The multi leaf spring design is based on the use of a flat trapezoid geometry, with cut leaves stacked one on top of the other. This leads to a multi leaf design with different lengths of each leaf. When a multi leaf spring deflects the leaves move relative each other. The contact between the leaves produces a friction between them, which leads to a difference in load between compression and decompression. The longest leaf has two eyes for assembling to the vehicle body. The strength of this leaf and the eyes are critical and sometimes the second leaf will have safety loops which hook around the main eye as a back up in case of failure. The centre of the spring is normally fixed to the vehicle axle with U-bolts.
The parabolic spring is designed for low internal friction and lowest possible weight and consequently has fewer leaves. The leaves are partly given a parabolic shape from the centre to the end of the leaf, which gives an equal bending stress level along the parabolic part. This ensures efficient use of material and minimum weight. The shape of the leaves are such that contact between the leaves only exists at the centre and the ends. And since the centre of the leaf spring is fixed to the axle with U-bolts, the only friction comes from the end contacts. Rubber pads are also sometimes used to lower the friction. As with multi-leaf springs, the strength of the eyes is a critical safety factor.
Leaf springs can be divided in to three broad areas:
– Preparing leaves
– Bending – heat treatment – shot peening
– Assembling – presetting – surface treatment
Leaves for multi-leaf springs are prepared by cutting and stamping whilst parabolic leaves are hot rolled in a purpose-made rolling machine. The ends of the main leaf are heated and the eyes are then hot rolled. The leaves are heated and individually bent to the correct arc radius and, whilst still clamped between the bending tool, each leaf is quenched in oil to receive high strength and the correct shape. Parabolic leaves also have an arc radius but are however normally straight in the thicker central section. The leaves are finally tempered for high material toughness and strength.
After cooling, the shot peening process is necessary for all parabolic leaves. For multi-leaf springs however, it is normally only the two safety-critical leaves that require shot peening. The bushes are pressed into the eyes and the stack of leaves is assembled. Positive stresses are then introduced in to the spring by a pre-stressing process.
The multi leaf surfaces are protected against corrosion by painting of the outer surfaces and by lubrication between each leaf. Parabolic leaves are individually painted on all surfaces.