A Mini Project on Design,Manufacturing & Failure Analysis of Leaf Spring

A Mini project report on

Design, Manufacturing and Failure Analysis of Leaf Spring submitted by : CH.RAKESH KUMAR

08471A0311

K.SAI KIRAN

08471A0320

K.SESHI REDDY

08471A0321

K.NAGA LAKSHMI

08471A0356

1

ACKNOWLEDGEMENT

We express our sincere thanks to Mr. K.VARA PRASAD RAO , Professor & HOD of  Mechanical Engineering, Narasaraopeta Engineering College, for his encouragement and valuable suggestions through out the miniproject. We profoundly thank  Mr.Dr B.V.RAMA MOHANA RAO , principle of Narasaraopeta Narasaraopeta Engineering College for having given us permission to carry out the miniproject. We express our sincere thanks and deep sense of gratitude for the inspiring guidance and kind encouragemet encouragemet with unfailing rendered by Mr.M.VENKAIAH, Asst Prof. Dept of  Mechanical Engineering, Narasaraopeta Engineering College. We are also thankful to the Staff of our our Mechanical Engineering Engineering Department Department for there valuable support, comments during the course of the miniproject. We are also thankful to all others who have directly or indirectly contributed to the success of the miniproject. We would like to express my heart-felt gratitude to my parents without whom we would not have been privileged to achieve and fulfill our dreams. We owe our gratitude to our Narasaraopeta Engineering College for providing an opportunity to do our project.

Our sincere sincere thank‟s to Mr.T.VINOD BABU, Managing Partner of  TRAILOR SPRINGS & the Working staff of TRAILOR SPRINGS for supporting us to complete our miniproject in their presence.

CH.RAKESH KUMAR

08471A0311

K.SAI KIRAN

08471A0320

K.SESHI REDDY

08471A0321

K.NAGA LAKSHMI

08471A0356

2

CONTENTS ABSTRACT LIST OF FIGURES SYMBOLS 1. INTRODUCTION TO LEAF LEAF SPRIN G

……………………………….….9

2. HISTORY OF LEAF SPRING

…………………………………………….…….10

3. DESGN OF LEAF SPRING

…………………………………………………..12

3.1. Materials for leaf spring.......................................……………………………… spring.......................................………………………………13 13 3.2. Steel……………………………………………………………………………..13 Steel……………………………………………………………………………..13 3.2.1. Introduction……………………………………………………………...13 Introduction……………………………………………………………...13 3.2.2. Types of steel…………………………………………………………….14 steel …………………………………………………………….14 3.2.3. Spring steel………………………………………………………………16 steel………………………………………………………………16 …………………………………………………...18 3.2.4. Role of carbon in steel steel…………………………………………………...18 3.3. Types of leaf springs…………………………………………………………….18 springs …………………………………………………………….18 3.4. Design of leaf spring…………………………………………………………….19 spring…………………………………………………………….19 3.5. Length of leaf spring leaves……………………………………………………..28 leaves ……………………………………………………..28 ………………………………29 3.6. Standard sizes sizes of automobile suspension suspension springs springs………………………………29 3.7. Dimensions for center bolts……………………………………………………..30 bolts……………………………………………………..30 3.8. Dimensions of clip,rivet and bolts………………………………………………30 bolts………………………………………………30

4. MANUFACTURING OF LEAF SPRING

………………………………………..31

4.1. Raw material…………………………………………………………………… material……………………………………………………………………..33 ..33 4.2. Shearing or cutting process……………………………………………………. process ……………………………………………………...33 ..33 4.3. Drilling operation……………………………………………………………… operation………………………………………………………………..34 ..34 4.4. Eye rolling of main blade……………………………………………………… blade………………………………………………………..35 ..35 4.5. Hardening (Quenching) ( Quenching)……………………………………………………… ………………………………………………………..…36 4.6. Tempering……………………………………………………………………… Tempering………………………………………………………………………..38 ..38 4.7. Hardness testing……………………………………………………………… testing ………………………………………………………………....40 ....40 4.8. Fitting………………………………………………………………………….... Fitting…………………………………………………………………………....42 42 4.8.1. Clamps………………………………………………………………… Clamps…………………………………………………………………......42 ......42 4.8.2. Bolts and nuts………………………………………………………… nuts ………………………………………………………….......44 .......44 4.8.3. Rivets…………………………………………………………………… Rivets……………………………………………………………………....45 ....45 3

4.8.4. MS pipe…………………………………………………………………… pipe……………………………………………………………………..45 ..45 4.8.5. Bushes…………………………………………………………………… Bushes……………………………………………………………………....45 ....45 4.9. Painting………………………………………………………………………… Painting…………………………………………………………………………...45 ...45 4.10. Labeling………………………………………………………………………… Labeling………………………………………………………………………….46 .46 ……..47 4.11. Inspection………………………………………………………………… Inspection…………………………………………………………………..……..47 4.12. Stock ready to supply…………………………………………………………… supply…………………………………………………………….47 .47

5. FAILURE ANALYSIS OF LEAF SPRING ……………………………………….48

5.1. Failure analysis……………………………………………………………………49 analysis……………………………………………………………………49 5.2. Quench cracks…………………………………………………………………….49 cracks …………………………………………………………………….49 5.3. Optical Metallurgical Microscope………………………………………………...50 Microscope………………………………………………...50 5.4. Scanning Electron Microscope………………………………………………........51 Microscope………………………………………………........51 5.5. Tensile test of leaf spring……………………………………………………........53 spring……………………………………………………........53 5.6. Fatigue test of leaf spring…………………………………………………….…..54 spring…………………………………………………….…..54 5.7. Increasing the fatigue strength of leaf spring………………………………….....56 spring ………………………………….....56 5.8. When to repair…………………………………………………………………….58 repair…………………………………………………………………….58 5.9. When to replace…………………………………………………………………...59 replace…………………………………………………………………...59 6. CONCLUSION BIBLOGRAPHY

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ABSTRACT

Although leaf springs are one of the oldest suspension components they are still frequently used, especially in commercial vehicles. Being able to capture the leafspring characteristics is of significant importance for vehicle handling dynamics studies. This report describes the theoretical design considerations that are used during the design of  leaf springs including the design of various accessories such as rivets,bolts,nuts,clips and main spring. spring. It also decribes decribes the manufacturing process of leaf springs springs and the failures failures that are caused in the leaf springs and there remedies. Various tests such as fatigue test, tensile test are carried out to determine the failure analysis of leaf springs and necessary modifications are described in order to increase the fatigue strength of leaf springs. This report also describes when to repair r epair and when to replace a deformed leaf spring.

