An Engine Driven Auxiliary Air Compressor Failed Engineering Essay

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In this project, the mission which needs to be accomplished is to investigate the relentless root cause of such failure and to give useful recommendations which can help to avoid such failures in the reciprocating compressor under study in the future. An engine driven auxiliary air compressor failed, the air compressor is located in the Hawke workshop. The first stage will be to establish details about the compressor, the manufacturer and how the compressor works. Classification of compressors and types will be also discussed and of course the theory of operation. Collecting components failed in order to be examined subsequently for the failure cause or causes this will be held using a variety of methods. NDE (non-distractive examination) or NDT (non-destructive testing) is valuable way to use for preliminary inspection of the failed parts without damaging it. DT (destructive test) is another way to check and inspect parts but this inspection method appears from its name it lead do destructive damage the tested parts which will be no longer able to be used. Fatigue principles also will be showed. Studying all the above will gave a clear idea about the cause of the damage.

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Discussion of the probable causes for the failure well be performed in the following chapters as well as the conclusion obtained from study conducted.

1.1 Introduction

As this project focuses on the how a compressor functions in general up to and including the specified compressor in question, it is important to identify the mechanism of which a compressor works so the reasons of failure for any compressor can be clear and hence future mistakes can be avoided.

An air compressor operates by converting mechanical energy into pneumatic energy via compression which includes subjecting the system to an increase of pressure to a point where the air starts converting the energy to a kinetic energy. The input energy could come from a drive motor, gasoline engine, or power takeoff.

The production of air compressors go back for a long time. The main idea of compressing air and discharge it used by the ancients thousands of years ago in deferent ways where the first usage for the air compressor is increasing the combustion of furnace, where this procedure was used for creating more powerful weapons and tools with high tolerance.

The air compressor is raising the heat of the furnace by blowing compressed air to it, moreover, the main products got from these furnaces are metals and weapons. Man muscles were the main power source for the old air compressor where the prime user for such a device back then was a blacksmith considering that that old compressor was mostly used for weapons. The blacksmith simply used the handles to push them against each other to exert compressed air out of the old compressor, where this hot air coming outside of it was hot enough to process most types of metals. The figure below shows the old air compressor. That kind of old air compressors still used in different productions.

Figure : Old air compressor (bellows)

Of course with the advance of civilization, it became harder to process air up to a desired temperatures and pressures knowing that this old compressor has only a finite limit to the air blown out of it, which was the reason behind the necessity of the existence of a more complicated compressor that was only complicated because each and every part of it had a specific task in which that old compressor cannot accomplish with its simple design.

In this time, there is a huge development in the manufacturing of air compressors, where it now uses pistons, vanes, and other pumping mechanisms to take air from the atmosphere, compress it, and discharge it into a receiver or pressure system so it can be processed later on for other purposes.

This project will investigate and analyze a failure that has occurred in an air compressor. This investigation will be done in several ways to figure out the cause of the failure so it can be avoided in the future. An air compressor is consisting of many components; these components basically include an electrical motor, pistons, air pump, air receiver, air drier, filters, air pressure regulator and pressure switch. And each one of these components has a specific task that defines the final outcome of the compressor.

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Those components, among other types of compressors will be discussed thoroughly later on in the coming chapters.

1.2 Background

The case which had happened here is a failure in an air compressor. From the first sight on the damage that has happened in it, it seems to be that the failure happened in one of the elementary parts included within the design of the air compressor which is the aluminium connecting rod, and it can be clearly seen in figure 2 below.

The aluminium connecting rod is part in the compressor that connects the piston head to the crank shaft in the compressor, and it serves as moving element where it is responsible for the movement of the piston in a mechanism that is called "The Skotch - Yoke Mechanism", The Skotch - Yoke Mechanism is responsible for transferring the rotational movement a certain part into a linear movement of a sliding element.

C:\Users\Aziz\Documents\reports\greenwich\MOHAMMAD\Air_Comp\IMG_0471.JPG

Figure : Failure of compressor connecting rod

The previous figure shows that the failure that has happened in the connecting rod from the ring part which is connected to a crank shaft. But the reason for this failure is unknown as there are several scenarios that could be adopted; the most common two are the following;

The fatigue is the reason for this failure.

