Views: 286 Author: ANEBON Publish Time: 2024-12-10 Origin: Site
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● Why do bolts become tighter as they are tightened?
● Analysis of bolt loosening causes
● Three common anti-loosening methods
Bolt loosening is a common issue that can lead to equipment vibration, component damage, and even injuries. The technique for securely tightening a small nut has always been a significant topic in mechanical design.
While we are familiar with various types of nuts, such as Japanese eccentric nuts, Tang nuts, and Chinese self-locking nuts, we won’t focus on these prominent fasteners today. Instead, let's discuss the causes of bolt loosening and three effective methods for preventing it.
Generally speaking, we analyze bolt fracture from the following four aspects:
The quality of the bolt
The pre-tightening torque of the bolt
The strength of the bolt
The fatigue strength of the bolt
Most bolt fractures occur due to looseness, which leads to damage. The conditions that cause bolts to loosen and break are similar to those that result in fatigue fractures. Ultimately, we can trace most issues back to the fatigue strength of the material. In fact, the fatigue strength of bolts is often underestimated; during use, this strength is typically not fully utilized.
1. Bolt breakage is not due to the tensile strength of the bolt
Consider an M20×80 8.8-grade high-strength bolt. This bolt weighs only 0.2 kg, yet it has a minimum tensile load capacity of 20 tons, which is 100,000 times its own weight. Typically, we use this bolt to fasten components that weigh around 20 kg, utilizing only one-thousandth of its maximum capacity. Even when accounting for other forces that may act on the equipment, it is unlikely that they would exceed a thousand times the weight of the CNC machining component. Therefore, the tensile strength of the threaded fastener is more than adequate, and it is improbable that it would fail due to insufficient bolt strength.
2. The fracture of the bolt is not due to the fatigue strength of the bolt.
Threaded fasteners can be loosened only a hundred times in a transverse vibration test, but they need to be vibrated repeatedly a million times in a fatigue strength test. In other words, the threaded fastener becomes loose when it uses ten-thousandth of its fatigue strength. We only use one ten-thousandth of its maximum capacity, so the loosening of the threaded fastener is not due to the fatigue strength of the bolt.
3. The real cause of damage to threaded fasteners is looseness
After the threaded fasteners are loosened, a huge kinetic energy mv2 is generated. This huge kinetic energy directly acts on the fasteners and Equipment, causing damage to the fasteners. After the fasteners are damaged, the Equipment cannot work under normal conditions, further causing the Equipment to be damaged.
When fasteners are subjected to axial force, the threads are damaged, and the bolts are pulled apart.
When fasteners are subjected to radial force, the bolts are sheared, and the bolt holes are made into ovals.
4. Choosing a thread anti-loosening method with excellent anti-loosening effect is the fundamental solution to the problem
Let's consider the hydraulic hammer as an example. The GT80 hydraulic hammer weighs 1.663 tons and features seven sets of 10.9-grade M42 side plate bolts. Each bolt has a tensile strength of 110 tons, and the pre-tightening force is typically calculated as half of the tensile strength, resulting in a pre-tightening force ranging from three to four hundred tons. However, bolts can still break. To address this issue, we are switching to M48 bolts because the problem of bolt anti-loosening remains unresolved.
When a bolt breaks, the most common assumption is that the strength is insufficient. Consequently, many people opt to increase the diameter and strength grade of the bolts. This approach can enhance the pre-tightening force and increase friction, thus improving the anti-loosening effect. However, this method is often considered non-professional due to its high investment costs, and the benefits may not justify the expense.
In short, the bolt is: "It will not break if it is not loose, and it will break if it is loose."
The threaded connection is designed based on the self-locking condition: ψ ≤ ρv. The friction generated between the threaded surfaces allows the bolt to remain self-locking and keeps it tightened, preventing the connection from loosening under static loads. However, under conditions such as impact, vibration, fluctuating loads, or significant temperature changes, the friction force (F) in the threaded pair may decrease or vanish suddenly.
If this situation occurs repeatedly, the connecting bolt may gradually loosen. When a threaded fastener becomes loose, it generates kinetic energy (mv⊃2;). For fasteners subjected to axial forces, this can lead to thread damage and the bolt being pulled out. In contrast, for fasteners under radial forces, the bolt may shear, causing damage to the bolt hole.
The principle behind the anti-loosening of bolts is to either limit the relative movement between the threaded pairs or to make it more difficult for that movement to occur.
There are three common methods for preventing bolt loosening: friction anti-loosening, mechanical anti-loosening, and permanent anti-loosening. Both mechanical and friction anti-loosening methods are removable, while permanent anti-loosening methods are considered non-removable.
1. Friction anti-loosening
(1) Spring washer anti-loosening
The principle behind preventing a spring washer from loosening is that once the spring washer is flattened, it generates a continuous elastic force. This force ensures that the threaded connection between the nut and the bolt maintains friction, creating resistance torque that helps keep the nut secure. Additionally, the sharp edges of the spring washer's opening embed themselves into the surfaces of both the bolt and the connected die casting aluminumpart, preventing any relative rotation between the bolt and the connected component.
(2) Anti-loosening of the top nut (double nut)
(3) Self-locking nut to prevent loosening
One end of the nut is made into a non-circular closing or a radial closing after slit. When the nut is tightened, the closing expands, and the elastic force of the closing is used to tighten the screw threads.
(4) Elastic ring nut anti-loosening
Fiber or nylon is embedded in the threaded part to increase friction. The elastic ring also prevents liquid leakage.
2. Mechanical anti-loosening
(1) Anti-loosening of slotted nuts and cotter pins
(2) Stop washers
After tightening the nut, bend the single or double ear stop washers against the sides of the nut and the connected part to prevent loosening.
(3) Series wire anti-loosening
Use low-carbon steel wire to penetrate the holes of each screw head, connect the screws in series, and brake each other.
3. Permanent anti-loosening
Common permanent anti-loosening methods include spot welding, riveting, and bonding. These methods typically damage the threaded fasteners during disassembly, making them unsuitable for reuse.
There are also other anti-loosening techniques, such as applying liquid adhesive between the screw threads, using nylon rings at the end of the nut, and employing riveting and punching to prevent loosening. Methods that allow for disassembly, such as mechanical and friction anti-loosening techniques, are referred to as removable anti-loosening methods. In contrast, permanent anti-loosening methods are known as non-removable anti-loosening methods.
(1) Punching method to prevent loosening
After tightening the nut, punch the end of the thread to break the thread.
(2) Adhesion to prevent loosening - nut anti-loosening liquid
Apply the nut anti-loosening liquid to the tightening part of the bolt, then screw on the nut. After self-curing, the anti-loosening effect is good.
Finally, let's take a look at the bolt manufacturing process.
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