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LIST OF FIGURES

CHAPTER 1

Figure 1(a): Leaf Springs CHAPTER 2

Figure 2(a): Leaf spring suspension CHAPTER 3

Figure 3(a): Types of lleaf eaf springs Figure 3(b): Flat spring (cantilever type) Figure 3(c): Stress diagram Figure 3(d): Flat spring (simply supported beam type) Figure 3(e): Traingular plate Figure 3(f): Laminated leaf spring Figure 3(g): Construction of leaf spring Figure 3(h): Nipping Figure 3(i): Spring clip CHAPTER 4

Figure 4(a) : Steel alloy Figure 4(b): 100 ton press cutting machine Figure 4(c): Vertical drilling machine Figure 4(d): Eye rolling of main spring Figure 4(e): Hardening 6

Figure 4(f): Hydraulic bending machine Figure 4(g): Tempering process Figure 4(h): Tempering furnace Figure 4(i): Rockwell hardness tester Figure 4(j): Clamps of different sizes Figure 4(k): Circular cutting die of clamp Figure 4(l): Bolts and rivets Figure 4(m): Bush Figure 4(n): Painting Figure 4(o): Labeling Figure 4(p): Stock ready to supply CHAPTER 5

Figure 5(a): Metallurgical Microscope Figure 5(b): Scanning Electron Microscope Figure 5(c): Failure Material Leaf Spring and Received Material Leaf Spring Figure 5(d): Universal Tensile Testing Machine Figure 5(e): Fatigue test

7

SYMBOLS t = Thickness of plate b = Width of plate  L = Length of plate or distancenof the load W from the cantilever end.

M = Maximum bending moment Z = Section modulus σ = Bending Bending stress δ = Maximum deflection E = Young‟s modulus W = Total load on the spring n = Number of leaves nG = Number Number of graduated leaves nF = Number of full length leaves C = Initial gap between the leaves Wb = The load on the clip bolts WG = Load taken up by graduated leaves WF = Load taken up by full length leaves σG = Stress in the graduated leaves σF = Stress in the full length leaves d = Inside diameter of eye

R = Radius of curvature y = Camber of the spring  L1 = Half span of the spring

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1. INTRODUCTION TO LEAF SPRING

Originally Leaf spring called laminated or carriage spring, a leaf spring is a simple form of spring, of  spring, commonly used for the suspension in wheeled vehicles. It is also one of the oldest forms of springing, dating back to medieval times. Sometimes referred to as a semi-elliptical semi -elliptical spring or cart spring, it takes the form of a slender arc-shaped arc-shaped length of  spring steel of rectangular of  rectangular cross-section. The center of the arc provides location for the axle, while tie holes are provided at either end for attaching to the vehicle body. For very heavy vehicles, a leaf spring can be made from several leaves stacked on top of each other in several layers, often with progressively shorter leaves. Leaf springs can serve locating and to some extent damping as well as springing functions. A leaf spring can either be attached directly to the frame at both ends or attached directly at one end, usually the front, with the other end attached through a shackle, a short swinging arm. The shackle takes up the tendency of the leaf spring to elongate when compressed and thus makes for softer springiness.

Figure 1(a): Leaf Springs

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2. History of Leaf Spring There were a variety of leaf springs, usually employing the word "elliptical". "Elliptical" or "full elliptical" leaf springs referred to two circular arcs linked at their tips. This was joined to the frame at the top center of the upper arc, the bottom center was joined to the "live" suspension components, such as a solid front axle. Additional suspension components, such as trailing arms, would be needed for this design, but not for "semi-elliptical" leaf springs as used in the Hotchkiss drive. That employed the lower arc, hence its name. "Quarter-elliptic" springs often had the thickest part of the stack of leaves stuck into the rear end of the side pieces of a short ladder frame, with the free end attached to the differential, as in the Austin Seven of the 1920s. As an example of non-elliptic leaf  springs, the Ford Model T had multiple leaf springs over its differential that were curved in the shape of a yoke. As a substitute for dampers (shock absorbers), some manufacturers laid non-metallic sheets in between the metal leaves, such as wood. Leaf springs were very common on automobiles, right up to the 1970s, when the move to front wheel drive, and more sophisticated suspension designs saw automobile manufacturers use coil springs instead. U.S. passenger cars used leaf springs until 1989 where the Chrysler M platform was the final production vehicle marketed. However, leaf springs are still used in heavy commercial vehicles such as vans and trucks, and railway carriages. For heavy vehicles, they have the advantage of spreading the load more widely over the vehicle's chassis, whereas coil springs transfer it to a single point. Unlike coil springs, leaf springs also locate the rear axle, eliminating the need for trailing arms and a Panhard rod, thereby saving cost and weight in a simple live axle rear suspension. A more modern implementation is the parabolic leaf spring. This design is characterized by fewer leaves whose thickness varies from centre to ends following a parabolic curve. In this design, inter-leaf friction is unwanted, and therefore there is only contact between the springs at the ends and at the centre where the axle is connected. Spacers prevent contact at other points. Aside from a weight saving, the main advantage of parabolic springs is their greater flexibility, which translates into vehicle ride quality that approaches that of coil springs. There is a trade-off in the form of reduced load carrying capability. Like most other fundamental mechanisms, metal springs have existed since the Bronze Age. Even before metals, wood was used as a flexible structural member in archery bows and 10

military catapults. Precision springs first became a necessity during the Renaissance with the advent of accurate timepieces. The fourteenth century saw the development of precise clocks which revolutionized celestial navigation. World exploration and conquest by the European colonial powers continued to provide an impetus to the clockmakers' science and art. Firearms were another area that pushed spring development. The eighteenth century dawn of the industrial revolution raised the need for large, accurate, and inexpensive springs. Whereas clockmakers' springs were often hand-made, now springs needed to be mass-produced from music wire and the like. Manufacturing methodologies were developed so that today springs are ubiquitous. Computer-controlled wire and sheet metal bending machines now allow custom springs to be tooled within weeks, although the throughput is not as hi high gh as that for dedicated machinery.

Figure 2(a): Leaf spring suspension

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DESIGN OF LEAF SPRING

12

3. DESIGN OF LEAF SPRING

3.1

Materials for leaf spring:

The material used for leaf springs is usually a plain carbon steel having 0.90 to 1.0% carbon. The leaves are heat treated after the forming process. The heat treatment of  spring steel produces greater strength and therefore greater load capacity, greater range of  deflection and better fatigue properties.

According to Indian standards, the recommended materials are : 1. For automobiles : 50 Cr 1, 50 Cr 1 V 23, and 55 Si 2 Mn 90 all used in hardened and

tempered state. 2. For rail road springs : C 55 (water-hardened), C 75 (oil-hardened), 40 Si 2 Mn 90

(waterhardened) and 55 Si 2 Mn 90 (oil-hardened). 3. The physical properties of some of these materials are given in the following table. All

values are for oil quenched condition and for single heat only. onl y.

Physical properties of materials commonly used for leaf springs :

3.2 STEEL 3.2.1

Introduction

Steel is an alloy of iron that contains the element iron as the major component and small amounts of carbon as the major alloying element.

The carbon contents in steel ranges from 0.02% to 2.0% by weight. Small amounts, generally on the order of few percent, of other alloying elements such as manganese, silicon, 13

chromium, nickel and molybdenum molybdenum may also be present, but it is the carbon content that turns iron into steel. Also the properties like toughness and ductility are obtained by the addition of  elements like manganese, chromium, nickel, molybdenum, tungsten, vanadium, silicon etc. Steel is the most common and widely used metallic materia l in today‟s society. It can be cast or wrought into numerous forms and can be produced with tensile strength exceeding 5GPa.

