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The History Of Tribology Engineering Essay

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philosophical change is in progress in manufacturing. Today, engineers look for means of reducing environmental and energy losses in machines and in their frictional components. As a result, overcoming frictional forces accounts for 1.5 - 2% of power consumption in a modern jet engine, 9% of power consumption in a piston airplane engine, up to 40-45% in a new auto engine (from the world lending manufacturers), more than 50% in railroads trains, and more than 80% of the installed power at textile enterprises[3].

Therefore, in an industrial world, frictional measurements are an essential matter. Since power could be saved by friction reduction, it is important to measure the friction of materials used in the industry. Frictional measurement methods in various machines differ from the application of that certain machine. In a Tribology Machine (Tribomachine) where friction is sensed using a load cell it is crucial to obtain an accurate and efficient frictional coefficient value. Tribology is the science and technology of interacting surfaces in relative motion. It incorporates the study and application of the principles of friction, lubrication and wear. Generally, a Tribomachine uses a weighing or frictional indicator to display frictional coefficient of the specimen used in kilogram (kg)[4]-[8]

Nowadays, there are high demands for tribological machines. This is due to the necessity of tribological investigation for new materials. As a result of the unreliability of mathematical models to predict the tribological characteristics of materials, experimental work becomes the key part in determining the tribological properties of materials. There are different types of tribological machines (tribo-machines) such as

1)Pin-on-disc

2) Block-on-ring

3) wet sand rubber wheel

4) dry sand rubber wheel,

5) sand/steel wheel, etc[9].

In this project, the knowledge of Tribology and Electronics is integrated to

develop a new method to measure frictional coefficient. A data acquisition system was fabricated and connected to a Block on Disc (BOD) Tribology Machine to test and experiment with different loads applied .Thus, the system was programmed and connected to an LCD to obtain frictional coefficient values in mili-Volts (mV).

The following chapters illustrate the details on the project done.

Example of a pin-on-disc tribo-machine is given below:

Fig. 1. Diagram of a pin-on-disc tribo-machine. [5]

AIMS/ OBJECTIVES

To design an electric board connecting the load cell with an LCD to monitor the frictional force(s) during sliding motion.

Able to rotate the motor from 0 to 2000RPM

Study and investigate the effect of load on the frictional coefficient using the new data acquisition system.

Capture frictional coefficient via an LCD integration during a

Tribological tests.

REPORT ORGANIZATION

This report contains sections of chapters. It is divided into five chapters. The chapters are as follows:

Introduction

Literature review

Methodology

Results and discussions

Conclusions

INTRODUCTION: It clearly states the purpose of the project, important background and/or theory, describes specialized equipment and justifies project's importance.

LITERATURE REVIEW: It provides general and relevant background, knowledge and founding from previous researches. It also serves as a great reference throoughout the project.

METHODOLOGY: The literature review discusses in detail the design process of the machine, preparation of the specimens and procedures of the experiments.

RESULTS AND DISCUSSIONS: The results obtained are processed and presented with charts, graphs, tables and explanations.

CONCLUSIONS: The conclusion gives the summary of the whole project with some recommendations suggested.

The flow chat (layout) of this report is given below.

CHAPTER 1

INTRODUCTION

CHAPTER 2

LITERATURE REVIEW

CHAPTER 3

METHODOLOGY

CHAPTER 4

DISCUSSIONS AND CONCLUSIONS

CHAPTER 5

CONCLUSIONS

Fig. report layout

CHAPTER 2

LITERATURE REVIEW

TRIBOLOGY

Tribology is the science and technology of two interacting surfaces in relative motion and of related subjects and practices. The word tribology is coined from two greek words tribos and -logy. Tribos simply means "rubbing" while -logy means study. Therefore, tribology can be literally defined as the study of rubbing[ ]. It comprises the entire field of friction and wear including lubrication. It includes corresponding interfacial interactions between solids as well as between solids and liquids. It is a new interdisciplinary approach to subjects previously treated separately (under various categories such as adhesion, lubrication, friction and wear, bearings, abrasion) by several disciplines e.g. physics, chemistry, applied mathematics, solid mechanics, fluid mechanics, thermodynamics, heat transfer, materials science, rheology, lubrication, machine design, performance and reliability., metallurgy and physics, chemistry ,solid mechanics ,fluid mechanics ,thermodynamics, heat transfer ,material science, lubrication ,machine design , [ ]. In any machine there are many component parts that apply tribology. They operate by rubbing or sliding against each other. Examples of such component parts include bearings, gears, cams and tappets, tires, brakes, and piston rings. All of these components have two surfaces which come into contact, support a load, and move with respect to each other. Sometimes it is advantageous to have low friction, to save energy, or high friction, as in the case of brakes. In order to prevent the components to wear, a prevention method is called lubrication is always applied.

