The most important design feature of the seal is the elastomeric sealing lip. The beam length is the axial distance from the thinnest portion of the lip (the flex thickness) to the point at which the lip contacts the shaft. For a given flex thickness, a short lip exerts more force on the shaft (with a corresponding increase in friction and wear) than a long lip. A short lip also has better resistance to deformation caused by high pressure than a long lip. A longer lip (with the same flex thickness) exerts less force on the shaft, thus reducing friction and wear. A longer lip is also more flexible and can thus more easily follow any shaft eccentricities, such as shaft-to-bore misalignment (STBM) or dynamic run-out (DRO). Increasing the flex thickness while maintaining the same lip length will increase force on the shaft; decreasing the flex thickness decreases force on the shaft.

Two other important lip-related variables are the angles that meet at the head of the lip (portion nearest the shaft) to form the contact point. The angle facing the fluid being sealed is known as the oil side (or scraper) angle. The angle facing away from the fluid being sealed is known as the air side (or barrel) angle. To prevent leakage, the oil side angle must always be greater (steeper) than the air side angle.

In order to ensure contact between the lip and the shaft, the lip must always be designed to have a smaller inside diameter (I.D.) than the diameter of the shaft. The difference between the shaft diameter and the seal lip I.D. is known as interference. Increasing the interference (e.g. making the lip I.D. even smaller relative to a given shaft diameter) increases the amount of force on the shaft, thus also increasing friction and wear. Decreasing the interference (e.g. enlarging the lip I.D. such that it is closer to the diameter of the shaft) reduces the force on the shaft but also reduces the lip’s ability to follow shaft dynamics.

ANATOMY OF A SHAFT SEAL MAIN PAGE