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.
