Anti-loosening measures for threaded connections selected by fasteners

The threaded connection may become loose under the action of variable load, vibration and shock, and when the working temperature changes greatly. The pre-tightening force of the connection is reduced or even lost due to the looseness, so that the tightness of the connection cannot be guaranteed, and even the connection is loosened or the parts in the connection are prematurely fatigued, and finally cause serious accidents such as machinery or equipment.

(1) Loosening principle

Generally, bolts and nuts use ordinary threads, because under static load, the thread lift angle is smaller than the equivalent friction angle of the thread pair. Therefore, the bolt-nut thread pair meets the self-locking condition, and the friction between the nut (or bolt) and the supporting surface of the connected part or washer after tightening is added. Therefore, if the supporting surface is not compressed too deeply, it can be under static load. Ensure that the connection is not loose.

When the nut starts to loosen, it is necessary to 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 preventing the loosening of the nut makes the torque of the nut loosening equal to this, and T is approximately equal to 80% of the tightening torque T. It can be considered that for a bolt connection tightened with a torque T, as long as the loosening torque acting on the nut or bolt head is not greater than 0.8T, the connection will not loosen. However, under the action of variable load, vibration and impact, the friction coefficient of the thread pair drops sharply, and the friction resistance between the thread pair and the nut supporting surface will disappear instantly. The thread pair cannot meet the self-locking condition and has a small amount of slip, which causes the nut to rotate. Such repeated repetitions will cause the bolt connection to loosen.

In the tension bolt connection where the nut is compressed, when the tension acts, the nut matrix expands radially due to the radial component of the positive pressure on the thread flanks; while the bolt rod shrinks radially, so the thread pair The soy sauce slips slightly. Experiments have proved that when there is such a radial relative sliding, the tangential sliding of the nut relative to the bolt rotation is easy to produce. Repeated occurrence of this phenomenon will cause the connection to loosen.

The above are two analyses on the principle of the bolts from sliding relative to rotation to rotating under the action of tensile load, and finally leading to the loosening of the connection.

In addition, even if the nut does not rotate, the bearing surface of the nut and bolt head and the connected part or washer will produce plastic ring compression due to excessive pressure, and this plastic deformation may continue to occur during use, resulting in The reduction of the pre-tightening force of the connection may also cause the connection to loosen.

There are also the mutual contact surfaces in the bolt connection (such as the thread flanks, the supporting surfaces, the mutual contact surfaces of the connected parts) due to roughness, corrugation, shape errors, etc., which are locally deformed during tightening, and are in use Due to the accumulation of external forces in the middle, a part of the deformation will continue to develop, which will also cause the connection to loosen. For a specific bolt connection, its loosening may be caused by a variety of factors, so there are several loosening principles involved, of which there are primary and secondary. Therefore, measures should be taken to prevent the loosening of the bolt connection from various factors, and it is basically to prevent the relative movement of the thread pair in the loosening direction. In general, increasing the preload is beneficial to prevent loosening. Another example is to overcome the above-mentioned two common methods of loosening due to plastic deformation by using large washers to reduce the pressure and re-tightening the nut after the connection has been working for a period of time to eliminate the loosening.