PRINT: Dynamic Seals

Dynamic Seals.

In contrast to static seals, dynamic seals exist where there is relative motion between the mating surfaces being sealed. In most instances, the dimensional variations inherent in dynamic seals make them more difficult to design and more expensive to construct than static seals. Nevertheless, dynamic O-ring seals are indispensable to a wide variety of applications. Here’s a closer look at the major types of dynamic seals:

RECIPROCATING SEALS
Reciprocating seals involve relative reciprocating motion along the shaft axis between the inner and outer elements. In reciprocating seal applications, the O-ring slides or rocks back and forth within its gland with the reciprocating motion.

Reciprocating seals are most often seen in cylinders and linear actuators. Some examples of reciprocating O-ring seals are shown in Figures 113 and 114. Gland design measurements for industrial reciprocating O-ring seals can be found in Table 48. Gland dimensions can be found in Table 49.

FLOATING PNEUMATIC PISTON SEALS
Floating pneumatic piston seals are reciprocating in nature, but the way in which the seals are effected is unique. Normal reciprocating designs rely on the O-ring being stretched over a piston and then squeezed radially (on the inside diameter, or I.D., and the outside diameter, or O.D.).

In floating O-ring designs, however, there is no radial squeeze on the seal’s cross-section. The O-ring’s O.D. is larger than the cylinder bore diameter. Peripheral squeeze is applied to the O.D. as the O-ring is installed into the bore. Incoming air pressure forces the O-ring against the groove wall, and a seal is effected as shown in Figure 115.

Floating designs offer a number of advantages, including greatly reduced breakout friction and longer seal life. Floating pneumatic piston seals are suited for applications in which the air pressure does not exceed 200 psi (or in hydraulic designs where a small amount of leakage is permissible). Floating O-rings are NOT suitable as rod seals. Gland design measurements for floating pneumatic piston O-ring seals can be found in Table 50. Gland dimensions can be found in Table 51.

ROTARY SEALS
Rotary seals involve motion between a shaft and a housing. Typical rotary seals include motor shafts and wheels on a fixed axle. Installation of a rotary O-ring seal is shown in Figure 116.

R.L. Hudson & Company recommends lip type shaft seals for most rotary applications. There are applications, however, where an O-ring will provide an effective rotary seal.

O-ring seals are NOT recommended for rotary applications under the following conditions:

• Pressures exceeding 800 psi.
• Temperatures lower than -40° C (-40° F) or higher than 107° C (225° F).
• Surface speeds exceeding 600 feet per minute (fpm).

Note: Feet per minute = .2618 X shaft diameter (inches) X rpm

When an elastomer is stretched and heated, it will contract. This is called the Gough-Joule effect. This is an important design consideration in a rotary application because if an O-ring is installed in a stretched condition, frictional heat will cause the O-ring to contract onto the shaft. This may cause the O-ring to seize the rotating shaft so that the dynamic interface becomes the O-ring O.D. and the groove I.D. The contraction will also cause more frictional heat, further exacerbating the situation and causing premature failure of the O-ring.

We designed our rotary O-ring seals so that the free O-ring I.D. is larger than the shaft onto which it fits. The gland I.D. is smaller than the free O-ring O.D. so that when it is placed into the gland, the O-ring is peripherally squeezed, and the I.D. is reduced so that a positive interference exists between the O-ring I.D. and the shaft. Because the O-ring is not in a stretched condition, it will not build up heat, seize the shaft, and rotate in the groove.

Rotary seals (such as the one shown in Figure 117) do not dissipate heat as well as reciprocating seals do, so provisions must be made to keep heat build-up to a minimum.

• The housing I.D. should not be used as a bearing surface.
• Bearings should be provided to ensure that the shaft runout does not exceed .002" TIR.
• The O-ring groove should be located away from the bearing and close to the lubricating fluid.
• The housing length should be 8 to 10 times the O-ring cross-section to provide for better heat transfer.

To prevent extrusion of the O-ring, we recommend the clearance gap (extrusion gap) to be no more than .005" per side. If pressures greater than 800 psi are encountered, it is recommended that an 80 durometer O-ring be used.

The minimum hardness for the section of shaft that comes into contact with the O-rings is Rockwell C30. To prevent excessive wear, scratches, nicks, and handling damage, a hardness of Rockwell C45 is recommended. A shaft finish of 10-20 micro-inches is recommended, and plunge grinding with no machine lead is the preferred finishing method. The shaft ends should be chamfered with a 15/30° chamfer to prevent installation damage.

Gland design measurements for rotary O-ring seals can be found in Table 52. Gland dimensions can be found in Table 53.

OSCILLATING SEALS
Oscillating seals are commonly used in faucet valves. In oscillating applications, the shaft or housing rotates back and forth through a limited number of turns around the axis of the shaft. An oscillating O-ring seal is shown in Figure 118.

Because the surface speed in oscillating seals is so slow, reciprocating design charts are used.

The gland dimensions for the above seals can be calculated with our online O-Ring Calculator.

“Dynamic seals exist where there is relative motion between the mating surfaces being sealed.”

Figures 113-114