Spool Overlap in Seat Valves and Spool Valves

If we want to classify valves by their construction, we can classify them in two ways: seat valves and spool valves.

Seat Valves

In seat valves, a ball, a conical element, or, less commonly, a disc, is pressed against the seating surface as the closing element.

This type of valve has a tight sealing characteristic. Due to the seating principle, at most three ports can be opened or closed by a single control element.

Here, the overlap is negative. This means that a valve with more than three ports needs to have more than one control element.
A 4/2 directional valve designed on the seating principle is internally made up of two 3/2 directional valves.

Spool Valves

Spool valves are divided into two types: axial spool valves and rotary spool valves. A rotary spool valve consists of one or more pistons rotating within a cylindrical housing. An axial spool valve consists of one or more pistons, arranged in a certain order relative to each other, that can be slid axially within a cylindrical housing.

In spool valves, as the pistons slide axially, multiple connection ports can be opened, connected to each other, or closed as desired.

Depending on the application, the shape of the spool overlap can be set differently for each control edge of the piston.

To prevent the piston from being pressed against the cylinder’s inner surface, the piston’s outer surface is fitted with circular grooves. This way, only fluid friction occurs during the piston’s motion.

If the hydraulic oil becomes contaminated, contaminant particles settle between the piston and the cylinder’s inner wall. Here, they act like an abrasive, causing the cylinder’s inner diameter to enlarge and, as a result, increasing oil leakage.

Comparing Spool Valves and Seat Valves

Spool Overlap

A valve’s behavior when closing or opening is determined by spool overlap. Spool overlap is evaluated as positive, negative, or zero. Each control edge of a piston can have a different overlap.

Spool overlap determines the tolerance between the piston and cylinder, and, related to this, the amount of leakage oil. The overlap effect is important for achieving various advantages. Depending on the application, the appropriate overlap type is chosen.

Positive Spool Overlap

During position change, all ports are disconnected from each other for a short time; there’s no pressure surge. There is a sudden pressure rise causing an abrupt transition and a hard start-up.

Negative Overlap

During position change, all ports are connected to each other briefly; pressure drops for a short time.

Tank-first opening: During position change, the connection between the actuator outlet and the tank is established before the connection between the actuator inlet and the pump.

Zero overlap: Refers to an edge-to-edge overlap condition. This is important for fast position changes in valves, and the switching travel is short.

Pump-first opening: During position change, the connection between the pump and the actuator is established first, followed by the connection between the actuator outlet and the tank.

In multi-position valves, different spool overlaps can exist within the same valve depending on the intended use.

In seat valves, the desired spool overlap is achieved by staggering when the valves engage at different times. When needed, the overlap can be reconfigured as desired by changing the timing of when the valves engage.

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