Learn more about leaf springs(一)

Section 1  Material requirements for leaf springs

 

A spring is a mechanical part that utilizes the elastic characteristics of a material. Its performance and service life depend largely on the choice of the spring. Therefore, the material required to manufacture the spring is to have a sufficiently high elastic limit. At the same time, since many springs are subjected to alternating loads during operation, their deformation varies with the load. In addition to requiring the material to have high elasticity, it also requires high fatigue strength. The use of springs is very extensive. Depending on the service conditions and working environment, various special requirements are also put forward for spring materials. For example, the spring material is required to be corrosion-resistant, heat-resistant, elastic, non-magnetic, and have good conductivity. In addition, the material is also required to have excellent public welfare. In short, the spring material should have the characteristics of easy processing, convenient manufacturing, stable performance, and low price.

 

1. Elastic limit

The material is deformed to a certain extent by external force within the elastic range. Once the external force disappears, the material returns to its original state. The maximum stress that can be borne without permanent deformation is called the elastic limit of the material. When a material with a high elastic limit is required, a material with high tensile strength is usually selected. The tensile strength is closely related to the chemical composition of the material and the organization after heat treatment. Therefore, the selection of suitable materials and the correct use of heat treatment processes are the most critical factors that determine the life of the spring.

 

2. Fatigue strength

Since the leaf spring is mainly subjected to repeated bending stress during operation, the material is required to have a high hardness fatigue strength.

The surface state of the material has a great influence on the fatigue strength. Therefore, the material notch fatigue strength is required to be high and the material notch sensitivity is required to be low to avoid early fatigue and damage. In addition, the fatigue strength of the material will also vary with its decarburization depth, cold working degree and heat treatment results. Even a very thin decarburization layer will lead to a sharp drop in fatigue strength.

 

3. good plasticity and toughness

During the spring manufacturing process, the material needs to undergo different degrees of processing deformation, so the material is required to have a certain degree of plasticity. At the same time, when the spring is subjected to impact loads or alternating loads, the material should have good toughness, which is very beneficial to improving the life of the spring and ensuring the safety, reliability and smoothness of the spring during service.

 

4. Hardenability

In order to give full play to the performance of the material, it is hoped that the material can be fully hardened, and at the same time, it must have low overheat sensitivity and not easy to decarburize.

 

Section 2  Commonly used leaf springs

 

At present, the commonly used leaf springs are mainly: silicon manganese steel (SiMn), chrome manganese steel (CrMn), chrome vanadium steel (CrV), chrome manganese boron steel (CrMnB), etc.

 

Section 3  Manufacturing process of automobile leaf springs

 

Cutting - straightening - ear forming - center hole drilling - cutting angle or short cone rolling - punching clip hole - quenching - tempering - shot peening - pressing hinge bushing - riveting clip - assembly - preloading - painting - Packing

 

I. Cutting

1. According to the shearing equipment, it is divided into press shearing method or punching and shearing machine shearing method and cutting machine cutting method.

2. According to the heating method, it is divided into cold shearing and hot shearing.

 

II. Common cutting defects and causes

A perfect fracture should be flat, without tilt, burr, and crack (including micro cracks).

1. Shear crack: The cause of the crack is that the shear blade is not sharp, the blade gap is improperly adjusted, or the material itself has interlayers, slag inclusions, etc.

2. Shear burrs: Burrs are the most common material breaking defects. Using blunt blades or blade gaps that are too large are the main causes of burrs. Especially for hot shearing, too large a gap may even cause cut stretching. If the burrs at the end of the spring leaf are too large (more than 0.5mm), they have a higher hardness after heat treatment and cannot be eliminated during shot peening. When the spring assembly is under load, it will increase the friction of the segments and produce noise, causing stress concentration and seriously affecting the life of the leaf spring. Therefore, the size of the burrs must be controlled or removed.

3. Bevel cutting in the thickness direction: The cut of a single piece is sloped along the thickness direction. This is caused by the large gap between the blades or the loose positioning and tightening, which causes the sheet to move.

4. Bevel cutting in width direction: A single piece is tilted in width direction, i.e. bevel head, which is caused by improper positioning or lack of clamping. In addition, the side bending of flat steel can also cause bevel head.

5. Uneven cuts: This is caused by blunt or worn blades. The blade gap is too large. The blade should be replaced, repaired or adjusted in time. In addition, the high hardness of the raw material itself or inclusions will also cause uneven cuts.