Rocker arm type tail undercarriage onboard connector and supporting structure design method

Abstract

The invention belongs to the field of structural design, and relates to a rocker arm type tail undercarriage onboard connector and a supporting structure design method. The method comprises the following steps: determining a front joint position and a rear joint position according to the overall layout of the helicopter, a tail undercarriage structure and intersection data; the front joint B and the front joint A have the same structure and are symmetrical; designing a front joint structure and a front frame position according to the front joint position and the front joint load; designing a rear joint structure and a rear frame position according to the rear joint position and the rear joint load; according to the front joint structure, the front frame position, the rear joint structure, the rear frame position and the load transmission path of the front joint and the rear joint, an initial finite element model of the front joint, the rear joint and the fuselage supporting structure is created.

Description

Technical Field

[0001] The invention belongs to the field of structural design and relates to a rocker arm type tail landing gear upper joint and a supporting structure design method. Background Art

[0002] In previous models, when determining the structural form of the connecting joint on the helicopter landing gear, the load conditions of the connecting joint under all landing and ground conditions were not considered. The unreasonable design of the joint structure will directly lead to more weight costs. In addition, when conducting the strength design of the on-board joint and fuselage connection structure of the helicopter tail landing gear, the traditional design idea is to take the connecting joint as the research object alone and create its solid unit finite element model, and will not take the connecting joint and the fuselage support structure as a whole as the research object. The stiffness of the fuselage support structure will directly affect the load distribution of the connecting joint and the load size of the connecting bolts. If the stiffness of the fuselage support structure is not designed, the load calculation accuracy of the connecting bolts between the joint and the fuselage support structure will be directly affected, and the accuracy and reliability of the strength calculation of the connecting bolts, on-board joints and fuselage support structures cannot be guaranteed, which will ultimately affect the safety of helicopter use. Summary of the invention

[0003] Purpose of the invention: To propose a standardized process for the integrated design of static strength and fatigue strength of the joints and supporting structures on a rocker arm landing gear.

[0004] Technical solution:

[0005] Provided is a method for designing a joint and a supporting structure of a rocker arm type tail landing gear, comprising:

[0006] Step 1: Determine the front joint position and the rear joint position according to the overall layout of the helicopter, the tail landing gear structure and the intersection data; the front joint includes a front joint B and a front joint A having the same structure and being symmetrical;

[0007] Step 2: Design the front joint structure and the front frame position according to the front joint position and the front joint load;

[0008] Step 3: Design the rear joint structure and rear frame position according to the rear joint position and rear joint load;

[0009] Step 4: Create an initial finite element model of the front and rear joints and the fuselage support structure based on the front joint structure and the front frame position, the rear joint structure and the rear frame position, and the load transfer paths of the front and rear joints.

[0010] Furthermore, the method further comprises:

[0011] Step 5: Use the initial finite element model to obtain the loads of the front and rear joints and the mounting bolts of the fuselage tail section structure, determine that the structural form of the front and rear frames in the support structure in the connection area is a double-angle joint, calculate the strength of the double-angle joint using engineering methods, and determine the size of the double-angle joint after repeated iterative optimization;

[0012] Step 6: Use the initial finite element model to obtain the stress distribution of the front and rear joints. According to the stress failure criterion, determine whether the front and rear joint structures meet the strength design requirements. After multiple rounds of optimization iterations, finally determine the size and material of the front and rear joints and complete the size definition.

[0013] Step 7. Consider the preload of the installation bolts, calculate the strength of the bolt bare rod and the minor diameter of the thread according to the fourth strength theory, and determine the bolt diameter and material based on the stress failure criterion and the design principle of retaining at least 20% safety margin of the connecting bolt strength after considering the joint coefficient of 1.15.

[0014] Furthermore, the method further comprises:

[0015] Step 8: Use the rocker-type tail landing gear and supporting structure obtained after the static strength design of steps 1-7 as the initial model for fatigue strength design;

[0016] Step 9: Modify the dimensions of the initial model of fatigue strength design to complete the integrated design of static strength and fatigue strength of the joint and supporting structure of the rocker arm tail landing gear.

