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Originally called laminated or carriage spring, a leaf spring is a simple form 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.

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-elliptical spring or cart spring, it takes the form of a slender arc-shaped length of spring steel 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. 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 manufacturers have experimented with 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 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 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.

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 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.

Maunfacturing

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
    1. Eye reaming / eye boring
    2. Bush insertion
    3. Bush reaming
  7. Assemble
    1. Presetting & load testing
    2. Paint touch-up
    3. Marking & packing