New material for pneumatic seals

Polyurethane’s elastic properties, mechanical strength and wear resistance make it a good material for pneumatic seals. When the polymer’s composition and design development are tuned to the specific demands of pneumatic applications, modern polyurethane pneumatic seals shift operating limits to new levels. This has been validated by rigorous testing and validation.

Pneumatic actuators are key elements of material-handling and automation systems used for moving, clamping and positioning goods, boxes, flaps, gates, etc. A typical pneumatic cylinder (Figure 1) contains several static and dynamic seals. The dynamic seals are:

• Double-acting piston seal (often two U-cup seals)
• Rod seal and dirt wiper (often combined in one element)
• Two cushioning seals for the adjustable pneumatic end cushioning.

Figure 1. This cutaway model of a pneumatic cylinder, cutaway model shows the basic components covered below.

The dynamic seals have to work well with:

• Oil-free compressed air using only the minimal grease lubrication that was applied during cylinder assembly
• Operating pressures to 100 psi with excursions to 360 psi during cushioning
• Creep velocity of 3+ ft./sec.
• A temperature range of -4°F to +176°F

With a typical cumulative lifetime travel of at least 4,000 miles, the area of dynamic contact must maintain grease lubrication. A flexible sealing lip geometry provides low radial forces, and a rounded contact area allows the lip to float on a grease film.

If hydraulic seals were used in pneumatic cylinders, the high radial force and sharp sealing edges would scrape the grease away from the contact area. The grease would collect at the ends and the ensuing poor lubrication would result in high friction, heavy wear and shortened useful life.

Seal design refined

The seal design follows the principles described above. Additional features ensure proper function under any operating condition.

Piston seal: U-cup-type piston seals have been used in pneumatic cylinders for many years. The design in Figure 2 has several special features. It has a thin connection between sealing lip and body to achieve low radial force. Toward the contact area, the lip is larger to give stability under pressure.

Figure 2. This piston seal provides stability in the contact area and minimizes radial forces that can affect cylinder movement.

Notches at both lips and on the outer diameter of the back side avoid malfunctions. The notches on the side of the outer sealing lip ensure proper activation when the pressure increases. Notches at the outer diameter of the back (not shown in Figure 2) and on the inner lip act as pressure-release channels in case pressure is trapped between the two U-cups in a double-acting piston. Without these notches, a pressure trap, together with friction, can tilt the seal in the groove, which results in an inoperable cylinder. The rounded contact area of the sealing lip was optimized by finite element analysis to ensure good function over the whole pressure range (Figure 3).

Figure 3: Finite element analysis result of the lip seal reveal its performance at 29 psi and 145 psi.

A polyurethane material was developed for the specific demands of pneumatic applications. The 83 Shore A hardness is relatively low to minimize the radial force. With its good compression set, low friction and extraordinary wear resistance, it provides the preconditions for good functional properties and long service life.

Rod seal: The rod seal (Figure 4) is designed to suit grooves for pneumatic cylinders to ISO 15552. The seal has a sealing lip and a dirt wiper lip, and is fixed by a retainer “nose” at the outer diameter. The seal can be mounted into an unsplit gland by pushing it into the bore until the retainer nose snaps into the mating groove of the housing bore. The outer, static sealing lip is chamfered for ease of installation.

Figure 4. The nose on the rod seal/scraper ensures proper installation.

As the seal is fixed only by its retainer nose, the seal material must be stiff enough to withstand the axial forces of pressure and friction. Therefore, a harder material (94 Shore A) is used to maintain the proper seal orientation.

Cushioning seals provide smooth piston movement at the dead-end positions. The cushioning seal (Figure 5) works better than O-rings. The specific design features are:

• Dynamic sealing lip gives good tightness and low friction
• Axial sealing edge with check valve functionality
• Combined notches at the lip side and on the outer diameter to allow air flow

Figure 5. The cushioning seal protects the cylinder from end-of-travel shocks.

The cushioning seal is active only at the end of the stroke. A pressure chamber, which is formed between cushioning seal and piston seal, works like an air pillow to absorb the kinetic energy smoothly. To ensure that the cushioning seal doesn’t affect movement when the cylinder reverses direction, the seal has an integrated check-valve function that forces the pressure to act on the full cylinder area. If O-rings are used as cushioning seals, the movement is delayed because the air pressure can’t act on the full cylinder area until the cushioning rod has moved out of the O-ring.