The very heavy pressure that is loaded to the compressor which led to that failure.

These are the most expected scenarios which will be investigated later in this project to figure out the reason of the failure.

1.3 Aims and objectives

1.3.1 Aims:

The main aim of this project is to investigate and discover the cause or the causes of failure of diesel engine driven auxiliary reciprocating air compressor made by Knorr-Bremse, model number is LP 4865. This will be achieved by carrying out the below objectives, once it's achieved, the main aim of the project will be achieved.

1.3.2 Project objectives:

Collecting valuable information about, model number (LP 4865), which will include the design of compressor, the design of each part assembled to form the compressor, the construction and selection of materials of each part, the operation principles and the start-up conditions, the working theory and mechanism, the production range..etc. collecting data will help to recognize the main failure causes, and accordingly preparing a new procedure to eliminate the problem in the future.

To decrease the opportunity of failure occurrence in the future, to use greater safety factor and to recognize the person or the entity responsible for failure and to make mistake proofing for future avoidance of the failure. Collecting this background information is not limited to numbers only but it is should be extended to everything such as pictures, charts and graphs and samples.

Investigating operational parameters, history of operation, nature of operation..etc in order to be able to determine precisely the root cause of such failure and which failure mode it follows.

To carry out a physical investigation or macroscopic inspection, where this mission will include photo capturing, code of the product, model number, batch number and serial number. This process will provide a better understanding to the compressor and hence better outcomes will be reached.

To perform compressor disassembly in order to dismantle each component, part, assemblies and sub-assemblies to perform thorough investigation and inspection of each one of them. It's should be mentioned that necessary tests and analyses should be performed; mainly the manufacturer usually has his own procedures and tests for test and inspection.

2.0 Air Compressor

2.1 Theory (2)

Before discussing what a compressor is and how it works, it will be helpful to consider some of the basic gas laws and the manner in which they affect compressors, especially that based on those laws, the concept of an enhanced and more effective compressor was formed in the first place.

The First Law of Thermodynamics

This law states that energy cannot be created or destroyed during a process, such as compression and delivery of a gas, but it always changes from one form into another indefinitely. In other words, whenever a quantity of one kind of energy disappears, an exactly equivalent total of other kinds of energy must be produced. A good example of that would be the construction of the human body, where the food consumed by a certain body will change its form from a chemical energy into mechanical energy, kinetic energy and potential energy, in addition to some energy lost into other forms.

The Second Law of Thermodynamics

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This law can be stated in several ways:

The available energy of the isolated system decreases in all real processes due to change in energy forms which will yield loss of energy from that same system, and it is called loss because the energy is lost from that isolated system to another system which could be the outside perimeter of that system.

Heat can transferred from a body at a lower temperature to one at a higher temperature only if external work is performed, knowing that in regular conditions, is goes the other way around.

Heat cannot pass from a colder to a hotter body from itself, because heat is a form of energy that only travels from hotter sources to a less hot source.

By itself, heat or energy (like water), will flow only downhill (i.e. from hot to cold).

These statements say that energy which exists at various levels is available for use only if it can move from a higher to a lower content level.

Ideal or Perfect Gas Laws

An ideal or perfect gas is one to which the laws of Boyle, Charles, and Amonton apply.

There are no truly perfect gases, but these laws are used and corrected by compressibility factors based on experimental data. And the expression used to identify an ideal gas is stated as follows;

Where;

P is the pressure of which the gas is subjected to

V is the volume of the air contained in the system

M is the mass of the air contained in the system

R is the ideal gas constant, where each gas has its own unique value

T is the temperature of the gas

is the specific volume where it can be calculated by simply dividing the volume of the gas by the mass of the gas

Boyle's Law

At constant temperature, the volume of an ideal gas varies inversely with the pressure.

This is the isothermal law. Where V is volume and P is pressure.

