Primary & Secondary Sealing
In the case of an expanding ring, primary sealing is established between the outside diameter (OD) of the ring’s surface and the cylinder bore. For contracting rings, the primary sealing is formed where the ring’s inside diameter (ID) makes contact with the shaft/rod. Contact is maintained by the inherent tension built into the ring until the pressure differential is sustained across the ring. This pressure differential keeps the ring "seated," forming an effective seal.
Secondary sealing is created by contact between the ring’s side face and the groove wall. The desired clearance between the ring and its groove forms a flow channel that carries fluid to the back of the ring, establishing a pressure differential. Once an ideal pressure differential is attained, the unbalanced reacting forces at the sides of the ring allow it to seat and to form an effective seal. Both primary and secondary sealing produce the desired sealing action for the application.
The same factors that contribute to seating a piston ring also create drag forces. Axial drag forces are a result of high pressure fluid acting on the I.D. surface of the ring and the coefficient of friction between mating surfaces. In addition to this force, drag forces must be considered due to the ring’s inherent tension. In very high pressure applications, these drag forces become almost negligible by comparison. For low pressure applications, rings must be carefully designed to compensate for drag forces.
Another type of drag force is caused by friction between the side face of the ring and the groove's side surface. If this side drag force is too large, the ring will not achieve primary sealing contact. Again, for low pressure applications, this drag force must be carefully balanced while designing the ring.