3.2.2

Types of steel

Steel can be classified in many ways, such as method of manufacturing, its final use and AISI standards, but mostly steel can be classified into following two categories:

1. Carbon Steel 2. Alloy Steel Carbon Steel

Carbon steel is steel that receives its distinctive properties, mainly form carbon it contains. Other elements, such as manganese, silicon, phosphorus and sulfur may be present in relatively small amounts and their purpose is not to modify the mechanical properties of  steel. The carbon content of this grade may ranges from a trace to 1.7%. The plain carbon steels are usually divided into three grades on the basis of their carbon content. These are give as under.

a)

Low Carbon Steel The plain carbon steel in which carbon content ranges from 0.08 to below

0.3% are known as low carbon steel or mild steel. Mild steel are not much affected by heat treatment processes specially hardening process. A decrease in carbon improves the ductility of mild steels. These steels posses good machine ability and weld ability. They are used for making wires, rivets, nuts and bolts, screws, sheets, plates, tubes, rods, shafts, structural steel sections and form general work shop purposes.

b) Medium Carbon Steel The plain carbon steel in which carbon content ranges from 0.3 to 0.8% are called medium carbon steel.

14

These steels are stronger than mild steel but are less ductile. Their mechanical properties can be much improved by proper heat treatment. They have lesser machine ability than mild steel. However, they can easily be welded and forged. They are used for making stronger nuts and bolts, shafts, various steel sections, high tensile tubes, springs, locomotive, large forging dies, hammer and agricultural tools etc.

c)

High Carbon steel The plain carbon steel in which carbon content range from 0.8 to 1.8% are

known as high carbon steel. The structure and hardness of steel increases with the increase of carbon content and the strength is almost maximum at about 0.8% carbon, there after hardness continuously increases while strength decreases. Ductility and machine ability of steel decrease with increase of carbon content from mild steel onwards. Their mechanical properties can be altered by proper heat treatment. They are mainly used for making springs, wood working tools, press work dies and punches. Alloy Steel:

Alloy steels are the type of steels in which elements other than iron and carbon are present in sufficient quantities to modify the properties. The utility of alloy steels lies i n the fact that they permit a much wider range of physical ph ysical and mechanical mechanical properties than is possible in plain carbon steel. Alloy steels may be classified in many ways but for the purpose of convenience we may divide it as follows:

a) Micro alloy steel:

Micro alloy steels are those steels in which the total alloying elements present in steel other than iron and carbon are below one percent. Micro alloy steels are widely used in many engineering industries such as manufacturing of  pipe lines, automobiles and in aircraft industries. Micro alloy steel is used in these industries because of its good strength, light weight and good weld ability. Some common micro alloy steels contain small amounts of vanadium m, columbium, copper, manganese. Micro alloy steels are only slightly heavier than pure iron but are as strong as some steels. 15

b) Low alloy steel: Low alloy steels are those steels in which the total alloying elements present in steel other than that of iron and carbon are above one percent and below 5%. Low alloy steels have superior mechanical properties than plain carbon steels. Superior properties usually mean higher strength, hardness, hot hardness, wear resistance and toughness.

3.2.3

Spring Steel Spring steel is a low alloy, medium carbon steel with a very high yield

strength. This allows objects made of spring steel to return to their original shape despite significant bending or twisting. Spring steel chemical composition which is given as under: ELEMENTS IN LEAF

% OF ELEMENTS

SPRING

CARBON

0.5 ~0.65

SILICON

0.15 ~0.35

MANGANESE

0.65 ~0.95

PHOPHOROUS

0.035

SILICON

0.035

CHROMIUM

0.65 ~0.95

Depending on the type of application, springs are made of carbon steels, silicon and manganese containing steels, silicon-manganese steels, alloyed steels, stainless steels. Springs must be capable of storing and releasing the energy. After repeated applications of  load, they must retain their original shape and dimensions. This property may be attained by the use of a highly elastic material and by proper design because the allowable stress values determine the choice of material and design. Major requirements of spring steels are: 2

1. Should have a highly yield strength (of the order of 21000kg/mm ) or more accurately, a high proportional limit, so that it will not show any appreciable permanent set 2. High fatigue strength under alternating and fluctuating stresses with a reserve for occasional occasional or more frequent overloads overloads (e.g. Vehicle springs when stressed in their resonance range) 3. Should have an adequate plastic range for the forming (winding) of the springs.

16

These desirable properties of springs can be achieved firstly by a higher carbon content or with suitable alloying elements, and secondly by heat treatment. The actual 2

springiness of steel is determined by its modulus of elasticity which is about 21000kg/mm , 2

the modulus 0f rigidity being about 8000kg/mm . It is possible to influence the modulus of  elasticity and shear by severe cold working treatments. Spring failure is almost invariably from fatigue, with some stress raisers are nucleus. Spring steels are used in hard, high strength condition. To attain these properties springs are hardened and tempered. In the hardened condition, the steel should have 100% martensite to attain the maximum yield strength to avoid excessive set in service. The presence of retained austenite in the hardened condition lowers the yield strength and produces excessive set. Use of carbon steel for smaller sections in martensite as against sections in which hardness will be less when carbon steel is used. However, harden ability of the steel may be increased by the addition of alloying elements. elements. To get get high yield strength strength on tempering tempering martensite needs to be high in carbon. The combination of high carbon along with alloying elements will posses the desired hardenability in different sections. Hence the desired properties of spring can be achieved firstly by higher carbon content or by addition may occur if the hardening temperature is too low. For many applications, where the working stresses are low, carbon spring steels are quite satisfactory for smaller cross- sections. But, for higher duty springs, steels of higher harden ability are used. In addition to carbon steels, mainly alloy steels having principle alloying elements such as manganese- silicon, chromium, vanadium, molybdenum, etc., are used for production of springs. Spring steels that are commercially available are carbon spring steel, silico-manganese steel, manganese alloyed steel, silicon alloyed steel, chromevanadium steel, etc. Drawn spring steels used for highly duty valve spring in the automotive and aircraft industries are coiled in the annealed condition, and finally hardened and oil-tempered, as this treatment imparts to them improved fatigue strength. Springs of small dimensions are also made from patented drawn spring wires coiled in the cold state. They posses a very high tensile strength and elastic properties compare to the normally drawn wire. They also posses good ductility and toughness. Springs made out of  patented wire do not require hardening. They are only subjected to tempering at 200 to O

250 C.

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3.2.4

Role of Carbon in Steel

Generally, carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite. This means carbon content can be both a blessing and a curse when it comes to commercial steel. And while there are steels that have up to 2 percent carbon content, they are the exception. Most steel contains less than 0.35 percent carbon. To put this in   perspective, keep in mind that‟s 35/100 of 1 percent. Now, any steel in the 0.35 to 1.86 percent carbon content range can be hardened using a heat-quench-temper cycle Carbon is the most important element in steel as slight variations in percentage cause very marked changes in physical and mechanical properties. When a small amount of carbon is added to iron, the properties which give steel its great value begin to appear. In plain carbon steels manganese, phosphorus and sulfur are present in amounts which do not interfere in any way with the effect that carbon variation has on the properties of the steel. Thus, steels may be considered binary alloys of iron and carbon. In alloy steels, the effects of  the alloying elements must be considered. The plain carbon steels represent the most important group of engineering materials known. They represent represent by far the major percentage of steel production production and the widest diversity of application of any of the engineering materials. These applications are so diversified that anything like a complete listing or even a classification on the basis of  application is not feasible.

