CHAPTER 4 FRICTION, WEAR AND LUBRICA TION In machinery design material and energy conservation is becoming more and more important.We are going to learn something basic about friction, wear and lubrication in this chapter. Basic Concept Friction——The twisting force that resists relative motion between two bodies in contact Friction force——The resistance because of friction wear——to exhaust or lessen the strength gradually by friction lubrication——appending grease in the interface of friction as a film between solid surfaces to reduce friction, heat and wear and make smooth or slippery . Tribology——a study that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion Several statistical data : 1. 1/3~1/2 of the energy in the world is used up in friction. 2. 30% of the failure in machine elements is caused by wearing. 3. Applying the knowledge and technique of tribology in the production , we will get the economic benefit which is about 2%~7% of GDP 4.1 FRICTION Friction is a principle cause of energy dissipation and the improved friction control can guarantee considerable savings. Classifications of Friction: internal friction :inside the material external friction: on contact surface 1.Classified by trend of relative movement static friction: sliding friction (eg: sliding bearing ,belt) kinetic friction: rolling friction (eg: rolling bearing, cam) 2. Classified by friction condition (or is there any lubricant) (1)Dry friction: no anti-friction material or safety film (2)Boundary friction : two parts separated by boundary film (3)Liquid friction: two parts separated by fluid film (4)Mixed fiction: mixture of the three (1)Dry friction friction mechanism ① a. coulomb's law in 18 century F=fFn Tra it: simpl e, powe rful utility , in additi on to liqui d fricti on the equation suit for every condi tion, but it only used in the open surface. eg: belt drive, flank of thread friction and working in interference, etc. b. Molecule---Machinery theory It is believed that the friction between two dry surfaces arises from two main sources, adhesion and deformation ,and the most important is adhesion. mathematical expression: F=Fa+Fm=(fa+fm)Fn among this fa——adhesion component of friction coefficient
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friction ,the only difference is the value of f is less.(falittle)
(3)Liquid frictionWhen friction is carry through the molecule of liquid F≈0.001~0.008,there is nearly no wearing
on the surface.
eg:the friction of dynamic or hydrostatic lubrication sliding bearing
(4)Mixed friction
Definition: when the friction condition is between the boundary friction and fluid friction, we
always use the thickness of the film to determine the friction condition.
thickness of the oil film
arithmetic average deviation of the two surface outlineλ =
Generally speaking:
λ<1 boundary friction
λ=1~3 mixed friction
λ 〉3 liquid friction
In the mechanical transmission, the majority condition of friction surface is mixed friction.
4.2 WEAR
Wear is a major cause of material wastage, so any reduction of wear can affect considerable
savings.
wear: The friction will cause the displacement and removal of surface material, so wear can causethe failure of the element, which is in 70 to 80 percent of the total failure.
The harm caused by wear:
influence the performance quality of machine① ;
eg: The wear of the gear tooth surface will destroy the involute tooth surface, and then cause the
impact and vibration.
The wear of the machinery mainshaft bearing will influence the machining precision of element.
impaired the efficiency of machine;②
eg: The wear of cylinder sleeve of the diesel engine’s will cause inadequate functioning of power.