Screws are often referred to as the "rice of industry"—small in size, but essential in function. Especially in high-tech applications like aircraft and rockets, even the smallest bolt must be carefully secured. A single loose bolt can lead to catastrophic consequences. So, how do engineers ensure that bolts stay in place?
In aircraft construction, anti-loosening measures are crucial. Common methods include using fuse wires, split pins, safety screws, and self-locking nuts. For example, some smaller nuts have elliptical holes that allow them to lock into place when installed. The nut is oval before installation and is clamped by a slot at its end.
A fuse wire is a thin wire used to connect two or more fasteners together. If a component starts to loosen, the fuse prevents it from moving further. These wires are typically made of low carbon steel, stainless steel, or brass, with low carbon steel being the most common. Fuses can be used as single-strand or double-strand wires, with double-twisted knots being the most widely used, as shown in the image below.
For small groups of closely spaced screws or hard-to-reach areas, single-strand fuses are often preferred.
When multiple threaded fasteners are grouped together, interlocking fuses are more effective than individual ones. The number of fuses used depends on the specific situation.
Split pins are another common method. They are inserted through a hole in the bolt and then bent to prevent rotation. This method is simple yet highly effective.
Safety pins work similarly, providing an additional layer of security by locking the nut in place.
Safety screws are designed with special features that prevent loosening, such as a locking mechanism or a unique thread design.
Thread self-locking is also widely used. It involves using threads with a smaller pitch, which increases the friction between the screw and the nut. However, this method relies heavily on the nut’s condition. If the nut is damaged, the self-locking effect may weaken, and replacing the nut is usually the solution.
Besides these methods, there are several other ways to prevent screws from loosening during work:
1. **Proper sizing**: Choosing the right screw size is critical. A slightly oversized or undersized screw might seem fine at first, but over time, it can either loosen or become stuck.
2. **Engineering plastics**: Some screws are coated or processed with engineering plastics that create strong friction when tightened. This increases the resistance against vibration and ensures a secure connection.
3. **Increasing friction**: Methods like using spring washers or double nuts help maintain pressure on the threads, preventing them from loosening under external forces.
4. **Mechanical anti-loosening**: This involves using mechanical components like split pins, slotted nuts, retaining washers, or wire locks to prevent relative motion between the screw and nut. This method is highly reliable and commonly used in critical applications.
5. **Permanent fixation**: In some cases, the nut is fixed permanently to the bolt or the connected part using methods like spot welding or riveting. This ensures the connection remains secure for the long term.
By employing these techniques, engineers can significantly reduce the risk of bolt loosening, ensuring the safety and reliability of complex systems. Whether in aerospace, automotive, or industrial settings, proper anti-loose measures are essential to avoid failures and accidents.
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