Chapter One .. Preview

1.1 Introduction :

Leaf springs are composed of flat bars of varying lengths ( laminated form ) to obtain greater efficiency and resilience . A common trouble with laminated springs is fatigue failure of the leaves . Some factors contributing to this type of failure are the weakening effect of the hole if the center bolt is used ; contact pressure produced by U-bolts and rebound clips ; stress concentration caused by improperly shaped leaf ends ; initial curvature and relative change of curvature of the leaves during loading ; vibration of the spring ends during rebound , which may cause breakage of a leaf at an unloaded location ; and improper heat treatment , especially surface decarburization . Each of these factors has at least a partial cure , which should be incorporated in leaf spring design. Leaf springs are widely used in railway wagons , coach and road vehicles . They are loaded at the ends and supported in the middle . They are essentially the beams of uniform strength . when the full load is applied , the central deflection disappears , and the plate becomes flat . The leaves are held together by a bolt passing through the centre or by a shrunk band around them (1) . Also leaf springs may be full elliptic , semi elliptic and cantilever as shown in fig. (1.1)

Fig.1.1. Semi-elliptic leaf springs

In these type of springs the major stresses are tensile & compressive . (2)

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Fig.1.2. forms of leaf springs

These type of springs are used in the automobile suspension system .

Following are the standard sizes for automobile suspension springs :

1- Standard nominal widths are : 32,40*,45,50*,55,60*,65,70*,75,80,90,100 and 125 mm.( Dimension marked * are the preferred widths )

2- Standard nominal thickness are : 3.2,4.5,5,6,5.6,7,7.5,8,9,10,11,12,14 and 16 mm.

3- At the eye , the following bore diameters are recommended :19,20,22,23,25,27,28,30,32,35,38,50 and 55 mm .

4- Dimension for the centre bolts , if employed , shall be as given in the following table .

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Standard sizes of automobile

suspension : :springs

Table 1.1.Dimension for centre bolts .

Width of leaves in mm

Dia. Of centre bolt in mm

Dia. Of head in mm

Length of bolt head in mm

Up to including 65Above 65

8 or 1012 or 16

12 or 1517 or 20

10 or 1111

5- Minimum clip sections and the corresponding sizes of rivets and bolts used with the clips shall be as given in the following table.(3)

Table 1.2. Dimension of clip , rivet , and bolts.

Spring width (B) in mm

Clip section (b×t) in mm

Dia. Of rivet (d1) in mm

Dia. Of bolt (d2) in mm

Under 5050,55 and 6065,70,75 and 8090,100 and 125

20×425×525×632×6

681010

68810

Fig.1.3. Auto-leaf spring

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Fig.1.4. bolt of leaf spring

1.2 purpose of project :

The purpose of the project is building a visual basic program to design leaf spring and use of all facilities of the computer to help the user to make his design by the simplest method of input the values and the simplest method that the user get from the program and trying to find relationships between design equations variables which describe the engineering behavior during the work of product .

1.3 Material for leaf springs :

The material used for leaf springs is usually a plain carbon steel having 0.9 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 .(3)

According to Indian standards , the recommended materials are :

1- For automobiles : 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 ( water – hardened), and 55 Si 2 Mn 90(oil-hardened) .

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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 .

Table 1.3. physical properties of materials commonly used for leaf springs.

Material Condition Ultimate tensile strength(Mpa)

Tensile yield strength (Mpa)

Brinell hardness number

50 Cr 150 Cr 1 v 2355 Si 2 Mn 90

HardenedAnd tempered

1680-22001900-22001820-2060

1540-17501680-18901680-1920

461-601534-601534-601

1.4 Characteristics :

1- The leaf spring acts as a linkage for holding the axle in position and thus separate linkage are not necessary. It makes the construction of the suspension simple and strong.(4)

2- As the positioning of the axle is carried out by the leaf springs so it makes it disadvantageous to use soft springs i.e. a spring with low spring constant.

