Spring is an elastic element widely used in the mechanical and electronic industries.
When loaded, the spring can produce large elastic deformation and convert the mechanical work or kinetic energy into deformation energy.
After unloading, the deformation of the spring disappears and returns to the original state, and the deformation energy is converted into mechanical work or kinetic energy.
The ratio of spring load to deformation is called spring stiffness.
The greater the stiffness, the harder the spring is.
1. Function of spring
Cushioning and damping.
Such as the damping spring under the car and train trunk, the damping spring of various buffers, etc;
Control the movement of the mechanism.
Such as valve spring in internal combustion engine and control spring in clutch;
Store and output energy.
Such as clock spring, gun latch spring, etc;
Measure the force.
Such as spring scale, spring in dynamometer, etc;
2. Classification of springs
According to the stress nature, the spring is divided into: tension spring, compression spring, torsion spring and bending spring.
Tension spring is a coil spring that bears axial tension.
Tension springs are generally made of circular section materials.
When not under load, the coils of the tension spring are generally tight without clearance.
Compression spring is a helical spring bearing upward pressure.
Its material section is mostly circular.
It is also made of rectangular and multi strand steel.
The spring is generally of equal pitch. There is a certain gap between the coils of the compression spring.
When subjected to external load, the spring shrinks and deforms and stores deformation energy.
Torsion springs are coil springs.
The torsion spring can store and release angular energy or statically fix a device by rotating the force arm around the central axis of the spring body.
The end of the torsion spring is fixed to the other components, and when the other components rotate around the center of the spring,
The spring pulls them back to their original position, generating torque or rotational force.
There are also two unusual air springs and carbon nanotube springs;
Air spring is a non-metallic spring that adds pressure air into the flexible closed container and uses the compressibility of air to realize the elastic effect.
When used in the suspension device of high-grade vehicles, it can greatly improve the ride comfort of vehicles, so air spring has been widely used in automobiles and railway locomotives.
Carbon nanotube spring: It is necessary to prepare carbon nanotube film first, and then spin carbon nanotube film into carbon nanotube spring by spinning technology.
The diameter can reach hundreds of microns and the length can reach a few centimeters.
It is expected to be used in the fields of retractable conductors, flexible electrodes, micro strain sensors, supercapacitors, integrated circuits, solar cells, field emission sources, energy dissipation fibers and so on. It is also expected to be used in medical devices, such as tension sensing bandages and so on.
3. Spring material and allowable stress
Spring is often subjected to alternating and impact load in work, and requires large deformation, so spring material should have high tensile strength, elastic limit and fatigue strength.
The process should have certain hardenability, not easy to decarburize and good surface quality.
Common spring materials and allowable shear stress
| Material Science | See shear stress [τ] / MP for details | Shear modulus of elasticity G / MPa | Recommended operating temperature / ℃ | |||
|---|---|---|---|---|---|---|
| category | Code | Type I spring | Type II spring | Type III spring | ||
| Carbon spring steel wire | Group I II, II and III | 0.3s | 0.45 | 0.5 | 80000 | -40~120 |
| 65Mn | 420 | 560 | 700 | 80000 | -40~120 | |
| Alloy spring steel wire | 60Si2Mn | 480 | 640 | 800 | 80000 | -40~200 |
| 65SiMnWA | 570 | 760 | 950 | 80000 | -40~250 | |
| 50CrVA | 450 | 600 | 750 | 80000 | -40~210 | |
| Stainless steel wire | 1Cr18Ni9 | 330 | 440 | 550 | 73000 | -250~300 |
| 4Cr13 | 450 | 600 | 750 | 77000 | -40~300 | |
Note:
1) According to the number of cycles under load, the spring is divided into three types: type I, n > 106 ; Class II n = 103 ~ 105 and under impact load; Class III n ＜ 103.
2) The allowable stress of shackle tension spring is 80% of the value in the table;
If the spring is subjected to strong pressure treatment, the allowable stress in the table can be increased by 20%
3) According to different mechanical properties, the carbon cable spring steel is divided into four groups, of which group 1 has the highest tensile strength, group II takes the second place, group III has the lowest, and group II4 has the same tensile strength as group II, but the plasticity is better.
Refer to the following table for Sb of carbon cable spring steel wire.
