Open center Closed center?
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xlr82v2
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A closed center valve, when in neutral, does not allow the hyd fluid to flow through it back to the reservoir... In a closed center system, fluid only flows when the valve is open, allowing fluid to flow to and from an actuator. When the valve returns to neutral, all flow stops and the system builds back up to "system pressure". Closed center systems typically have an accumulator to hold system pressure, and a variable displacement pump.
In an Open Center system, fluid is always flowing, either to and from an actuator when a control valve is open, or just flowing through the control valve back to the reservoir when the control valve is in neutral. Open center systems typically don't have an accumulator, and have a fixed displacement pump.
In an Open Center system, fluid is always flowing, either to and from an actuator when a control valve is open, or just flowing through the control valve back to the reservoir when the control valve is in neutral. Open center systems typically don't have an accumulator, and have a fixed displacement pump.
johndeerefarmer
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Closed center systems offer better fuel economy as the pump doesn't run unless it has a demand placed on it.
Better tractors offer closed center systems.
Better tractors offer closed center systems.
RonMar
Elite Member
Closed center systems always maintain system pressure so there is always a load on the primary mover. An open center system has very little load circulating unpressurized oil about the system till that fluid is diverted in a spool valve and pressure is built to perform work.
As mentioned, closed center systems typically employ variable displacement pumps which reduce the flow thru the pressure regulating circuit when no work is being performed. I have seen them used in cranes and hoists where the delay to build system pressure when shifting loads is undesireable. I have also seen them used in some control schemes such as a shipboard controllable pitch propeller. Because of the extra sensors and controls to maintain desired flow and control of the fluid as well as the variable displacement pumps, they are far more expensive to build and maintain so are typically reserved for systems such as cranes that require precise control.
As mentioned, closed center systems typically employ variable displacement pumps which reduce the flow thru the pressure regulating circuit when no work is being performed. I have seen them used in cranes and hoists where the delay to build system pressure when shifting loads is undesireable. I have also seen them used in some control schemes such as a shipboard controllable pitch propeller. Because of the extra sensors and controls to maintain desired flow and control of the fluid as well as the variable displacement pumps, they are far more expensive to build and maintain so are typically reserved for systems such as cranes that require precise control.
AndyinIowa
Silver Member
We need to be careful about terminology here. Closed center systems DON'T always maintain system pressure. Load sense pumps only maintain a margin pressure. Backhoe loaders, wheel loaders, skid steer loaders, etc.. are common applications for load sense piston pumps.
AndyinIowa
AndyinIowa
Farmerford
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Open center systems cost much less to build and maintain, primarily because the typical fixed displacement pump is a gear pump that is less complicated to machine, easier to rebuild, and much more tolerant of fluid quality and particle contamination than variable displacement piston pumps in closed center systems.
But gear (and all other fixed displacement) pumps require that the fluid have a path to flow through all the time since the pump produces a fixed volume of flow per revolution and will be damaged if the fluid is completely blocked. Thus the name "open center", which refers to the open passage (gallery, core) through a valve when the spool is centered that allows the pump flow to pass through the valve unimpeded. When the spool is shifted to direct fluid from the open center passage to a work port, the pump output flows out the work port to the cylinder or motor.
The main shortcoming of an open center system is that to direct the fluid to a motor or cylinder two things have to happen: 1. a path has to be opened from the open center gallery to the desired work port and thus out to the device, and 2. the open center gallery has to be blocked to force the fluid to flow into the work port against the resistance of the device (cylinder) rather than out the other end of the center gallery and back to the pump
(via the reservoir).
This arrangement works pretty well if there is only one valve in the circuit, since blocking the center gallery and directing all fluid out the work port doesn't affect any other hydraulic device (since there are none other in the circuit). However, if the fixed displacement pump is to operate more than one device, the valves must be put in series so that blocking the open center gallery in either valve will stop the flow through both valves (otherwise, no fluid would flow to the work port because it would follow the path of least resistance through the open center of the other valve). But when the valves are in series, the upstream valve will have "priority" over the downstream valve: that is, if the upstream spool is shifted all the way to, say, work port "A", it also blockes the flow through the open center gallery in the upstream valve, and no fluid flows to the downstream valve. Thus, the downstream valve will not function while the upstream valve is fully shifted.
