Mecanismo de vedação dinâmica de vedações de eixo

Dynamic sealing mechanism of shaft seals

shaft oil seals

The operation of the oil seal is based on the principle that the lip slides on the rotating surface of the shaft. The sealing lip is pressed radially against the shaft surface due to the inner diameter of the lip at rest being smaller than the shaft diameter. This difference is called preload. The radial force acting on the contact area is further supported by a metal spring to counterbalance the decrease in radial force due to the aging of the elastomer material (tension relaxation).

The lip sealing effect may vary depending on the shaft position.

Seal with stationary shaft

When the shaft is stationary, the sealing action is based on the radial pressure exerted by the lip on the shaft surface; the deformation suffered by the sealing lip means that it adapts perfectly to the shaft surface. This generates a radial force that acts on the shaft. Contact pressure is the result of preload and spring action. The reduction of the elastomer preload in operation depends on the operating conditions. The radial force FR is thus composed of the FE component due to the action of the elastomer and the FF component exerted by the spring.

Rotary shaft seal

When the shaft rotates, a hydrodynamic effect is created that causes the sealing lip to float over the lubricating film formed by the fluid to be retained. This prevents premature wear and thermal destruction of the sealing lip. On the one hand, the lubricating film, useful to prevent wear, must be maintained in the contact area and, on the other hand, the fluid to be retained must be prevented from escaping to the air side, with consequent leaks. Immediately after putting into operation a new oil seal, the sealing lip, in the contact area, undergoes a settling phenomenon, caused by the relative movement between the sealing lip and the shaft, which generates microscopic protrusions on the sealing lip. in the axial direction. The orientation of these protrusions depends on the pressure distribution in the contact area and the direction of rotation of the shaft.

This seating deformation of the elastomer surface creates a pumping effect from the air side towards the oil side in the contact zone, which is similar to that of a microscopic screw pump. The pumping effect is obtained only if the pressure distribution on the contact surface is asymmetric; Only then will the “micropump” be able to act in the right direction. The asymmetric distribution of contact pressure is achieved partly with different contact angles (a>ß) and partly by moving the spring axis towards the air side (spring distance).

Due to the orientation of the microprotrusions, the fluid in the contact area is pushed not only peripherally but also axially. Furthermore, the influence of surface tensions has an effect on the direction of loss. As a result of capillary forces, fluid is attracted to the meatus and orifice, but to a separating surface on the air side called the “meniscus.” There is a balance between the forces that cause leakage (pressure difference and capillary force), on the one hand, and the pumping effect generated by the microprotrusions in the elastomer, on the other.

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