stiffness

Definition: refers to the ability of a component or part to resist elastic deformation or displacement under the action of external forces, that is, the elastic deformation or the only should not exceed the scope permitted by the project.

Stiffness is a parameter that reflects the relationship between structural deformation and force, that is, the amount of deformation produced by the structure by much force. Simply put, it is a spring, and the tension divided by the elongation is the stiffness of the spring. The unit of stiffness is usually N/m.

2.1

Stiffness type

When the load is a constant load, it is called static stiffness; Is alternating load, it is called dynamic stiffness. Static stiffness mainly includes structural stiffness and contact stiffness. Structural stiffness refers to the stiffness of the member itself, mainly including bending stiffness and torsional stiffness.

Bending stiffness is calculated according to the following formula:

Where, P is the static load (N) and δ is the elastic deformation (μm) in the direction of load.

The torsional stiffness is calculated according to the following formula:

Where, M is the acting torque (N· M), L is the distance (M) from the acting torque point to the fixed end, and θ is the torsion Angle (°).

The two contact

3

Based on the above theoretical understanding of strength and stiffness, relative to stiffness, the definition of strength is for the damage under the action of external forces; The failure types are classified as plastic yield and brittle fracture, which is associated with the stress-strain curve during tension. This is shown here.

The curve in the figure can be divided into four stages:

I. Elastic deformation stage;

II. Yield stage;

III. Strengthening stage;

IV. Local necking stage.

The definition of stiffness is to resist elastic deformation, which is carried out in the first stage. Hooke’s law is met under elastic action. The calculation formula of bending stiffness and torsional stiffness under static load is observed, which is similar to Hooke’s law, and it can be inferred that the measurement of stiffness is only carried out in the elastic deformation stage.

After entering the next stage, the plastic strain or residual strain will not disappear in the tensile process. Under the stress-strain curve, the stress is almost constant, while the strain increases significantly. At this time, the stress is the yield limit, and the material enters the failure stage of plastic yield. After entering the strengthening stage, the strain increases with the increase of stress and finally reaches the strength limit. Thus, the strength measurement is after the elastic deformation of the material and before the strength limit.

To sum up, it can be concluded that both stiffness and strength are measured at the failure stage of parts, while stiffness can be measured by stress and strength can be measured by deformation. In the process of strain, the stiffness is in the former stage while the strength is in the later stage. Therefore, in the failure condition measurement of parts, as long as the stiffness requirement is met, enough stress can be resisted in the elastic deformation stage, and the strength can also meet the requirements of parts under such premise. According to this relationship, there will be all kinds of design in the actual production, such as the shaft in mechanical equipment, usually according to the strength conditions to determine the size of the shaft, and then according to the stiffness conditions for stiffness check. Precision machinery for shaft stiffness requirements are therefore set very high, its section size design is often controlled by stiffness conditions.

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