Of course, if the upstream spool is not fully shifted, the open center gallery is only partially blocked and some fluid flows downstream to the next valve, but even the excess flow is at a reduced pressure since the pressure drop across the center valve in the spool will be the same as the pressure drop in the work circuit to which part of the fluid is directed.
The series arrangement is generally not a serious impairment if there are only two or three valves in the system, but if there are several valves in the system (say five circuits in a typical excavator with thumb) and two or three of those valves are often used at the same time, even a skilled operator will have trouble working the valves to keep the right amount of fluid flowing to each valve. Furthermore, the system pressure will be divided among the shifted valves, and no work port will receive full system pressure (in a simple case; there are exceptions).
The answer is a variable displacement pump with closed center valves plumbed in parallel. Fluid does not flow through the center gallery of a valve when the spools are centered. Fluid either flows through the work ports when the spool is shifted, or it does not flow at all. This does not create a problem for the variable displacement pump because it senses the pressure drop in the system when a valve is opened (and fluid flows out of the valve) and increases its flow (up to the pump capacity) as required to provide system pressure to the opened valve. If another valve is opened at the same time, the pump senses the pressure drop caused by the open valve and further increases the flow until system pressure is available at both valves. Thus if the pump has enough capacity (in gpm) it may fully serve every valve in the system at the same time.
Piston pumps and the associated flow regulating devices are much more expensive to manufacture than gear pumps. And piston pumps have many more metal surfaces rubbing each other than gear pumps, so the fluid quality must be very high and the level of particulate contamination kept very low to obtain a reasonable pump life.
In closing (if anyone is still awake), the basic drawback to a fixed displacement system is the difficulty in dividing the flow between two or more work devices. A partial solution is flow dividers, that divide the pump flow between two (or more) circuits, either by giving priority (in gpm) to one circuit or by dividing the flow proportionately (ie, 1/3 to one circuit and 2/3 to the other circuit). Flow dividers are of two types: spool type and gear type. Spool type flow dividers are relatively cheap, but very inefficient because they force the desired amount of fluid to flow out of one port by partially (or totally) blocking the flow of fluid out the other port. For example, my Kubota B2400 has one 5 gpm (approx) gear pump for the power steering and the three point hitch/FEL with a priority valve that sends the first 1 gpm to the power steering pump. If I turn the front wheels to full lock with a load in the FEL, it takes full system pressure to turn the wheels because of the weight on the front end. The priority valve forces 1 gpm to the power steering pump by reducing the orifice by which the excess fluid flows to the TPH/FEL to such a small size that the pressure drop across the orifice is the full system pressure. Therefore, the excess 4 gpm that flows through the priority valve when the power steering is working hard gives up all its energy in the priority valve, and since it doesn't move something (like a piston or motor vane) all the energy is turned into heat. The fluid heats up very quickly.
Of course, the power steering doesn't work hard much of the time, and when it is not working (when the wheels are not turning) the orifice opens up and allows the 4 gpm excess flow to go to the TPH/FEL without a significant pressure drop (because the power steering valve is open and its 1 gpm flows through at very low pressure drop).
But a spool type flow divider on a two valve system where one valve is often open (say to a hydraulic motor that runs constantly) keeps pressure in the system all the time to insure that the open valve receives the proper flow. In effect, it is just a fluid heater.
Gear type flow dividers solve some of the problems of the spool type, but they are expensive (because each section is like a separate "pump") and not very flexible.
But gear (and all other fixed displacement) pumps require that the fluid have a path to flow through all the time since the pump produces a fixed volume of flow per revolution and will be damaged if the fluid is completely blocked. Thus the name "open center", which refers to the open passage (gallery, core) through a valve when the spool is centered that allows the pump flow to pass through the valve unimpeded. When the spool is shifted to direct fluid from the open center passage to a work port, the pump output flows out the work port to the cylinder or motor.
The main shortcoming of an open center system is that to direct the fluid to a motor or cylinder two things have to happen: 1. a path has to be opened from the open center gallery to the desired work port and thus out to the device, and 2. the open center gallery has to be blocked to force the fluid to flow into the work port against the resistance of the device (cylinder) rather than out the other end of the center gallery and back to the pump
(via the reservoir).
