Directional Control Valves - Part 2

Spool Selection:

Spools in Directional Control Valves come in various configurations depending on the applications they are required for. The extreme positions usually have the same configuration where P is connected to A and B is connected to T at one extreme and vice versa for the opposite side. Some of the more common spool types are listed below with examples of some of the applications they serve.

Spools for 2-Position Valves:

2-Position spool valves have only one actuator and the spools are usually spring offset. Although they have all the four ports ( P, T, A and B) on their interface, they may or may not necessarily use all of these ports. All 2-position spools can either be normally-open or normally-closed.

This is a misnomer, however. It is not necessary for the valve to be “closed” or “open” it may just be redirecting flow to a different direction. It simply means that the solenoid and the spring switch positions so the neutral position changes.

On-Off Spool: 

 On-Off spools are the most basic of all spools. They allow flow to pass in one position and restrict flow path in the second position. If the neutral position allows for flow, the valve is called “normally-open” and if flow is restricted in the neutral position, the valve is “normally closed”. The valve is most commonly used for switching on or switching off flows to different sub circuits or as a main on-off valve. It can also be used as a manual drain valve to unload pumps or accumulators. It should be noted that since the valve is of the spool type, it may have certain leakages. For simple on-off valves, it is more plausible to use Solenoid Poppet valves.

Selector Spools (Exhaust Spool): 

Selector Spools are used when one inlet has to feed two outlets depending upon the actuation. Depending upon the connections made to the various ports of the valves, they are also called Exhaust Spools. Exhaust spools are used with Pressure Gauges or Hydraulic Clutches or Parking Brakes or any other accessory that needs to be referenced to tank when not in use. For a normally closed valve, when actuated, the pressure line is connected to the accessory at the outlet; let us say it is a pressure gauge, which gives us the pressure reading. On releasing the actuator, the pressure gauge is reference back to tank, which is connected to the third port, and hence shows zero pressure. For a normally open valve, in the neutral position, the valve is referenced to tank, which, in the case of Parking Brakes, keeps them engaged. Using a hydraulic pressure line as an actuator, the pressure in the two lines of the motor is sensed and opens the valve allowing the pressure line to be connected to the parking brake cylinder, releasing it.

Parallel and Cross Spool: 

Parallel and Cross spools are spools that connect P to A and B to T in the parallel position and vice versa in the cross position. Parallel and cross- positions are more commonly the extreme positions in 3 position valves. These are versatile spools that can be used for any of the applications above simply by blocking the port not required on the subplate.

Spools for 3-Position Valves:

3-Position spool valves have two actuators and the spools are usually spring centered (although they can be detented). They usually use all the four ports ( P, T, A and B) on their interface.

Closed Spool (Cylinder Spool): 

Closed spools have their ports isolated from each other in the central position. The lands are fairly large which keeps leakages down to a minimum. This spool is also the easiest to manufacture. These valves are not to be used as load holding valves in the puritan sense since a certain amount of leakage can expected in this configuration (5 to 10 cc/ min from A and B to T). The transition positions can be either closed as well or completely open.

In the central position, pump flow has no recourse to the tank line due to the blocked position. Hence the pump either needs to be pressure-compensated where the system pressure is at the pump compensator setting until all pump flow is going to the actuators at their working pressures or unloaded to tank either with a relief valve which will relieve it at the set pressure or an unloading valve which unloads the valve at a minimum pressure and conserves power and electricity. The spool is used in cylinders which cannot be drained to tank and need to be kept pressurized (but only for a short while) and the P port is needed in a secondary system to perform other operations.

Float Spool (Motor Spool): 

Float spool valves have their A and B ports connected to T in the central position. Again in this spool, the lands are sufficiently long to prevent massive leakages between ports. The valves are used to relieve pressures at the A and B ports while keeping the P port isolated in order to service a secondary system or be separately unloaded to tank. The valve is commonly used with hydraulic motors (hence the colloquial term “motor spool”). This is because when the oil supply to motor is cut off, the momentum of the motor keeps it rotating. Oil is picked up from the inlet port due to rotational inertia and is deposited to the outlet port. Hence, instead of the system driving the motor, the motor drives the system. 

Connecting A and B to T serve two purposes; the first is any spike at the outlet due to excess oil being deposited is prevented since it connects to tank (unlike in blocked centre where the oil would have nowhere to go). The second is that the inlet has a direct line to tank in the case oil is required in the prevention of cavitation which may permanently damage the pump.

Another common use of this spool is with single or double Pilot Operated Check Valves or Overcentre Valves. These valves, which serve as means to lock cylinders in place, need their downstream ports connected to tank. For Pilot Check Valves this is essential since any pressure entrained in the line may serve as a pilot pressure and open the pilot check valve in the opposite line. 

For Overcentre Valves, they act as load holding valves as well as thermal relief valves. Not only can a wrong pilot signal be given to the opposite line as in the case of Pilot Check Valves, but the quick draining of the Pilot port as well as the Valve port makes it easy for the poppet to sit quickly and give a leak free performance.Also in the event of a pressure spike caused by an external load or temperature increase, the relieved oil needs to be given a direct pathway to tank.system. Connecting A and B to T serve two purposes; the first is any spike at the outlet due to excess oil being deposited is prevented since it connects to tank (unlike in blocked centre where the oil would have nowhere to go). The second is that the inlet has a direct line to tank in the case oil is required in the prevention of cavitation which may permanently damage the pump.

