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open center/ closed center
5

open center/ closed center

open center/ closed center

(OP)
When using a closed center valve what is the normal practice used to keep pressure off pump when valves are in center posistion? Doing no work.
I used air to check if fluid would circulate through the valve when spools are in center posistion. No circulation which leads me to beleive the pump would be under a High load when doing no work.
Using a Husco 5000 series double action motor valve in tandem and wish to controll 2 motors in forward, reverse and one forward while the other in reverse. Like in a zero turn mower. Built this thing from scratch. Only a fool like me would do this. I have a million hours in it and enough $ to buy a cheap one. Don't recomend anyone to build your owne. I understand fluid circuts untill I get to the question above. Thanks.

RE: open center/ closed center

You need a pump unloader valve that directs the pump flow to tank when your directional valve blocks pump flow.

Ted

RE: open center/ closed center

3
You'll still want to direct all fluid returning to the tank through a heat exchanger.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: open center/ closed center

closed center is usually matched with a variable displacement pressure compensated pump. The pump destrokes and does not go across relief valve when flow is not required.

It can also be used with a fixed displacement pump if there is a load sensing unloading valve (externally or in the valve inlet section) but it requires a load sense signal from each of the spool sections. Essentially, that load sense signal controls the unloading of the pump when  work is or is not being done. If it is not already in the valve stack I think you would be better off getting a new valve. To convert the spools, or add external load sense shuttles, is beyond the normal back yard skills.

If the valve is commonly available and not obsolete, you might be able to get open center cylinder or motor spools and convert the spools only, without trying to add load sensing. Check with the rep for  the valve brand and part numbers.

Changing to variable pump may be possible, but the control and heat generation under most operation at light loads make this a less desireable way to go IMO.

kcj

RE: open center/ closed center

(OP)
Thanks Hydtools. I know of underloader valves in pressure washers however cannot find one for hydraulics. These valves can easly be converted  to open center which will solve the pressure problem however I don't quite understand how it would get pressure if it is freely circulating. Must be done in the spool. I will atempt to call manufacture to learn more. MIKE I plan on installing a large oil cooler made from 3/4" hydronic heating element and was going to install on suction side to prevent the possiability of high pressure. They are only good to about 60 psi. and remove heat very eficently. I will post my findings. Thanks again

RE: open center/ closed center

(OP)
KCJ I was replying as your post came through. That makes sense to me. The Husco 5000 series are very adaptable and looking at drawings and specs it apears all need to be done is drill for sae fitting at end of aray for power beyond and problem solved. Please what is your thought on this.
Thanks again.

RE: open center/ closed center

You get pressure in an open center system by blocking the flow from pump to tank, which is what an open center valve does as soon as you move it off center.  Just like in automobile power steering systems, where the pump pressure is essentially zero until the servovalve directs flow to the steering cylinder.  

  
 

Mike Halloran
Pembroke Pines, FL, USA

RE: open center/ closed center

Do not put the cooler in the suction line.  You run the risk of cavitating the pump especially on cold startup.  Put the cooler in the return line just before the filter to the tank.  This a low pressure section of the circuit and should present no overpressure risk for your cooler.

Ted

RE: open center/ closed center

Sun makes QCDB-*** unloading cartridges that work in conjunction with a RVEA-*** valve for a close center circuit.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: open center/ closed center

Power beyond is different than open/closed center description. The return flow from a function goes to tank, the unused flow continues in the PB circuit. You would have to provide a sleeve to separate the tank flows, not just drill an additional port.

Mike described the center of spool condition.

Open center descibes the P & T connection only. A & B connections are separate terms. It can have blocked work ports A&B (usually called cylinder spools) that hold position, or can have AB connected to P & T (motor spools) that float.

You should be able to exchange the spools to open center, but it may get pricey.

+1 on the cooler, no restrictions on the suction side. Typically put a 50 psi or so check/releif valve around the cooler so if ther are surges or at cold start the maximum pressure is limited.

ed, I think the sun numbers for check and unloading valves require an accumulator circuit to store energy and unload the gear pump. without an accumualtor they would short cycle or chatter on and off, ormaybe act as a stright relief valve converting all the flow to heat.

kcj

RE: open center/ closed center

kcj,

You are correct, an unloader circuit with a gear pump must have an accumulator. This type of system is best for functions that cycle from high flow to no flow.

For a wheel drive steering system you need a tandem center spool for each wheel. This will create more heat than a hydrostatic pump and motor an each wheel but will be less expensive.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: open center/ closed center

(OP)
This site is awesome; I can't believe all the great people sharing knowledge. I hope I can be of some use in time.
 After reading all the input I will redesign the drive system (the whole system). As of now I am designing around parts I can get at surplus center which is overstocks, takeoffs, and discontinued, most are new and the prices seem good. There descriptions are such I can understand. I have looked for a real hydraulic supply house online and they all seem incomplete.  I will explain new design.
I have calculated using motors I have (6.2cu in CHAR-LYNN 104-1062) that 10gpm (5 to each) will speed with tire size to almost 14 mph which is plenty enough speed. Starting at the tank to pump(0.73cu in gear pump) (through suction filter) than to a pair (in parallel) of motor speed (flow) controls  (pressure compensated) with adjustable relief then sending controlled flow to a electric reversing valve, sending  un-used flow to 4 way joystick (for mower deck to up/down and side to side movement) then back to tank through cooler. (Bypass around cooler with pressure valve good idea jck) Electric reversing valve will forward and reverse drive motors.  Steering accomplished by varying speed controls.  Zero turn by reversing one or the other reversing valves. Going to do all the steering and drive with foot pedal and steering wheel, can't be the same as all the rest of zero turn mowers out there.
What are the things to look for as far as this circuit is concerned?  I find hydraulics is likened to a combination of plumbing and electronics. Accumulators/capacitor, flow valve/potentiometer and must be considered.
This board is great, thanks again.
 

Louisville,Ky

RE: open center/ closed center

Yeah, hydraulics is like electronics, except:

- Kirchoff's Laws apply, but the resistors are nonlinear.

- you can get killed in different ways.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: open center/ closed center

The Husco 5000 valve should be able to be converted to an open center valve assembly by changing or modifying the power beyond adapter in the outlet cover. Any local hydraulic shop should be able to help you identify the solution if you take the valve to them and show them.

Always put the cooler in the return line before the filter.

The cooler will generally handle more back pressure than the filter will.

RE: open center/ closed center

Use a check valve to bypass the cooler when the oil is cold.  Use a check valve that cracks open at about 60psi.  The check valve will open when the resistance to flow through the cooler reaches or exceeds 60psi.  Cold oil will flow through the check valve around the cooler.  As the oil temperature rises the pressure drop through the cooler will drop and the check valve will close.  Plumb the check valve to flow around the cooler from cooler inlet to cooler outlet; check to block reverse flow from cooler outlet to inlet.  Simpler setup than a pressure control valve.

Ted

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