The skeleton oil seal is the quintessential example of an oil seal; generally speaking, when people refer to an "oil seal," they are specifically referring to the skeleton oil seal. The primary function of an oil seal is to isolate the components within a transmission system that require lubrication from the output components, thereby preventing the leakage of lubricating oil. The skeleton acts much like the steel reinforcement bars within a concrete structure: it provides structural reinforcement and enables the oil seal to maintain its shape and tension. Based on structural design, skeleton oil seals can be classified into single-lip and double-lip types. In a double-lip skeleton oil seal, the secondary lip serves a dust-proofing function, preventing external dust, impurities, and other contaminants from entering the interior of the machinery. Based on the type of skeleton, they can be categorized into inner-encased skeleton seals, exposed-skeleton seals, and assembled-type seals. Based on operating conditions, they can be classified as rotary skeleton seals or reciprocating skeleton seals.
Different classification criteria yield different categories of oil seals. Today, our primary focus is on the skeleton oil seal. The structure of a skeleton oil seal consists of three main components: the seal body, the reinforcing skeleton, and the self-tightening helical spring. The seal body itself is further subdivided into various sections, including the base, the waist, the sealing edge, and the sealing lip. Typically, when in a free (uninstalled) state, the inner diameter of a skeleton oil seal is slightly smaller than the diameter of the shaft; this difference constitutes a specific amount of "interference fit."
Consequently, once the oil seal is installed into its housing and onto the shaft, the contact pressure from the sealing edge-combined with the contractile force of the self-tightening helical spring-exerts a specific radial clamping force against the shaft. After a period of operation, however, this pressure may diminish rapidly or even dissipate entirely; the addition of the spring serves to continuously compensate for this loss, thereby maintaining the oil seal's self-tightening force.
Therefore, during installation, it is essential to apply a thin layer of oil to the sealing ring and to ensure that the skeleton oil seal is positioned perfectly perpendicular to the shaft's axis. If the seal is not perpendicular, the sealing lip will effectively scrape the lubricating oil away from the shaft surface, which can also lead to excessive wear on the sealing lip itself. During operation, a minute amount of lubricant is allowed to seep out from within the housing, thereby establishing a thin oil film at the sealing interface.