3.3 Types of leaf springs springs

Figure 3(a) Types of leaf springs 18

3.4 Design of leaf spring

Leaf springs (also known as flat springs) are made out of flat plates. The advantage of leaf spring over helical spring is that the ends of the spring may be guided along a definite path as it deflects to act as a structural member in addition to energy absorbing device. Thus the leaf springs may carry lateral loads, brake torque, driving torque etc., in addition to shocks. Consider a single plate fixed at one end and loaded at the other end as shown in Fig. 23.25. This plate may be used as a flat spring. Let t = Thickness of plate, b = Width of plate, and  L = Length of plate or distancenof the load W from the cantilever end.

Figure 3(b): Flat spring (cantilever type). We know that the maximum bending moment at the cantilever end  A,

 M = W  L . L

and section modulus,

∴

            

Bending stress in such a spring,

            19

……… (I) We know that the maximum deflection for a cantilever with concentrated load at the free end is given by

                   

………(II) ………(II)

∴   It may be noted that due to bending moment, top fibres will be in tension and the bottom fibres are in compression, but the shear stress is zero at the extreme fibres and maximum at the centre, as shown in Fig. Hence for analysis, both stresses need not to be taken into account simultaneously. We shall consider the bending stress only.

Figure 3(c): Stress diagram

If the spring is not of cantilever type but it is like a simply supported beam, with length 2 L and load 2 W in the centre, as shown in Fig., then Maximum bending moment in the centre,as shown in fig, then Maximum bending moment in the center, M=W.L

20

Sectionmodulus,



Figure 3(d): Flat spring (simply supported beam type)

    ∴              We know that maximum deflection of a simply supported beam loaded in the centre is given by

               (Since in this case, W 1=2W,and L1=2L) From above we see that a spring such as automobile spring (semi-elliptical spring) with length 2L and loaded in the centre by a load 2W, may be treated as a double cantilever. If the plate of cantilever is cut into a series of n strips of width b and these are placed as shown in Fig then equations (i) and (ii) may be written as

   

and

       

…… (III)

….. (IV)

21

Figure 3(e): Traingular plate The above relations give the stress and deflection of a leaf spring of uniform cross-sectionThe stress at such a spring is maximum at the support. If a triangular plate is used as shown in Fig 3(e), the stress will be uniform throughout. If this triangular plate is cut into strips of uniform width and placed one below the other, as shown in Fig to form a graduated graduated or laminated leaf leaf spring, then

Figure 3(f): Laminated leaf spring

and

Where

   

         

………..(V)

…………(VI)

n=number of graduated leaves. A little consideration will show that by the above arrangement, the

spring becomes compact so that the space occupied by the spring is considerably reduced. When bending stress alone is considered, the graduated leaves may have zero width at the loaded end. But sufficient metal must be provided to support the shear. Therefore, it becomes necessary to have one or more leaves of uniform cross-section 22

extending clear to the end. We see from equations (iv) and (vi) that for the same deflection, the stress in the uniform cross-section leaves (i.e. full length leaves) is 50% greater than in the graduated leaves, assuming that each spring element deflects according to its own elastic curve. If the suffixes F and G are used to indicate the full length (or uniform cross section) and graduated leaves, then

                        ∴   

or

………..(VII) Adding 1 to both sides we have

                    ()     ()  ……….. (VIII)

where

W= total load on the spring=WG+WF. WG=load taken up by graduated leaves, and WF= load taken up by full length leaves.

From equation (VIII), we may write

or

                       23

∴   ()     ( )  therefore bending stress stress for full length length leaves,

   (  )                    since

                        ,therefore

The deflection in full length and graduated leaves is given by equation (VI), i.e.

              [    ]     Construction of Leaf Spring

A leaf spring commonly used in automobiles is of semi-elliptical form as shown in Fig.

Figure 3(g): Construction of leaf spring It is built up of a number of plates (known as leaves). The leaves are usually given an initial curvature or cambered so that they will tend to straighten under the load. The leaves are held together by means of a band shrunk around them at the centre or by a bolt passing 24

through the centre. Since the band exerts a stiffening and strengthening effect, therefore the effective length of the spring for bending will be overall length of the spring minus width of  band. In case of a centre bolt, two-third distance between centres of  U -bolt -bolt should be subtracted from the overall length of the spring in order to find effective length. The spring is clamped to the axle housing by means of  U -bolts. -bolts. The longest leaf known as main leaf or master leaf has its ends formed in the shape of an eye through which the bolts are passed to secure the spring to its supports. Usually the eyes, through which the spring is attached to the hanger or shackle, are provided with bushings of some antifriction material such as bronze or rubber. The other leaves of the spring are known as graduated leaves. In order to prevent digging in the adjacent leaves, the ends of the graduated leaves are trimmed in various forms as shown in Fig. 23.30. Since the master leaf has to with stand vertical bending loads as well as loads due to sideways of the vehicle and twisting, therefore due to the presence of stresses caused by these loads, it is usual to provide two full length leaves and the rest graduated leaves as shown in Fig.

.

Rebound clips are located at intermediate positions in the length of the spring, so that the graduated leaves also share the stresses induced in the full length leaves when the spring rebounds. Equalised Stress in Spring Leaves (Nipping)

We have already discussed that the stress in the full length leaves is 50% greater than the stress in the graduated graduated leaves. In order to utilise the material to the best best advantage, all the leaves should be equally stressed. This condition may be obtained in the following two ways : 1. By making the full length leaves of smaller thickness than the graduated leaves. In this way, the full length leaves will induce smaller bending stress due to small distance from the neutral axis to the edge of the leaf.

