U-Cups are popular due to the small gland space required. They are usually specified in low speed, low pressure environments for both pneumatic and hydraulic cylinders. The seal is obtained by means of the flared, flexible lips coming in contact with the rod or cylinder wall. Clearance is provided at the heel for minimum friction. Seal effectiveness is increased as internal pressure increases against the inner surface of the lips.
Since the shape is symmetrical it is possible this type of piston seal could also be used as a rod seal if required.