What happens to the fluid in the neutral position on an open center valve?

[bjcsc]

Why not just use a valve with a motor spool?

 

[J_J]

Why not just use a valve with a motor spool?

Of course you would use a motor spool valve with a hydraulic motor, but the situation was, can you use an open center valve on the tractor to run a hydraulic motor. The answer is, yes, if plumbed right. All tractors don't have motor spools, so one has to be innovative. We were discussing a hydraulic motor turning a large bush-hog blade . If you pull the open center valve back to neutral, or it pops out of detent, at full speed, bad things will happen.

 

[john_bud]

We were discussing a hydraulic motor turning a large bush-hog blade . If you pull the open center valve back to neutral, or it pops out of detent, at full speed, bad things will happen.

Exactly! Loud expensive noises will occur...

jb

 

[SPYDERLK]

Of course you would use a motor spool valve with a hydraulic motor, but the situation was, can you use an open center valve on the tractor to run a hydraulic motor. The answer is, yes, if plumbed right. All tractors don't have motor spools, so one has to be innovative. We were discussing a hydraulic motor turning a large bush-hog blade . If you pull the open center valve back to neutral, or it pops out of detent, at full speed, bad things will happen.

You are referring to a tandem center valve. Its a variation of a true open center that closes off the work ports while routing the supply flow on thru. These are what is used for our remotes. A true open center connects all lines to the return port when centered - perfect for a motor.
larry

 

[Farmerford]

One disagrees with Larry at his own risk, but I think "tandem" means the same thing in hydraulics that it does in other contexts: one behind the other.

In multi-spool directional control valves (monoblock or sectional), the spools are organized in "tandem", "parallel", or a combination of both (with certain limits).

With tandem spools, the pressurized fluid from the pump port flows through the center section of first the spool nearest the inlet and then through the center sections of the other downstream spools in turn. When a spool is shifted fully, it blocks the center power core through which the fluid flows so that fluid can not pass to the downstream spools. The fluid is available only to the work ports of the shifted spool. These are also called "series pools" and "priority spools". Tandem spools can be open center or closed center (which refers to the status of the power core when the spools are in neutral) and they can be cylinder spools (the work ports are blocked in neutral to keep the cylinder from shifting) or motor spools (the work ports are connected to tank so the motor can free wheel).

With parallel spools, when any spool is shifted in an open center spool, the flow of pressurized fluid out of the valve body is blocked, but "parallel" cores in the valve body make the fluid now trapped in the body available to all spools. Thus, the spools are in parallel, and any or all of them can be shifted to send fluid to the work ports. Of course, if more than one spool is shifted the fluid will flow out the work port with the least resistance.

Like tandem spools, parallel spools can be open center or closed center (which refers to the status of the power core when the spools are in neutral) and they can be cylinder spools (the work ports are blocked in neutral to keep the cylinder from shifting) or motor spools (the work ports are connected to tank so the motor can free wheel).

Most valves are parallel spool. In the FEL, this lets you both lift the booms and curl the bucket at the same time. Shifting either the boom or bucket spool blocks all fluid flow from the valve body, and the fluid will flow to the shifted spool's work ports or both shifted spools if that is the case.

If the FEL valve were a tandem (series) spool configuration, once you shifted the valve nearest the inlet (the upstream spool), all fluid would be directed out of its work port and no fluid would be available for the downstream spool (unless you feather the upstream spool).

So, the best arrangement for a hydraulic motor is a motor spool (with the work ports open to tank in neutral so the motor can coast to a stop), which protects against a sudden closing of the valve and allows the motor to start and stop without jerking, and, if there is a chance of the pump stalling with a high inertial load on the motor, a check valve across the motor ports to provide a source of fluid (from the motor exhaust) to replace the fluid no longer flowing from the pump.

 

[SPYDERLK]

Oil Hydraulic Systems: Principles ... - Google Book Search

One disagrees with Larry at his own risk, but I think "tandem" means the same thing in hydraulics that it does in other contexts: one behind the other.

In multi-spool directional control valves (monoblock or sectional), the spools are organized in "tandem", "parallel", or a combination of both (with certain limits).

With tandem spools, the pressurized fluid from the pump port flows through the center section of first the spool nearest the inlet and then through the center sections of the other downstream spools in turn. When a spool is shifted fully, it blocks the center power core through which the fluid flows so that fluid can not pass to the downstream spools. The fluid is available only to the work ports of the shifted spool. These are also called "series pools" and "priority spools". Tandem spools can be open center or closed center (which refers to the status of the power core when the spools are in neutral) and they can be cylinder spools (the work ports are blocked in neutral to keep the cylinder from shifting) or motor spools (the work ports are connected to tank so the motor can free wheel).

With parallel spools, when any spool is shifted in an open center spool, the flow of pressurized fluid out of the valve body is blocked, but "parallel" cores in the valve body make the fluid now trapped in the body available to all spools. Thus, the spools are in parallel, and any or all of them can be shifted to send fluid to the work ports. Of course, if more than one spool is shifted the fluid will flow out the work port with the least resistance.

Like tandem spools, parallel spools can be open center or closed center (which refers to the status of the power core when the spools are in neutral) and they can be cylinder spools (the work ports are blocked in neutral to keep the cylinder from shifting) or motor spools (the work ports are connected to tank so the motor can free wheel).

Most valves are parallel spool. In the FEL, this lets you both lift the booms and curl the bucket at the same time. Shifting either the boom or bucket spool blocks all fluid flow from the valve body, and the fluid will flow to the shifted spool's work ports or both shifted spools if that is the case.

I think tandem is used in a looser sense as -moving or acting together. If you draw a valve as a square with pressure and return on the top and bottom and the workports on the sides you see a tandem motion of the arrows depicting flow as the spool is shifted. That may be where the name "tandem center" comes from. Dont know. Interesting re // valves. FEL valves seem to behave as tandem center, but with 2 spools being supplied from the center.
larry