Figure 3(h): Nipping

25

2. By giving a greater radius of curvature to the full length leaves than graduated leaves, as shown in Fig. 23.31, before the leaves l eaves are assembled to form a spring. By doing so, a gap or clearance will be left between the leaves. This initial gap, as shown by C in Fig. 23.31, is called nip. When the central bolt, holding the various leaves together, is tightened, the full length leaf will bend back as shown dotted in Fig.

and have an

initial stress in a direction opposite to that of the normal load. The graduated leaves will have an initial stress in the same direction as that of the normal load. When the load is gradually applied to the spring, the full length l eaf is first relieved r elieved of this initial stress and then stressed in opposite direction. Consequently, the full length leaf will be stressed less than the graduated leaf. The initial gap between the leaves may be adjusted so that under maximum load condition the stress in all the leaves is equal, or if desired, the full length leaves may have the lower stress. This is desirable in automobile springs in which full length leaves are designed for lower stress because the full length leaves carry additional loads caused by the swaying of the car, twisting and in some cases due to driving the car through the rear springs. Let us now find the value of initial gap or nip C . Consider that under maximum load conditions, the stress in all the leaves leaves is equal. Then

at maximum maximum load, load, the the total total deflection deflection of of the graduated leaves will

exceed the deflection of the full length leaves by an amount equal to the initial gap C . In other words,

                       …………(I)

Since the stresses are equal,therefore

And

                        26

         Substituting the values values of WG and WF in equation (I), we have

           

………….(II) ………….(II)

The load on the clip bolts ( Wb) required to close the gap is determined by the fact that the gap is equal to the initial deflections of full length and graduated leaves.

                                        ∴    

…………… (III) The final stress in spring leaves will be the stress in the full length leaves due to the applied load  minus the initial stress.

∴

Final stress

                        

                                                                    27

…….(substituting n =n F+nG ) ……… . (IV) Notes : 1. The final stress in the leaves is also equal to the stress in graduated leaves due to the applied load plusthe initial stress. 2. The deflection in the spring due to the applied load is same as without i nitial stress. Design considerations of Main spring

The master leaf of a laminated spring is hinged to the supports. The support forces induce, induce, stresses due to longitudinal longitudinal forces and stresses arising due to possible twist. Hence, the master master leaf is more stressed compared to other other the graduated leaves. Methods to reduce additional stresses could be,

1.) Master leaf is made of stronger material than the other leaves. 2.) Master leaf is made thinner than the other leaves. This will reduce the bending stress as evident from stress equation. 3.) Another common practice is to increase the radius of curvature of the master leaf than the next leaf.

3.5 Length of Leaf Spring Leaves

The length of the leaf spring leaves may be obtained as discussed below : Let

2L1 = Length of span or overall length of the spring,



= Width of band or distance between centres of U-bolts. It is the

ineffective length of the spring, nF = Number of full length leaves, nG = Number of graduated leaves, and n = Total number of leaves = nF + nG. We have already discussed that the effective length of the spring,

  

…….. …….. ...(When band is used) 28



=

………..... ………..... (When U-bolts are used)

It may be noted that when there is only one full length leaf (i.e. master leaf only), then the number of leaves to be cut will be n and when there are two full length leaves (including one master leaf), then the number of leaves to be cut will be (n  – 1).  –  1). If a leaf spring has two full length leaves, then the length of leaves is obtained as follows :

Similarly,

                       

                        

The nth leaf will be the master leaf and it is of full length. Since the master leaf has

eyes on both sides, therefore Length of master leaf = 2  L1 + π (d + t ) × 2 d = Inside diameter of eye, and

Where

t = Thickness of master leaf.

The approximate relation between the radius of curvature ( R) and the camber ( y) of the spring is given by

      

The exact relation is given by 2

 y (2 R + y) = ( L  L1)

t he spring is equal where L1 = Half span of the spring. Note : The maximum deflection ( δ ) of the to camber ( y) of the spring.

3.6 Standard Sizes of Automobile Suspension Springs 

Standard nominal widths are : 32, 40*, 45, 50*, 55, 60*, 65, 70*, 75, 80, 90, 100 and 125



mm. (Dimensions marked* are the preferred widths)

Standard nominal thicknesses thicknesses are : 3.2, 4.5, 5, 6, 6.5, 7, 7.5, 8, 9, 10, 11, 12, 14 and 16 mm.



At the eye, the following bore diameters are recommended : 19, 20, 22, 2 3, 25, 27, 28, 30, 32, 35, 38, 50 and 55 mm.

29



Dimensions for the centre bolts, if employed, shall be as given in the following table.



Minimum clip sections and the corresponding sizes of rivets and bolts used with the clips shall be as given in the following table (See Fig. 23.32).

3.7 Dimensions for centre bolts

3.8 Dimensions of clip, clip, rivet and bolts.

Notes :

For springs of width below 65 mm, one rivet of 6, 8 or 10 mm may be used. For springs of  width above 65 mm, two rivets of 6 or 8 mm or one rivet of 10 mm may be used. 1. For further details, the following Indian Standards may be referred : (a) IS : 9484  –  1980 (Reaffirmed 1990) on „Specification for centre bolts for leaf 

springs‟. springs‟. (b) IS : 9574  –  1989 (Reaffirmed 1994) on „Leaf springs spring s assembly-Clips-

Specification‟.

Figure 3(i): Spring clip 30

MANUFACTURING OF LEAF SPRING

31

4. Manufacturing of leaf spring

Raw material

Shearing or cutting process

Drilling

Eye rolling of main blade

Hardening

Tempering

Hardness testing

Fitting

Painting

labeling

inspection

stock ready for supply

32

4.1 Raw material

Generally leaf springs are made of various fine grade alloy steel. The most commonly used grades grades of steel are 55 Si 7,60 Si Cr7,50 Cr V4. The others are En 45 A, 65 Si 7,55 Si Cr 7,65 Si cr7,En 42 60 s 87. In our project we are going to use En 42 60 S87 grade of steel alloy. Generally the width of the raw material varies from 40-100 mm and thickness varies from 4 to 20mm.

Figure 4(a) : Steel alloy 4.2 Shearing or cutting process

Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock. The raw material is cut into different sizes with the help of the 100 ton cutting press machine. 1) At first the raw material is placed on the roller bed so that it will be easy to move the material towards the machine. 2) Required length of the material to be cut is measured with a tape and marking is done on the raw material. 3) Now move the material in to the cutting area of the machine so that the mark is placed exactly at the cutting edge of the blade. 4) Now lock the material with the help of the lock nut provided. 33

5) Allow the lubricant to flow for free action of cutting and for reducing friction. 6) Now apply the load on the material by pressing the brake provided. 7) Now the required length of the material piece can be obtained.

Figure 4(b): 100 ton press cutting machine

4.3 Drilling operation

Necessary holes are provided provided on the strips of leaf springs to hold all the plates together. So drilling operation is performed. Generally vertical drilling machine is used for this operation. 1.) The given material piece is placed on the table of the drilling machine.

34

2.) The diameter of the required hole is choosen and the required drill bit is connected to the spindle. 3.) The workpiece is placed in the required position for drilling and the coolent oil is sprayed over the work piece. 4.) Now by applying the hand lever the required hole diameter is drilled. Required number of holes are drilled.

Figure 4(c): Vertical drilling machine 4.4 Eye rolling of main blade

The master blade is heated at its two ends for eye formation, these are done to attach with the frame of the vehicle. The heating is done in a end heating furnace

35

at a temperature of 1000 degree centigrade. The heating is done only at the ends so that it will be easy to bend at the ends. Furnace oil and the air are used for heating the furnace. The furnace is first allowed to heat freely for 45 mins. Now the master blades are placed in the furnace such that only ends are heated. After heating is done for one end it is bend to form the eye e ye and again it is placed in the furnace to heat the other end. After heating the main blade is bent slightly to form curve at the end with the help of 50 ton punching machine so that it will be easy to roll to form eye formation. In the eye rolling machine the master blade end is placed between the circular wheel and the die. after placing the hand lever is moved so that the end of the master blade rolls over the die thus forming eye shape.