Open Centre Spool: 

Open Centre spools have all ports open to each other in the central position. However, due to the smaller land width that needs to be maintained in order to allow flow, in the actuated positions, cross port leakages are highly probable.

One of the advantage of this spool is the pump directly unloads to tank. A circuit normally using a fixed-volume pump is used in conjunction with this type of valve. Ideally, the open center allows all of the pump’s flow return to tank with little or no back-pressure. This saves energy and reduces heat to the point that a heat exchanger is not necessary on most circuits. But the advantages end there.

 These spools find their use in applications where the valves are not taken up to the maximum flow and the cylinder is horizontal. Since the pump is constantly unloads to tank in the central position, clogging of the filter and restrictions in the line are possible which may lead to higher back pressures. Using this spool for vertical cylinder is impossible without holding valves as the ports are connected to tank and would bring the cylinder down. Even the use of Pilot Operated Check Valves or Overcentre Valves is debatable as the Pilot port may end up being pressurized due to restrictions in the tank line. 

Furthermore, even in horizontal cylinders, the restrictions in the tank line would put pressure on both the sides of cylinder (i.e. the head end as well as the rod end). Ultimately, in the neutral position the cylinder would gradually extend; a funny but true situation! This is because, the head end, having a greater area, would create a greater force forward for the same pressure as would the rod end. With this force imbalance, the cylinder would end up extending even in the central position!

These spools are also extensively used in vehicles where any external load, such as a pull or a push, can be adjusted without any cavitation. Equipment such as front-end loaders when they need to crawl without load or road paving machines where asphalt has to be laid in an even fashion find use of Open Centre Spools. In Front End Loaders or Side Dump Loaders needing to push material on the ground, the cylinder has to be biased to extend without there being any real pressure from the pump. At the same time, an obstacle such as a bump on the road causing the implement to get stuck may cause severe damage to the system. With Open Centre Spools, the cylinder is allowed to follow the contour of the path without being too rigid with minimal effort on the part of the pump thus reducing the power needed as well as protecting the implement and cylinders. 

Tandem Spool:

Tandem Spools connect the P port to the T port in the neutral position while blocking the A and B ports. Flow is directed from the P to the T port, ideally at a low pressure. Since there is no Use of the pump in the neutral position, two or more Tandem Centre valves can be connected in series to be sequentially operated. This may incite a lot of designers to opt for series connected Tandem Centre Valves, but beware! A circuit may look good on paper, but can run hot because of wasted energy. The spool is hollow, and ports P and T have cross drilled holes. This substantially increases the pressure drop of the valve for oil flowing from P to T. Because of the higher pressure drop most manufacturers’ catalogs show a lower nominal flow or higher pressure drop curve for tandem-center valves. Connecting these valves in series will only amplify the pressure drop since it is additive. Two or three valves connected in series could have pressure drops as high as 20-30 bar for large flows!

Tandem Centre Spools can beconnected in series to actuate systems one after the other using the same pump. The back pressure at T will be the cumulative back pressures of Valves 1, 2, 3 and 4

Another cause for concern is when the second spool is in operation, the load on the second P port is felt on the tank line of the first T port. In many Directional Control Valves, the tank line pressure is limited to 140 or 210 bar while the load on the P, A and B ports could be as high as 350 bar. This may cause damage to the valve if it is not used in a sensibly designed circuit.

Pressure Spool:

Pressure Spools have the P line connected to A and B to allow the actuator ports to remain pressurized in the neutral position. Equal pressures on both the actuator ports would not affect rotary actuators such as hydraulic motors. Liner actuators, however, would extend in the neutral
position due to unequal forces unless double rod cylinders are used. 

Although not very commonly seen in regular practice, they are used in machine tools with dual clamping systems where single acting cylinders are used. Here either one of the clamps has to be engaged with the work piece when it is being fed into the machine. While the operation is being performed, both clamps need to be engaged with the work piece for better stability.

Regenerative Spool:

Regenerative Spools have the P line connected to T in the center or neutral position to allow the pump to unload into the tank. In one extreme position, the A and B ports are connected to the P Line. This allows the flow coming out of the rod end of the cylinder (the ‘B’ line in this case) to feed oil into the cap end of the cylinder. This type of a system is quote common in hydraulics and is referred to as a “Regenerative Circuit”. 

(Left) Regeneration using two check valves. (Middle) Regeneration using a sequence valve and a check valve 

(Right) Regeneration using regenerative directional control valves.

Regenerative Circuits are used to save on the cost of the pump since oil is reused and a smaller capacity pump can be used for the same circuit (In cases of cylinders with 1:2 area ratios, the pump capacity will be slighter greater than half the original size required). Although these spools are slightly more expensive than the other spools mentioned earlier, there are huge savings on external valves to create the same regenerative circuit.

In the case shown in the spool allows the actuator ports to connect in the retract position. Cylinders would extend quicker than in other valves in this position due to outgoing flow being put back into the cap side. Although not very commonly seen in regular practice due to unawareness, they are used in many applications where the force of the cylinder is not in question but simply the speed. This is because pressurizing both ends of the cylinder would  mean a resultant extension, albeit reduced, force due to the pressure on the rod end acting against the pressure on the cap end. As in all cases, the regenerative spool also has its pro’s and cons!

For a view at our full range of Directional Control Valves, click here.

Click Here for Directional Control Valves - Part 1 
Click Here for Directional Control Valves - Part 3