Why choose a Soft-Sealing System on a Piston?

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

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.

Category 2: Soft-sealing systems

A technical alternative to hard-sealing systems is to use rubber gaskets (O-rings or moulded elastomer seals) on the piston.

The problem with this alternative is that the rubber gaskets may be worn down fast when in contact with the channel edge – the so-called control edge. The control edge must therefore be carefully designed, as only optimal planning of the control edge, the moulded elastomer seal and the piston guide can guarantee a soft passage with a minimum degree of wear.

Piston_Spool_Valves (006)

Figure 6: Soft-sealing system with seal in the housing

There is, however, yet another alternative. As illustrated in figure 6, the seals can be mounted directly in the valve housing. This looks like a great option, but there are two disadvantages.

Firstly, the recesses required are difficult to produce and, secondly, the Bernoulli effect will cause the seal to be pulled out at a pressure of approx. 8 bar or higher. This again causes greater wear on the seal.

Figure 7 shows the process in detail:

Figure 7: Increased seal wear due to the Bernoulli effect at a pressure of approx. 8 bar or higher.

Figure 7: Increased seal wear due to the Bernoulli effect at a pressure of approx. 8 bar or higher.

1. The high pressure and the restriction of the channel produce a higher air flow rate through the valve.
2. The pressure in the valve is reduced due to the increasing air flow rate.
3. The greater static pressure on the groove base forces the seal slightly upwards.
4. When switching positions, the valve is squeezed harder and is potentially worn faster.

The cartridge principle for piston spool valves
One solution to the problem described in figure 7 is to use moulded seals in metal cages. The seals are thus retained in recesses in the metal housing; the so-called cartridge principle (figures 8 and 9).

Figure 8: Piston spool valve with cartridge principle

Figure 8: Piston spool valve with cartridge principle

The advantage with this solution is that the valve will not be pulled out of its position even at an operating pressure of up to 16 bar, thanks to the recesses in the metal cage. This ensures a significantly longer valve life. In addition, the quality of the sealing between the piston and the elastomer seal is good enough for the cartridge principle valves to be used for vacuum operations without any problems.

Figure 9: Cartridge seal

Figure 9: Cartridge seal

In his next blog post Mr. Wahl will share a valve selection aid to help us determine which valve type suits us best.

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