Figure 4(d): Eye rolling of main spring

36

4.5 Hardening (Quenching)

Hardening is carried out to achieve the maximum hardness. The main blades after the eye formation are heated to a temperature of 800-1000 degree centigrade in a furnace to increase i ncrease the hardness of the material. The other blades along with main blades are heated in the furnace. The furnace is heated by using air and furnace oil through conventional air flow system. The conventional air flow system is used to mix both air and furnace oil for heating purpose. A pump is provided for the air to go out. After heating the blades in the furnace for 45 minutes they are taken out and bent to the required angle on the hydraulic bending machine. The required angle can be obtained by using required angle dies. The blades after making the required angle they are immersed in the Quenchngon oil to increase the hardness. The hardness at the end of this stage is about 50 to 60.

Figure 4(e): Hardening

37

Figure 4(f): Hydraulic bending machine Following are the factors of affecting the hardening process. 

Chemical composition of steel.



Size and shape of steel part.



Hardening cycle, i.e. heating rate, hardening temperature, holding time and cooling rate.



Homogeneity and grain size of austenite.



Quenching media.



Surface condition of steel part.

4.6 Tempering

Quenched steel, while very hard and strong, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. For most steels, tempering involves heating to between 250 and 500 °C, holding that temperature (soaking) for an appropriate amount of time (on the order of seconds or hours), then cooling slowly over an appropriate length of time (minutes or hours). This heat treatment results in higher

38

toughness and ductility, without sacrificing all of the hardness and tensile strength gained from rapid quenching. Tempering balances the amount of hard martensite with ductile ferrite and pearlite. In some applications, different areas of a single object are given different heat treatments. This is called differential hardening. It is common in high quality knives and swords.

Figure 4(g): Tempering process

Figure 4(h): Tempering furnace 39

Purpose of tempering 

Improve ductility



Improve toughness



Reduce hardness



Increase% elongation



Relieve residual; stresses

After the hardening process the blades are dried and again heated in the furnace for tempering process. In the tempering process the blades are heated to nearly 6oo degree centigrade for nearly 80 minutes to decrease the hardness up to 30 to 40. Here also air along with furnace oil is used for heating process. After tempering is done the blades are removed from the furnace and they are dried in air.

4.7 Hardness testing

Hardness is usually defined as resistance of a material to penetration. It also refers to stiffness or resistance to scratching, abrasion or cutting. In the most general accepted tests, an indenter is pressed into the surface of the material by a slowly applied known load, and the extent of the resulting impression is measured mechanically or optically. A large impression for a given load and indenter indicates a soft material, and the opposite is true for a small impression.

Following are the testing methods used to determine hardness. The scratch hardness test (one material scratches another or not)



The Brinnell hardness tester (ball indenter)



The Rockwell hardness tester (diamond cone or steel ball indenter)



The Vicker hardness tester ( diamond pyramid indenter)



The shore Scleroscope ( the height of bounce of diamond tipped hammer)

We generally use Rockwell hardness tester for testing the t he hardness of the steel.

40

Rockwell Hardness Test

This test determines the hardness of metals by measuring the depth of impression, which can be made by a hard test point under a known load. The penetrator may be either a steel ball or diamond sphero-conical. Soft metals will be indicated by low hardness numbers. Hard metals will be indicated by high hardness numbers. Harder metals permit less of an impression to be made, resulting in higher hardness numbers. s.no A

Indenter

diamond

load

60

dial

application

black

Carbides,thin steel,shallow

case

hardenedsteel,case carburized surfaces. B

diamond

150

black

Hard

cast

iron,pearlitic malleable

iron,

steel,deep casehardened steel,titanium. C

diamond

150

Blackpearlitic

Pearlitic malleable

malleable iron,

iron,thin steel and medium

case

hardened steel,titanium

41

Figure 4(i): Rockwell hardness tester 4.8 Fitting

In this section the leaf springs along with the main spring are fitted together with the help of clamps,bolts,nuts and rivets. 4.8.1 Clamps

Clamps are the devices which are used for holding the leaf springs to gether. Clamps are made of Mild Steel. Procedure for making clamps

At first the material is cut into the required length on the 50 ton cutting machine by using blades. Next the cutted material is punched on the 5o ton punching machine to obtain required circular shape at the edge by b y using suitable die. Now after measuring the correct size the other end is also punched on 50 ton punching machine to obtain circular shape by using the same die. This circular shape at the ends is provided for clamps of width greater than 20 mm.

42

After making the reqired size the clamp materials are heated in the end heating furnace at 1000 degree centigrade by using furnace oil and air. The clamps are heated for 15 minutes in the furnace. After heating they are are taken out and placed on the 50 ton punching machine to make the clamp of „U‟ shape by using clamp dies. Different dies are used used for making clamps of  different widths. Generally 4 clamps are used for 12 plates and 2 clamps are used for 7 leaf plates.the clamps are arranged at equal distance from the centre.

Figure 4(j): Clamps of different sizes

43

Figure 4(k): circular cutting die of clamp 4.8.2 Bolts and nuts

Bolts and nuts of different sizes are used in the holes provided on the leaf springs to hold them tightly.

Figure 4(l): Bolts and rivets 44

4.8.3 Rivets

Rivets are used for holding the t he clamps and leaf springs together. to gether. 4.8.4 MS pipe

Ms pipes are provided for the bolts that are attached to the clamps to reduce the wear of the bolts. 4.8.5 Bushes

Bushes made of MS with gold coating are inserted in to the eye rolling end of the main spring in order to the wear.

Figure 4(m): Bush 4.9 Painting

After fitting operation the leaf springs are allowed for painting operation.

Figure 4(n): Painting 45

Two types of paints are generally used for leaf springs (1) Black  (2) Green. Black paint is generally generally used for high quality quality leaf springs and Green is used for low quality quality leaf springs. The leaf springs are placed on the painting table and now the required paint is sprayed over it with the help of Spray gun. 4.10 Labelling

After drying the painted leaf springs, labelling is done on the leaf springs. Generally the name of the manufactured company and the dimensions are used as labelling.

Figure 4(o): labeling

46

4.11 Inspection

Inspection is carried out by authorities regularly to check the quality, performance of the leaf springs. The leaf springs with bad quality or with low performance are rejected and once again allowed for manufacturing. 4.12 Stock ready to supply

After inspection the leaf springs with good quality and performance are allowed to stock storage. Now the stock is are ready to supply.