Tribology in itself aims at the functional, economical and ecological optimization of motion systems. The implementation of tribological knowledge results in a bringing down of wear and an optimization of friction systems.

The operational reliability of machines and installations is raised (increased), production costs are declined, resources and energy are saved and emissions are also declined []. For the cause of friction and wear, the respective national economies of the industrialized countries suffer losses yearly amounting to approx. 5 % of the gross national product (GDP) which are, e.g., for Germany approx. 35 billion Euros per year. When carrying out the available tribological knowledge, it would be possible to save 5 billion Euros in a year of such losses [ ]. It is also noted that losses resulting from ignorance of tribology amount in the United States to about 6% of its

gross national product (or about $200 billion dollars per year in 1966). Also approximately one-third of the world's energy resources in present used emerge as friction reduction and wear control cannot be overemphasized for economic reasons and long-term consistency [].The intensified consideration of tribological knowledge results in important savings regarding energy and material consumption, production and maintenance. Energy and raw material resources are saved, environmental damage is avoided and protection of labour is improved.

THE HISTORY OF TRIBOLOGY

Tribology has always been around to assist man achieve his technological triumphs, from the use of animal fats to grease the axles of his chariots, the use of 'squeeze films' of sand and mortar to accurately position the huge building blocks in the pyramids, the use of the bow-drill to help in the manufacture of furniture and equipment, to the use of sound tribological principles for moving the huge statues (e.g in ancient egypt).

Fig. Painting from el-bersheh, circa 1880 bc, showing transportation of a giant statue.

The diagram above shows a man in fron of the statue pouring liquid from a jar. some historians suggested that the liquid served ceremonial purposes, whereas some engineers suggested that this is one of the first recorded cases of lubrication.

There were very few advances in tribology from ancient times until the Renaissance, due mainly to the stagnation of civilization during the Dark Ages. Around the turn of the 15th century, Leonard0 da Vinci experimentally deduced the laws of friction.

These laws remained unpublicized until Amontons rediscovered them in the 17th century. Apart from these two events the history of tribology had to wait until the discovery of oil wells in the 19th century, together with the expansion of railways, before the next 'big step forward' could be recorded. This was the formulation, by Osborne Reynolds, of the basic principles of fluid film lubrication based on the

'physical wedge' condition. From the turno f the century everything was again fairly satisfactory from the tribological poinot f view until the 1940s, when the war imposed severe conditions upon equipment involving rotating machinery. The development of the gas turbine introduced a whole new range of tribological problems, involving surfaces which have to rotate at high speeds, at high temperatures and in extremely corrosive atmospheres. By 1964 the demands of modern internal combustion engines, nuclear reactors, space probes , continuous plant production, etc, indicated that not enough was known about the limits of lubrication, in particular about wear.

TYPES OF TRIBOLOGY MACHINES ( TRIBOMACHINES)

In order to carry out a tribology test, a tribology machine is used. There are different types of tribology machine. These includes;

Block on ring machine (BOR)

Pin on disc or Block on disc machine (BOD)

Block on flat machine(BOF)

ASTM G99

ASTM G77

ASTM G105

ASTM G137-95

ASTM B611 (sand/ wheel test under wet/dry conditions)

Dry sand rubber wheel (DSRW)

Wet sand rubber wheel (WSRW)

BLOCK ON RING MACHINE (BOR)

This test method covers a laboratory procedure for determining the wear of materials during sliding using a pin-on-disk apparatus. Materials are tested in pairs under nominally non-abrasive conditions. A load is applied to the ring as rotates. This pneumatically loaded adapter permits tests to b done using the classic pin on block test geometry and also using twin block on the geometry, with a test block loaded on either side of rotating ring. The block specimen used has a flat shape or conforming shape. The conforming shaped specimen allows journal bearing type tests to be performed. The principal areas of experimental attention in using this type of apparatus to measure wear are described. The coefficient of friction may also be determined.

