Loaded U-cups are named for their "U" shape and the elastomeric energizer. They offer more effective low pressure sealing and they seal on both the inside and outside diameters. U-cup seals are often used for piston or rod seals due to the small gland space they require. Typically, U-cups are specified for low speed, low pressure environments. They seal through the flared, flexible lip in contact with the cylinder or rod wall. Dynamic contact surfaces should have a finish of 8 - 16 RMS.