Strength of carbon spring steel wire
| Code | MP | |||
|---|---|---|---|---|
| Group I | Group II | Group III | ||
| Wire diameter d / Mn | 0.2 | 2700 | 2250 | 1750 |
| 0.3 | 2700 | 2250 | 1750 | |
| 0.5 | 2650 | 2200 | 1700 | |
| 0.8 | 2600 | 2150 | 1700 | |
| 1 | 2500 | 2050 | 1650 | |
| 1.5 | 2200 | 1850 | 1450 | |
| 2 | 2000 | 1800 | 1400 | |
| 2.5 | 1800 | 1650 | 1300 | |
| 3 | 1700 | 1650 | 1300 | |
| 3.6 | 1650 | 1550 | 1200 | |
| 4 | 1600 | 1500 | 1150 | |
| 4.5 | 1500 | 1400 | 1150 | |
| 5 | 1500 | 1400 | 1100 | |
| 5.6 | 1450 | 1350 | ||
| 6 | 1450 | 1350 | 1050 | |
| 7 | 1250 | 1000 | ||
| 8 | 1250 | 1000 | ||
4. Manufacture of spring
The manufacturing process of coil spring includes rolling, making of hook or finishing of end face ring, heat treatment and process performance test.
In mass production, it is rolled on the universal automatic spring coiling machine;
For single piece and small batch production, it is made by ordinary lathe or hand. When the diameter of spring wire is less than or equal to 8mm, cold coiling method is commonly used.
Heat treatment is required before coiling and low-temperature tempering is required after coiling.
When the diameter is greater than 8mm, the hot coil (hot coil temperature 800 ℃ ~ 1000 ℃) method shall be adopted.
After the hot coil, it shall be quenched and tempered at medium temperature.
After the spring is formed, the surface quality inspection shall be carried out, and the surface shall be smooth, free of scars, decarburization and other defects;
The spring subjected to variable load must also be subject to surface treatment such as shot peening to improve the fatigue life of the spring.
5. End structure of spring
In addition to the effective number of turns N participating in the deformation of the compression spring, in order to make the compression spring work evenly and ensure that the center line of the spring is perpendicular to the end face, there are 3 / 4 ~ 7 / 4 turns at both ends of the spring and play a tight supporting role.
It does not participate in the deformation during work, so it is called dead circle or supporting ring.
The end of the tension spring is provided with a hook for installation and loading. There are four types of end structures commonly used;
Semicircular shackle and circular shackle are easy to manufacture and widely used. However, due to large bending stress at the transition of the hook, they are only suitable for springs with spring wire diameter d ≤ 10mm.
The adjustable and rotatable hooks are in good stress condition, and can be turned to any position for easy installation.
6. Stress calculation of spring
Stress analysis of compression spring
Fig. (a) shows the cylindrical helical compression spring, which bears the axial working load F.
Through the analysis of the section method, it is known that the section of the spring wire is subject to the shear force F and torque T = FD / 2, and the shear stress caused by torque is:
If considering the influence of shear stress caused by shear force F and the spiral curvature of spring wire, the maximum shear stress t occurs in figure (b) on the inner side of spring, and its value and strength conditions shall be:
Where,
C – winding ratio,
C = D / D, which can be selected according to table 1
K — spring curvature coefficient,
K can also be found directly from table 2.
It is known from the table that the greater C, the smaller the influence of K on T;
F — working load of spring, N;
D — pitch diameter of spring, mm;
D – material diameter mm.
Table 1 recommended values of winding ratio
| Steel wire dia. D | 0.2~0.6 | 0.5~1 | 1.1~2.2 | 2.5~6 | 7~16 | 18~50 |
| C=D/d | 7~14 | 5~12 | 5~10 | 4~9 | 4~8 | 4~6 |
Table 2 curvature coefficient K
| Winding ratio C | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 12 | 14 |
| K | 1.4 | 1.31 | 1.25 | 1.21 | 1.18 | 1.16 | 1.14 | 1.2 | 1.1 |
In equation 1, the formula for calculating the diameter of spring steel wire according to the strength condition can be obtained by replacing f with the maximum working load F2 of spring:
The strength calculation method of tension spring is the same as that of compression spring.
7. The spring is not in place and the failure reason
In practical work, we often encounter that the spring can not push the moving object to the set position, that is, the calculated free length of the spring becomes shorter.
The main reason is that there is no initial compression treatment, that is, the operation of compressing a manufactured spring to its compression height or tightening height with a large force (if necessary), and it cannot return to its original free length after release.
The shortening amount is called “initial compression shrinkage”.
Generally, after 3-6 times of compression, the length will not be shortened, that is, the spring “positioning”.
The spring is permanently deformed after initial compression.
8. Spring precautions
In practical work, the compression spring should be able to maintain its working length even if it is subjected to a force beyond the elastic limit of the material.
Therefore, the length of the finished spring should be equal to the calculated length of the spring plus the initial compression shrinkage, which can avoid the spring not in place, so as to avoid dangerous stress when the spring coil is tightened together, resulting in abnormal indication line of the spring.
During the heat treatment of the finished spring, especially the hardening and tempering process, the workpiece must be placed horizontally (lying) in the furnace to prevent the spring from becoming shorter due to its own weight, resulting in improper operation.
