Leaf springs constitute a universal spring, used in a vehicle suspension, because they can transmit not only vertical forces resulting during a ride over uneven surface of the road. Leaf springs can also transmit shearing, longitudinal forces and torque moments. Leaf springs are a bundle of flat bars of spring steel having different length connected by a contracting bolt. Such a design results from the intention of using the material in the best way. Because stress in the central part of a spring is the highest, the thickness of stack of leave is the highest there.
Spring leaves in a free state are initially formed in such way so that longer leaves had bigger radius of curvature. After spring assembly, a uniform spring radius of curvature is achieved, which results in initial stress in the leaves, which reduces working stress in the longest leaves. Owing to that, spring fatigue life is increased.
Characteristic qualities of leaf spring design:
Spring eyes
Main spring leaf is normally finished with eyes, which serve as fitting to vehicle frame. Into spring eyes, bushes are pressed in, which ensure better fitting quality. In heavy leaves bronze bushes are used, which reduce friction between the pin and the bush. In lighter leaf springs rubber and metal bushes are used, which give more elastic coupling.
Slides
Main leaves of heavy springs often end in one or two slides instead of eyes. This allows to reduce active spring length during increasing load. Owing to that, such a spring has a progressive characteristic, that is, spring rate increases when load increases. This insures advantageous vehicle vibration frequency from the point of view of human physiology.
Progressive springs
So as to ensure advantageous vehicle vibration frequency from the point of view of human physiology, spring rate should increase when load increases. For this reason springs of progressive characteristic are used in vehicles. In case of linear characteristic of the main spring, progressive characteristic is achieved by the use of additional spring, which joins in operation when load increases.
Separators between leaves
Sliding friction between leaves has a highly detrimental influence on leaf spring operation. Friction increases spring rate, which results in increase of the frequency of vehicle body vibration. It also causes leaf surface seizure and cavities on their surface, which reduce their fatigue life.
Sliding friction between leaves has a highly detrimental influence on leaf spring operation. Friction increases spring rate, which results in increase of the frequency of vehicle body vibration. It also causes leaf surface seizure and cavities on their surface, which reduce their fatigue life.
In order to increase friction forces leaf surface is covered with zinc paint or graphite grease. In case of light leaf springs plastic separators are used.
Tapered ends
Straight ending of leaf ends, particularly thicker ones, result in increase of pressure between leaves, which leads to the increase of friction and abrasion of leaf surface. For this reason some leaf ends are tapered. Due to increased elasticity of tapered leaf ends, contact stress distribution between leaves is more advantageous.
Tapering also decreases spring weight. It also results in the fact that its design approximates theoretical beam of constant strength.
Transverse spring
Leaf springs are usually used as longitudinal suspension springs of front and rear vehicle axle. However, in some cases they are used as transverse springs in suspension of independent front and rear wheels. This considerably simplifies the design of suspension, but requires higher precision of spring make. It is particularly important in front suspension, as spring parameters influence vehicle steerability.
