Leaf spring

leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a laminated or carriage spring, and sometimes referred to as a semi-elliptical spring or cart spring, it is one of the oldest forms of springing, appearing on carriages in England after 1750 and from there migrating to France and Germany. [1]

 A traditional semi-elliptical Hotchkiss leaf spring arrangement. On the left, the spring is connected to the frame through a shackle.

Leaf springs front independent suspension, front-wheel-drive Alvis1928

Independent front suspension by transverse leaf spring Humber 1935

Independent front suspension by semi-elliptical springs Mercedes Benz 230 W153 1938

Leaf spring on a German locomotive built by Orenstein-Koppel and Lübecker Maschinenbau

A leaf spring takes the form of a slender arc-shaped length of spring steel of rectangular cross-section. In the most common configuration, the center of the arc provides location for the axle, while loops formed at either end provide for attaching to the vehicle chassis. 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 stiction in the motion of the suspension. For this reason, some manufacturers have used mono-leaf springs.

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. Some springs terminated in a concave end, called a spoon end (seldom used now), to carry a swiveling member.

The leaf spring has seen a modern development in cars. The new Volvo XC90 (from 2016 year model and forward) has a transverse leaf spring in high tech composite materials, a solution that is similar to the latest Chevrolet Corvette. This means a straight leaf spring, that is tightly secured to the chassis, and the ends of the spring bolted to the wheel suspension, to allow the spring to work independently on each wheel. This means the suspension is smaller, flatter and lighter than a traditional setup.

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 arms, would usually 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.

Elliptic Carriage with elliptic springs
Semi-elliptic Vehicle suspension with semi-elliptic springs
Three quarter-elliptic Car suspension with three-quarter elliptic springs
Quarter-elliptic Vehicle suspension with quarter-elliptic springs
Transverse Front suspension with transverse leaf spring

Leaf springs were very common on automobiles, right up to the 1970s in Europe and Japan and late 1970s in America when the move to front-wheel drive, and more sophisticated suspension designs saw automobile manufacturers use coil springs instead. Today leaf springs are still used in heavy commercial vehicles such as vans and trucks, 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 arms and a Panhard rod, thereby saving cost and weight in a simple live axle rear suspension. A further advantage of a leaf spring over a helical spring is that the end of the leaf spring may be guided along a definite path.

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, 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 among others, is the move to composite plastic leaf springs. Nevertheless, due to missing inter-leaf friction and internal dampening effects, this type of spring requires more powerful dampers or shock absorbers.

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 use a Watts link (or a Panhard rod) and radius arms to locate the axle and do not have this drawback. Such designs can use softer springs, resulting in better ride. The various Austin-Healey 3000’s and Fiat 128‘s rear suspension are examples.

Characteristics

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

Because the positioning of the axle is carried out by the leaf springs, it is disadvantageous to use soft springs i.e. springs with low spring constant.

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

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

The inter-leaf friction damps the spring’s motion and reduces rebound, which until shock absorbers were widely adopted was a great advantage over helical springs.[2]

Manufacturing process

Multi-leaf springs are made as follows:

  1. Shearing of flat bar
  2. Center hole punching / Drilling
  3. Heating process (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 / end rolling
    4. Center hole punching / Drilling / nibbing
  4. Heat Treatment
    1. Heating
    2. Hardening
    3. Cambering
    4. Quenching
    5. Tempering
  5. Surface preparation
    1. Shot peening / stress peening
    2. Primary painting
  6. Eye bush preparation process
    1. Eye reaming / eye boring
    2. Bush insertion
    3. Bush reaming
  7. Assemble
    1. Presetting & load testing
    2. Finish painting
    3. Marking & packing

Other uses

By blacksmiths

Because leaf springs are made of relatively high quality steel, they are a favorite material for blacksmiths. In countries such as India, Nepal, Bangladesh, the Philippines 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.[3] They are also commonly used by amateur and hobbyist blacksmiths.

In trampolines

Leaf springs have also replaced traditional coil springs in some trampolines (known as soft-edge trampolines), which improves safety for users and reduces risk of concussion. [4] The leaf springs are spaced around the frame as ‘legs’ that branch from the base frame to suspend the jumping mat, providing flexibility and resilience. [5]

Clutches

The “diaphragm” common in automotive clutches is a type of leaf spring.