This law originates from the use of the ideal gas constant, it can be explained as follows;

So between two states;

And since the temperature was set to be constant, and the mass of the gas enclosed will not change knowing that it is a closed system, then;

Charles' Law

The volume of an ideal gas at constant pressure varies directly as the absolute temperature.

Where V is volume and T is temperature. And this law also can be originated to the ideal gas equation knowing that the difference is that this time the volume is constant instead of the temperature.

Amonton's Law

At constant volume, the pressure of an ideal gas will vary directly with the absolute temperature.

And the derivation of this expression can also be traced back to the ideal gas equation.

2.2 What is the air compressor?

An air compressor operates by converting mechanical energy into pneumatic energy via compression where this energy comes from pressurizing and compressing air in terms of increasing the applied pressure on it. After that, the compressed air goes into a chamber, and the air is kept in the chamber by the use of a unidirectional valve that acts as a gate that stops the air from flowing outside of the system.

The most basic types of air compressors are designated as positive displacement compressors and dynamic compressors. The characteristic action of a positive displacement compressor is thus a distinct volumetric change a literal displacement action by which successive volumes of air are confined within a closed chamber of fixed volume and the pressure is gradually increasing by reducing the volume of the space. There are many types of compressors but the main types are reciprocating compressor, rotary screw compressor and centrifugal compressor. These types will be explaining in details below.

Compressor_Types.png

Figure : Compressors Types

2.2.1 Reciprocating compressor

Reciprocating air compressor is positive displacement compressor. This compressor functions by applying vacuum suction to suck a volume of air and then apply high pressure in order to presses it up to a certain point that is specified within the design on the compressor itself. This pressure is done by using a piston and cylinder as movement part and displacement part.

The compressor could be single acting or double acting according to what is required from it to do and it carries that out by using one side or both sides. When the pressure difference between the cylinder and the receiver becomes proper in terms of the desired pressure and temperature, the valves start to open. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure, and the discharge valves open when the pressure in the cylinder is slightly above the discharge pressure. The compressor could be a single stage when the compression process done parallel.

Advantages:

Simple design that doesn't require extra attachments.

Relatively lower initial cost.

Easy to install.

High efficiency with 2 stages.

No need for lubrication.

Large range of horsepower.

Disadvantages:

Maintenance cost is high.

Many moving parts.

Vibration problems.

Foundation maybe required.

Not running at full capacity all the time.

Figure : Reciprocating air compressor

2.2.2 Rotary screw compressor

Rotary air compressor is also a positive displacement compressor, the single stage helical or spiral lobe oil flooded screw is the most common in rotary air compressor.

This compressor consists of 2 rotors located in a case where the air is compressed internally without any valves. The cooling for these compressors is done by the use of oil. As the cooling for the working parts happens inside the compressor, this type of compressor will not experience over heat due to operation, so it could operate without stop as long as it is well lubricated more often and whenever needed to.

Due to the simple design of the rotary screw compressor and its parts, it can be maintained easily and installed anywhere that could handle its static weight. The 2 stages rotary air compressor uses 2 rotors with a combined air end. These 2 rotors are installed in series to share the compression, this raise the efficiency up to 50% more than the originally designed efficiency.

This 2 stage rotary compressor combines the maximum profit from the rotary screw with its simplicity and flexibility and the reciprocating compressor with its effective energy when it functions in 2 stages with double acting. The 2 stages of this compressor could be cooled by water, air or oil.

Advantages:

Simple design.

Low initial cost.

Low maintenance cost.

Good efficiency at 2 stages.

Easy to install.

Few moving parts.

Common in use.

Disadvantages:

Limited life for airend.

Service for airend couldn't be done at field.

High rotational speeds.

Must be oil lubricated.

Single stage has low efficiency.

Figure : Single screw air compressor

Figure : Double rotary screw air compressor

2.2.3 Centrifugal compressor:

The centrifugal air compressor is a dynamic compressor that depends on transfer of energy from a rotating impeller to the air. This process is done by changing the air's momentum and pressure by slowing the air in stationary diffuser the momentum converted to pressure.

This compressor is oil free compressor, and there is a separation between the lubricated oil and air by shaft seals and atmospheric vents, where the high volume of dry air is required.

