Introduction to tribology

By- Kumar Ankur

Mechanical Engineering Department

What is Tribology ?

• Tribology is derived from the Greek word “Tribos”. Meaning ofTribos is Rubbing.

• Tribology is a science that deals with friction, lubrication andwear in all contacting pairs.

• Tribological knowledge helps to improve service life, safety andreliability of interacting machine components; and yieldssubstantial economic benefits.

Consider few failed machine components, failure of which could had been avoided usingtribological knowledge.

Example 1 CAM

Example 1 is related to pitting wear on the cam surface. Cams are used to transmit rotarymotion in reciprocating motion. These components are subjected to jerks in slidingdistance, which leads to form some pits on the cam surface. Creation of pits on cam surfaceincreases noise pollution and reduces mechanical performance. Understanding themechanism of pit formation helps to estimate the life of component and find methods toreduce such pitting failures.

Example 2 JOURNAL BEARING

In these examples of journal bearing, wear increases the clearance between shaft andbearing and leads to reduction in load support capacity of the bearing. Often suchfailures occur in absence of sufficient lubricant hydrodynamic film thickness due torelatively low speed. Learning tribology cultivates an understanding that at lowspeeds, the main purpose of oil is the lubrication and high viscosity oil will bepreferred to low viscosity oil, while at high speeds the major purpose of oil is to act asa coolant and low viscosity lubricants are preferred to carry away frictional heat ofoperation. Here lubrication is a secondary consideration.

Example 3 Multi-row Roller Bearing

Here cracking means deep cracks which breaks outer ring in number of pieces. Suchfailure occurs due to faulty manufacturing and wrong assembly of roller bearing.Tribological relations help estimating increase in contact stresses due to misalignmentof shaft and improper mounting of bearing surfaces. Hence an approximation onreduction in service life can be estimated.

Example 4 Gear

The pit generally occurs due to excessive contact stress. Understanding the effect of contact stress helps in developing an equation for estimation of perspective gear life.

Need of Tribology as subject

• Most tribological phenomenon are inherently complicated and interconnected, making it necessary to understand the concepts of TRIBOLOGY in details.

• Integration of knowledge from multifaceted disciplines(solid mechanics, fluid mechanics, material science, chemistry etc) is essential and therefore a separate subject is required.

• Solid Mechanics: Focus is on expressions of contact stresses/deformations and surface temperatures due to rolling/sliding.

• Fluid Mechanics: Study of lubricant film formed between various geometric shapes of rolling/sliding surfaces.

• Material Science: Focus is on atomic and micro scales mechanisms whereby solid surface degradation or alteration occurs during relative motion.

• Chemistry: Deals with reactivity between lubricants and solid surfaces.

• Thermodynamics: Heat and mass transfer in fluids and bounding solids.

Fluid Film Lubrication

Fluid Mechanics Concepts : Fluid is defined as a substance that easily moves and changes its relative position without a separation from the bulk mass.

• Lubrication makes relative motion between two surfaces very smooth. It reduces the friction and minimizes the wear.

• In general, real area of contact between tribo-pair is hardly ten percent of apparent area.

• Here, stress-state always exceeds the elastic/fracture point of all known solids. It results in high plastic deformation in ductile materials, while generates cracks in brittle materials, which causes excessive wear.

• Therefore, it is essential to reduce the normal stresses imposed by applied load and to reduce the shear stresses induced due to relative motion.

There are some ways to reduce the normal and shear stresses shown on next slide.

In hydrostatic lubrication that separates two surfaces by an external pressure source. It is suitable for extremely high load carrying capacity at low speed or at highly controlled precision works.

• As pressure is generated and supplied by external sources, it is one of the expensive approaches to separate two surfaces.

• If applied load is reduced, the film thickness (separation between tribo-pair) will increase. Similarly if more load is added to the moving surface, the film thickness will decrease.

• To compensate cost, often a hybrid concept of hydrodynamic + hydrostatic is used to achieve best of both the mechanisms of fluid film lubrications.

In a hydrodynamic lubrication mechanism, a fluid is drawn into the region between the relatively-moving surface by the virtue of its viscosity and adhesion to the surfaces.

Due to the converging geometry of the surfaces, a pressure is generated within the fluid that separates the tribo-surfaces. The separating film is only generated when the there is relative motion.

• Reynolds’ Equation :

Fluid film lubrication between two plates.

In 1886, Reynolds derived an equation for estimation of

pressure distribution for “Fluid Film Lubrication”.

• Quantification of fluid film lubrication can be made by

solving Reynolds’ Equation, which provides fluid film

pressure as a function of coordinates and time. Reynolds

equation helps to predict hydrodynamic, squeeze, and

hydrostatic film mechanisms.

Derivation of Reynolds’ Equation :

To model the pressure as a function of angle of inclination, let us consider a fluid element subjected to pressure and viscous forces, assuming gravity and inertia forces acting on fluid element can be neglected.

Fig: Fluid element subjected to pressure & viscous forces.

On balancing forces shown in figure-

For laminar flow of Newtonian fluid,

On substitution

Equation is based on following assumptions : 1. Negligible inertia terms2. Negligible pressure gradient in the direction of film thickness3. Newtonian fluid

Asaas

Assuming no slip at liquid-solid boundary;

y = 0, u = U2

y = h, u = U1

Utilizing these boundary conditions, values of integration constants can be evaluated.

This equation is applicable for following assumptions :

1. Negligible inertia terms.

2. Negligible pressure gradient in the direction of film thickness.

3. Newtonian fluid.

4. Constant value of viscosity.

5. No slip at liquid solid boundary.

In that equation, on right hand side there are three terms, two velocity terms and one pressure term. Velocity terms represent “shear flow” also known as “Couette flow”. Flow due to pressure gradient is termed as “Poiseuille flow”. similarly flow velocity in z direction.