# Spring finger tips

1、 Common spring stiffness
Elastic modulus is one of the most important and characteristic mechanical properties of elastic materials. It is a representation of the difficulty of deformation. Denoted by E. It is defined as the ratio of stress to corresponding strain of ideal material with small deformation. E is expressed as the force per unit area in N / m ^ 2. The properties of modulus depend on the properties of deformation. The modulus of shear deformation is called shear modulus, expressed by G; the modulus of compression deformation is called compression modulus, expressed by K. The reciprocal of the modulus is called compliance, expressed by J.

The yield limit, B, and strength limit, s, obtained from the tensile test, reflect the material’s ability to bear the force, while the elongation δ In order to show the material’s ability to resist deformation within the elastic range, the meaning of the elastic modulus E is usually reflected by the stiffness of the parts in the actual engineering structure. This is because once the parts are designed according to the stress, they are produced by the load they bear during the service process within the elastic deformation range The stiffness is determined by the amount of deformation.

Where λ is the strain caused by stress
F — stress
D is the average value of inner and outer diameter of spring coil
G — shear modulus of elasticity
D — spring diameter
N — effective number of turns
Generally, the load causing unit strain is taken as the stiffness of the part. The unit of spring stiffness is n / mm, so it can be simplified as: stiffness = (g * D4) / (8 * n * D3), and the calculation formula of series spring stiffness is 1 / k = 1 / (K1) + 1 / (K2), where k is the stiffness.

In order to improve the rigidity e of the parts, that is to reduce the elastic deformation of the parts, the materials with high elastic modulus and the cross-sectional area of the bearing can be appropriately increased. The importance of the stiffness lies in that it determines the stability of the parts in service, especially for slender members and thin-walled members. Therefore, for the theoretical analysis and design calculation of the component, the elastic modulus E is often used as an important mechanical performance index.

The calculation formula of cylindrical spring stiffness can also be simplified as follows:
F’=GD/8C4n;
Where G is the shear modulus of elasticity;
D — spring pitch diameter;
C — winding ratio, C = D / D;
D — diameter of spring wire
The winding ratio C = (D / D) * (B / a) 2,
among
D — median diameter of long axis of elliptic circle
A — the middle radius of the long axis of the elliptic circle
B is the middle radius of the minor axis of the elliptic circle
D — diameter of spring wire
The expanded length of the spring with elliptical cross section is L = n1d π ζ 2, where
N1 — total number of spring coils
D — median diameter of long axis of elliptic circle
ζ 2 — length correction coefficient, selected from the following table according to B / A:

2、 Spring finger
The oblique coil spring is a circular wire winding spring. Its winding coil is elliptical and inclined. When compressed, each coil will deform independently. No matter which part of the coil deforms, the whole spring will react, so as to realize the uniform load at each contact point

For connectors, the ends of the spring will be welded together to form a complete ring. For the specific stress requirements of insertion or pull-out on each standard coil size, the spring can be put into the groove in the design of socket and plug. When the convex groove is inserted into the groove, the inclined coil spring of the spring will deform under the action of continuous spring stress, In this case, the load of the spring coil and the design of the slot will produce continuous spring stress. In this way, a corresponding pull-out force is required to disconnect the two components. On the clasp of the micro connector, the radial inclined coil spring has two key functions and can reduce the number of components in the system

The helical coil spring provides dual functions. It can be used as a mechanical joint, and can also maintain the circuit between the external and internal screw joints, so as to minimize the RF interference. The compact design for electronic interconnection is more complex. The compact design with simple installation provides a new test for the interconnection design of military, industrial and commercial applications. The speed of the circuit is increasing, As a result, the connector needs to process more signals in a smaller space at a higher speed, or more contacts in a more compact space, and the smaller the size of the connector. Its unique elastic deformation curve, wide working range – the allowable compression of the spring is up to 35%, and the relatively stable elastic force reduces the temperature difference, For wear caused by tolerances and other deviations, each coil can function independently.
Stress state analysis
Stress strain curve
Due to the particularity of the spring finger structure, the stiffness assessment of the ordinary spring is transformed into the evaluation of the pulling force (or stress-strain) of the spring contact finger. The test of this index is carried out in the simulated working state: firstly, a force is applied to the spring contact finger, and then its deformation is observed to assess the “soft” and “hard” degree of the spring. This index is directly related to the high-voltage switch Whether the opening and closing process is smooth. To test this index, a dynamometer and a set of mould are needed to simulate the working state of spring contact finger. Due to technical confidentiality, the manufacturer does not provide more specific inspection scheme at present.

Through the research of contact system, three detection schemes are proposed as follows:
1. Disconnect the spring contact finger from the welding point, fix one end, and connect the other end with the dynamometer. Apply different forces to the dynamometer, record the spring elongation under each force, and draw the stress-strain curve, so as to select the spring finger that is most suitable for our products;
2. Simulate the working state of the spring contact finger, design a set of mould, as shown in the figure below. Apply force F on the shaft of the simulated conductor to make the shaft drop. The contact with the same height h and different stiffness needs different force. Select the one suitable for our company