Leaf Springs

INTRODUCTION TO LEAF SPRINGS
In order to conserve natural resources and economize energy, weight reduction has been the main focus of automobile manufacturer in the present scenario. Weight reduction can be achieved primarily by the introduction of better material, design optimization and better manufacturing processes .The suspension leaf spring is one of the potential items for weight reduction in automobile as it accounts for ten to twenty percent of the un-sprung weight.
This helps in achieving the vehicle with improved riding qualities. It is well known that springs, are designed to absorb and store energy and then release it. Hence, the coefficient of friction and frictional losses of the material becomes a major factor in designing the springs. It can be easily observed that material having lower coefficient of friction will have a greater load carrying capacity at same spring rate.
Therefore by reducing the frictional losses between the leaves, the load carrying capacity can be increased and weight of the assembly can be reduced further without any modification in stiffness. The advantage of leaf spring is that it carries lateral loads, braking torque, driving torque in addition to shock absorbing.
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 capability.
Generally conventional leaf springs are stacked and clamped together with pins and clips. The stacked leaf spring generally has high inter leaf frictional losses at various vehicle dynamic load conditions. In order to compromise the inter leaf frictional losses and to withstand vehicle static and dynamic load conditions, more no of leaves are currently used in semi elliptical leaf spring assemblies.
The increase in number of leaves leads to the massive leaf spring assembly structure with increase weight. And due to the inter leaf frictions the fatigue life of the semi elliptical leaf spring assembly is considerably reduced.
Currently during the development of the existing semi elliptical leaf spring assembly following parameters are introduced to reduce the friction in spring steel materials (spring steels of SUP 9 and SUP 11 are currently used for manufacturing). The shot peening is done on the tension side of the leaves to reduce material wear rate and to reduce residual stress.
And as an additional aspect a lubricating substance of graphite grease or graphite primer is coated on the tension side of the leaves to reduce inter leaf friction. But due to the operating conditions and weather the coating materials degrades leading to the increased friction between the spring leaves.

CONSTRUCTION OF LEAF SPRING:

MAIN PARTS:
 Eye,
 Full leafs & graduated leafs,
 Centre U-bolts,
 Rebound clip etc.,
DESCRIPTION:
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 band shrunk around them at the centre or by a bolt passing through centre. Since, the band exerts stiffening and strengthening effect, therefore effective length of the spring for bending will be overall length of the spring minus width of the band.
In case of a centre bolt, two-third distance between centres of U-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.
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. 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. Rebound clips are located at intermediate positions in the length of the spring, so that the graduated leaves also share the stress induced in the full length leaves when the spring rebounds.
TYPES OF LEAF SPRING:
 Elliptic leaf spring,
 Semi elliptic leaf spring,
 Three quarter elliptic leaf spring,
 Quarter elliptic leaf spring,
 Transverse elliptic leaf spring.
ELLIPTIC 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 centre of the upper arc, the bottom centre was joined to the “live” suspension components, such as a solid front axle.
SEMI ELLIPTIC LEAF SPRING:
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.
THREE QUARTER ELLIPTIC LEAF SPRING:
Three 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.
QUARTER ELLIPTIC LEAF SPRING:
“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.
TRANSVERSE LEAF SPRING:
Transverse leaf springs, the Ford Model T had multiple leaf springs over its differential that was 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.
CHARACTERISTICS OF LEAF SPRING:
 Sometimes it is also called as a semi-elliptical spring; as it takes the form of a slender arc shaped length of spring steel of rectangular cross section.
 The centre of the arc provides the location for the axle, while the tie holes are provided at either end for attaching to the vehicle body.
 Supports the chassis weight.
 Controls chassis roll more efficiently-high rear moment centre and wide spring base.
 Controls rear end wrap-up.
 Controls axle damping.
 Controls braking forces.
 Regulates wheelbase lengths (rears steer) under acceleration and braking.

MATERIALS OF 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 products greater strength and therefore greater load capacity, greater range of deflection and better fatigue properties.
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 products greater strength and therefore greater load capacity, greater range of deflection and better fatigue properties.
Carbon/Graphite fibres:
Their advantages include high specific strength and modulus, low coefficient of thermal expansion and high fatigue strength.
Graphite, when used alone has low impact resistance.
Its drawbacks include high cost, low impact resistance and high electrical conductivity.
Glass fibres:
The main advantage of Glass fibre over others is its low cost. It has high strength, high chemical resistance and good insulating properties.
The disadvantages are low elastic modulus poor adhesion to polymers, low fatigue strength and high density, which increase leaf spring weight and size.
MANUFACTURING OF LEAF SPRING:
Leaf springs are made as follows:
1. Shearing of flat bar
2. Centre hole punching / Drilling
3. End Heating process forming (hot & cold process)
3.1 Eye Forming / Wrapper Forming
3.2 Diamond cutting / end trimming / width cutting / end tapering
3.3 End punching / end grooving / end bending / end forging / eye grinding
3.4 Centre hole punching / Drilling / nibbing
4. Heat Treatment
4.1 Heating
4.2 Camber forming
4.3 Hardening
4.4 Quenching
4.5 Tempering
5. Surface preparation
5.1 Shot peening / stress peening
5.2 Painting
6. Eye bush preparation process
6.1 Eye reaming / eye boring
6.2 Bush insertion
6.3 Bush reaming
7. Assemble
7.1 Pre-setting & load testing
7.2 Paint touch-up
7.3 Marking & packing etc.,
ADVANTAGES OF LEAF SPRING:
1. Less unsprung weight. Coil springs contribute to unsprung weight; the less there is, the more quickly the wheel can respond at a given spring rate.
2. Less weight. Weight is positioned lower. Coil springs and the associated chassis hard mounts raise the centre of mass of the car.
3. Superior wear characteristics. The Corvette’s composite leaf springs last longer than coils, though in a car as light as the Corvette, the difference is not especially significant.
4. Due to laminate structure and reduced thickness of the mono composite leaf spring, the overall weight would be less.
5. Due to weight reduction, fuel consumption would be reduced.
6. They have high damping capacity; hence produce less vibration and noise.
7. They have good corrosion resistance.
DISADVANTAGES OF LEAF SPRING:
1. They have less specific modulus and strength.
2. Increased weight
3. Conventional leaf springs are usually manufactured and assembled by using number of leafs made of steel and hence the weight is more.
4. Its corrosion resistance is less compared to composite materials.
5. Steel leaf springs have less damping capacity.
APPLICATION OF LEAF SPRINGS:
Generally leaf springs are used in automobiles suspension systems.




0 Comments

Leave a reply

CONTACT US

We're not around right now. But you can send us an email and we'll get back to you, asap.

Sending

All newly proposed concepts/methods/techniques are property of respective contributor and mechanical.in. Any content from third party are property of respective third party, governed by third parties legal information, published here with third party's grant and are available only for reference purpose.

COPYRIGHT ©2013-2016 MECHANICAL.IN. ALL RIGHTS RESERVED.

Privacy Policy | Terms of Use | User Content Policy | Report Problem/Bug/Abuse | FAQ | Contact us

Log in with your credentials

or    

Forgot your details?

Create Account