U-Cups derive their name from their "U" shape. They seal on both the inside and outside diameters. 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. Dynamic contact surfaces should have a finish of 8 - 16 RMS.