[0017] Furthermore, according to the front joint position and the front joint load, the front joint structure and the front frame position are designed, including:

[0018] Establish a coordinate system, where the positive direction of the X axis is the direction from the midpoint between the front left joint and the front right joint to the rear joint, the positive direction of the Z axis is the vertical upward direction, and the positive direction of the Y axis is the rightward direction perpendicular to the XOZ plane according to the right-hand rule;

[0019] Determine that there are three directional loads (F x 、F y 、F z ), for any front joint, the X-direction load F of a front joint in all landing and ground conditions x There are positive and negative Z-direction loads F on the front joint. z is always positive, so according to the Z-direction load F z With X-axis load F x When the ratio of is used to determine the boundary of the front joint, two boundaries are obtained, which are named boundary 1 and boundary 2;

[0020] Design of the bottom surface of the front joint: The bottom surface of the front joint needs to fit with the tail section skin, and the bottom surface and boundary 1 and boundary 2 enclose the front joint ear area. At the same time, the bottom surface of the front joint is designed according to the skin shape; the length of the bottom surface of the front joint is determined according to the boundary 1 and boundary 2 of the front joint. The width of the bottom surface of the front joint is determined according to the preset number of rows, general row spacing, and margins of the bolts installed on the bottom surface;

[0021] Design of the number of bolts for the front joint installation: Determine the number of bolts in a single row based on the length of the bottom surface of the front joint and the general spacing of the bolts;

[0022] Since the connecting bolts on the front joint ear and the tail landing gear rocker arm need to bear the Y-direction load F y , and there is a distance between the front joint connection point a and the root of the front joint lug, which generates a bending moment on the root of the front joint lug, so the cross section of the root of the front joint lug needs to be strengthened;

[0023] The position of the front frame is determined based on the principle that the line between the connection point a and the middle surface of the front frame web is parallel to the boundary 1.

[0024] Furthermore, according to the rear joint position and the rear joint load, the rear joint structure and the rear frame position are designed, including:

[0025] The height of the rear joint is determined based on the fact that the axial direction of the rear joint single ear piece is the same as the axial compression direction of the buffer strut and the position of the rear intersection of the tail landing gear;

[0026] The position of the rear frame is determined based on the principle that the mid-surface position of the rear frame web is located on the extension line of the axial direction of the single ear piece of the rear joint.

[0027] Furthermore, the initial finite element model of the front and rear joints and the fuselage support structure was created, including:

[0028] According to the load size of the front and rear intersection points of the tail landing gear in all landing and ground conditions, select the working conditions where the front joint and rear joint are subjected to severe loads;

[0029] Shell units are used to simulate the fuselage support structure, solid units are used to simulate the front and rear joints, beam units and MPCs are created in the connection area between the front and rear joints and the fuselage support structure to simulate the bolt connection, and connecting rivet elements are created to simulate the connection between the fuselage support structures;

[0030] Constrain the translational freedom of the front end node of the tail section structure in three directions;

[0031] Create a working condition where the front and rear joints are severely loaded, and apply loads corresponding to the working condition to the front and rear intersections of the tail landing gear.

[0032] Furthermore, considering the preload of the installation bolts, the strength of the bolt polished rod and the minor diameter of the thread are calculated according to the fourth strength theory, and the bolt diameter and material are determined according to the design principle of retaining at least 20% of the strength safety margin of the connection bolts after considering the joint coefficient of 1.15, including:

[0033] The axial load of the mounting bolt without considering the preload is defined as P ext , define the shear load of the mounting bolt as Q;

[0034] According to the relationship expression between the tightening torque and the preload force, the bolt preload force P0 is calculated;

[0035] The tightening torque consists of two parts: M = M T +M P , M P It is the torque generated to overcome the friction of the thread; M T It is the torque generated by the friction force of the nut bearing surface;

[0036] The bolt connection stiffness coefficient is k, and the separation load of the mounting bolt is: P sep =P0 / (1-k); Considering the actual axial load of the bolt under the working state of the bolt preload: P b =P0+kP ext (P ext <P sep ), P b =P ext (P ext >P sep );

[0037] The bolt rod stress includes the actual axial load P of the bolt. b The normal stress σ1 generated by the bolt shear load Q, the shear stress τ1 generated by the moment M P The generated shear stress τ2, the cross-sectional area is the cross-sectional area of ​​the bolt polished rod, and the equivalent stress σ of the bolt polished rod is calculated according to the fourth strength theory eq1 ;

[0038] The thread minor diameter stress includes the actual axial load P of the bolt. b The normal stress σ2 generated by the moment M P The shear stress τ'2 generated, the cross-sectional area is the cross-sectional area of ​​the thread minor diameter, and the equivalent stress σ of the thread minor diameter is calculated according to the fourth strength theory eq2 .

