Simply defined, friction is the resistance to motion that develops when two objects are in contact. Friction must be minimized in order to reduce O-ring wear and prolong seal life. There are two main types of friction to be considered: break-out friction and running friction.

Also known as static friction or stiction, break-out friction is the static frictional force which must be overcome to initiate movement either at startup or after a period of inactivity. The amount or degree of break-out friction depends largely on how long the seal and the gland have been in contact. The longer the contact, the more of an opportunity the seal has had to flow into the gland’s metallic micropores. A magnified view of what this interface between an O-ring and a mating surface might look like can be seen in Figure 81. A combination of rubber-to-metal adhesion and the shearing force generated by the micropore irregularities must then be overcome before movement can begin.

Running friction is the dynamic frictional force which must be overcome to maintain movement. Running friction is perhaps most problematic when soft metals (such as aluminum, copper, or brass) are in use. Other factors that affect both break-out and running friction include the hardness of the O-ring, the gland’s surface finish, the amount of O-ring squeeze, the fluid temperature and pressure, and the amount and type of lubrication.

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