Smooth-running cylinders and seal-less cylinders are generally less common in pneumatics, but are indispensable in some areas of technology.
But where are the big differences to the standardized pneumatic cylinders on the market?
A smooth-running cylinder differs from a standard cylinder primarily because the breakaway force required to actuate the cylinder is significantly smaller. While a standard cylinder needs about 300-600mbar of pressure to overcome the breakaway force, smooth-running cylinders require only about 10-100mbar.
Through various property changes, an attempt is made to keep the so-called “stick-slip” effect as low as possible. The stick-slip effect always occurs when the static friction is significantly greater than the sliding friction. Due to the piston seal and the rod seal, comparatively strong static and sliding friction occurs on the pneumatic cylinder.
How can this friction be minimized?
- Optimization of material properties
- Special elastomers with friction-optimized shapes and materials can be used for smooth-running systems. The smaller the area on which the elastomer rests on the surface, the lower the static friction.
- The hardness of the seals also has a significant impact on friction.
- Coatings of sliding partners
- Coatings can greatly reduce the sliding properties of both the seals and the metal counter surfaces. Common coatings are usually based on PTFE or similar anti-friction coatings.
- Adaptation of the preload of the seals
- Decreasing the preload of the seal also leads to a significant reduction in friction. The contact pressure of the piston seal on the cylinder tube is reduced until it is just securely sealed. This is achieved by reducing the contact area of the piston seal on the piston to a desired level.
- The same procedure applies for the rod seal.
A seal-less cylinder goes a significant step further in reducing friction. As the name suggests, the seal-less cylinder does not use any elastomer seals at all. However, the main problem with these systems is also obvious. A seal between the piston and the cylinder tube is only achieved via a perfectly executed minimal air gap in the range of < 0.01mm.
The larger the air gap, the more “leaky” a seal-less cylinder is.
However, since it does not have an elastomer seal, the seal-less cylinder is almost friction-free and extends at a pressure of just a few mbar. In our example, the seal-less cylinder already extends at 2-3mbar. This means that a standard cylinder needs approximately 200 times more pressure to extend than a seal-less cylinder.
