bearing   335 / 392



■Friction and Wear There are two contradictory theories that attempt to explain mechanism of friction between the two solid surfaces. One is a theory based on concavity and convexity of solid surface. The other is a theory based on adhesion. This theory accounts adhesion which takes place between the two convex parts as a cause of friction. Recently, the adhesion of two solid surfaces is considered to be the main cause of the friction. According to the theory of adhesion, contacts of convex parts of two solid bodies consists of minute real contact points as shown in Fig.4. The very tops of convex parts are constantly squashed and generate adhesion to each other forming a junction. A relative motion breaks this junction. As such, adhesion and disjunction repeatedly occurs. Sum of the force needed to detach this adhesion is friction force. The generation of wear particles at this moment as "free wear pieces" results in wear. Boundary Lubrication Lubricating oil film between the two contact surfaces is extremely thin and no viscous hydrodynamic oil film exists between the two. As shown in the Fig.3, only film of absorbed oil molecules exists. Absorptive oil film is arranged oil molecules that are adhered onto the solid surfaces and its shear resistance is greater than hydrodynamic oil film. Frictional force in this area, compared to hydrodyanamic lubrication, is greater (coefficient of friction ; 5 × 10−2 to 5 × 10−1). In the frictional contact points such as real contact points, oil film is frequently broken. The lubrication condition that generates a frictional condition such as this is called "Boundary Lubrication." In order to decrease the friction under this condition, selection of self-lubricating bearing may be desirable. Solid lubrication (Dry friction area) This is a condition in which two solid surfaces come into contact directly with each other. In this condition, there is no lubricating film such as hydrodynamic film or absorptive oil film. An empirical law by Amonton (1663-1705) and Coulomb (1736-1806) explains the principles of dry friction as below. Fig.3 Modle of absorption of oil molecule in boundary lubrication Oxide layer Sliding direction Load W Absorbed oil molecule Metal Metal Metal to metal contact area "Amonton and Coulomb's Law" 1. Frictional force is proportional only to the vertical load applied to the contact surface of the solid body and is independent of apparent contact area. 2. Coefficient of friction is independent of the sliding velocity. 3. Under the same condition, the static friction (force required to generate sliding) is greater than kinetic (force required to maintain sliding) friction. Mechanism of friction by adhesive theory Adhesive area Junction Free wear particle J A A A B B B Sliding direction Fig.4 Mode of producing wear particles by adhesive friction 334 Selection Guide Product Information Plastic Bearing Multi-layer Bearing Metallic Bearing Air Bearings Technical Slide Shifter Information Corporate Profi le Technical Information