Figure: Block on Ring.

PIN ON DISC/ BLOCK ON DISC (BOD)

The pin-on-disk tribology machine is a versatile laboratory apparatus for measuring the friction and wear properties of combinations of metals and lubricants under selected conditions of load, speed and temperature. The tester consists of a stationary "pin" under an applied load in contact with a rotating disc. Either the pin or the disc can be wear- and friction-tested using the pin on disc tester. The pin is usually a sphere however it may be any geometry that simulates the actual application counter surface. The pin carriage is electrically isolated from the load beam and thus from the disc specimen. This allows a small potential to be applied across the contact from a Lunn-Furey Contact Resistance Circuit. The connection to the disc spindle is via a slip-ring. .Variations in the voltage across the contact are indicative of the amount of contact between the pin and disc specimens provided that both are conductors of electricity. Maximum voltage (typically 40 mV) corresponds to no contact (open circuit) while zero voltage corresponds to full contact (closed circuit). The voltage signal will naturally vary

quickly during a test so an RMS signal is used for recording purposes. In other words a load cell which converts a mechanical force to an electrical signal attached to the pin on disc tester is used to measure the evolution of the friction coefficient with sliding distance. Sliding wear of the disc can be measured after the pin on disc test using a simple piece of equipment called a Calo tester.

Figure: Pin on Disc.

BLOCK OF FLAT MACHINE (BOF)

FRICTION

As stated earlier, tribology is the study of friction, wear and lubrication. This means that friction, wear and lubrication are the constituents of tribology. In this round of this project, the frictional measurement is the main focal emphasis. Friction is the force that opposes the relative motion of two objects in contact. Friction may be advantageous like the traction needed to walk without slipping and also disadvatageous because friction produces heat in various parts of machines. In this way some useful energy is wasted as heat energy. Due to friction we have to exert more power in machines, noise is produced in machines and engines of automobiles consume more fuel which is a money loss.

The frictional force can be calculated using the formula which was brought by Amonton who tried to define the mathematical correlation between the frictional force and the normal load. The formula is given below:

………………………………………………………….. Equation ….

Where:

F_\mathrm{f}\,is the force exerted by friction (in the case of equality, the maximum possible magnitude of this force).

\mu\,is the coefficient of friction, which is an empirical property of the contacting materials,

F_\mathrm{n}\, is the normal force exerted between the surfaces.

In most applications the aim is to diminish friction in order to reduce energy consumption and improve efficiency. About 20 percent of the engine power of automobiles is consumed in overcoming frictional forces in the moving parts. Related aspects are surface engineering (the modification of a component's surface to advance its function, for example by applying a surface coating), surface roughness, and rolling contact fatigue (where repeated contacts cause fatigue to occur) [ ].

The proportionality between friction force and normal load has led to the definition of "kinetic" and "static" coefficients of friction. They are referred to as "properties" of certain combinations of materials. This approach is extremely basic since the coefficients of friction are dependent on parameters such as temperature and sliding speed and, in some cases there is no exact proportionality between friction force and normal load. Recently, it has been found that much of the characteristics of friction are a result of the properties of rough surfaces in contact. Also, the concentration of frictional energy (heat) over small localized areas has a significant influence on friction and wears [ ].

FRICTION MEASUREMENT

STRAIN GAUGE

A strain gauge is a device used to measure the strain of an object. It was invented in 1938 by by Edward E. Simmons and Arthur C. Ruge. It is a fundamental sensing element for many types of sensors, including pressure sensors, load cells, torque sensors, position sensors, etc. It is a sensor whose resistance varies with applied force. It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured. The majority of strain gauges are foil types, available in a wide choice of shapes and sizes to suit a variety of applications. They consist of a pattern of resistive foil which is mounted on a backing material. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change.

Figure: Foil Type of Strain Gauge.

When a force is applied to a structure, it results to stress and strain that causes the length of the structure changes. Stress is a measure of the average force per unit area of a surface within a deformable body on which internal forces act. In other words, it is a measure of the intensity of the internal forces acting between particles of a deformable body across imaginary internal surfaces.

stress2

Fig: stress of a body

While strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length of a body when an external body is applied.