This arrangement works pretty well if there is only one valve in the circuit, since blocking the center gallery and directing all fluid out the work port doesn't affect any other hydraulic device (since there are none other in the circuit). However, if the fixed displacement pump is to operate more than one device, the valves must be put in series so that blocking the open center gallery in either valve will stop the flow through both valves (otherwise, no fluid would flow to the work port because it would follow the path of least resistance through the open center of the other valve). But when the valves are in series, the upstream valve will have "priority" over the downstream valve: that is, if the upstream spool is shifted all the way to, say, work port "A", it also blockes the flow through the open center gallery in the upstream valve, and no fluid flows to the downstream valve. Thus, the downstream valve will not function while the upstream valve is fully shifted.
Of course, if the upstream spool is not fully shifted, the open center gallery is only partially blocked and some fluid flows downstream to the next valve, but even the excess flow is at a reduced pressure since the pressure drop across the center valve in the spool will be the same as the pressure drop in the work circuit to which part of the fluid is directed.
The series arrangement is generally not a serious impairment if there are only two or three valves in the system, but if there are several valves in the system (say five circuits in a typical excavator with thumb) and two or three of those valves are often used at the same time, even a skilled operator will have trouble working the valves to keep the right amount of fluid flowing to each valve. Furthermore, the system pressure will be divided among the shifted valves, and no work port will receive full system pressure (in a simple case; there are exceptions).
The answer is a variable displacement pump with closed center valves plumbed in parallel. Fluid does not flow through the center gallery of a valve when the spools are centered. Fluid either flows through the work ports when the spool is shifted, or it does not flow at all. This does not create a problem for the variable displacement pump because it senses the pressure drop in the system when a valve is opened (and fluid flows out of the valve) and increases its flow (up to the pump capacity) as required to provide system pressure to the opened valve. If another valve is opened at the same time, the pump senses the pressure drop caused by the open valve and further increases the flow until system pressure is available at both valves. Thus if the pump has enough capacity (in gpm) it may fully serve every valve in the system at the same time.
Piston pumps and the associated flow regulating devices are much more expensive to manufacture than gear pumps. And piston pumps have many more metal surfaces rubbing each other than gear pumps, so the fluid quality must be very high and the level of particulate contamination kept very low to obtain a reasonable pump life.
In closing (if anyone is still awake), the basic drawback to a fixed displacement system is the difficulty in dividing the flow between two or more work devices. A partial solution is flow dividers, that divide the pump flow between two (or more) circuits, either by giving priority (in gpm) to one circuit or by dividing the flow proportionately (ie, 1/3 to one circuit and 2/3 to the other circuit). Flow dividers are of two types: spool type and gear type. Spool type flow dividers are relatively cheap, but very inefficient because they force the desired amount of fluid to flow out of one port by partially (or totally) blocking the flow of fluid out the other port. For example, my Kubota B2400 has one 5 gpm (approx) gear pump for the power steering and the three point hitch/FEL with a priority valve that sends the first 1 gpm to the power steering pump. If I turn the front wheels to full lock with a load in the FEL, it takes full system pressure to turn the wheels because of the weight on the front end. The priority valve forces 1 gpm to the power steering pump by reducing the orifice by which the excess fluid flows to the TPH/FEL to such a small size that the pressure drop across the orifice is the full system pressure. Therefore, the excess 4 gpm that flows through the priority valve when the power steering is working hard gives up all its energy in the priority valve, and since it doesn't move something (like a piston or motor vane) all the energy is turned into heat. The fluid heats up very quickly.
Of course, the power steering doesn't work hard much of the time, and when it is not working (when the wheels are not turning) the orifice opens up and allows the 4 gpm excess flow to go to the TPH/FEL without a significant pressure drop (because the power steering valve is open and its 1 gpm flows through at very low pressure drop).
But a spool type flow divider on a two valve system where one valve is often open (say to a hydraulic motor that runs constantly) keeps pressure in the system all the time to insure that the open valve receives the proper flow. In effect, it is just a fluid heater.
Gear type flow dividers solve some of the problems of the spool type, but they are expensive (because each section is like a separate "pump") and not very flexible.
xlr82v2
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Farmerford,
Excellent explanation of open/closed center. Thanks!
Excellent explanation of open/closed center. Thanks!
Farmerford
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Brian:
Thanks for the complement. If anyone is still interested, when I finish work today I will fill in a gap by explaining (or at least trying to explain) why a multi-spool valve (monoblock or sectional) such as those on small front end loaders, which would appear at first glance to be series open center valves (since virtually all small tractor pumps are fixed displacement gear pumps), exhibit the characteristics of both series and parallel valves.
