How to calculate the shear strength of the screw?

The shear strength of a bolt refers to its ability to withstand maximum shear force without breaking when subjected to an external lateral force. We can understand the calculation method, standards and some specific shear strength values ​​of screws.

Firstly, the formula to calculate the shear strength of the bolt is W=P/F=P/ab, where P is the load at break (N), F is the overlap area (cm^2), a is the overlap length (cm), and b is the overlap width (cm). This implies that the shear strength of the bolt is related to its size and shape. By measuring these parameters and applying the above formula, the shear strength of the bolt can be calculated.

Secondly, the shear strength of a bolt is also related to its material type. For example, a grade 4.8 bolt has a shear strength of approximately half its tensile strength, while a grade 12.9 bolt has a shear strength of 900 MPa. This indicates that screws of different classes have different shear strength standards.

Furthermore, the design value of bolt shear strength is determined by statistical connection test data, which implies that the actual shear strength of bolts in use may vary based on specific conditions and test results.

What methods are there to calculate the shear strength of bolts?

The calculation methods of bolt shear strength mainly include finite element analysis and experimental tests.

Finite element analysis is a method that predicts the behavior of materials or structures under force by establishing a mathematical model. It can evaluate the shear performance of bolts without actual destruction. This method can be used in the design and optimization of specific types of bolts, such as blind bolts, by simulating the strength behavior of bolts under pure shear force to study their load-bearing performance.

Experimental tests evaluate by loading and observing the failure mode of the bolt, which can directly reflect the actual shear strength of the bolt.

Each of the application scenarios of these two methods has its own emphasis. Finite element analysis is suitable for the early stage of bolt design, necessitating evaluating the impact of different design schemes on the shear performance of bolts, as well as predicting and optimizing bolt performance during the design process.

Experimental tests are most often applied in the later phase of product development, used to verify the accuracy and reliability of the design, ensuring that the screw product can meet performance requirements in real applications.

Bolted connection under working shear force

As shown in Figure 1-8, this connection uses a bolt to resist the working load F through a drilled hole. Assuming that each bolt receives an equal working load, the shear force received by each bolt is F.

Therefore, the compressive strength condition between the screw shank and the hole wall is:

The conditions for the shear strength of screws are:

In the formula:

F represents the operational shear force exerted on the screw, in Newtons;

d ₀ represents the diameter of the shear face of the screw, which can be taken as the diameter of the screw hole, in millimeters;

(τ) represents the allowable shear strength of the thread, in MPa, for steel

Where:

(S) _τ is the safety factor according to Table 1-9;

l _min represents the minimum height of the screw shank compressed by the hole wall, in millimeters.

During the project, L _min must be greater than or equal to 1.25d; i represents the number of shear faces on the screw shank. In Figure 1-1b, i=2, and in Figure 1-8, i=1.

The required shear stress for bolts is normally selected as 60Mpa.

How to adjust the shear strength of bolts according to operating conditions

In practical applications, adjusting the shear strength of bolts to meet specific requirements based on conditions of use can be accomplished in several ways:

Choose the appropriate screw type: Depending on the usage scenario, choose high-strength screws or common screws. High strength bolts are suitable for connecting important structures such as bridges and steel structures where the reliability and security of connections need to be guaranteed. Common screws are most widely used in mechanical and furniture equipment in general.

Calculate the shear stress of the bolt: Simplified from the formula F = A × (F/A), the shear force of the bolt is equal to the force to which the bolt is subjected. This step helps you understand the maximum shear force a bolt can withstand under a specific load, providing a basis for subsequent selection.

Consider screw deformability: To solve the problem of insufficient deformability of screw shear connectors, shear connectors with high deformability can be used to improve their performance. This design can improve the bolt's adaptability and reliability without sacrificing much strength.

Adjust the safety factor: Depending on the strength of the bolt and the anticipated conditions of use, the safety factor must be adjusted accordingly. For example, the allowable shear stress Ít can be calculated by dividing the shear stress by the safety factor. Adjusting the safety factor can optimize screw performance while ensuring safety.

Consider environmental factors: For high-strength fasteners that are long-term exposed to aggressive environments, such as those used in wind energy, special attention must be paid to their maintenance conditions and stability. In this case, special materials or coatings may be required to increase the corrosion resistance and durability of the screw.

Follow technical procedures: Referring to the technical procedures for high-strength bolted connections in steel structures, measures such as increasing the friction surface of the coating and its slip resistance coefficient, increasing the tensile connections and connection joints of the end plate can effectively improve the performance and reliability of bolted connections.