In addition to the primary sealing lip, many designs also incorporate a smaller, secondary lip to exclude dust, dirt, and other contaminants. Unlike the primary lip, this secondary lip typically faces the application’s air side (since dirt and other unwanted matter may try to migrate in from outside the assembly). If present, a secondary lip generally originates away from the primary lip, at the opposite end of the elastomeric beam (in what is known as the heel, rather than the head). Depending on the needs of the application, a secondary lip can be oriented either radially (facing the shaft; known as a radial dirt lip) or axially (facing away from the shaft; an axial dirt lip). An axial dirt lip will require a vertical component against which to seal.

ANATOMY OF A SHAFT SEAL MAIN PAGE