Thanks for the complement. If anyone is still interested, when I finish work today I will fill in a gap by explaining (or at least trying to explain) why a multi-spool valve (monoblock or sectional) such as those on small front end loaders, which would appear at first glance to be series open center valves (since virtually all small tractor pumps are fixed displacement gear pumps), exhibit the characteristics of both series and parallel valves.
xlr82v2
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That would be good... I'm more interested in how a gear type flow divider works.
I've been around hydraulics most all of my life, starting with the tractors on the farm, and now in the airplanes I fly. But it's always been one of those vodoo/PFM kind of things. I know the basics pretty well... but when you get into the science of it, I know just enough to be dangerous. Never had any formal schooling on hydraulics.
To me it's fascenating how closely it appears to be related to electricity (in theory anyway.)
Cool Stuff... keep it coming
I've been around hydraulics most all of my life, starting with the tractors on the farm, and now in the airplanes I fly. But it's always been one of those vodoo/PFM kind of things. I know the basics pretty well... but when you get into the science of it, I know just enough to be dangerous. Never had any formal schooling on hydraulics.
To me it's fascenating how closely it appears to be related to electricity (in theory anyway.)
Cool Stuff... keep it coming
RickB
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A load sensing system is different from a pure closed center system which does always maintain high standby pressure. Load sensing systems are desribed as Closed Center Load Sensing (CCLS), Pressure & Flow Compensated (PFC), or other descriptive term other than closed center. I can think of no manufacturer that uses the term Closed Center to describe a closed center load sensing system.AndyinIowa said:
Farmerford
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To anyone who is still awake:
When I posted late last week I talked about open center and closed center systems. And a couple of favorable responses lured me into promising to try to explain why typical FEL valves don't seem to follow the series/parallel rules. Then I promptly got an eye infection that kept me from following up. Here goes.
I (hope that I) explained how in the typical small tractor open center system the valves had to be both open center (to allow fluid to flow through the valve when the spools were centered) and plumbed in series so that the fluid has to pass through the centers of both valves to get back to the pump (reservoir). That way, when a spool on either the upstream or downstream valve is shifted and blocks the open center gallery the fluid is forced out an open work port because it has no other path back to the pump.
This arrangement is most common on small tractors with a front end loader and a three point hitch. Fluid flows from the pump, through the FEL valve, then to the TPH valve, and finally through the TPH valve to the reservoir (usually the transmission case). When a spool in the upstream valve for the FEL is shifted fully to, say, lift the booms, flow through the open center of the FEL valve is blocked and directed out the work port to the base end of the boom cylinders. As long as the boom spool is fully shifted, no fluid flows through the center of the FEL valve to the TPH. Therefore, if you move the TPH control the TPH does not move until the FEL boom spool is shifted back toward center and some of the fluid from the pump then flows through the now open center of the FEL valve and on to the TPH valve.
Move now to the typical two spool valve for the FEL. Each spool is clearly open center because fluid from the fixed displacement pump flows freely through the valve on to the TPH when both spools are centered. But, you can raise the booms and curl the bucket at the same time. How can that be if the two spools must be in series? If they are in series, when you fully shift the upstream spool (say for the booms) don't you block the center gallery? If so, then how can the downstream spool (for the bucket) curl the bucket at the same time you move the booms? The downstream spool should not have any fluid to direct to the bucket cylinders if the upstream spool (booms) has blocked the open center at the upstream spool.
The answer (as you might suspect) lies in the internal plumbing of the FEL valve. Because in a multi-spool valve (monoblock or sectional) the spools are in the same valve body and less than an inch or so apart, it is possible to arrange the internal passages so that the spools act like series spools in their function of blocking pump flow through the open center gallery and at the same time to act like parallel spools in their function of directing fluid from the pump to a work port.
As I sat in the ophthalmologist's office yesterday I drew the attached sketch maybe that's why it is so crappy). It shows a three spool open centered valve with power beyond (but pb does not change the result). Note that fluid from the pump flows in at one end of the center gallery. Note also that the center gallery has two additional branches just after it enters the valve body that are usually called "power cores". Thatｴs because when the center gallery is blocked by a shifted spool, the fluid from the pump under pressure flows into the power cores where it travels through the appropriate open work port to move a cylinder or turn a motor. In the sketch, spool 1 is shifted to (1) connect its "A" work port to a power core, (2) block the center gallery so that fluid can't pass through the valve body and out the power beyond port to the TPH valve, therefore pressuring the fluid in the power core and forcing it out the open spool 1 A work port to move a cylinder, and (3) connect its B work port to the exhaust core where the fluid returning from the other side of the cylinder can return to the tank.