3- Therefore, this type of suspension does not provide good riding comfort. The inter-leaf friction between the leaf springs affects the riding comfort.

4- Acceleration and braking torque cause wind-up and vibration. Also wind-up causes rear-end squat and nose-diving.

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Fig.1.5. solid axle, leaf spring suspension

1.5 Manufacturing process :

Multi-leaf springs are made as follows ..:

1. Shearing of flat bar 2. Center hole punching / Drilling 3. End Heating process forming (hot & cold process) :

1. Eye Forming / Wrapper Forming 2. Diamond cutting / end trimming / width cutting / end

tapering 3. End punching / end grooving / end bending / end

forging / eye grinding 4. Center hole punching / Drilling / nibbing

4. Heat Treatment :1. Heating 2. Camber forming 3. Quenching 4. Tempering

5. Surface preparation: 1. Shot peening / stress peening 2. Painting

6. Eye bush preparation process :

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1. Eye reaming / eye boring 2. Bush insertion 3. Bush reaming

7. Assemble : 1. Presetting & load testing 2. Paint touch-up 3. Marking & packing

1.6 Advantages :

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 .(4)

Fig.1.6.leaf spring

1.7 uses of leaf springs :

1- To apply forces and to control motion as in brakes and clutches.

2- To measure forces as in spring balance.

3- To store energy as in clock spring.

4- To reduce the effect of shock or impact loading as in carriage springs .

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5- To change the vibrating of characteristics of a member as inflexible mounting of motors.

Fig.1.7. leaf spring

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Chapter Tow .. calculation

2.1 .Thickness of leaves :

The equation used to calculate thickness of leaves is :

T = √18w . l /σb(2n g+3nf ) …(2.1)

Where : w : Load of pin L : Effective length of the spring

σ: Maximum of leaves

b : width of leaves

ng : Number of graduated leaves

nf :Number of full length leaves

2.2.Deflection of spring :

The equation used to calculate deflection of spring is :

δ = 12w .l3

E .b .t 3(2ng+3nf ) …(2.2)

Where :

E: young modulus

2.3 Diameter of eye :

The equation used to calculate diameter of eye is :

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To calculate inner dia.

d= l1×pb / w …(2.3)

To calculate outer dia.

D=3√ Mσb∗z

…(2.4)

Where :

L1 :length of the pin which is equal to the width of the eye or leaf

Pb :bearing pressure on the pin

σb:bending stress

M : maximum bending moment of the pin

Z : section modulus

2.4 length of leaves :

The equation used to calculate length of leaves is :

Length of the smallest leaf =

effective lengthn−1

+ ineffective length …(2.5)

Length of the 2nd leaf =

effective lengthn−1

∗2 + ineffective length …(2.6)

Length of the 3rd leaf =

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effective lengthn−1

∗3 + ineffective length …(2.7)

Length of the 4th leaf =

effective lengthn−1

∗4+ ineffective length …(2.8)

Length of the 5th leaf =

effective lengthn−1

∗5+ ineffective length …(2.9)

Length of the 6th leaf =

effective lengthn−1

∗6+ ineffective length …(2.10)

The 6th and 7th leaves are full length leaves and the 7th leaf ( i.e the top leaf ) will act a master leaf .

Length of the master leaf = 2L1+π (d+t )∗2 …(2.11)

2.5 Radius to which the leaves should be initially bent :

The equation used to calculate radius is :

R= ( l1 )2+ y2

2 y …(2.12)

Where :

Y : camber of the spring

References : 11

1-Paul H. Black K , Machine design ,McGra W-Hill book company .

O.Eugene adams GR. , Machine design ,McGra W-Hill book company .

2- R.B. Guta , Mechanical Engineering Design , Shri R.P. Handa prop Satya Prakashan , First addition , July 1947.

3- Guta , A Textbook of Machine Design .

4- Www.google.com

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