CHAPTER 1

INTRODUCTION TO

TRIBOLOGY

1

2

1.1 Definition of Tribology

Contents

1.2 History of Tribology

1.3 Importance of Tribology

1.4 Research in Tribology

1.5 Tribological Components and Applications

3

Upon completing this chapter, you should be able to:

Learning objectives

1. Define tribology.

2. Understand the history and importance of tribology.

3. Identify the tribological components /machine elements/ and their applications in machine design and operation.

1.1 Definition of tribology • “Tribology” is derived from the Greek word ‘tribos’ meaning rubbing or sliding.

• It is the study of rubbing or sliding. • Tribology focuses on friction, wear and lubrication of interacting surfaces in relative motion (1967)

• Tribology is an applied art and as such should be based upon, or requires background knowledge in, in many topics.

In mechanical engineering (the seat of most design instruction):

• Fluid mechanics, • Elasticity,

4

• Material science, • Dynamics, • Heat transfer, Methods of mechanical design and the like.

But the complete tribologist will have added some knowledge

• Plasticity • Contact mechanics • Surface Chemistry, (oxidation, adhesion, adsorption), • Lubricant Chemistry and the like. Tribology is also defined as the science and technology of interacting surfaces in relative motion and of related subjects and practices.

5

1.2 Historyof tribology a) During the Paleolithic period: • Drills were fitted with bearings made from antlers or bones

• Potter’s wheels of stones for grinding cereals, etc., clearly had a requirement for some form of bearings.

• A ball thrust bearing about AD40 was found in Lake Nimi near Rome.

b) Records show the use of wheels from 3500BC (illustrates concern with reducing friction in translationary motion)

6

c)The transportation of large stone building blocks and monuments required the know-how of frictional devices and lubricants, such as water-lubricated sleds. fig-chp1\chp1-fig1.pptx

d) Leonardo da Vinci (1452-1519), deduced the rules governing the motion of a rectangular block sliding over a flat surface. fig-chp1\chp1-fig2.pptx

• the areas in contact have no effect on friction. • if the load of an object is doubled, its friction will also be doubled. e) In 1699, the French physicist Guillaume Amontons rediscovered the rules of friction after he studied dry sliding between two flat surfaces.

7

f) During 1500, many developments occurred in the use of improved bearing materials. Essential laws of viscous flow were postulated by Sir Isaac

Newton in 1668 g) The principle of hydrodynamic lubrication was made by the experimental studies and the theoretical interpretations of Reynolds. Hydrodynamic bearing theory and practice were extremely

rapid in meeting the demand for reliable bearings in new machinery.

8

h) Wear much more younger subject than friction and bearing

development, and it was initiated on a largely empirical basis. Scientific studies of wear developed little until the mid-

twentieth century. i) Since the beginning of the 20th century, knowledge in all areas of tribology has expanded. enormous industrial growth leading to demand for better

tribology,

9

1.3 Importance of tribology Tribology is crucial to modern machinery which uses sliding and rolling surfaces. Examples of productive friction are:

• brakes, clutch, • driving wheels on trains and automobiles, • bolts and nuts and the like Tribology is not only important in industry, it also affects day-to-day life. Examples:

• writing is a tribological process, • shaving cream is used as a lubricant to minimize friction

between a razor and the skin, • friction is helpful during walking and driving,

10

• The consequence of (unproductive) friction and wear:

1) Friction and wear usually cost money In the form of energy loss and material loss as well

as the social system using the mechanical device. 2) Friction and wear can decrease national productivity Less desirable products (wear faster and require

maintenance) Worn machine tool (require skilled machinist and

affect products)

11

3) Friction and wear can affect national security Decreased efficiency of military hardware (wear of

aircraft engines and barrels of large guns) decreases the ability to perform a military mission.

4) Friction and wear can affect quality of life Tooth fillings, artificial teeth, artificial skeletal joints

improve the quality of life. Worn cars and worn tires require lower driving speeds

on wet roads. 5) Wear causes accidents Traffic accidents are sometimes caused by worn

brakes or other worn parts. 12

1.4 Research in tribology The purpose of research in tribology is: the minimization and elimination of losses (resulting

from friction and wear at all levels of technology where the rubbing of surfaces is involved). leads to greater plant efficiency, better performance,

fewer breakdowns, and significant savings. Available literature in tribology: 1. Journals and Periodicals Wear, Journal of Tribology (formerly the Journal of Lubrication

Technology), Tribology Transactions of the Society of Tribologists and

Lubrication Engineers(STLE)

13

2. Books : About 5 new books appear each year in the field, some books contain the following words in the title

tribology, contact mechanics, lubricant chemistry etc 3. Conferences on friction, lubrication and wear USA, Europe, Japan and the like

14

1.5 Tribological components and applications Common tribological components: sliding-contact and rolling contact bearings piston rings gears cams and tappets electrical brushes seals cutting and forming tools

Some of the common industrial applications: internal combustion engines (automotive applications) gas turbine engines (aerospace applications) material processing Railroads magnetic storage.

15

a) Bearings • Machine element that support a moving shaft against a stationary housing.

• In general, bearings as either sliding-contact (plain bearings or bushings) or rolling-contact bearings (antifriction bearings or journal bearings ) fig-chp1\chp1-fig3.pptx

b) Piston rings • An open-ended ring that fits into a groove on the outer diameter of a piston in a reciprocating engine (IC engine) fig-chp1\chp1-fig4.pptx

16

C) Gears • Gears used for transmission of rotary motion from one shaft to another shaft and change in rotational speed.