Note that each power core runs to all three spools. Therefore, blocking of the center gallery by upstream #1 spool pressurizes the power cores for all three spools. So if, as in the sketch, downstream spool #3 is shifted to connect its B work port to a power core, fluid will also be forced by pump pressure out of the spool #3 B work port to activate another cylinder. Therefore, the two cylinders at spools 1 and 3 will be connected to the pump in parallel. Each cylinder will get the full system pressure because it is connected directly to the pump via a power core, but the flow will be divided between the two cylinders with the cylinder with the least resistance getting practically all the fluid until its resistance increases to the same amount as the other cylinder, without regard to which one is upstream and which one is downstream.
When you purchase multispool directional control valves, both monoblock (a single casting houses all spools) and sectional (each spool has a separate housing and all housings bolt together), you have to choose whether the spools will be in series or parallel (as in my sketch). Sometimes the arrangement I drew is called "series/parallel". Since most small valve banks work best if parallel, it is unusual to find an off the shelf series monoblock valve because the demand is low. But sectional valves, which are made up from the menu, often let you choose between parallel and series valves. For example, the Surplus Center catalog gives you that choice. Within limits you can mix series and parallel types in the same group.
When I posted late last week I talked about open center and closed center systems. And a couple of favorable responses lured me into promising to try to explain why typical FEL valves don't seem to follow the series/parallel rules. Then I promptly got an eye infection that kept me from following up. Here goes.
I (hope that I) explained how in the typical small tractor open center system the valves had to be both open center (to allow fluid to flow through the valve when the spools were centered) and plumbed in series so that the fluid has to pass through the centers of both valves to get back to the pump (reservoir). That way, when a spool on either the upstream or downstream valve is shifted and blocks the open center gallery the fluid is forced out an open work port because it has no other path back to the pump.
This arrangement is most common on small tractors with a front end loader and a three point hitch. Fluid flows from the pump, through the FEL valve, then to the TPH valve, and finally through the TPH valve to the reservoir (usually the transmission case). When a spool in the upstream valve for the FEL is shifted fully to, say, lift the booms, flow through the open center of the FEL valve is blocked and directed out the work port to the base end of the boom cylinders. As long as the boom spool is fully shifted, no fluid flows through the center of the FEL valve to the TPH. Therefore, if you move the TPH control the TPH does not move until the FEL boom spool is shifted back toward center and some of the fluid from the pump then flows through the now open center of the FEL valve and on to the TPH valve.
Move now to the typical two spool valve for the FEL. Each spool is clearly open center because fluid from the fixed displacement pump flows freely through the valve on to the TPH when both spools are centered. But, you can raise the booms and curl the bucket at the same time. How can that be if the two spools must be in series? If they are in series, when you fully shift the upstream spool (say for the booms) don't you block the center gallery? If so, then how can the downstream spool (for the bucket) curl the bucket at the same time you move the booms? The downstream spool should not have any fluid to direct to the bucket cylinders if the upstream spool (booms) has blocked the open center at the upstream spool.
The answer (as you might suspect) lies in the internal plumbing of the FEL valve. Because in a multi-spool valve (monoblock or sectional) the spools are in the same valve body and less than an inch or so apart, it is possible to arrange the internal passages so that the spools act like series spools in their function of blocking pump flow through the open center gallery and at the same time to act like parallel spools in their function of directing fluid from the pump to a work port.
As I sat in the ophthalmologist's office yesterday I drew the attached sketch maybe that's why it is so crappy). It shows a three spool open centered valve with power beyond (but pb does not change the result). Note that fluid from the pump flows in at one end of the center gallery. Note also that the center gallery has two additional branches just after it enters the valve body that are usually called "power cores". Thatｴs because when the center gallery is blocked by a shifted spool, the fluid from the pump under pressure flows into the power cores where it travels through the appropriate open work port to move a cylinder or turn a motor. In the sketch, spool 1 is shifted to (1) connect its "A" work port to a power core, (2) block the center gallery so that fluid can't pass through the valve body and out the power beyond port to the TPH valve, therefore pressuring the fluid in the power core and forcing it out the open spool 1 A work port to move a cylinder, and (3) connect its B work port to the exhaust core where the fluid returning from the other side of the cylinder can return to the tank.
