Calculation of bolt preload
Pre tightening torque calculation of screw connection
MT = k × P0 × d × 10-3kgf. M
k: tightening force coefficient D: nominal diameter of thread
P0: preload (also refer to the table below) P0= σ 0 × As
As= π× DS / 4 ds: calculated diameter of dangerous section of thread part
DS = (D2 + D3) / 2 D3 = d1-h / 6 h: nominal working height of thread tooth
DS = (D2 + D3) / 2 D3 = d1-h / 6 h σ 0=0.5～0.7 σ s σ S -- yield limit of bolt material kgf / mm (related to strength grade, determined by material)
extended data:
the size of preload is limited not only by the strength of screw material, but also by the material strength of connected parts. When the material of internal and external thread is the same, only check the strength of external thread
for the screw connection with short screw length, non-standard thread parts and large difference in strength between internal and external thread materials under axial load, the strength of thread teeth should also be checked. For example, for the fixation of elastic elements of a certain type of proct, because the base material of screw connection is die-casting aluminum alloy yl113, and its strength is far lower than that of high-quality carbon structural steel 20, the strength of thread profile on aluminum alloy should be checked, mainly the shear stress and bending stress of thread material
Pre tightening torque MT = k × P0 × d × 0.001 n.mk: tightening force coefficient D: nominal diameter of thread P0: pretightening force P0= σ 0 × As can also be found from the table below= π× ds × DS / 4 ds: calculated diameter of dangerous section of thread DS = (D2 + D3) / 2D3 = d1-h / 6 h: nominal working height of thread σ 0 =0.5～0.7 σ s σ S -- yield limit of bolt material n / mm2 (related to strength grade, determined by material) k value look up table: (k value calculation formula is omitted) k value friction surface condition with or without lubrication finishing surface 0.100.12 general machining surface 0.13 ~ 0.150.18 ~ 0.21 surface oxidation 0.200.24 galvanized 0.180.22 dry rough machining surface 0.26 ~ 0.3 σ S: thread performance grade 3.64.64.85.65.86.88.89.810.912.9 σ S or σ 0.2n/mm2 180 240 300 320 400 480 640 720 900 1080 as: nominal diameter of thread D / mm 3.5 4 5 6 7 8 10 12 14 16 18 20 22 24 27 30 33 36 nominal stress sectional area as / mm2 5.03 6.78 8.78 14.2 20.1 28.9 36.6 58 84.3 115 157 192 245 303 353 459 561 694 817 bolt preload and gasket tightness in flange connection In this paper, the calculation method of bolt force and preload in pressure pipeline flange connection is analyzed, and the sealing performance of gasket is studied, including basic sealing characteristics, pressure rebound characteristics, thickness and width effect of gasket. It is concluded that the leakage of flange connection point is related to bolt pre tightening force, sealing surface state, working condition and gasket
Bolt pre tightening force is the pre tightening force along the axis of bolt generated between bolt and connected parts under the action of tightening torque in the process of tightening bolt. For a specific bolt, the pre tightening force is related to the tightening torque of the bolt, the friction between the bolt and the nut, and the friction between the nut and the connected part
In addition to the strength of screw material, the pre tightening force is also limited by the material strength of the connected part. When the material of internal and external thread is the same, only check the strength of external thread. The strength of thread teeth should also be checked for the thread connection with short screw length, non-standard thread parts and large strength difference between internal and external thread materials under axial loadfor the fixation of elastic elements of a certain type of proct, because the base material of screw connection is die-casting aluminum alloy yl113, and its strength is far lower than that of high-quality carbon structural steel 20, the strength of thread profile on aluminum alloy should be checked, mainly the shear stress and bending stress of thread material
extended data
method of controlling screw preload
method 1: the preload is controlled by tightening torque
the relationship between tightening force and bolt preload is linear. If the tightening torque is controlled, the preload value can be obtained by experiment or theoretical calculation. But in practice, because of the influence of friction coefficient and geometric parameter deviation
under a certain tightening torque, the preload changes greatly, so the accuracy of controlling bolt preload by tightening torque is not high, and the error is about 0 ± Up to 25% ± Generally speaking, the tools with higher tightening torque accuracy in control area are torque measuring wrench and force limiting wrench
method 2: control the pretightening force through the nut angle