[0039] Furthermore, the bolt diameter and material are determined by considering the design principle that the strength safety margin of the connecting bolts is at least 20% after the joint coefficient is 1.15, including:

[0040] The bolt diameter and material are determined based on the stress failure criterion and the design principle that the installation bolt strength safety margin is retained by retaining at least 20% after considering the joint coefficient of 1.15.

[0041] Beneficial effects:

[0042] A standardized process for the integrated design of static strength and fatigue strength of the onboard joints and supporting structures of a rocker arm landing gear is provided; the design of the onboard joints and supporting structures completed in accordance with this patent is more reasonable, avoiding paying more weight costs and making it easier for the helicopter body structure to meet the minimum weight design principle; a method for calculating the static strength of connecting bolts is provided, and the strength of the bolt bare rod and the thread minor diameter is calculated respectively after considering the bolt preload. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of the intersection where the tail landing gear connects to the fuselage.

[0044] Figure 2 It is a schematic diagram of the double-angle joint structure in the front and rear frame connection area. DETAILED DESCRIPTION

[0045] The present invention determines the load envelope and structural form of the joint according to the load at the connection point between the tail landing gear and the onboard joint under all landing and ground conditions. In order to accurately simulate the real stress state, load transfer path and diffusion form of the onboard joint of the tail landing gear in actual installation, the onboard joint and the connected fuselage structure as a whole are taken as the research object to create a finite element model. According to the load at the connection point between the tail landing gear and the onboard joint under all landing and ground conditions, the onboard joint is screened for severe load conditions. The finite element model is used to calculate and obtain the load of the connecting bolt, and the strength of the bolt bare rod and the thread diameter is calculated when the bolt preload is considered. Based on the load of the connecting bolt, the form of the fuselage support structure is determined, and its static strength evaluation is completed by applying engineering methods. After multiple rounds of calculation iterations, the structure that meets the static strength design requirements is determined, and this is used as the initial structure for fatigue strength design, and fatigue strength calculation iterations are started. Determine the onboard joint and support structure that meet the static strength and fatigue strength design requirements at the same time, so as to achieve the purpose of integrated design.

[0046] The present invention provides a design method for a rocker-arm tail landing gear on-board joint and a supporting structure, comprising:

[0047] [1] Design of onboard joints and supporting structures;

[0048] The rocker-arm tail landing gear of a certain type of aircraft is connected to the tail section structure of the fuselage through the front left joint, the front right joint and the rear joint, and the connection points are defined as a, b, and c respectively. The tail section structure of the fuselage located between the front joint and the rear joint is defined as the supporting structure, and the supporting structure includes the front frame, the rear frame, the left longitudinal beam, the right longitudinal beam, the middle angle material and the skin. The positions of the front left joint, the front right joint, the rear joint, and the front and rear frames connecting the tail landing gear to the fuselage are determined according to the overall layout of the helicopter and the intersection data. In order to ensure the continuous and effective transmission of the X-direction load of the connecting joint, the fuselage connection structure contains the left longitudinal beam, the right longitudinal beam and the middle angle material; the Y-direction load is transmitted and diffused by the tensile and compressive loads of the frame flange; the Z-direction load is borne and diffused by the longitudinal ribs of the frame. According to the load size and direction of the connection points a, b, and c between the tail landing gear and the fuselage (as shown in Table 1), the structural form of the front left joint, the front right joint, and the rear joint, and the positions of the front and rear frames are determined, as shown in Table 1. Figure 1 and Figure 2 As shown,

[0049] Table 1 Load diagram of the connection point between a tail landing gear and the aircraft

[0050]

[0051] The specific steps are as follows:

[0052] 1) Determine the positions of the front left joint, front right joint and rear joint according to the overall layout of the helicopter, the tail landing gear structure and the intersection data;

[0053] 2) Front joint structure design and front frame position design:

[0054] Front joint boundary design: There are three directional loads (F x 、F y 、F z ), where the X-direction load F of each front joint in all landing and ground conditions is x There are positive and negative loads, and Z loads F z It is always in the positive direction. Therefore, taking the left joint A as an example, according to the Z-direction load F z With X-axis load F x When the ratio of is used to determine the boundary of joint A, two boundaries will be obtained, named boundary 1 and boundary 2; the front left joint A and the front right joint B have the same structure and are arranged symmetrically along the Y axis;

[0055] Design of the bottom surface of joint A: The bottom surface of joint A needs to fit with the tail section skin, so the bottom surface of joint A is designed according to the skin shape. Determine the length of the bottom surface of joint A based on the boundary 1 and boundary 2 of joint S. Determine the width of the bottom surface of joint A based on the preset number of rows, general row spacing, and margin of the mounting bolts;

[0056] Design of the number of front joint mounting bolts: Determine the number of single-row mounting bolts based on the length of the bottom surface of the front joint A and the general spacing of the mounting bolts;

[0057] Since the connecting bolts at the connection point a of the front joint A need to bear the Y-direction load F y , and there is a distance between the connection point a of the front joint A and the root of the ear piece of the front joint A, which generates a bending moment on the root of the ear piece of the front joint A. Therefore, the cross section of the root of the ear piece of the front joint A needs to be strengthened;

[0058] The position of the front frame is determined based on the principle that the line between the center point of the connecting bolt hole and the middle surface of the front frame web is parallel to boundary 1.

[0059] 3) Rear joint structure design and rear frame position design:

[0060] Rear joint ear structure design: The rear joint C is the connection joint between the tail landing gear buffer strut and the fuselage. In all landing and ground conditions, the rear joint connection point c only has X-direction and Z-direction loads (F x 、F z ), the direction of the combined force of the two is always along the axial compression direction of the buffer strut, so the axial direction of the rear joint single ear piece is the axial compression direction of the buffer strut. According to the position of the rear intersection of the tail landing gear, the height of the rear joint is determined;

[0061] The position of the rear frame is determined based on the principle that the mid-surface position of the rear frame web is located on the extension line of the axial direction of the single ear piece of the rear joint.

[0062] [2] Create finite element models of the front and rear joints and fuselage support structure. The specific steps are as follows:

[0063] 1) Select the working conditions where the front joint and rear joint are subjected to severe loads according to the load magnitude of the front and rear intersection points of the tail landing gear in all landing and ground working conditions;

[0064] 2) Use shell units to simulate the fuselage support structure, use solid units to simulate the front and rear joints, create beam units and MPCs to simulate bolt connections in the connection areas between the front and rear joints and the fuselage support structure, and create connecting rivet elements to simulate the connection between the fuselage support structures;

[0065] 3) Constrain the translational freedom of the nodes at the front end of the tail section structure in three directions;

[0066] 4) Create the severe loading conditions of the front and rear joints selected in step 1) of [2], and apply the loads of the corresponding conditions to the front and rear intersections of the tail landing gear.

[0067] [3] Apply the finite element model created in step [2] to obtain the load of the bolts connecting the joint and the fuselage tail section structure, so as to determine that the structural form of the front and rear frames in the support structure in the connection area is a double angle joint, such as Figure 2As shown, the engineering method is applied to calculate the strength of the double-angle joint, and the size of the double-angle joint is determined after repeated iterative optimization;

[0068] [4] Apply the finite element model created in step [2] to obtain the stress distribution of the front and rear joints. Based on the stress failure criterion, determine whether the front and rear joint structures meet the strength design requirements. After multiple rounds of optimization iterations, finally determine the joint size and material, and complete the size definition.

[0069] [5] Considering the bolt preload, the strength of the bolt polished rod and the minor diameter of the thread are calculated according to the fourth strength theory. The bolt diameter and material are determined according to the stress failure criterion and the design principle that the safety margin of the connecting bolt strength is retained by at least 20% after considering the joint coefficient of 1.15. The specific process is as follows:

[0070] 1) Define the axial load of the connecting bolt as P ext , define the shear load of the connecting bolt as Q;

[0071] 2) According to the relationship expression between the tightening torque and the preload force, the bolt preload force P0 is calculated;

[0072] 3) Tightening torque consists of two parts: M = M T +M P , M P It is the torque generated to overcome the friction of the thread; M T It is the torque generated by the friction force of the nut bearing surface;