Note that each power core runs to all three spools. Therefore, blocking of the center gallery by upstream #1 spool pressurizes the power cores for all three spools. So if, as in the sketch, downstream spool #3 is shifted to connect its B work port to a power core, fluid will also be forced by pump pressure out of the spool #3 B work port to activate another cylinder. Therefore, the two cylinders at spools 1 and 3 will be connected to the pump in parallel. Each cylinder will get the full system pressure because it is connected directly to the pump via a power core, but the flow will be divided between the two cylinders with the cylinder with the least resistance getting practically all the fluid until its resistance increases to the same amount as the other cylinder, without regard to which one is upstream and which one is downstream.
When you purchase multispool directional control valves, both monoblock (a single casting houses all spools) and sectional (each spool has a separate housing and all housings bolt together), you have to choose whether the spools will be in series or parallel (as in my sketch). Sometimes the arrangement I drew is called "series/parallel". Since most small valve banks work best if parallel, it is unusual to find an off the shelf series monoblock valve because the demand is low. But sectional valves, which are made up from the menu, often let you choose between parallel and series valves. For example, the Surplus Center catalog gives you that choice. Within limits you can mix series and parallel types in the same group.
Bob_Young
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I'm happy to have found this thread and to have read Farmer Ford's write-ups. I don't understand it all at this point, but a few re-reads and some study will probably help.
I've been going over the specs of some new Ag tractors in the 100 to 130HP range. Have noticed that Case-IH offers three levels within its MX and Maxxum lines: standard, deluxe and Pro....or something like that. I've also noticed that the standard models often come with open center hydraulics while the better models, especially the "Pro" come with closed center. All have pretty hefty hydraulic flows.
I'm wondering what Ag applications benefit the most from the closed center design? Does it have anything to do with running hydraulic motors from tractor hydraulics while maintaining 3PH or FEL functionality? From what Farmer Ford has related, it appears that open center would be the clear choice unless an application specifically demands a closed center design.
Bob
I've been going over the specs of some new Ag tractors in the 100 to 130HP range. Have noticed that Case-IH offers three levels within its MX and Maxxum lines: standard, deluxe and Pro....or something like that. I've also noticed that the standard models often come with open center hydraulics while the better models, especially the "Pro" come with closed center. All have pretty hefty hydraulic flows.
I'm wondering what Ag applications benefit the most from the closed center design? Does it have anything to do with running hydraulic motors from tractor hydraulics while maintaining 3PH or FEL functionality? From what Farmer Ford has related, it appears that open center would be the clear choice unless an application specifically demands a closed center design.
Bob
AndyinIowa
Silver Member
Closed center hydraulics offer better efficiencies and other similar advantages on larger machines. It also is a better fit for a variable piston pump (vs. a gear pump), allowing you to go with higher pressures.
AndyinIowa
AndyinIowa
CraigM
Silver Member
Farmerford, thank you for the sketch and excellent discussion. Maybe you should change your handle to professor_ford. What you described answers most of the questions I have about what goes on inside my loader valve, and adding another hydraulic function to my Kubota B2150 to drive the lift cylinder of a sickle bar mower. I have a feeling that asking one question is going to spawn a few more at first until I can get my brain wrapped around it all.
Is the pb port where you have the 'out' from the center gallery, or is it connected to the power cores? If I connect a valve to pb, where do I return the exhaust flow to? Is there normally a pb-return port also?
On my joystick (photo attached), there are 3 ports in line across the valve body. One is conveniently marked pb, the others are plugged and not marked. Any idea what they may be?
Thank You
Is the pb port where you have the 'out' from the center gallery, or is it connected to the power cores? If I connect a valve to pb, where do I return the exhaust flow to? Is there normally a pb-return port also?
On my joystick (photo attached), there are 3 ports in line across the valve body. One is conveniently marked pb, the others are plugged and not marked. Any idea what they may be?
Thank You
Farmerford
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Craig:
Thanks for the compliment.
The power beyond port (more correctly named "high pressure carry over") is indeed the "out" port on the sketch. The PB is connected to the power cores in the sense that high pressure fluid comes into all three, but the two power cores "dead end" in the valve body: otherwise the fluid would flow right on through when the center gallery is blocked.
