How to Prevent Screws from Loosening?

1. Reason for Connection Loosening of Threaded Fasteners

The essence of a threaded fastener connection is to hold the connected parts together through axial force. When the bolts are tightened, the axial force attenuation is called threaded connection loosening. The main reasons for its loosening are as follows.

1.1 Design Deficiency

Bolt improper selection in the bolt tightening process, connecting threaded fasteners between the clamping force and rapid increase with the increase of pre-tightening force, began to occur plastic deformation when the yield point is reached, the clamping force as the bolt pre-tightening force increase with increasing small constant, even when adding loosen bolts pre-tightening force its clamping force until fracture gradually smaller. Therefore, designers need to accurately analyze and calculate the tightening torque, and fully consider the weight of locking nuts and bolts, bearing load, safety standards, and other factors, to choose the right bolt connection.

Failure to consider the poor anti-loosening design or anti-loosening method of the bolt connection may cause the connection to loosen or even the bolt to fall off due to vibration, high and low load changes, impact, and other reasons during the operation. Therefore, effective anti-loosening measures must be implemented in how to prevent screws from loosening design to avoid loosened bolts or nuts falling off due to bad working conditions.

Threaded Fasteners

1.2 Insufficient Preload

The pre-tightening force of bolt tightening directly determines the clamping force between two connecting parts. The insufficient pre-tightening force will inevitably lead to the loosening of connecting bolts and the loosening of connecting parts. The pre-tightening force of the bolt shall be close to or equal to the yield strength of the bolt material. However, in the actual assembly process, due to the limited arm strength of the operator or the mismatch of the selected tool types, the output tightening torque is insufficient, and the bolts cannot meet the required pre-tightening force.

The design and process do not put forward specific requirements on the torque of tightening bolts, and there is no corresponding torque wrench at the assembly station. Operators often judge whether to tighten bolts by feeling and experience when tightening, resulting in an insufficient pre-tightening force of locking nuts and bolts at some parts with large vibration and loosening phenomena.

1.3 The Supporting Surface is Deformed and Loosened

When the supporting surface of threaded fasteners is under a lot of pressure, the supporting surface of threaded fasteners and the contact surface of the connected parts will collapse and deform, leading to the decrease or even loss of the pre-tightening force of threaded fasteners, and thus the connection loosening phenomenon occurs.

Threaded Fasteners

1.4 Improper Assembly Process

For the tightening of multiple threaded fasteners with regular distribution, the assembly process did not formulate reasonable tightening process documents, and the operators did the tightening operation entirely according to their personal experience. As a result, some bolts were loosened with inconsistent tightness due to the incorrect tightening sequence and uneven force. For example, the installation bolts with a square distribution that are common in the assembly process, the diagonal cross-tightening method is generally adopted to ensure the bolt’s balanced force as far as possible, otherwise, the bolt will be loosened, or even the connected parts will be deformed due to the uneven force.

1.5 Machining Quality Defect

The dimension precision of a screw hole or bolt hole is particularly important when locking nuts and bolts are connected. The size of the screw thread directly affects the pre-tightening force obtained by bolts. If the size of the bolt mounting hole is too small, it will be difficult to assemble. If the size is too large, the contact place between the surface of the part and the supporting surface of the bolt or nut will produce compression deformation, which will lead to the loosening of the bolt or nut.

Threaded Fasteners

Threaded Fasteners Connection is Commonly Used to Prevent Loose Structure

According to the working principle, there are mainly three types, the first is friction anti-loose; The second is mechanical loosening; The third is permanent containment.

Two identical nuts are tightened to the top so that the bolts are pulled in the screw section and the nuts are compressed so that the double nuts are generally used for longitudinal pressing of the threaded connection pair with stable load or low speed and heavy load. The anti-loosening effect is general, and the pressure is compressed after the gasket assembly at a low cost. The rebound force and notch insertion keep the pressing force and friction between the threads.

Spring Washer

Rub force and achieve the anti-loosening effect, the anti-loosening effect is general, mostly used for non-key processes to connect the variable tooth type anti-loosening nut. There is a bevel at the bottom of the internal thread. When the bolts and nuts are tightened, the tines of the bolts are tightly placed on the level of the thread, so as to produce a large locking force. The anti-loosening effect is very good.

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