[0073] 4) The bolt connection stiffness coefficient is k, and the separation load of the connecting bolt is: P sep =P0 / (1-k); Considering the actual axial load of the bolt under the working state of the bolt preload: P b =P0+kP ext (P ext <P sep ), P b =P ext (P ext >P sep );

[0074] 5) The bolt rod stress includes the actual axial load P of the bolt. b The normal stress σ1 generated by the bolt shear load Q, the shear stress τ1 generated by the moment M P The generated shear stress τ2, the cross-sectional area is the cross-sectional area of ​​the bolt polished rod, and the equivalent stress σ of the bolt polished rod is calculated according to the fourth strength theory eq1 ;

[0075] 6) The minor diameter stress of the thread includes the actual axial load P of the bolt. b The normal stress σ2 generated by the moment M PThe shear stress τ'2 generated, the cross-sectional area is the cross-sectional area of ​​the thread minor diameter, and the equivalent stress σ of the thread minor diameter is calculated according to the fourth strength theory eq2 ;

[0076] 7) Determine the bolt diameter and material based on the stress failure criterion and the design principle that the strength safety margin of the connecting bolts is retained by retaining at least 20% after considering the joint coefficient of 1.15.

[0077] [6] The rocker-arm tail landing gear and supporting structure obtained after the static strength design are used as the initial model for fatigue strength design. According to the fatigue load spectrum, the fatigue strength of the connecting bolts, onboard joints and fuselage supporting structure is evaluated, and the unreasonable structure is modified. After multiple rounds of iterative calculations, the onboard joints and supporting structures that meet both the static strength and fatigue strength design requirements are determined;

[0078] [7] Combining the above steps, the integrated design of static strength and fatigue strength of the joint and supporting structure of the rocker arm tail landing gear is finally completed.

Claims

1. A method for designing a joint and a supporting structure of a rocker arm tail landing gear, characterized in that: include: Step 1: Determine the front joint position and the rear joint position according to the overall layout of the helicopter, the tail landing gear structure and the intersection data; the front joint includes a front joint B and a front joint A having the same structure and being symmetrical; Step 2: Design the front joint structure and the front frame position according to the front joint position and the front joint load; Step 3: Design the rear joint structure and rear frame position according to the rear joint position and rear joint load; Step 4: Create an initial finite element model of the front and rear joints and the fuselage support structure based on the front joint structure and the front frame position, the rear joint structure and the rear frame position, and the load transfer paths of the front and rear joints.

2. The method according to claim 1, characterized in that: The method further comprises: Step 5: Use the initial finite element model to obtain the loads of the front and rear joints and the mounting bolts of the fuselage tail section structure, determine that the structural form of the front and rear frames in the support structure in the connection area is a double-angle joint, calculate the strength of the double-angle joint using engineering methods, and determine the size of the double-angle joint after repeated iterative optimization; Step 6: Use the initial finite element model to obtain the stress distribution of the front and rear joints. According to the stress failure criterion, determine whether the front and rear joint structures meet the strength design requirements. After multiple rounds of optimization iterations, finally determine the size and material of the front and rear joints and complete the size definition. Step 7. Consider the preload of the installation bolts, calculate the strength of the bolt bare rod and the minor diameter of the thread according to the fourth strength theory, and determine the bolt diameter and material based on the stress failure criterion and the design principle of retaining at least 20% safety margin of the connecting bolt strength after considering the joint coefficient of 1.

15.

3. The method according to claim 2, characterized in that The method further comprises: Step 8: Use the rocker-type tail landing gear and supporting structure obtained after the static strength design of steps 1-7 as the initial model for fatigue strength design; Step 9: Modify the dimensions of the initial model of fatigue strength design to complete the integrated design of static strength and fatigue strength of the joint and supporting structure of the rocker arm tail landing gear.