Figure 4(p): Stock ready to supply

47

FAILURE ANALYSIS OF LEAF SPRING

48

5. Failure analysis of leafspring

5.1 Failure analysis

Spring failures may be categorized into three types : Early Life Failures

• These type of failures occur generally due to a spring defect, installation problem or overload. This may be due to the material used, the manufacturing processes or improper installation techniques. This type of failure may also be caused by a short-term overloadcondition. Midlife Failures

• Once the spring has passed the time in service which would expose early life failures, a very low failure rate should be observed, assuming the spring is subjected to normal service. Late Life Failures

• At this point, the frequency frequ ency of spring failures will tend to increase rapidly as the useful life of the spring has been reached. By this time the spring steel has been fatigued and corroded to a point where its useful life is over. Failures occurring in early and midlife of the spring are usually most economically handled by repairing the broken leaf rather than replacing the spring. Failures in older springs occur at a point when all leaves l eaves have reached reached their fatigue life lif e the spring should now be replaced. The difficulty, of course, is determining what type of failure the spring has experienced. Basically, the condition of the spring, as well as its service history, will indicate if the spring should be repaired or replaced. 5.2 Quench cracks:

Quench cracks, which are aligned normal to the length of the leaf, have been identified in shot peened and polished surfaces. These cracks have been attributed to an improper quenching process. It is established by theoretical analysis that the leaf thickness is smaller than the critical plate thickness required for this composition of steel, and that leads to an increase in quench severity. It appears that some of these quench cracks have propagated by a 49

fatigue mode which is confirmed by the presence of beach marks on the fractures surface. Observation of intergranular cracking and the presence of FeS inclusions at the prior austenite rain boundaries implies that some sort of grain boundary embrittlement might have facilitated crack growth and led to failure. It has been suggested that quenching should be carried out by recommended procedures guided by the thickness of the component and chemical composition of the steel. Careful inspection of the surface after quenching must also be carried out to maintain quality assurance in order to avoid premature failure. The failure analysis of a leaf spring which failed prematurely during service was carried out using 1.) Optical and scanning electron microscopy 2.) hardness and tensile testing, 3.) residual stress evaluation by X-ray diffraction.

5.3 Optical Metallurgical Microscope

Microstructures were visualized and photographed by using optical metallurgical microscope shown in fig6.

Metallurgical microscope is a value able apparatus for

metallurgist which is used to investigate the microstructure of metal or alloys usually at magnification ranging from 40X to 600X. Since a metallographic sample is opaque to light, the sample is illuminated by reflected light. A properly polished and etched specimen is placed under the objective lens and when a particular combination of objective lens and eye piece is used at the proper tube length, the total magnification is equal to the product of the objective and the eyepiece.

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Figure 5(a): Metallurgical Microscope 5.4 Scanning Electron Microscope

The scanning electron microscope (SEM) is a type of  electron microscope that creates various images by focusing a high energy beam of electrons onto the surface of a sample and detecting signals from the interaction of the incident electrons with the sample's surface. The type of signals gathered in a SEM varies and can include secondary electrons, characteristic x-rays, and back scattered electrons. The sample for SEM do not require any special preparation, except for cutting to appropriate size that can fit into the specimen chamber. Non-conductive solid specimen are coated with a layer of conducting material. A very thin coating of electrically conducting material is deposited over the specimen by vacuum evaporation. Such coatings include gold, platinium, tungsten, graphite etc. As these elements are quite expensive therefore, while preparing the sample for SEM, we inserted a thin copper wire inside the sample during mounting, thus making an electrical contact between the metal piece and base of specimen holder.

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Figure 5(b): Scanning Electron Microscope

Microstructure of Failure & Received Material

As we know that the microstructures have a significant influence on the mechanical and physical properties of the materials. Therefore, we examined the microstructures of samples in different conditions, viz. As rolled, hardened tempered shot penned. The changes in microstructure with respect to the treatment given to the material are very clear in the figures given below. For the best fatigue resistance, tempered martensite structure is obtained, because it is an ideal structure with low mean free path available that is extremely small, uniformly dispersed cementite in uniform ferrite (similar to spheroid, but much smaller).

Figure 5(c): Failure Material Leaf Spring and Received Material Leaf Spring 52

5.5 Tensile Test of Leaf Spring

Tensile test is most frequently performed to determine the certain mechanical properties. Tensile strength can give an idea of fatigue strength, as fatigue strength is almost half of tensile strength in most cases. The standard tensile strength of in tempered condition according to JIS G 4801 2

2

is 125 kgf/mm (126N/ mm ) minimum. We performed the tensile test of material in two t wo conditions, i.e.: as rolled, and tempered.

The results obtained are tabulated below:

S.No

Condition

Tensile Strength

1

Failure Material

156 kg/mm

2

Received Material

126.75 kg/mm

2 2

This test can be performed on following type of Machine

Figure 5(d): Universal Tensile Testing Machine 53

5.6 Fatigue Test of Leaf Spring

The fatigue testing machine in Landhi Engineering Works is imported from Japan and is specially designed for the fatigue test of the Leaf Spring. The picture of  this machine has been shown below:

Figure Leaf Spring Sup-9 Sup-9 during Fatigue Test Test LEW Karachi The Leaf spring shown in above diagram is the rare leaf spring of IMV; an upcoming model by Toyota Motors. This vehicle would be type of a heavy load carriage.

The fatigue testing machine at the Landhi engineering works was installed by the Toyota motors for the tasting of the leaf spring to be used in IMV; their new product. Machine was set at the speed of 60 cycles per minute. The test was conducted by applying load of 1545Kg and it took 153761 numbers of cycles to break the Leaf on which the test was carried out.

Standard Mechanical Test results of Leaf Spring Give by People Steel Mills (PSM) Karachi

Yield Strength 2

Tensile Strength 2

Elongation

Hardness

N/mm

N/mm

%

HB

1079

1226

9%

363-429

Test Results of Prepared Specimen

But we have received following results during the experimental work 

Yield Strength 2

Tensile Strength 2

Elongation

Hardness

N/mm

N/mm

%

HB

1181

1305

12 %

388

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Figure 5(e): fatigue test

Springs are a limited life component. Regardless of how well a spring is maintained or how favorable the operating conditions are, all springs will eventually fail from fatigue caused by the repeated r epeated flexing of the spring. Once the spring life limit is reached a fatigue failure will or has occurred.

