The name "U-cup" comes from their "U" shape. U-cups seal both the outside and inside diameters. U-cups are particularly popular because the required gland space is small. Typically, they're specified for low speed, low pressure environments for both hydraulic and pneumatic cylinders. The seal is secured by the flared, flexible lips in contact with the rod or cylinder wall. To minimize friction, clearance is at the heel. The effectiveness of the seal is increased as internal pressure increases against the lip's inner surface. Dynamic contact surfaces should have a finish of 8 - 16 RMS.