# Manufacture of spiral extension springs and torsion spring

Manufacture of spiral tension springs and torsion springs
1. Cylindrical spiral spring structure

According to the nature of the force, the cylindrical coil spring has three types:

1. Cylindrical spiral compression spring

1) The distance between each coil of the spring

Suppose the pitch of the spring is p; the diameter of the spring wire is d; there should be an appropriate spacing δ between the turns in the free state. In order to maintain a certain elasticity of the spring after compression, it should also be ensured that under the maximum load, there is still a certain distance δ1 between the rings. The size of δ1 is generally recommended as: δ1=0.1d≥0.2mm

The two end face rings of the spring are tight with the adjacent ring (no gap), and only play a supporting role without participating in deformation, so it is called a dead ring. When the number of working turns of the spring n≤7, the dead loop at each end of the spring is about 0.75 turns; when n>7, the dead loop at each end is about 1 to 1.75 turns.

3) End structure

YI type: Both end face rings are tightly tied to the adjacent rings, and they are ground on a special grinder;

Type YII: Both ends of the spring wire are forged flat and tightly tied to the adjacent ring during heating and winding (the end ring can be ground or not);

Type YIII: Both end rings are tightly aligned with the adjacent rings and are not flattened

In important occasions, the YI type should be used to ensure that the two supporting end faces are perpendicular to the axis of the spring, so that the spring will not skew when compressed. When the spring wire diameter d≤0.5mm, the two supporting end faces of the spring do not need to be ground flat. For springs with d>0.5mm, both supporting end faces need to be ground flat. The flattened part should be no less than 3/4 of the circumference. The end thickness is generally not less than d/8, and the end surface roughness should be less than

2. Cylindrical spiral tension spring

1) End hook type

In order to facilitate connection, fixation and loading of the tension spring, hooks are made at both ends.

The LI type and LII type hooks are easy to manufacture and are widely used. However, due to the large bending stress generated at the transition of the hook, it is only suitable for springs with a spring wire diameter d≤l0mm.

The LVII and LVIII type hooks are not integrated with the spring wire, so there is no shortcoming of the transition part mentioned above, and the hook can be turned to any direction for easy installation. In situations where the force is large, it is best to use the LVII type hook, but its price is more expensive.

2) Prestressed tension spring

When the cylindrical spiral tension spring is unloaded, the coils should be close to each other. In addition, in order to save the axial working space and ensure that the coils of the spring are pressed against each other when there is no load, often during the winding process, the spring wire is twisted around its own axis. The spring made in this way has a certain pressing force between each coil, and a certain prestress is also generated in the spring wire, so it is called. This kind of spring must only start to separate after the applied tension is greater than the initial tension F0, so it can save axial working space compared with tension springs without prestress.

3. Spiral torsion spring

The torsion spring, in order to facilitate connection, fixation and loading, there are lever arms at both ends:

(2) Manufacturing of cylindrical coil springs

The manufacturing process of the coil spring includes: a) rolling; b) hook making or finishing of the end face ring; c) heat treatment; d) process test and pressure treatment.

Rolling is to wind the spring wire that meets the technical requirements on the mandrel. In mass production, it is rolled on a universal automatic spring coiling machine; in single-piece and small batch production, it is rolled on an ordinary lathe or manual winding machine.

There are two types of rolling: cold rolling and hot rolling. Cold coil is used for spring wire with a diameter d<(8～10)mm drawn after pre-heat treatment; hot coil is used for strong spring made of spring wire with larger diameter. The temperature during hot coiling is selected within the range of 800-1000℃ depending on the thickness of the spring wire. Regardless of whether cold coil or hot coil is used, the pitch of the spring should be adjusted as necessary after coiling.

After completing the above-mentioned procedures, the spring should be heat treated. No significant decarburization layer should appear on the spring surface after heat treatment. The cold rolled spring is only tempered to eliminate the internal stress generated during the rolling.

In addition, the spring must be subjected to process tests and precision, impact, and fatigue tests according to the technical conditions of the spring to check whether the spring meets the technical requirements. It should be particularly pointed out that the durability and impact strength of the spring depend to a large extent on the surface condition of the spring wire, so the surface of the spring wire must be smooth and free of defects such as cracks and scars. Surface decarburization will seriously affect the durable strength and impact resistance of the material. Therefore, the depth of the decarburized layer and other surface defects should be specified in the technical conditions of the acceptance spring. Important springs must also be surface-protected (such as galvanized); ordinary springs are generally coated with oil or paint.