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How much do you know about the seal principle of oil seals?

February 7, 2026
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Why can oil seals seal fluids?

Lubrication Characteristics

After an oil seal is installed in a mechanical device, it mainly functions to seal fluids both when stationary and when the shaft is rotating. In addition, low friction and minimal wear at the lip are among its key characteristics.

What about the lubrication characteristics of the lip sliding surface, which affects the service life of the oil seal?

Here, we explain the lubrication characteristics through macroscopic phenomena.

To understand the lubrication state of the oil seal, it is important to evaluate its friction characteristics.

For this purpose, we mounted the oil seal on a testing machine under the conditions shown in Figure-1, rotated the shaft under various conditions, and measured its friction force.
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From Figure-2, we can obtain the relationship between the dimensionless characteristic number G (determined by the shape of the oil seal and its operating conditions) and the friction coefficient f under these conditions.

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Here, the relationship between the friction coefficient and the dimensionless characteristic number is expressed by Equation (1):


Where:

= Friction coefficient

= Constant determined by the oil film state

= Dimensionless characteristic number ()

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As shown in Figure-2, the phenomenon where the friction characteristic exhibits a positive slope in lubrication theory illustrates the properties of fluid lubrication. In this lubrication state, the friction characteristic of the oil seal is identical to that of a bearing, governed by the fluid's viscosity and sliding velocity, with an oil film present in the sliding area. From a macroscopic perspective, the mutual sliding surfaces of the oil seal and shaft move under fluid lubrication, separated by an oil film, which is why the oil seal maintains its characteristics of low friction and minimal wear.