calculate the nut angle according to the required pretightening force, and measure the nut angle when tightening to achieve the purpose of controlling the pretightening force. The simple way to measure the nut angle is to carve a zero line and measure the nut angle according to the number of Lu Mu's turns. The measurement accuracy of the nut angle can be controlled within 10 °- fifteen ° Inside
method 3: control the preload through the bolt elongation
because the bolt elongation is only related to the bolt stress, the influence of variable factors such as friction coefficient, contact deformation and connected part deformation can be eliminated. Therefore, high precision can be obtained by controlling the preload through bolt elongation. This method is widely used in important occasions
method 4: control the pre tightening force through the hydraulic stretcher
use the hydraulic stretcher to apply the tension force to the bolt to make the bolt extend, and then screw the nut to be unloaded. Because the bolt shrinks, the pre tightening force equal to the tension force can be generated in the connection. This method can improve the control precision of preload
There is no friction when the hydraulic stretcher applies the preload to the bolt, so this method is suitable for any size bolt, and can apply the preload to a group of bolts at the same time to compress the nut and gasket evenly, so as not to affect the precise control of the preload e to inclinationmethod 5: use the torque angle to control the pretightening force
use the relationship between tightening torque and angle to control the pretightening force, which is to apply a certain torque to the bolt, then make the nut turn a certain angle, and check whether the torque and angle meet the e relationship, so as to avoid insufficient or excessive pretightening
The torque angle method to control the pretightening force is to control the tightening process with the tightening torque until the tightening torque reaches enough value to ensure that the nut, bolt and connected parts are really tight, then the nut angle can be measured Then the tightening process is controlled by the nut angle and tightening torque at the same time. By using the information given by tightening torque and nut angle, this method can accurately control the pre tightening force of bolts, and find out the possible under tightening or over tightening phenomenon in the installation processcorrection: it's a hexagon socket head screw, not a bolt. Fasteners that cannot be tightened with an open-end wrench cannot be called bolts.
common is the force of screwing with a wrench.
calculation method
pre tightening torque MT = k × P0 × d × 0.001 N.m
k: tightening force coefficient D: nominal diameter of thread
P0: preload
P0= σ 0 × As can also be found in the table below= π× ds × DS / 4 ds: calculated diameter of dangerous section of thread
DS = (D2 + D3) / 2
D3 = d1-h / 6 h: nominal working height of thread
DS = (D2 + D3) / 2 σ 0 =0.5～0.7 σ s
σ S -- yield limit of bolt material n / mm2 (related to strength grade, determined by material)
k value look up the table: (k value calculation formula is omitted)
pre tightening can improve the reliability, anti loosening ability and fatigue strength of bolt connection, and enhance the tightness and rigidity of connection. In fact, a large number of tests and use experience have proved that higher preload is beneficial to the reliability of the connection and the life of the connected, especially for the connection with sealing requirements. Of course, as the saying goes, "things go against the extreme." if the preload is too high, improper control or accidental overload will often lead to the failure of the connection. Therefore, it is very important to accurately determine the bolt preload.
Bolt pre tightening force of M8:
5.6 bolt: 12 ~ 15N. M; Grade 8.8 bolt: 19 ~ 24N. M; Grade 10.9 bolt: 22 ~ 29n. M
pre tightening can improve the reliability, anti loosening ability and fatigue strength of the bolt connection, and enhance the tightness and rigidity of the connection. In fact, a large number of tests and use experience have proved that higher preload is beneficial to the reliability of the connection and the life of the connected, especially for the connection with sealing requirements
At present, there are mainly two ways to measure the stress with resistance strain gauge, which are force measuring bolt and ring washer. The force measuring bolt is a sensor that directly replaces the existing bolt and measures the bolt preload. It can accurately measure the bolt preload, which can be accurate to kilogram. Especially, it is more suitable for the detection of bolt preload before air tightness test of large pressure vessel The calculation method of bolt stress and preload in flange connection of pressure pipeline is analyzed, and the sealing performance of gasket is studied, including basic sealing characteristics, pressure rebound characteristics, thickness and width effect of gasket. It is concluded that the leakage of flange connection point is related to bolt pre tightening force, sealing surface state, working condition and gasket