Factors influencing fatigue life :

Overloading

• The higher the loads or deflections seen by a spring, spri ng, the lower its fatigue life. Shock Absorbers

• A properly functioning shock absorber will tend to reduce the spring deflection as the vehicle hits a bump. Lower spring deflections mean lower operating stresses on the spring which in turn gives longer fatigue life. This is especially true for full taper springs which do not have the high interleaf friction to help dampen spring deflections. Worn or missing shock  absorbers must be replaced to maximize spring life. Brake Adjustments

• Improperly adjusted brakes can also reduce spring l ife. Under braking, springs are expected to absorb some of the braking forces. If the brakes on an axle are unevenly adjusted one spring will have to absorb more than its share of braking force which can reduce its fatigue life. 55

Protective Coatings

• Corrosion is one of the major factors in reducing spring life. Proper paints and care during handling and installation can help to slow the spread of spring corrosion. On full taper springs the only acceptable coating is the individual painting of each leaf with zinc-rich paint. This paint may be recognized by its characteristic gray color. Surface Condition

• The condition of the spring surface also has an effect on fatigue fat igue life. Generally, a fatigue crack will start at some sort of surface defect on the spring leaf. Therefore, care needs to be used when manufacturing and installing springs to reduce these defects to a minimum. Shot Peening

• Extensive testing indicates that shot peening can increase the life of springs by a factor of  three or more. It is not enough, however, to simply shot peen the first one or two leaves in an assembly-all leaves must be shot peened. All major vehicle manufacturers specify that their OEM springs have each leaf shot peened. Decarburization and Steel Quality

• Improper manufacturing methods can also reduce fatigue life. For example, poorly controlled heat-treat furnaces can excessively decarburize the leaf surface. Decarburization is the loss of carbon from the steel surface which will result in a soft leaf  surface once heat-treating is complete. This soft layer will not be able to handle the spring stresses and will lead to early failure. Poor steel quality can also influence spring life. If the steel has excessive impurities in it, the fatigue life will be reduced. Maintenance • Finally, Finally, improper maintenance will affect spring life. • Spring eyes and other suspension components should be regularly greased to prevent binding. • U-bolts U-bolts should never be reused. • Axle seats, top plates and other components should be peri odically inspected and replaced as required.

5.7

Increasing the fatigue strength of leaf spring:

In order to increase the fatigue strength of leaf spring you have to do t he following:

Carefully polish the surfaces of the springs to remove any surface defects or machining marks that will be the location for crack initiation. 56

Shot- Peen the steel. Bombard the surfaces of the spring with steel shot, which cold works the steel, which hardens the surface and puts it into a state of compression, which impedes crack propagation.

Temper the steel. When initially forming the piece, cool the outer surface quickly to put the surface into a state of compression, which will also impede crack propagation. (Theres another definition for Tempering steel, which is heating it to stabilize and de-stress the crystal structure.) Quench the steel, when forming the piece the faster you cool it down, the more steel gets locked into a Martensite phase, which has better fatigue properties. However, if you quench it too quickly, you develop thermal stresses and cracks, which will obviously weaken the steel. (Another method for increasing the Martensite percentage is to heat treat the steel before tempering it.) Case is thus expended more than the core because,

(Austenite

Martensite) volume increase by 4% and

(Austenite

Ferrite + pearlite) volume increase by 3%,

This is lesser than that. During fabrication, prevent air pockets or contaminates (non-metallic inclusion) from getting into the steel. These contaminates will provide starting points for cracks to form.

Increase the carbon content from 0.5% to about 0.95%. This will harden your steel and improve the fatigue properties.

Finally, you could just decrease the applied load on your leaf spring steel has the nifty  property of a “Fatigue Limit”, which means that if your loads are less than a certain value, then you can cycle the part as many times as you want without ever seeing fatigue fractures. Most non-ferrous alloys such as aluminum or copper will not have a fatigue limit.

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Fatigue cracks that have begun to propagate can sometimes be stopped by drilling holes, called drill stops, in the path of fatigue crack. However, it is not recommended because a hole represents a stress concentration factor of about 2. There is thus the possibility of a new crack starting in the side of the hole. It is always for better to replace the cracked part entirely. Several disasters have been caused by botched repairs to cracked structures.

In the end we are able to conclude that fatigue life of leaf spring can be increased if:

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The vehicle is loaded not greater than load for which it has been manufactured.

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Proper cleanliness of leaf spring is done and also it should be avoided from water and mud etc. as much as possible to minimize the corrosion of leaf springs.

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The government takes appreciable steps to improve the smoothness of the road network in the country.

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Shot-peening must be done to create residual compressive stresses as fatigue failure always occurs in the tensile stresses.

5.8 When To Repair

• If the spring has not been repaired or repaired only once. Stamping a 1 in the clip for the first repair and a 2 for a second repair will help identify the number of previous repairs. • If the spring mileage is less than half of normal li fe. • If the repair cost is less than 1/2 the cost of a new spring. • If no more than two or three leaves are broken. • If the failure is not of a fatigue type. type. For example, a leaf broken through the center hole is caused by improper spring clamping brought on by loose U-bolts or worn axle seats, not fatigue. This spring should be repaired, if possible, and the cause of failure corrected. Even when it appears to make sense to repair, the following should be kept in mind : 1. Repair leaves are usually not shot peened and must often be heavily hand-fit to match the old spring. Therefore, the repair leaf will not be as durable as a l eaf in a new spring would be. 2. Since the remaining leaves have lost some of their strength, the replaced leaves will be carrying more of the load than they were originally designed for. 58

3. When the leaves first broke the remaining leaves in the spring had to carry more load and were probably overstressed. 4. Replacing the broken leaves does nothing to restore the fatigue life of the reused leaves. These leaves will continue to fail since their fatigue life is essentially over.

5.9 When To Replace

• The spring has already been repaired once or, at most, twice. • The spring service mileage has exceeded 1/2 its normal life. • The repair cost exceeds 1/2 the cost of a new spring. • More than two or three leaves are broken. • If small fatigue cracks can be seen running across the leaf width near the U-bolts on the unbroken leaves. • If the leaf tips have separated awa y from the leaf above. • Never attempt to repair a full taper spring.

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CONCLUSION: This project work has provided us an excellent opportunity and experience to use our limited knowledge. We gained a lot of practical knowledge regarding the “DESIGN, MANUFACTURING AND FAILURE ANALYSIS OF LEAF SPRING”. In “TRAILOR SPRINGS” our intensions were to observe the process deeply and to point out any deficiency that is taking place in the process. We have found that the process and steps are followed by them with many safety measurements and they manufacture leaf  springs with all necessary, viz. Heating, Quenching, Tempering and Shot peening. Since the manufacturing process is difficult skilled workers are generally employed. Finally we conclude that we clearly observed the design,manufacturing and failures of leaf  spring and completed our mini project project successfully. successfully.

working staff of TRAILOR SPRINGS

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BIBLIOGRAPHY/REFERENCES/WEBSITES

Books: Design ” by R.S.KHURMI_AND_J.K.GUPTA. R.S.KHURMI_AND_J.K.GUPTA. 1. “A Textbook of Machine Design”

2. William D Callister. “Fundamentals of Material Science and Engineering”. Fifth Edition. John Wiley & Sons, Inc 1985

3. Mikell P. Groover, “Fundamentals of Modern Manufacturing” Second Edition. John Wiley & Sons.Inc.

4. Metals Handbook. Volume 1 Microsturcture 5. Heat Treatment Principle and Technique by T.V Rajan, C.P Sharma, Ashok Sharma 6. Steel and Its Heat Treatmentby K.E Theling

Websites: 1. http://www.wikipedia.org 2. http://www.howstuffworks.com 3. http://www.tpub.com 4. http://www.google.com 5. http://www.answer.com 6. http://www. freepatentsonline.com freepatentsonline.com 7. http://www. procarcare.com

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