Why choose a Piston Spool Valve?

This blog is the next in a series of blog posts contributed by Philipp Wahl, Festo AG & Co. KG. Mr. Wahl introduces valve technologies, what we need to know about poppet valves, piston spool valves and the cartridge principle.  He will also give us a valve selection aid to help us determine which valve type suits us best.

Poppet valves, piston spool valves and the cartridge principle — what you need to know

The piston spool valve
Whereas the actuation forces in poppet valves are relatively great, they are much smaller in piston spool valves, because the valves do not have to be switched against any forces generated by the operating pressure. The problem with overlap is also easily solved, as can be seen in figure 3 above.

However, there are drawbacks with this valve type too. For example, the strokes for switching operating positions are longer than with poppet valves. These longer strokes and the placement of several pistons mean that the piston spool valves on the whole have to be longer. In terms of manufacturing, the piston spool valves are also more challenging, particularly when it comes to sealing technology. Two categories have evolved in this area.  I will cover category 1 today:

Category 1: Hard-sealing systems
These sealing systems are more durable, but also more demanding. The durability is a result of the absence of soft rubber sealing elements in the piston spool valve. This means that valves using this sealing technology are immediately ready for full dynamic use even after longer periods of inactivity. In soft-sealing systems, there may be adhesive effects, which cause the sealing elements to wear out faster. The greatest challenge with the system is that the air gap surrounding the moving piston must not be larger than a few micrometres. A metal sleeve is often used, in which the valve spool moves. The sleeve then works both as a seal and as a guide for the valve spool. It is crucial that the sleeve and the spool are made with the same material, since different materials have different thermal expansion coefficients. If this is not taken into consideration, greater leakage will occur, and even jamming of the spool. The combination of the sleeve and the spool therefore has to comply with many technical specifications, since even the smallest deformation in the housing, e.g., due to tightening of screws, can damage sleeves and affect the life and leakage values of the valve.

Figure 4: Hard-sealing system

In conclusion, hard-sealing systems are very robust and durable. The greatest drawback is that there is always a certain leakage with these systems because of the air gap. Furthermore, their flow capacity is lower than that of other valves with the same constructional dimensions.

In his next blog post Mr. Wahl will talk about soft-sealing systems and the cartridge principle for piston spool valves.