The surfaces in contact with a seal determine what type to use. The surface can either be static or dynamic, in motion or without movement. Static seals are usually used when there is no relative motion between mating surfaces. Dynamic seals are the opposite. They are used when there is motion between surfaces. This can be either reciprocating or oscillating motions.
A static seal functions against mating surfaces that have no relative motion between each other. Depending on the direction of compression, a static seal can be classified as either axial or radial.
Axial static seals act by applying compression, or squeeze, between the upper and lower surfaces of a seal. In pressure applications the dimensions of the seal need to be considered to fully utilize the part. Seals should be seated on the low pressure side surface to limit motion and wear within the part. Radial static seals are compressed between the inner and outer surfaces of the part. Cap and plug type configurations commonly utilize radial seals. O-rings are common static seals and are used for either axial or radial depending on its application.
Dynamic seals exist when there is motion between surfaces. Typical motions include reciprocating, oscillating, and rotation. Operational factors can greatly affect how dynamic seals perform. Factors such as swelling of seals in fluids, surface roughness of mating surfaces, lubrication, internal pressure, compression, elasticity, and friction from surfaces.
Reciprocating dynamic seals are utilized when reciprocating motion along an axis between inner and outer surfaces. Reciprocating seals are seated within glands where relative motion exists. Applications of reciprocating seals include pistons in internal combustion engines, linear actuators, and hydraulic cylinders.
Rotary seals involve rotational motion from a shaft and housing. Important factors to consider in designing rotary seal glands are frictional heat buildup, material stretch, compression (squeeze), operational temperature limits, shaft and glandular machining.
Oscillating seals are commonly use in faucet valves. The shaft rotates through a limited number of turns around the axis causing oscillation. Due to twisting in the shaft, self-lubricated seals with higher hardness ratings are recommended for these applications.
Understanding which category an application falls into determines what type of seal to use. After understanding which class, next you need to know what type of motion or what your sealing requirements are. This will ultimately determine which seal will be appropriate for you specific application. Still not sure? That is why we are here to help you!