When the clamping force caused by thread tightening is attenuated enough to resist the external load, we consider the threaded connection to be loose.
Loosening of threaded connections can be divided into two categories. One is rotational loosening, that is, the loosening between the inner and outer threads that occurs when the relative rotation of the loosening direction occurs; The other type is non-rotational loosening, that is, loosening that occurs when the internal and external threads do not rotate relative to each other.
Today, Screw Jun will take you to learn more about the mechanism and countermeasures of these two loosenings.
1. Non-rotating loosening
1.The connection surface is rough
As shown in the figure below, the contact surface between the nut and the fastener, the contact between the two fasteners, and the contact surface between the bolt and the fastener are not completely smooth, and after being subjected to alternating load, the non-smooth surface will become smooth because of friction, which will cause the clamping thickness of the connected piece to become thinner, as shown in the right figure of the following figure, the reduction of the thickness of the clamped part corresponds to the attenuation of the axial force.
Countermeasures: reduce the roughness of each contact surface as much as possible to avoid gaps between fasteners; Tightening the bolt again after a period of use restores the axial force of the bolt.
2.There is a soft connection between the connected parts
As shown in the figure below, due to the requirement that the connector has a sealing function, a gasket will be added to the connector, and the gasket will rebound after tightening, resulting in a large attenuation of axial force. This type of structure is collectively referred to as soft connection, and the axial force and torque attenuation of the soft connection are large, so it should be avoided for important locations.
Countermeasure: Optimize the structure and avoid the use of soft connections. If it must be used, after the connector springs back, tighten the bolt again to reduce the attenuation ratio of axial force and torque.
3.Collapsed by connectors
The fasteners used for tightening are usually heat-treated, the yield strength is generally above 640 MPa, and the hardness is also high, while the yield strength of the connected parts is generally lower than the strength of the fastener, when the tightening torque is high, the connected surface may be plastically deformed, and then collapse, as shown in the figure below. Because the clamping length of the connected part is reduced, the axial force and torque are also attenuated.
Countermeasures: Increase the contact area between the screw and the fastener by using washers or flange bolts; Increase the strength and hardness of the connected parts.
4.Thermal expansion
When the coefficient of thermal expansion of the bolt and the connected part is different, the axial force of the bolt also changes when the temperature of the coupling system changes after the bolt is tightened. Depending on the difference between the connector and the fastener, the axial force may or may not be larger.
Solution: Bolts and fasteners can be made of materials with the same coefficient of thermal expansion.
Second, the rotation is loose
Rotational loosening is another form of axial force attenuation, which is mainly manifested in the relative rotation between the inner and outer threads in the loosening direction, and we can confirm whether there is rotational loosening by marking the color code.
Below, Screw Jun analyzes the rotation loosening and coping strategies according to the different stress modes of the connecting pairs.
1.BearTorsionLoad
When the center of the rotating shaft is the center of the bolt, and the rotating shaft rotates in the direction of the bolt disassembly, the stress form of the bolt and the nut is torsional load. If the bolt moves clockwise and counterclockwise alternately, the threaded connection pair is also prone to rotational loosening. Rotation loosening is more sensitive to torsional load, and if there is no corresponding anti-loosening measures, the bolt is prone to rotation loosening;
Countermeasures: Slotted nuts, bolts with pin holes and stop washers can be used to prevent the bolts and nuts from loosening.
2.Withstand shear loads
If the connected part does not slide under the action of shear load, it means that the contact surface pressure and displacement of the bolt, nut and connected part have not changed, and in this state, the fastener will not be rotated and loosened.
When there is a slight slippage between the connected parts, this will cause the axis of the bolt to tilt, and there will be a change in pressure between the thread surface and the support surface, but this will not produce the force that causes the bolt to rotate.
When the relative movement between the thread surfaces occurs, because the thread surface is a spiral-shaped inclined plane, the resistance of sliding on the upper side is high, and the resistance of sliding on the downward side is low, and the thread is prone to sliding in the direction of loosening of the thread on the downward side. Rotational loosening is very sensitive to shear load, and rotational loosening is easy to occur under shear load, so anti-loosening measures must be added.
Countermeasures: use double nuts to prevent loosening, use coated chemical glue to prevent loosening, and use self-locking nuts to prevent loosening.
3.Withstands axial loads
Rotational loosening is not sensitive to axial loads, and rotational loosening rarely occurs in connection structures that are only subject to axial loads, so anti-loosening measures are usually not necessary. In this kind of stressed structure, the axial force is insufficient, and the connected parts are separated. In alternating axial loads, fasteners are prone to fatigue problems.
Countermeasures: In order to solve the problem of separation of connected parts and fastener fatigue caused by insufficient axial force, the number of threaded connections can be increased, the torque of threaded tightening can be increased, the size of bolts can be increased, and higher grade bolts can be used to increase the axial force of threaded connections.
Third, the experience of Screws Jun
01. There are two forms of axial force attenuation of threaded connections: one is non-rotating loosening (loosening occurs when the internal and external threads do not rotate relatively), and the causes of non-rotating loosening include rough contact surface, soft connection between connected parts, collapse of connected parts, thermal expansion, etc.
The other is rotational loosening (loosening between the inner and outer threads that occurs when the relative rotation occurs in the loosening direction), and the load forms that lead to rotational loosening include torsional load, shear load, and axial load. Among them, the rotational loosening is the most sensitive to the shear load, followed by the torsional load, which is basically not affected by the axial load.
02. Measures to deal with non-rotating loosening and rotational loosening, on the one hand, can start from the material and structure of the connecting pair itself (such as reducing the surface roughness of the connector, avoiding soft connection, increasing the contact surface area of the connector, etc.).
On the other hand, you can start from the anti-loosening method (such as the use of double bolts, gluing, self-locking nuts, slotted nuts, etc.).
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