The exhaust flow in a valve configured for power beyond must be returned to tank by a separate line from the outlet port of the valve. Therefore (ignoring the hoses from the work ports to the cylinder), the power beyond valve has one hose in for high pressure fluid from the pump, and two hoses out: one out hose to carry the high pressure fluid from the center gallery on to the next valve; and a second hose to carry the low pressure exhaust flow from the cylinders back to the tank.
In a valve not configured for PB, the outlet end of the center gallery and the exhaust cores are connected inside the valve and then flow into a single "out" hose to return to tank. That is, the unused (potentially) high pressure fluid from the pump and exhaust fluid from the cylinders flow through the same hose back to tank.
Here is why you need power beyond if you want another valve downstream. Remember that if the valve does not have power beyond, the downstream end of the center gallery and the exhaust cores are connected. If a downstream spool is shifted, the downstream center gallery is blocked to send fluid to the applicable cylinder. If, say, 1500 psig of pressure is required to move the cylinder, then the pressure in the upstream valve will naturally increase to 1500 psig as the downstream valve forces fluid into the cylinder. But remember that the upstream valve is not PB, so the upstream valve exhaust cores are connected to the center gallery. Therefore, the exhaust cores also "see" 1500 psig.
That pressure is a problem because in typical valves the spools pass through the exhaust cores just before the spools exit valve body. One end of the spool goes to the activating handle, and the other end goes to the spring chamber. Where the spool passes through the valve body the interface between the spool and the valve body is sealed by a simple o-ring. An o-ring is sufficient because in a properly configured valve the exhaust core never sees more than the very nominal pressure required to make the fluid flow back to the tank (and perhaps through the filter). But if a non PB valve is placed upstream of another valve, as in the above example, the upstream exhaust cores (and therefore the o-ring seals) may see full system pressure if the downstream valve is shifted fully and the cylinder allowed to deadhead. This pressure will almost surely damage the o-ring seals and, I have heard but never seen, can split the valve body if there are thin spots in the exhaust core.
So, you need PB mainly to keep unacceptable pressure out of the exhaust core of the upstream valve. Of course, it is possible to seal the ends of the spools where they pass through the valve body. The hydraulic cylinder rods are sealed against full pressure where they exit the cylinder body. But those seals are much longer (usually with several layers), more expensive, and create considerable friction (try moving a cylinder sometime by hand when the lines are disconnected).
PB also is required to make upstream "in valve" relief valves work properly, but that is another subject.
I can't open your picture while I am typing this answer, but I suspect the ports you mention are the ports for the "load checks" (they may or may not have the actual load checks in them). These keep fluid from flowing out of a cylinder backwards through the work ports and into the center gallery towards the pump if you shift the spool when the pump pressure is less than the pressure in the cylinder. Say you have a heavy load on the loader and it is raised half-way up. You want to raise it further and shift the boom spool to send high pressure fluid to the base end of the boom cylinders. But the engine is idling and the pump pressure at idle speed is not sufficient to overcome the pressure that is required to hold the load (much less raise it further). If there were no load checks, the load could fall when you shifted the spool because fluid could flow out of the base end of the cylinder, through the work port, into the center gallery, and either back through the pump (usually very slowly) or through the center gallery and out another work port if another spool were shifted.
Good luck with your project.
Thanks for the compliment.
The power beyond port (more correctly named "high pressure carry over") is indeed the "out" port on the sketch. The PB is connected to the power cores in the sense that high pressure fluid comes into all three, but the two power cores "dead end" in the valve body: otherwise the fluid would flow right on through when the center gallery is blocked.
The exhaust flow in a valve configured for power beyond must be returned to tank by a separate line from the outlet port of the valve. Therefore (ignoring the hoses from the work ports to the cylinder), the power beyond valve has one hose in for high pressure fluid from the pump, and two hoses out: one out hose to carry the high pressure fluid from the center gallery on to the next valve; and a second hose to carry the low pressure exhaust flow from the cylinders back to the tank.
In a valve not configured for PB, the outlet end of the center gallery and the exhaust cores are connected inside the valve and then flow into a single "out" hose to return to tank. That is, the unused (potentially) high pressure fluid from the pump and exhaust fluid from the cylinders flow through the same hose back to tank.
