When thinking about creating seals to keep fluid in and contaminates out, there are two main categories. A static seal is when two components come together and are fixed against each other. A dynamic seal means one or both components are in motion relative to each other. While both types of sealing have their challenges, the unique nature of sealing against a moving surface brings challenges to dynamic environments that require special solutions. A common dynamic application requiring sealing is a rotating shaft in a static housing. This application is common in motors, transmissions, pumps, axles, and countless other familiar situations. Some seals need only keep lubricating fluid in and dirt out while the shaft turns, while others must handle rotation at high speeds and pressures while also maintaining low friction on the shaft to give the seal longevity. The requirements drive the design and the need for unique elements for each application.
One of the most common uses for a shaft seal is typically referred to as an oil seal. While that broad name can characterize many varieties of sealing, a typical oil seal is made up of a rigid can, rubber sealing lip, and spring to energize the lip against the shaft. Most common varieties of oil seals are not intended to seal high pressures, instead sealing in no pressure to slight elevations of nearly 50 psi at most. Instead, these truly just seal in the fluid and maintain a squeeze on the shaft using the spring to aid.
At low to no pressure, an oil seal can be pressed into a housing and requires no additional structure to stay installed in application. Depending on the gland characteristics, the can may either be bare metal or rubber covered. The choice to rubber coat or use a bare can is typically driven by the finish provided by the gland. The need to include a spring is also application dependent. A spring can help maintain sealing pressure at the lip when the seal is static, or to overcome forces that pull the lip away from the shaft.
In more demanding applications, shaft seals may need reinforcement to stay in the application environment. Commonly, this is achieved through use of a retaining or snap ring. In other cases, the seal might be installed in a groove, then an additional housing component might be installed, or the seal is pressed into an internal groove before final assembly. These methods ensure the seal stays where it should be in relation to the outer diameter.
The lip of the shaft seal is designed in a way to maintain contact on the shaft while minimizing the contact pressure and potential wear on the shaft or of the seal. Excessive contact with the shaft can cause the sealing element to machine a groove into the shaft material, or wear the sealing element, creating contamination in the system.
The design of seals involving moving components involves a great many variables. Ensuring each element is accommodated is critical to ensure proper sealing and allow the seal to meet the requirements of component manufacturers and the end users of the final products. A functioning seal means the customer does not even contemplate its existence, while a drop of oil landing on the ground under a finished good leaves a stain on more than just the concrete.