A centrifugal compressor is the proper compressor as it has a few moving parts and could operate continuously.

Figure : Centrifugal air compressor

2.2.4 Rotary Sliding Vane Compressors:

The rotary sliding vane compressor is also a positive displacement compressor. This type of compressor is consisting of a rotor, a stator and eight blades. Between the intake and exhaust valves, a crescent shape is formed by the location of the rotor and stator.

The compression is achieved by one complete rotation as the volume changes from the maximum to minimum, and vanes are pushed outward against the stator walls. The injecting oil from the intake valve and along the stator wall will cool the air and form a sealing between the stator wall and vanes, not to mention that it will also lubricate the bearings. So that a filtration system must be installed to separates the oil from the compressed air.

Advantages:

Simple design.

Easy to install.

Low cost.

Maintenance cost is low.

Reliable.

Few moving parts.

Disadvantages:

Single stage.

Low efficiency.

Must lubricate with oil.

Difficult with over pressure.

Figure : Rotary sliding vans compressor

2.5 Main Component of Air compressor in general:

Figure : Air compressor system 1

Figure : Air compressor system 2There are several components that air compressor system consist of, these components are:

Figures above (9, 10) shows the main component of the air compressor systems. The system could be installed in 2 methods; the first method is where air drier before receiver tank, and the second method is where the air drier after the receiver tank. These components will be explained in details in the below paragraph.

Receiver tank: From its name, the receiver tank is a tank that receives the discharged air and it helps preventing rapid compressor cycling by providing a storage capacity. The advantage for the receiver tank is reducing the wear and tear in motor, inlet control system and compression module. It also eliminates the flow pulsing.

Figure : Receiver tank

Air Dryers: there are 3 types of air dryer. The main objective for air dryers is to dry the air to prevent rust and wearing in the air compressor components and discharge line. The types of air dryers are:

Refrigerated air dryers: This dryer uses a mechanical technique to remove moisture from compressed air and hence cooling it to form condensate water.

Desiccant dryers: This dryer usually absorbs water vapour by using utilizing chemical drops that are called desiccant in order to remove the moisture content in the air. The common desiccants used in this dryer are silica gel, molecular sieve and activated alumina.

Deliquescent air dryers: This type of dryer uses desiccant to dry compressed air. The wetness in the compressed air reacted with the absorption material and turned into liquid which is drained from the dryer. This process could be corrosive so the corrosion check has to be done to avoid future failures due to fatigue because of the weak tolerance due to the corrosive and erosive effects.

Figure : Air dryer

Filters: Filters are installed to remove the lubricants and water from the compressed air, and it could be installed downstream at refrigerated air dryer or upstream at desiccant dryer. The most common in filters is coalescing filter. But this filter it only can remove liquid and water which have been previously condensed.

Figure : Filters

Piping Distribution systems: Piping distribution system is controlling the process of how the compressed air goes out to the tools and also determined the required energy for the air compressor.

2.6 The Reciprocating Air Compressor (Knorr-Bremse, model LP 4865)

After identifying the tasks and types of compressors, it is inevitable now to discuss the compressor in question and the reasons of its failure.

The main study object in this project as said before is to study the reciprocating air compressor (Knorr-Bremse LP 4865). By searching the internet database and trying to get information about the air compressor which will be needed to complete this study (please refer to appendices), but unfortunately the required information couldn't be found except for the following;

It has a maximum pressure of 12 bar

The volume is 460

So the general information about the reciprocating air compressor will be mentioned instead of the specific air compressor.

There are 9 main parts for the reciprocating air compressor these parts are:

Crank case: Crank case is a close rigid body where the crank shaft and the bearing house where located. This body could by rectangular or square shaped. Mainly crank case manufactured by using cast iron.

Figure : Crank Case

Crank Shaft: Crank shaft in one of the most important parts in air compressor, it is a motion transferor that is designed as one piece which has balances in its dynamics and tries to avoid any twisting, and to ensure a long life for bearings, the polishing and crank pin will be done. Generally crank shafts have a fly wheel. This crank shaft is manufactured mainly of high grade S.G iron.

