Semi elliptic leaf springs are almost universally used for suspension in light and heavy commercial vehicles. For cars also, these are widely used for rear suspension, but now replaced, completely by coil springs.



Construction :-

The spring consists of a number of leaves called blades. The blades vary in length. The composite spring is based upon the theory of a beam of uniform strength. The lengthiest blade has eyes on its ends. This blade is called master leaf. All the blades are bound together by means of steel straps.

The spring is supported on the axle, front or rear by means of a U-Bolt. One end of the spring is mounted on the frame with a simple pin, while on the other end, connection is made with a shackle. When the vehicle comes across a projection on the road surface, the wheel moves up, deflecting the spring. This changes the length between the spring eyes. If both the ends are fixed, the spring will not be able to accommodate this change of length. This is provided by means of a shackle at one end which gives a flexible connection.

The spring may be either cambered initially or flat. Highly cambered springs provide a soft suspension, but they also increase the tendency to yaw (movement about vertical axis). Flat springs reduce the tendency of the vehicle to dip that is pitching when braking or accelerating suddenly. Use of longer springs gives a soft suspension. Generally rear springs are kept longer than the front springs. This causes them to vibrate at different frequencies, which prevents exccessive bounce.

When the shackle is employed at the rear, the axle movement will be around the front rigid frame spring connection, while the drag link of the steering system will have its motion about the drag link drop arm joint. This results in wheel wobble. If, however, the shackle is mounted on the front end, the axle will have centre of rotation at the spring eye. This gives approximately the same arc as the radius action of the drag link. It is thus seen that by putting the shackle at the front end, the tendency of the front wheels to wobble is reduced. The position and number of shackles in case of rear axle depends upon the drive used.

Spring eyes for heavy vehicles are usually bushed with phosphor bronze bushes. However, for cars and light transport vehicles like vans, the use of rubber has also become a common practice. This obviates the necessity of lubrication as in the case of bronze bushes, where it was a necessity. The rubber bushes are quite in operation and also the wear on pin or the bush is negligible. Moreover, they allow for slight assembly misalignment, 'Silentbloc' is an example of this type of bushes.

Fatigue strength and hence the life of spring can be increased by shot peening the top surface of each leaf, which introduces a compressive residual stress, Rounding the edges of the leaves also avoids stress concentration, thereby improving the fatigue strength.

When the leaf spring deflects, the upper side of each leaf tip slides or rubs against the lower side of the leaf above it. This produces some damping which reduces spring vibrations, but since this available damping may change with time, it is preferred not to avail of the same. Moreover, it produces squeaking sound. Further if moisture is also present, such interleaf friction will cause fretting corrosion which decreases the fatigue life strength of the spring. Phosphate paint may reduce this problem fairly. Occasionally, this liners of zinc or any other soft metal are also inserted in between the blades, to prevent sqeaking. Zinc leaves also help to keep the value of the friction coefficient constant.

In some springs special inserts are provided at the end of each leaf, excepting however the master leaf. The material for the inserts maybe rubber or waxed cloth, or even some soft bearing metal impregnated with oil. This gives efficient spring operation.

Sometimes the springs are provided with metallic or fabric covers to exclude dirt. The covers also serve to contain the lubricant used in between the spring leaves. In case of metal covers, the design has to be telescopic type to accommodate the length of the cover after the change of spring length.

The leaves of the leaf spring require lubrication at periodic intervals. The vehicle is jacked up so that the weight of the axle opens up the leaves. The spring is then cleaned thoroughly and sprayed with a graphited penetrating oil. However, it is important to remember that in some vehicles example Ambassador it is specified that the replication of spring leaves should not be done. In such cases instruction must be followed.

The lubrication of shackle pins at regular intervals, say 1000 km, should also be done with S.A.E 140 oil. However, no lubrication is required when rubber bushes are used, as in case of Hindustan Ambassador cars.