intended to refer to a swash plate,  not a squash plate.  My introduction to a swash plate was as a helicopter repairman many moons ago.  The swash plate is tilted and/or lifted/lowered to transfer pilot inputs into cyclic and/or collective pitch control inputs to the rotating blades.Similar thing happens in a variable flow rate, pressure regulated hydraulic piston pump in which the shoes of the multiple pistons in the rotating piston/cylinder assembly slide around the non-rotating,  tiltable swash plate.  With the swash plate normal to the axis of rotation of the cylinder assembly the pistons do not move and there is no fluid pumped.  As the swash plate is tilted (sometimes by operator input but usually by a feedback pressure control piston) variable amounts of fluid are pumped to maintain the desired system pressure.  A truly ingenious mechanism.awrightLast edited by awright; 09-28-2008 at 02:12 PM.
Reply:In my mass of hydraulic salvage I found a filter unit with an inlet and outlet and a screw-on filter. I would like to put this in the system. It would be mounted on the tank. This may be a dumb question but would this go on the return line to the tank? Thanks RON
Reply:Yes.  You never want to place any restriction in the supply line to the pump.  You also want to place the pump as low relative to the fluid level in the reservoir as your physical layout will allow in order to maintain the highest possible pressure at the pump inlet.To do it right, you might want to check the recommended filtering specs on your pump and buy a filter cartridge with the appropriate micron spec.awright