Figure : Crank Shaft

Connecting rod: This part is responsible of transfer the motion from the crank shaft to the piston and changing the direction from the rotating motion that is coming from the crank shaft into reciprocating motion in piston. This part is mainly manufactured by the process forging that is applied to alloy steel or aluminium.

Figure : Connecting Rod

Bearings: Bearings are made to provide the rotating gear with rigidity. They are mainly manufactured of copper lead alloy.

Figure : Bearings

Cross slide: to get a perfect running for cross head. This part is responsible for reducing the inertia, and it is commonly manufactured of high grade S.G. iron.

Figure : Cross slide

Cylinder: This part is where the air flow passes and compressed. Cylinders are manufactured with water jacket to reduce the generated heat from the compression process. This cylinder mainly made of cast iron.

Figure : Cylinders

Pistons: Piston is the main part in the compression process, and it moves forward to compress the air and then backward to intake a new amount of air to compress. There are 2 types of pistons according to its lubrication system; the first type is non-lubricated which is made from aluminium alloy, and the second type is lubricated and made from cast iron. Due to moving of piston in the cylinder, so there must a space exist between them, to ensure there is no leak happened for air a piston rings located on piston.

Figure : Piston

Piston rod: Piston rod is this rod which connects the piston to the connecting rod, it is also an important part in the operation of the Skotch-Yoke Mechanism that was discussed earlier. The piston rod is manufactured of alloy steel.

Figure : Piston rod

Intake and discharge valves: These valves are responsible for the amount of air sucked or discharged. When the piston in the retraction position, the suction valve opens to allow the air to get in. And when the piston move forward and reach the maximum pressure the discharge valve open. These 2 valves are adjusted to be opened and closed according to the pressure difference between inside the cylinder and the outside. These valves are made from stainless steel even a plate type or spring type.

Figure : Valves Types

2.7 Air Compressor Lubrication

Due to the operation of the air compressors which have different moving parts, friction must be occurring. This friction generates heat, and the problem is that it leads to part wearing which will definitely lead to a failure. And sometimes, a certain type of sealing needs to be installed in order to prevent the compressed air from leaking.

All of those reasons will lead to using lubrication. Oil lubrication is a must in air compressors to overcome all of the above problems. The lubrication method could be different from one type to another, but the main idea is to prevent the friction between moving parts, and the best proposal to prevent such thing is lubrication, like lubricating the bearings, or at the same time in some cases like reciprocating air compressor it also works like sealing between the cylinder wall and the piston. The problem in lubricating the air compressor is the selection of the proper oil, where the selected oil must be an industrial oil with high quality and matching the operation requirements.

The environment of where the compressor will be installed is one of the parameter of selecting the lubrication oil, if the environment is too cold and the temperature is low the freezing point, an anti-freeze oil must be used. If the environment is too hot, then the oil with the highest viscosity and ability to heat resistance must be used. All of these restrictions must be considered when selecting the lubricant.

These are the main ideas of lubrication but the implementation is different according to each type of air compressor. The lubrication system changes from one type of air compressor to another according to the theory of its operation.

3.0 Quality assurance for the manufacturing process

Air compressors are designed to work in both the industrial and the private fields; it is designed to press gas or pressured air.

During the air compressor operations leak or failure chances of happening, this will lead to a serious damage. The quality assurance purpose is to guarantee if the production is safe and ready to use or if rejected and have defects. This operation is done by two methods as explained below;

The first one is the destructive test, which means that the tested object will be destructed and no longer could be used or tested on, and this type is used in the initial design and on the first production to be sure of it.

The second test is called the non-destructive test (NDT) which means we can make our test and be sure if the production is in good shape and is can be used and tested again if needed without suffering from destructing the product. This type (NDT) is what we are going to focus on because it's the most common method in inspection.

