Tuesday 11 October 2016

LEAF-SPRING

A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a laminated or carriage spring, sometimes referred to as a semi-elliptical spring or cart spring, it is one of the oldest forms of spring.
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.

Historical Background

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 Pan hard 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. 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 Pan hard 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.


Explanation of Diagrams
Diagram shows a laminated semi- elliptic spring. The top leaf is known as the master leaf. The eye is provided for attaching the spring with another machine member. The amount of bend that is given to the spring from the central line, passing through the eyes, is known as camber. The camber is provided so that even at the maximum load the deflected spring should not touch the machine member to which it is attached. The camber shown in the figure is known as positive camber. The central clamp is required to hold the leaves of the spring. However, the bolt holes required to engage the bolts to clamp the leaves weaken the spring to some extent. Rebound clips help to share the load from the master leaf to the graduated lea Design Features When the springs are manufactured, each leaf is curved or given a camber set. The smallest leaf receives the maximum set, which is progressively reduced with the increase of the span of the leaf, so that the main leaf has the least set.

A centre bolt is used to align and clamp the various leaves together. For holding the leaves together along their span, they are clamped with steel clamps (sometimes rubber-lined) at about halfway between the centre bolt and the spring eyes. During multi-leaf spring deflection, the upper side of each leaf tip slides or rubs against the underside of the blade above it.

 This inter-leafs action creates friction, which may be useful under certain conditions, as it reduces the amount of bounce, but normally it does not match the ride characteristics required and it makes the suspension too stiff, so that harsh riding over light road irregularities is experienced. Inter-leaf rubbing in the presence of moisture causes fretting corrosion, which decreases the fatigue strength, so that the oscillating life of the spring is also reduced. This problem can be reduced to some extent by applying phosphate paint between the blades.

 Also by fitting a thin layer of lead or an anti-friction disc between the blades, the fiction in the interference and hence fretting can be reduced. The top surface of each leaf is shot-peened or work-har­dened to prolong its life. This process changes the stresses on the upper side of the blade from a normal tensile to a compres­sive state so that the fatigue life of spring-blade is greatly improved. Rounding the edges of the blades also reduces risk of fatigue failure. Further by changing from a straight cropping of the blade ends to a tapering of the leaves near their ends, the stresses within the blade are more evenly distributed along each blade span. This reduces the peaking of stress level so that spring life is increased. At present, leaf springs are mostly made from silicon manganese steel





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