• There are different type of gear including spur, helical, bevel and worm gears. fig-chp1\chp1-fig5.pptx

d) Cams and tappets The most common applications for cams is valve actuation in an internal combustion engine. fig-chp1\chp1-fig6.pptx

17

Application of Tribology

18

• In the early 1960s, there was a dramatic increase in the reported failures of plant and machinery due to wear and associated causes, some causing heavy financial losses. Continuous processes made machinery breakdowns more costly and serious than ever.

• This trend was recognized by specialists involved in the subjects of friction, wear, and lubrication, and several research studies reported on the impact of wear, corrosion, and friction on machinery, manufacturing productivity, and costs.

• As a result, tribology became a mainstream field of science, and many universities offer it as part of their mechanical engineering department curriculum.

Continued

19

The early focus of tribology was on improving operation

and extending the lifecycle of industrial machinery. Today,

those principles and design benefits are making a major

impact in a variety of modern applications, such as

biomedical, nanotechnology, alternative energies, and

“green” methodologies. Here are some examples.

1. Biomedical (Biotribology)

20

• The application of tribology in biological systems is a rapidly growing field and extends well beyond the conventional boundaries.

• Biomedical tribological systems involve an extensive range of synthetic materials and natural tissues, including cartilage, blood vessels, heart, tendons, ligaments, and skin.

• Biotribologists incorporate concepts of friction, wear, and lubrication of these biological surfaces in various applications, such as the design of joints and prosthetic devices, the wear of screws and plates in bone fracture repair, wear of denture and restorative materials, wear of replacement heart valves, and even the tribology of contact lenses.

Continued The aim of bio tribology is to gather information about friction, adhesion, lubrication and wear of biological systems and to apply this knowledge to technological innovation as well as to development of environmentally sound products. This new interdisciplinary field of research combines methods and knowledge of physics, chemistry, mechanics and biology.

Continued • Tribology is omnipresent in biology.

• Surfaces in relative motion occur e.g. in joints, in the blinking with the eye, in the foetus moving in the mothers womb.

• Example :Systems with reduced friction

– joints and articular cartilage

22

2. Nanotribology • The commercialization of microelectromechanical systems

(MEMS)/nanoelectromechanical systems (NEMS), such as disk drives and other magnetic storage systems in the early 1990s, along with the development of new materials with nanoscale thicknesses, have presented new tribological challenges.

• The emergence of sophisticated scanning probe technologies and computational techniques has given rise to the field of nanotribology for investigations of processes at the atomic, molecular, and microscopic scale.

• Nanotribological studies are helping to develop fundamental understanding of surface interfaces in micro/nanostructures used in a variety of modern applications.

23

Continued

• Some of these applications include chemical and biodetectors, advanced drug delivery systems, information recording layers, molecular sieves, systems on a chip, nanoparticle reinforced materials, and a new generation of lasers.

24

Why biomicro and -nanotribology ?

• Continuous miniaturization of technological

devices like hard disk drives and biosensors

increases the necessity for the fundamental

understanding of tribological phenomena at the

micro- and nanoscale.

• Biological systems excel also at this scale and

therefore their strategies can serve as templates for

new engineering devices.

Micro- and Nanotribology Micro- and nanotribology deals with tribology on length scales of functional elements from 100 micrometers down to a few nanometers. The boom of microsystem technology (silicon technology, MEMS) and the development of novel nano-electromechanical systems (NEMS) calls for detailed understanding of tribological phenomena also at this scale.

Nano Tribology • Study of adhesion, friction, wear, and

lubrication at the nano scale. • The distinction between nano tribology and tribology is primarily due to the involvement of atomic forces in the determination of the final behavior of the system.

27

3. Tribology in Automobile • According to recent study cost of friction and wear is from 55-60

% of equipment damage is caused by poor lubrication and severe

wear.

• Better fuel efficient and higher energy conserving engine oils will

become increasingly important in the face of saving of natural

resources.

• The application of tribological principles is necessary for

automobile, in order to reduce friction and wear with minimum

adverse impact on environment. This task is not with the

consideration of operating condition, speed, load and temperature. 28

4. Tribology application in mechanical system

most common elements, most common components, which are used

in number of machines, which have relative motion, any rotary

motion, any sliding motion, take an example of car, which is an

automobile, aircraft, turbines, any compressor all these require tribo

elements,

they require some sort of sliding motion, some sort of rotational

motion. And that is why we require, application of tribologies to

design those components, which are utilized for relative motion

under load.

29

5. Green Tribology • The concept of “green tribology” was also introduced by

Jost, who defined it as, “The science and technology of the

tribological aspects of ecological balance and of

environmental and biological impacts.”

• There are a number of problems that can be addressed by

green tribology. The specific field of green or environment-

friendly tribology emphasizes the aspects of interacting

surfaces in relative motion, which are of importance for

energy or environmental sustainability or which have

impact upon today’s environment. 30

• Wind turbines continue to hold promise as a viable alternative

energy source. While they have made gains in reliability in the past

decade, they are subject to tribological problems that are difficult

and costly to repair and can drastically reduce their expected

lifecycles.

• Two key areas of concern are reliability of gearboxes and turbine

lubrication.

31

Continued

32

To address these problems, there have been extensive studies of

lubricants, which are used in wind-turbine gearboxes, hydraulic

systems such as the blade pitch control, drive train brake

subsystems, and bearings, to improve turbine reliability in extreme

environments.

33

34

Fig. 1-1 Use of a sledge to transport a heavy statue by Egyptians circa 1880BC

35

Fig. 1-2 Laws of friction by Leonardo Da Vinci (1452-1519)

36 Fig. 1-3 Types of bearings

a) Sliding bearings b) Anti-friction bearings

37 Fig. 1-4 Piston rings

38 Fig. 1-5 Types of gears

a) Spur gears b) Helical gears

c) Bevel gears d) Worm gears

39 Fig. 1-6 Cams and tappets in vale gear mechanism