The definition of the NDT is it's a test methods used to examine an object without impairing its future usefulness. There are a several methods in NDT used like:

Visual

Microwave

Thermograph

Tap testing

X-ray

Magnetic particles

Acoustic Microscopy

Acoustic emission

Magnetic measurements

Liquid penetrate

Ultrasonic

Flux Leakage

Eddy current

Replication

Laser interferometer

All of the above methods are used in NDT, but in our case we will go to use the common methods and try to clarify its operation and how to use.

Visual inspection: It the most common and basic inspection method. In this method, we can use fiberscope, bore scopes, magnifying glasses and mirrors. Another technology could be used as electronic microscopes have the ability to enter the narrow places inside the air compressor.

Figure : Visual inspection

Magnetic Particle Inspection: This inspection could be done by magnetizing the inspection parts. After that, coloured particles of iron covered the inspection area. These particles of iron will spread to the magnetic flux and show the discontinuity area. By applying the proper light this inspection could be checked by visual.splinedflour

Figure : Magnetic inspection

Radiography: To perform this check, shot waves with high energy is used. Radioactive source (or X-ray machine) could generate such waves. The part which will be tested will be located between the radiation source and a film. This part will prevent some of the radiation. Thick areas will stop more waves. This will lead to variation on the film darkness where the thick part will be less dark and the thin or cracked part will be more darker.RAD1Casting

CastingRad

Figure : Radiography inspection (NDT Manual)

Figure : Eddy current inspectionEddy Current Testing: This type of tests is used to discover the cracks on the surfaces. It also could be used to test the thickness of coating of the surfaces.eddy.PNG

Ultrasonic Inspection: To perform this test, a high frequency sound wave is applied to the surface of the part. These waves are reflected waves which will be reflected from any crack inside the material. These sound waves are reflected into the device which will show the reflection time on a gauge, and this gauge could show the depth of the crack by time of reflection. It could also be connected to a computer by using a computer program which will draw a diagram of the part and determine the crack location.

Figure : Ultrasonic inspection ()ultra.PNG

Liquid Penetrate Inspection: To apply this type of tests, a liquid with high moistening is spread over the tested part and is given time to leak into the crakes. After that, the rest of liquid is removed from the surface.

After that a special powder is spread over the surface to get the stuck liquid out from the crack. The crack will appear and could be simply examined and noticed by eye. To increase the sensitivity and accuracy of this test, it could be done under ultra violet light in order to discover the remaining cracks.

FPI_PowDevGitsFtop1

Figure : Liquid penetrate inspection

For the above types of NDT, there is some common applications where they also could be used and they are listed below:

Inspection of Raw Products:

Forgings.

Castings.

Extrusions.

Inspection Following Secondary Processing:

Machining.

Welding.

Grinding.

Heat treating.

Plating.

In-Services Damage Inspection:

Cracking.

Corrosion.

Erosion/Wear.

Heat Damage.

3.1 Fatigue

Fatigue is a common types of failure that occurs due to a cyclic and repeated small load that is exerted on a certain device, and because the strength of the product is the most important safety factor, we have to be sure that the product will be safe and could stand the applied loads, which is why the safety factor must be added to the design.

During the operation, parts experience a non-constant loads which lead failure along with time. Fatigue could be defined as a failure happened due to a rapid variation in loads, but without any of this load variations exceed the maximum load.

This type of failure happens commonly in metallic materials not only but also in different materials. There are 3 stagers for Fatigue, these stages are:

1. Initial crack.

2. Propagation of crack.

3. Final fracture.

3.2 How to analysis fatigue and prevent it

Component material and stress field are the main 2 principals that govern the amount of time needed for crack to start and spread which lead into causing a failure. There are 3 main methods to calculate the fatigue time, these method are listed below:

SN Method: To predict a fatigue life for a part, stress life approach could be used. This method gave the fatigue life for the object and it is depending on the calculation of varying elastic strength so it couldn't be applied to low fatigue cycle. But this type of tests is very accurate at high fatigue cycles.

EN Method: This method is basically using plastic stress to calculate not only the fatigue life but also the crack penetration to reveal the extent of the damage on the part itself. The advantage of this method is that it could be used for calculating the fatigue in low life cycle and plastic life cycle also.

