Learn more about leaf springs(二)

Section 4 Center hole and clamp hole processing

 

I. Processing methods of center hole and clamp hole

1. Drilling: Use a standard drill bit to drill on an ordinary drilling machine. For clamp holes, drill first and then countersink, or use a small drill bit to drill and countersink at one time. However, this processing method is very inefficient and prone to burrs. It is suitable for small batch production, but its application range is wide and is not limited by material thickness.

2. Punching: Use a press to punch the center hole and use a large gap to punch rivet holes with sufficient slope. The production efficiency is high and the product quality is stable. It is suitable for mass production, but the punch is easy to break when punching small holes and thick materials. For thick reeds, local heating (750~800℃) and hot punching are used to improve efficiency.

3. Bump: The leaf spring adopts the center positioning bulge structure, which can reduce the occurrence of the phenomenon that the leaf spring with the center hole structure is easily damaged at the center hole due to the loosening of the riding bolt. However, due to the disadvantages of complex processing and easy fragmentation during transportation, the center positioning bulge is gradually reduced in multi-leaf springs and replaced by the center hole structure. In small batch production, the center positioning bulge is used to simplify the process. Before the single piece is formed and heat treated, the heated spring is first formed and positioned, and then formed and heat treated. Because the center positioning bulge is formed and heat treated, the position accuracy of the center positioning bulge is required to be high, so the scrap rate of the heat treatment forming process is increased.

 

Section 5 End Processing

 

1. End Angle Cutting: Generally, the cold cutting process is used on the press. When the thickness is greater than 16mm or the cutting angle is small, the hot cutting process (heating temperature 700~800℃) can be considered. When cutting the angle, attention should be paid to the symmetry of the angle, and the end should not be cut off, otherwise additional torsional force between the leaves will be generated, which is unfavorable.

2. End extension: The end extension of a single leaf is divided into two forms: short extension (the extension length is generally less than 2 times the leaf width) and long extension (full-field conical spring leaf). It is rolled and formed by a roll forging machine under heating (the leaf temperature is 950~1000℃). The manufacturing process is complex and the cost is high.

 

Section 6 Ear Rolling

 

When the leaf spring has a guiding function, the main leaf must have ears to transmit braking and driving forces.

There are three types of ears at the end of the first leaf of the leaf spring: upper ear ears, flat ear ears, and lower ear ears.

At present, the most widely used is the upper ear, which is simple to manufacture, and the ear part does not bear vertical load; the lower ear type is generally used to meet the requirements of layout and motion trajectory. Since the ear has to bear vertical load, the ear thickness should be larger, otherwise it is easy to open the plastic weaving; in addition to meeting the factors of layout and motion trajectory, the flat ear type can eliminate the additional bending moment of the main leaf when transmitting longitudinal force. Due to the complexity of manufacturing and the addition of the heating and forming process, the strength of the ear part is reduced, and it is not often used on trucks.

For safety, the ear of the main plate is often strengthened and protected; the most common way is to make the second plate into a wrap ear for protection, that is, after the ear of the main plate is broken, the second ear is used to ensure that the leaf spring does not fall off the bracket.

The ears are all processed by heat. The spring steel of the ear part is heated to 900~1000℃ in a medium-frequency induction furnace or a coking gas furnace, and the first ear is rolled in three stations on a special ear. The forming process is: 1. Pressing, bending, cutting 2. Ear forming 3. Shaping.

Because the inner hole of the ear needs to be pressed into the bushing, the aperture tolerance and cylindricity of the inner hole of the ear are required to be high. Sometimes, in order to improve the accuracy of the inner hole of the ear and ensure the matching requirements of the bushing, the inner hole is processed after the ear is formed. The specific requirements of the ear are as follows:

1. The inner hole size tolerance is ±0.15mm.

2. The opening gap at the interface of the rolled ear hole is allowed to be 1±0.7mm.

3. Except for the 10~15mm arc surface of the interface, the cylindricality of the inner surface of the remaining apertures is 0.1mm.

 

Section 7 Forming Heat Treatment

 

The forming heat treatment process of the steel leaf spring consists of three parts: bending, quenching, and tempering.

1. Bending deformation: The steel leaf spring is usually heated to a certain temperature and placed in a special forming fixture for bending. Due to design requirements, the curvature of each single leaf of the steel leaf spring assembly is different, so the forming fixture is divided into two types: one is a forming fixture (hard mold) with a fixed curvature, that is, a set of forming fixtures can only produce spring leaves with one curvature, which is suitable for mass production of a single variety. The other is a forming fixture with adjustable curvature, that is, a set of forming fixtures can be used to form spring leaves with multiple curvatures, which is generally used in mass production of multiple varieties.

2. Quenching: Quenching is to heat the steel leaf spring blank to AC3 or above 50℃, and keep it at this temperature for a sufficient time to make the material of the entire cross-section of the spring leaf austenite, and then quickly cool it in oil to obtain a stable martensite structure, thereby improving the hardness and strength of the spring leaf. This process is called quenching. The hardness of the spring leaf after quenching can generally reach about HRC59.