Originally called
laminated or
carriage spring, a
leaf spring is a simple form of
springA spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
, commonly used for the
suspensionSuspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...
in
wheelA wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...
ed
vehicleA vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
s. It is also one of the oldest forms of springing, dating back to
medievalThe Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
times.
An advantage of a leaf spring over a helical spring is that the end of the leaf spring may be guided along a definite path.
Sometimes referred to as a
semi-ellipticalIn geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
spring or
cart spring, it takes the form of a slender
arcIn geometry, an arc is a closed segment of a differentiable curve in the two-dimensional plane; for example, a circular arc is a segment of the circumference of a circle...
-shaped length of
spring steelSpring steel is a low alloy, medium carbon steel or high 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.-Grades:...
of
rectangularIn Euclidean plane geometry, a rectangle is any quadrilateral with four right angles. The term "oblong" is occasionally used to refer to a non-square rectangle...
cross-section. The center of the arc provides location for the
axleAn axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where 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. While the interleaf friction provides a damping action, it is not well controlled and results in
stictionStiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the term "static friction", perhaps also influenced by the verb "stick"....
in the motion of the suspension. For this reason manufacturers have experimented with mono-leaf springs.
A leaf spring can either be attached directly to the
frameA frame is the main structure of the chassis of a motor vehicle. All other components fasten to it; a term for this is design is body-on-frame construction.In 1920, every motor vehicle other than a few cars based on motorcycles had a 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. Some springs terminated in a concave end, called a
spoon end (seldom used now), to carry a swivelling member.
History
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 armthumb|220px|Trailing arm rear suspension of [[Front-engine, front-wheel drive layout|FF]] carsA trailing-arm suspension is an automobile suspension design in which one or more arms are connected between the axle and the chassis. It is usually used on rear axles...
s, would be needed for this design, but not for "semi-elliptical" leaf springs as used in the
Hotchkiss driveThe Hotchkiss drive is a system of power transmission. It was the dominant form of power transmission for front-engine, rear-wheel drive layout cars in the 20th century...
. 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 SevenThere were four Austin Motor Company cars to use the Seven name:* A single cylinder car produced in 1909* The 1922-1939 Austin 7* The original Mini* The launch title of the Austin A30...
of the 1920s. As an example of non-elliptic leaf springs, the
Ford Model TThe Ford Model T is an automobile that was produced by Henry Ford's Ford Motor Company from September 1908 to May 1927...
had multiple leaf springs over its differential that were curved in the shape of a
yokeA yoke is a wooden beam, normally used between a pair of oxen or other animals to enable them to pull together on a load when working in pairs, as oxen usually do; some yokes are fitted to individual animals. There are several types of yoke, used in different cultures, and for different types of oxen...
. As a substitute for dampers (
shock absorberA shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...
s), 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 in Europe and Japan and late 70's in America when the move to front wheel drive, and more sophisticated
suspensionSuspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...
designs saw automobile manufacturers use
coil springsA spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
instead. Today leaf springs are still used in heavy commercial vehicles such as
vanA van is a kind of vehicle used for transporting goods or groups of people.In British English usage, it can be either specially designed or based on a saloon or sedan car, the latter type often including derivatives with open backs...
s and
truckA truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...
s, SUVs, 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 armthumb|220px|Trailing arm rear suspension of [[Front-engine, front-wheel drive layout|FF]] carsA trailing-arm suspension is an automobile suspension design in which one or more arms are connected between the axle and the chassis. It is usually used on rear axles...
s and a
Panhard rodA Panhard rod is a component of a car suspension system that provides lateral location of the axle...
, thereby saving cost and weight in a simple
live axleA live axle, sometimes called a solid axle, is a type of beam axle suspension system that uses the driveshafts that transmit power to the wheels to connect the wheels laterally so that they move together as a unit....
rear suspension.
A more modern implementation is the parabolic leaf spring. This design is characterised by fewer leaves whose thickness varies from centre to ends following a
parabolic curveIn mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...
. 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 qualityRide quality refers to the degree of protection offered vehicle occupants from uneven elements in the road surface, or the terrain if driving off-road. A car with very good ride quality is also a comfortable car to ride in. Cars which disturb vehicle occupants with major or minor road...
that approaches that of coil springs. There is a trade-off in the form of reduced load carrying capability, however. The characteristic of parabolic springs is better riding comfort and not as "stiff" as conventional "multi-leaf springs". It is widely used on buses for better comfort. A further development by the British GKN company and by Chevrolet with the Corvette amongst others, is the move to composite plastic leaf springs.
Typically when used in automobile suspension the leaf both supports an axle and locates/ partially locates the axle. This can lead to handling issues (such as 'axle tramp'), as the flexible nature of the spring makes precise control of the unsprung mass of the axle difficult. Some suspension designs which use leaf springs do not use the leaf to locate the axle and do not have this drawback. The
Fiat 128The Fiat 128 is a small family car manufactured by the Italian manufacturer Fiat from 1969 to 1985. The engine was designed by the famous Ferrari racing engine designer Aurelio Lampredi.-History:...
's rear suspension is an example.
It has the following 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.
2. As the positioning of the axle is carried out by the leaf springs so it makes it deific to use soft spring
i.e. a spring with low spring constant. Therefore,
3. 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 rend squat and nose-diving.
Manufacturing process
Multi-leaf springs are made as follows:
- Shearing of flat bar
- Center punching
- End Heating process forming (hot & cold process)
- Eye Forming / Wrapper Forming
- Diamond cutting / end trimming / width cutting / end tapering
- End punching / end grooving / end bending / end forging / eye grinding
- Heat treatment
- Center hole punching / nibbing
- Camber forming
- Quenching
- Tempering
- Surface preparation
- Shot peening / stress peening
- Painting
- Eye bush preparation process
- Eye reaming / eye boring
- Bush insertion
- Bush reaming
- Assemble
- Presetting & load testing
- Paint touch-up
- Marking & packing
Use by Blacksmiths
Because leaf springs are made of relatively high quality steel, they are a favorite material for blacksmiths. In countries such as Nepal, Bangladesh and Pakistan where traditional blacksmiths still produce a large amount of the country's tools, leaf springs from scrapped cars are frequently used to make knives, kukris, and other tools. They are also commonly used by amateur and hobbyist blacksmiths in Western countries & India.
External links