Fig : strain of a body

Strain is calculated by dividing the total deformation of the original length by the original length (L): …………………Eqn

Essentially, all strain gauges are designed to convert mechanical motion into an electronic signal. A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor. If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the gauge factor (GF). The gauge factor is the strain gauge sensitivity to strain. It is the ratio of fractional change in electrical resistance to the fractional change in length (strain)

……………..Eqn

= …………………………Eqn

= ………………………….Eqn

Where ΔR is the change in resistance caused by strain, R is the resistance of the unreformed gauge, and ε is strain.

The ideal strain gauge would change resistance only due to the deformations of the surface to which the sensor is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gauge to the surface, and the stability of the metal all affect the detected resistance. Strain gauges are frequently used in mechanical engineering research and development to measure the stresses generated by machinery.

CIRCUIT MEASUREMENT

A typical strain gauge resistance is 120 Ohms (unstressed). This resistance may change only a fraction of a percent for the full force range of the gauge, which makes it very difficult to use the strain gauge alone as a measurement system Thus. In order to make it possible to use the strain gauge as a practical instrument, extremely small changes in resistance must be measured with high accuracy. Hence, the strain gauge is connected into a Wheatstone bridge circuit with a combination of four active gauges (full bridge), two gauges (half bridge), or, less commonly, a single gauge (quarter bridge). In the half and quarter circuits, the bridge is completed with precision resistors.

THE WHEATSTONE BRIDGE

The Wheatstone bridge (figure ) is one of the useful electrical bridge circuits that may be used to measure resistance, capacitance or inductance[]. When the Wheatstone bridge is balanced, the resistors at the right are identical to the resistors at the left (R1=R3 and R2=R4), this makes the voltage across the bridge to be equal to zero. However, a slight change in resistance on one of the resistors will cause the bridge to become unbalanced thereby causing a voltage difference appears.

In the circuit shown below it is apparent that the bridge can be imagined as two ballast circuits (composed of R1, R2 and R3, R4) connected so that the initial steady state voltages are cancelled in the measurement [ ]-[].

By employing voltage divider rule;

V1 = VEX ) ………………………………………….. Equation

V2 = VEX ( ………………………………………………..Equation

Vo = V1 - V2

Vo = VEX ( - VEX (

VO = VEX ( ……………………………………….Equation

Where VEX represents the excitation or the input voltage to the load cell while Vo represents the output voltage from the load cell which is also called the Signal.

Figure: Wheatstone bridge Circuit Diagram.

LOAD CELL

HISTORY

Before the invention of the load cell, the mechanical weighing instrument that was used is called the lever. The lever operates in a form of an equal arm balance that has a bar with two pans clinging from each end and a fulcrum at the middle of the bar upon which the bar can be in equilibrium (balance). It also notably used by the ancient Egyptians for measuring things like people and gold.

In 1843, an English physicist called Sir Charles Wheatstone devised a bridge

circuit that could measure electrical resistances. The Wheatstone

bridge circuit is ideal for measuring the resistance changes that

occur in strain gauges. Although the first bonded resistance wire strain

gauge was developed in the 1940s, it was not until modern electronics

caught up that the new technology became technically and economically

feasible. Since that time, however, strain gauges have proliferated both

as mechanical scale components and in stand-alone load cells.

Today, except for certain laboratories where precision mechanical

balances are still used, strain gauge load cells dominate the weighing

industry. Pneumatic load cells are sometimes used where intrinsic safety

and hygiene are desired, and hydraulic load cells are considered in

remote locations, as they do not require a power supply. Strain gauge

load cells offer accuracies from within 0.03% to 0.25% full scale and

are suitable for almost all industrial applications. [].

LOAD CELL FRICTION MEASUREMENT

A load cell (figure ) is a transducer that converts a mechanical force into an electrical signal. This conversion is indirect and occurs in two phases. When dealing with a mechanical arrangement, the force or load being sensed deforms a strain gauge. The strain gauge converts the strain to electrical signals.

There are different configurations of a load cell. There are configurations of a single strain gauge, double strain gauges and four strain gauges. The electrical signal output is usually very small. It is mostly measured in mill-volts and needs amplification by an instrumentation amplifier before it can be used.

CHAPTER 3

METHODOLOGY

The aim of this circuit system design is to bring about a scheme that captures frictional forces via LCD integration during Tribological tests and to increase

accuracy and efficiency while reading and recording the friction measurements from a Tribology machine. The new system uses a PIC microcontroller to connect with the Wheatstone bridge from strain gauges (load cell) which acts as the input given to the circuit in frictional force (R) form and reads the output using an LCD that eventually reads measurement results, as you can see in (Figure)

Figure 3.1 System Layout Design.

