Bolt ball joints shall consist of high-strength bolts, steel balls, screws (or pins), sleeves and cones (or seals) (Figure 1). Suitable for grids, double-layer net shells, space trusses, flat truss structures, etc.
1. Stress characteristics and construction requirements of bolt ball joints
The bolt ball joints are different depending on the force of the rod (pull or pressed), and the force transmission path and the function of the parts are also different.
When the rod is pulled, its force transmission route is: pulling force → steel pipe → cone head or sealing plate → bolt → steel ball; at this time, the sleeve is not subjected to force.
When the rod is pressed, its force transmission route is: pressure → steel pipe → cone head or sealing plate → sleeve → steel ball; at this time, the bolt is not subjected to force.
The material of the steel ball, sealing plate, cone head and sleeve used to manufacture the bolt ball joints shall be adopted in accordance with the provisions of (Table 1) and shall comply with the technical conditions of the corresponding standard. Product quality should comply with the current industry standard "Steel Grid Bolt Ball Joints".
2. Calculation of bolt ball nodes
1) Steel ball diameter
The diameter of the steel ball should be calculated according to the requirements of the adjacent bolts that do not touch the ball body and meet the contact surface of the sleeve (Fig. 2), and are calculated according to the following formulas, and the larger ones are selected according to the calculation results:
When the angle θ of adjacent bars is small, the diameter of the bolt ball should be calculated according to the requirements of adjacent bars and related sealing plates, cones, sleeves and other components that do not touch each other. At this time, the calculation can be performed by checking the condition that the sum of the angle between the touch point and the center of the ball and the axis of the adjacent member is not greater than.
2) High strength bolt design
The performance level of high-strength bolts should be selected according to the thread specifications. For the M12-M36 high-strength bolts, the strength rating is 10.9; for the M39-M64 high-strength bolts, the strength rating is 9.8s. The form and size of the bolts shall comply with the requirements of the current national standard "High-strength bolts for steel mesh bolts and ball joints".
The diameter of the high-strength bolt should be controlled by the internal force of the rod. The design value of the tensile load capacity of each high-strength bolt shall be calculated as follows:
The diameter of the connecting bolt of the pressure bar can be calculated according to the absolute value of the internal force of the design, and then the bolt diameter is reduced by 1~3 steps according to the bolt diameter series of Table 56, but it must be ensured that any cross section of the sleeve has sufficient compression resistance. strength.
3) sleeve
The sleeve is a hexagonal, non-ribbed nut. The outer dimensions should conform to the wrench opening series and the end requirements should be flat. In order to transmit the axial pressure of the rod. The inner diameter can be 1mm larger than the bolt diameter.
For the sleeve with the chute, the distance from the end of the sleeve to the end of the chute should be checked, so that the shear resistance of the effective section is not lower than the shear resistance of the pin and not less than 1.5 times the width of the chute.
The end of the structural member shall be a tapered head (Fig. 4a) or a sealing plate connection (Fig. 4b). The joints of the welded joint and the cone shall be stronger than the connected steel pipe. The width b of the bottom of the weld may be based on the wall of the welded steel pipe. Thickness is 2-5mm. The thickness of the sealing plate shall be determined according to the actual force, and shall not be less than 1/5 of the outer diameter of the steel pipe. The thickness of the base of the cone should not be smaller than the thickness of the cone
1/4 of the bottom inner diameter. The thickness of the sealing plate and the bottom of the cone can be used according to (Table 4).
4) Pin or screw
High-strength steel should be used for the pin or screw. The diameter of the bolt can be 0.16-0.18 times the diameter of the bolt and should not be less than 3mm. The screw diameter can be M5~M10.
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