Threaded connections may be loose under varying loads, vibrations and shocks, as well as when operating temperatures vary widely. The looseness of the connection pre-tightening force is reduced or even lost, so that the tightness of the connection cannot be ensured, and even the connection is loose or the parts in the connection are fatigued prematurely, and finally a serious accident occurs in the machine or equipment.
(1) Loosening principle
Generally, the bolts and nuts are made of ordinary threads, because under the static load, the angle of the thread is smaller than the equivalent friction angle of the thread pair. Therefore, the bolt-and-nut thread pair satisfies the self-locking condition, and the friction between the nut (or bolt) and the supported surface of the connected member or the washer after tightening, therefore, if the support surface is not too deep, the static load can be Make sure the connection is not loose.
When the nut starts to loosen, it must overcome the obstacle of the thread torque and the torque of the nut supporting surface. Under the action of the bolt pre-tightening force Qp, the total torque T' that prevents the nut from loosening is equal to the torque of the nut loosening, and T' is approximately equal to 80% of the tightening torque T. It can be considered that the bolt connection tightened by the torque T does not loosen as long as the loose torque acting on the head of the nut or the bolt is not more than 0.8T. However, under the effects of load, vibration and impact, the friction coefficient of the thread pair drops sharply, and the frictional resistance at the thread pair and the nut support surface disappears instantaneously. The thread pair can not meet the self-locking condition and has a slight sliding, which causes the nut to rotate, so that repeated times will cause the bolt connection to loosen.
In the tensioned bolt connection of the nut under pressure, when the tensile force acts, the radial component of the positive pressure on the side of the thread rib expands the nut body radially; and the bolt bolt is radially contracted, so the micro-sliding of the soy sauce between the thread pairs . Experiments have shown that the tangential sliding of the nut relative to the rotation of the bolt is easy to occur when there is such a radial relative sliding. Repeated occurrences of this phenomenon can cause loose connections.
The above is two analysis of the principle that the bolt slides from the relative amount to the rotation under the tensile load and finally causes the connection to loosen.
In addition, even if the nut does not rotate, the nut and bolt head and the supporting surface of the connected member or the gasket are plastically annularly collapsed due to excessive pressure, and this plastic deformation may continue to occur in use, resulting in the connection of the pre-preparation. The tightening force is reduced, which may also cause the connection to be loose.
In addition, in the bolt connection, the mutual contact surfaces (such as the thread side, the support surface, and the contact surface of the connected parts) are locally deformed during tightening due to roughness, corrugation, shape error, etc., and external force in use. Under the cumulative effect, some deformation will continue to develop, which will also lead to loose connections. For a specific bolt connection, its looseness may be caused by a variety of factors, and thus several loosening principles involved, including the main one. Therefore, loosening of the loosening bolt connection should take measures for various factors, and it is basically to prevent the thread pair from moving relative to the loose direction. In general, increasing the pre-tightening force is conducive to loosening. Another common way to overcome the above two loosening due to plastic deformation is to use a large washer to reduce the pressure and to tighten the nut after the connection for a period of time to eliminate loosening.
(2) Anti-loose method
According to the principle of anti-loose, it can be divided into three types: friction, direct locking and destruction of thread movement.
1. Friction and anti-loose: A positive pressure is generated between the thread pairs that does not change with the external force to generate a frictional force that can prevent the relative rotation of the thread pair. This positive pressure can be achieved by axially or laterally or simultaneously pressing the thread pair in both directions.
2, direct lock: use the stop directly limit the relative rotation of the thread pair.
3. Destroy the relationship of the movement of the thread pair: after tightening, the method of punching, welding, bonding, etc. is adopted, so that the thread pair loses the motion pair characteristics and the connection becomes a non-detachable connection.
The use of fine thread, using its small angle of lift to achieve friction and loosening or at the same time take two anti-loose methods, complement each other, etc., are good design ideas. One of the reasons for the use of fine-toothed Rottweiler is that bolts and nuts for aerospace machinery have a thread diameter of more than 10 mm and many mechanical bolts and nuts that are subjected to variable load.
Common bolting methods for locking, also for screw connections and stud connections.
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