Here is why you need power beyond if you want another valve downstream. Remember that if the valve does not have power beyond, the downstream end of the center gallery and the exhaust cores are connected. If a downstream spool is shifted, the downstream center gallery is blocked to send fluid to the applicable cylinder. If, say, 1500 psig of pressure is required to move the cylinder, then the pressure in the upstream valve will naturally increase to 1500 psig as the downstream valve forces fluid into the cylinder. But remember that the upstream valve is not PB, so the upstream valve exhaust cores are connected to the center gallery. Therefore, the exhaust cores also "see" 1500 psig.
That pressure is a problem because in typical valves the spools pass through the exhaust cores just before the spools exit valve body. One end of the spool goes to the activating handle, and the other end goes to the spring chamber. Where the spool passes through the valve body the interface between the spool and the valve body is sealed by a simple o-ring. An o-ring is sufficient because in a properly configured valve the exhaust core never sees more than the very nominal pressure required to make the fluid flow back to the tank (and perhaps through the filter). But if a non PB valve is placed upstream of another valve, as in the above example, the upstream exhaust cores (and therefore the o-ring seals) may see full system pressure if the downstream valve is shifted fully and the cylinder allowed to deadhead. This pressure will almost surely damage the o-ring seals and, I have heard but never seen, can split the valve body if there are thin spots in the exhaust core.
So, you need PB mainly to keep unacceptable pressure out of the exhaust core of the upstream valve. Of course, it is possible to seal the ends of the spools where they pass through the valve body. The hydraulic cylinder rods are sealed against full pressure where they exit the cylinder body. But those seals are much longer (usually with several layers), more expensive, and create considerable friction (try moving a cylinder sometime by hand when the lines are disconnected).
PB also is required to make upstream "in valve" relief valves work properly, but that is another subject.
I can't open your picture while I am typing this answer, but I suspect the ports you mention are the ports for the "load checks" (they may or may not have the actual load checks in them). These keep fluid from flowing out of a cylinder backwards through the work ports and into the center gallery towards the pump if you shift the spool when the pump pressure is less than the pressure in the cylinder. Say you have a heavy load on the loader and it is raised half-way up. You want to raise it further and shift the boom spool to send high pressure fluid to the base end of the boom cylinders. But the engine is idling and the pump pressure at idle speed is not sufficient to overcome the pressure that is required to hold the load (much less raise it further). If there were no load checks, the load could fall when you shifted the spool because fluid could flow out of the base end of the cylinder, through the work port, into the center gallery, and either back through the pump (usually very slowly) or through the center gallery and out another work port if another spool were shifted.
Good luck with your project.
hydroholic
New member
Farmerford,
New guy here. Retired electronics tech and agree, hydraulics is a lot like an electronic circuit. I read with GREAT interest your explanation regarding "open/closed" center valves (spools) (bear with me here I'm still learning the ropes)
I have a JD401-C loader and 3 point hitch. The bucket and lift valves are of the "closed" type. (I have the manual) I am trying to adapt an "auxiliary" valve to control a grappling hook on my bucket. No joy so far. I tried 'inserting' my GB (grappling bucket) valve in 'series' with the bucket valve and while the bucket would work ok the GB did not. I believe the GB valve is of the 'open' center type. (when I pump compressed air into the feed ports and switch the lever to either an up or down position, air freely blows through those ports. Then when in neutral air goes in and then out through the hi side and the low side ports)
At this point I'm not sure of what question to ask next other than I would like to weld a plate just above my bucket valve and mount my grappling hook 'gadget' to my bucket in order to pick up tree limbs etc..) HELP, please.
J_J
Super Star Member
- Joined
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- JACKSONVILLE, FL
- Tractor
- Power-Trac 1445, KUBOTA B-9200HST
Closed center valves are mounted in parallel.
Some valves can be converted to closed center by selecting the correct plug .
If your valve can not be converted to closed center, then purchase a valve that can.
Maybe one like this.
Single spool valve, add the closed center plug and you are good to go.
Surplus Center - 1 SPOOL 14 GPM PRINCE WVS11B5C1 DA VALVE
http://www.surpluscenter.com/item.asp?catname=hydraulic&item=9-7998-PB
Some valves can be converted to closed center by selecting the correct plug .
If your valve can not be converted to closed center, then purchase a valve that can.
Maybe one like this.
Single spool valve, add the closed center plug and you are good to go.
Surplus Center - 1 SPOOL 14 GPM PRINCE WVS11B5C1 DA VALVE
http://www.surpluscenter.com/item.asp?catname=hydraulic&item=9-7998-PB