References

  1. Leaf Springs: Their Characteristics and Methods of Specification. Wilkesbarre, PA: Sheldon Axle Company. 1912. p. 1.
  2. “Springs – A simple study of car suspension”, The Automotor Journal, August 10, 1912, pp936-937
  3. “Kamis, Khukuri makers of Nepal”. Himalayan-imports.com. Retrieved 2011-11-06.
  4. “Joe Andon’s leap of faith”. The Australian. Retrieved 2013-07-04.
  5. “Trampolines WO 2012167300 A1”. Retrieved 2013-07-04.
Transverse leaf spring and solid axle front suspension of early Ford cars Transverse leaf spring and solid axle front suspension of early Ford cars is a type of automotive front suspension that has been most common in early Ford Motor Company products. "Suicide front axle" is a term that has been used for it. The configuration consists of a one-piece axle (solid front axle), to the ends of which the steerable front wheels are mounted. The axle receives its vertical and transverse support from a transverse leaf spring (leaf springs were often used for support in more than one direction), and its longitudinal support from fore-aft links sometimes called "radius rods" which are attached (via pivots) to the ends of the axle at their forward end and to the sides of the chassis (again via pivots) at their aft end. The ends of the transverse leaf spring can either tie to the top of the rods, or to the top of the solid axle. The transverse leaf spring is attached at its center to the center of the chassis's front cross member. 1919 Ford Model T Advantag...
Coil spring A coil spring, also known as a helical spring, is a mechanical device which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces. They are made of an elastic material formed into the shape of a helixwhich returns to its natural length when unloaded. Under tension or compression, the material (wire) of a coil spring undergoes torsion. The spring characteristics therefore depend on the shear modulus, not Young's Modulus. A coil spring may also be used as a torsion spring: in this case the spring as a whole is subjected to torsion about its helical axis. The material of the spring is thereby subjected to a bending moment, either reducing or increasing the helical radius. In this mode, it is the Young's Modulus of the material that determines the spring characteristics. Metal coil springs are made by winding a wire around a shaped former - a cylinder is used to form cylindrical coil springs. Coil spri...
MacPherson strut The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles and is named for American automotive engineer Earle S. MacPherson, who originally invented and developed the design. A simple MacPherson strut suspension on the left front wheel of a rear-wheel drive vehicle. The front of the vehicle is at bottom right of the image. Upper green: Vehicle body/strut interface Red: Steering knuckle or hub carrier Blue: Lower control arm or track control arm Light blue: Steering gear tie rod or track rod Lower purple: Radius rod Upper purple: Coil spring Yellow: Tubular housing containing shock absorber or damper Lower green: Vehicle frame or unibody member History Earle S. MacPherson was appointed the chief engineer of Chevrolet's Light Car project in 1945, to develop new smaller cars for the immediate post-war market. This gave rise to the Chevrolet...
Twist-beam rear suspension The twist-beam rear suspension (also torsion-beam axle or deformable torsion beam) is a type of automobile suspension based on a large H or C shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each stub-axle assembly, on each side of the car. The cross beam of the H holds the two trailing arms together, and provides the roll stiffness of the suspension, by twisting as the two trailing arms move vertically, relative to each other. Twist-beam rear suspension of a Volkswagen Golf Mk3 About The coil springs usually bear on a pad alongside the stub-axle. Often the shock is colinear with the spring, to form a coilover. In many cases the damper is also used as a restraint strap to stop the arm descending so far that the coil spring falls out through being completely unloaded. This location gives them a very high motion ratio compared with most suspensions, which improves their performance, and reduces their we...
Air suspension Air suspension is a type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. The air pressure inflates the bellows, and raises the chassis from the axle. Overview Pneumatic spring on a semitrailer Air suspension is used in place of conventional steel springs in passenger cars, and in heavy vehicle applications such as buses and trucks. It is broadly used on semi trailers and trains (primarily passenger trains). One application was on EMD's experimental Aerotrain. The purpose of air suspension is to provide a smooth, constant ride quality, but in some cases is used for sports suspension. Modern electronically controlled systems in automobiles and light trucks almost always feature self-leveling along with raising and lowering functions. Although traditionally called air bags or air bellows, the correct term is air spring (although these ...