Details on PIC Microcontroller

Peripheral Interface Controllers (PICs) are famous with developers due to their low cost, wide accessibility, large user base, wide-ranging collection of application notes, availability of low cost or free development tools and serial programming (and re-programming with flash memory) ability. All PICs feature Harvard architecture, so the code space and the data space are separate. PIC code space is generally implemented as EPROM, ROM, or flash ROM []. In general, external code memory is not directly addressable due to the lack of an external memory interface. The exceptions are PIC17 and select high pin count PIC18 devices [].

The PIC microcontroller used in this system is PIC16f877A (figure ). The PIC16F877A features 256 bytes of EEPROM data memory, self-programming, an ICD, 2 Comparators, 8 channels of 10-bit Analog-to-Digital (A/D) converter, 2 capture/compare/PWM functions, the synchronous serial port can be configured as either 3-wire Serial Peripheral Interface (SPIâ„¢) or the 2-wire Inter-Integrated Circuit (I²Câ„¢) bus and a Universal Asynchronous Receiver Transmitter (USART). All of these features make it ideal for more advanced level A/D applications in automotive, industrial, appliances and consumer applications.

Figure

In this project, the code used is in C programming. This microcontroller has 40 pins and the pin arrangements are as follows (Figure 3.2) [28]-[30];

- Pin 2 (RA0/AN0) is the input with a voltage range of 0 to 5V and maximum input voltage that can be fed is 5.5V.

- Pin 13 and 14 are connected to an oscillator. The function of an oscillator is to provide an accurate and stable periodic clock signal to a microcontroller. The frequency of this clock signal is 20MH and determines how quickly the microcontroller executes it's its instructions. And two capacitors are needed to be connected to the oscillator for voltage regulation.

- Pin 19, 20, 21, 22, 27, 28 29 and 30 are the data bus pins of the LCD (pin 7 to pin 14)

- Pin 11 and 32 function as power supplies of 5V. It is also connected to pin 2 and 15 of the LCD.

- Pin 12 and 30 function as power supplies of 0V (Ground). And it is connected to pin 1 and 5 of the LCD.

- Pin 37(RB4) is connected to pin 4 of the LCD (RS) . This selects register, instruction or data register.

- Pin 38 (RB5) is connected to pin 6 (E) of the LCD. The function of this LCD pin is to enable reading or writing of data.

-

Diagram

CHAPTER 4

The circuit was first fabricated on a breadboard at first (figure). The breadboard is also known as the protoboard. The breadboard used is the solderless breadboard. It does not require soldering. It is reusable and thus can be used for temporary prototypes and experimenting with circuit design more easily []. After it was confirmed that the schematic can be use, the fabrication was transferred from the breadboard to the stripboard (figure ). The stripboard has parallel strips of copper track on one side. The tracks are 2.54mm apart and there are holes every 2.54mm. the stripboard is used to make a permanent soldered circuit. However, it is very easy to connect components in the wrong place since the holes are very small and it is easy to make mistakes.

As there were some connection errors on the circuit board while soldering and the fabrication did not look professional , as can be seen in (figure ), the circuit was fabricated again using a different breadboard (figure ). However, a problem occurred while trying to integrate the LCD to the circuit. When the LCD was connected to the circuit, the LCD cannot be displayed. So the circuit was cross-checked and found no fault. It was then given to the lab technicians to help and troubleshoot but they found no problem nor error from the soldering. Two weeks were spent in trying to figure out the problem. So an "Enhanced 40 pins PIC start-up kit" (figure) also known as SK40C was bought to work in place of the failed fabricated circuit. The SK4OC has a dimension of 8.5cm x 5.5cm and is designed to offer an easy to start board for PIC microcontroller user. The kit is design to offer:

Industrial grade PCB

Load program 

• perfectly fit for 40 pins 16F and PIC18F PIC

• With UIC00A (not included), program can be loaded in less than 5 seconds

•   More convenient to use as it is smaller than SK40B.

•   Maximum current is 1A.

2 x Programmable switch 

2 x LED indicator 

Finally, the circuit works and the LCD integrates perfectly with the circuits.


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