Linear Elastic Fracture Mechanics (LEFM) Method: The LEFM is used to calculate the crack growth rates. This approach assumes that a crack is already present and predicts crack growth with respect to the stress intensity at the crack tip.

3.3 Fatigues in Aluminium

From previous fatigue studies done on aluminium, it seems to be that aluminium is a very good material with high resistance to fatigue because of its high values of Young's Modulus and its ability to join with other materials in the form of composite material so a better tolerant material can be obtained.

These studies were conducted on a certain alloy of aluminium, but in general the heat treatment for aluminium leads to a significant enhancement in the fatigue life. Heat treatment also gives aluminium high strength, excellent toughness and reasonable ductility. The crack initial behaviour of aluminium took place from the surface just like many metals and alloys.

3.4 Aluminium Properties

To be familiar with Aluminium some properties must be considered and known, these properties are:

Young's Modulus: 70 GPa.

Ductility & Malleability: High.

Hardness: 420 MPa.

Density: Low 2700 kg / cubic metre.

Melting Point: 660.32 °C Boiling Point: 2519 °C.

Electrical Resistivity: Low 2.65 x Ohm metres.

Reflectivity: High 71% unpolished and when polished: 97%.

All of these properties give a clue that Aluminium is the metal of the modern world. With its strong, lightweight, ductile and reflective it could be used for several industries and accomplish great results.

4.0 Problem Analysis

It is a clear fact that while working on any project, problems and challenges will be faced. There is a variety of challenges in this project; the first problem in this study is that damage which happened in the aluminium connecting rod from ring which connected with crack shaft. As said in the background, there are 2 scenarios which might lead to this damage.

The first scenario is a fatigue accrues in the connecting rod with lead to this damage as it couldn't stand the motion and the pressure any more.

The second scenario is an excessive pressure more than the maximum pressure which could be hold by the compressor. At the same time a malfunction in the discharge happened, and this could be taken in consider as it must opened at slightly below the maximum pressure.

In both scenarios, the damage in the connecting rod lead to another greater damage in the piston and the cylinder, as the piston starts to move in a non-liner motion and hitting the cylinder wall. This will lead into a serious damage in the compressor's piston and cylinder. The second problem is gathering the information about the air compressor Knorr-Bremse LP 4865.

This problem occurred as the manufactured company keep the information about this compressor not listed and its material and properties to figure out the reason of the damage, maybe by knowing its maximum pressure or material type to check the fatigue on it.

4.1 Problem Solution

After studying all of the circumstances and what happened during the failure that took place, it seems that the failure happened due to a serious fatigue, which was not noticed for the previous time.

This fatigue happened to the aluminium alloy which the connecting rod was made of. An analysis was done for the particles and it showed that the particles are consisting of 88% aluminium (please refer to appendices). This gave a thought about how serious was the fatigue in this movable part.

As the connecting rod is responsible for transferring the power and motion from crank shaft to the piston, it experiences a load variation all through the operation time. A possible reason could be that a crack must have happened a long time ago on the surface of the connecting rod and then it started to spread, and over time it came to choose the weakest area, which is the ring where the crank shaft is hinged. This particular area is the weakest area in connecting rod as the thickness reduced to attach the crank shaft.

Figure & Figure 30: Damage in connecting rod and hole in compressor case

To overcome the second problem, an intense internet search is made with trying to contact with the manufactured company through their website www.knorr-bremse.com , but with no responses. So another technique was implemented, which is gathering common information about reciprocating air compressor and its common parts and materials, this could be useful but not accurate. There are 2 appendices attached to the project which have some data about the parts and how to maintain it gathered from the company itself.

4.2 Results

As a final result of this project, the damage which happened in the connecting rod accusing damage on the rest of parts is happened due to a fatigue. After the fatigue weak the connecting rod it reached a phase that the connecting rod couldn't stand any load. Just after the failure the piston starts to hit the cylinder wall, which accuses the obvious damage on the piston and the piston rings. To prevent such a failure in the future a regular check must be done for the concerning parts as scheduled in the producer's manual. A life time calculation for all parts must also be done and make the replacement in its time. These procedures are going to prevent such a failure to happen again.