4. The method according to claim 1, characterized in that: According to the front joint position and front joint load, design the front joint structure and front frame position, including: Establish a coordinate system, where the positive direction of the X axis is the direction from the midpoint between the front left joint and the front right joint to the rear joint, the positive direction of the Z axis is the vertical upward direction, and the positive direction of the Y axis is the rightward direction perpendicular to the XOZ plane according to the right-hand rule; Determine that there are three directional loads (F x 、F y 、F z ), for any front joint, the X-direction load F of a front joint in all landing and ground conditions x There are positive and negative Z-direction loads F on the front joint. z is always positive, so according to the Z-direction load F z With X-axis load F x When the ratio of is used to determine the boundary of the front joint, two boundaries are obtained, which are named boundary 1 and boundary 2; Design of the bottom surface of the front joint: The bottom surface of the front joint needs to fit with the tail section skin, and the bottom surface and boundary 1 and boundary 2 enclose the front joint ear area. At the same time, the bottom surface of the front joint is designed according to the skin shape; the length of the bottom surface of the front joint is determined according to the boundary 1 and boundary 2 of the front joint; the width of the bottom surface of the front joint is determined according to the preset number of rows, general row spacing, and margins of the bolts installed on the bottom surface; Design of the number of bolts for the front joint installation: Determine the number of bolts in a single row based on the length of the bottom surface of the front joint and the general spacing of the bolts; Since the connecting bolts on the front joint ear and the tail landing gear rocker arm need to bear the Y-direction load F y , and there is a distance between the front joint connection point a and the root of the front joint lug, which generates a bending moment on the root of the front joint lug, so the cross section of the root of the front joint lug needs to be strengthened; The position of the front frame is determined based on the principle that the line between the connection point a and the middle surface of the front frame web is parallel to the boundary 1.

5. The method according to claim 4, characterized in that According to the rear joint position and rear joint load, design the rear joint structure and rear frame position, including: The height of the rear joint is determined based on the fact that the axial direction of the rear joint single ear piece is the same as the axial compression direction of the buffer strut and the position of the rear intersection of the tail landing gear; The position of the rear frame is determined based on the principle that the mid-surface position of the rear frame web is located on the extension line of the axial direction of the single ear piece of the rear joint.

6. The method according to claim 5, characterized in that Create an initial finite element model of the forward and aft joints and fuselage support structure, including: According to the load size of the front and rear intersection points of the tail landing gear in all landing and ground conditions, select the working conditions where the front joint and rear joint are subjected to severe loads; Shell units are used to simulate the fuselage support structure, solid units are used to simulate the front and rear joints, beam units and MPCs are created in the connection area between the front and rear joints and the fuselage support structure to simulate the bolt connection, and connecting rivet elements are created to simulate the connection between the fuselage support structures; Constrain the translational freedom of the front end node of the tail section structure in three directions; Create a working condition where the front and rear joints are severely loaded, and apply loads corresponding to the working condition to the front and rear intersections of the tail landing gear.

7. The method according to claim 6, characterized in that Considering the preload of the installation bolts, the strength of the bolt polished rod and the minor diameter of the thread are calculated according to the fourth strength theory. Based on the stress failure criterion and the joint coefficient of 1.15, the bolt strength safety margin is calculated to retain at least 20% to determine the bolt diameter and material, including: The axial load of the mounting bolt without considering the preload is defined as P ext , define the shear load of the mounting bolt as Q; According to the relationship expression between the tightening torque and the preload force, the bolt preload force P0 is calculated; The tightening torque consists of two parts: M = M T +M P , M P It is the torque generated to overcome the friction of the thread; M T It is the torque generated by the friction force of the nut bearing surface; The bolt connection stiffness coefficient is k, and the separation load of the mounting bolt is: P sep =P0 / (1-k); Considering the actual axial load of the bolt under the working state of the bolt preload: P b =P0+kP ext (P ext <P sep ), P b =P ext (P ext >P sep ); The bolt rod stress includes the actual axial load P of the bolt. b The normal stress σ1 generated by the bolt shear load Q, the shear stress τ1 generated by the moment M P The generated shear stress τ2, the cross-sectional area is the cross-sectional area of ​​the bolt polished rod, and the equivalent stress σ of the bolt polished rod is calculated according to the fourth strength theory eq1 ; The thread minor diameter stress includes the actual axial load P of the bolt. b The normal stress σ2 generated by the moment M P The shear stress τ'2 generated, the cross-sectional area is the cross-sectional area of ​​the thread minor diameter, and the equivalent stress σ of the thread minor diameter is calculated according to the fourth strength theory eq2 .

8. The method according to claim 7, characterized in that After considering the joint coefficient of 1.15, the strength of the connecting bolts is calculated. The safety margin is kept at least 20%. The bolt diameter and material are determined according to the design principle, including: The bolt diameter and material are determined based on the stress failure criterion and the design principle that the installation bolt strength safety margin is retained by retaining at least 20% after considering the joint coefficient of 1.15.

Images