It is common for bolts to loosen, but if you don't pay attention, it will often cause equipment vibration, component damage, and even casualties. How to tighten a small nut has always been a long-lasting topic in mechanical design. Let's talk about the most basic method of fixing nuts at work.
No.01
Why do bolts get tighter and tighter?
Generally speaking, we analyze bolt breakage from the following four aspects:
First, the quality of the bolt
Second, the preload torque of the bolt
Third, the strength of the bolt
Fourth, the fatigue strength of the bolt
In fact, most bolts break because of looseness, and they are broken because of looseness. Because the situation of bolt loosening and breaking is roughly the same as the situation of fatigue fracture, in the end, we can always find the reason from the fatigue strength. In fact, the fatigue strength is so great that we can't imagine that the fatigue strength of the bolt is not used at all during use.
01
Bolt fracture is not due to the tensile strength of the bolt
Take an M20×80 8.8 grade high-strength bolt as an example. Its weight is only 0.2 kg, and its minimum tensile load is 20 tons, which is as high as 100,000 times its own weight. Under normal circumstances, we will only use it to tighten 20 kg parts, and only use one thousandth of its maximum capacity. Even if other forces in the equipment are applied, it is impossible to exceed a thousand times the weight of the parts. Therefore, the tensile strength of the threaded fastener is sufficient and it is impossible to be damaged due to insufficient bolt strength.
02
Bolt fracture is not due to the fatigue strength of the bolt
Threaded fasteners only need one hundred times to loosen in the transverse vibration loosening test, while they need to be repeatedly vibrated one million times in the fatigue strength test. In other words, the threaded fastener loosened when using one ten-thousandth of its fatigue strength. We only used one ten-thousandth of its maximum capacity, so the loosening of the threaded fastener is not due to the fatigue strength of the bolt.
03
The real reason for the damage of threaded fasteners is looseness
When threaded fasteners loosen, 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 in a normal state, further causing damage to the equipment.
For fasteners subjected to axial force, the threads are damaged and the bolts are pulled off.
For fasteners subjected to radial force, the bolts are sheared off and the bolt holes are punched into ellipses.
04
The selection of thread anti-loosening methods with excellent anti-loosening effect is the fundamental solution to the problem
Take the hydraulic hammer as an example. The weight of the GT80 hydraulic hammer is 1.663 tons, and its side plate bolts are 7 sets of 10.9-grade M42 bolts. The tensile force of each bolt is 110 tons, and the pre-tightening force is calculated as half of the tensile force, and the pre-tightening force is as high as three or four hundred tons. However, the bolts will break as well. Now we are going to change to M48 bolts. The fundamental reason is that the bolt anti-loosening cannot be solved.
When a bolt breaks, the easiest conclusion people draw is that the strength is not enough, so most people use the method of increasing the bolt diameter and strength grade. This method can increase the preload force of the bolt, and its friction force is also increased. Of course, the anti-loosening effect can also be improved, but this method is actually a non-professional method, and its investment is too large, and the benefits are too small.
In short, the bolt is: "It will not break if it is not loose, and it will break if it is loose."
No.02
Analysis of the cause of bolt loosening
The threaded connection is designed according to the self-locking condition: ψ ≤ρv. The friction pair generated in the threaded pair makes the bolt self-locking and tightens the bolt, so the connection will not loosen by itself under static load. However, under impact, vibration, variable load, and large temperature changes, the friction force F of the spiral pair will decrease or disappear instantly. If this phenomenon occurs repeatedly, the connecting bolt will gradually loosen. After the threaded fastener is loosened, the kinetic energy mv2 generated, the fastener under the action of axial force, the thread is damaged, and the bolt is pulled off. For fasteners under radial force, the bolt is sheared and the bolt hole is damaged.
Principle of bolt anti-loosening: limit the relative movement between thread pairs, or increase the difficulty of relative movement.
Introduction to common anti-loosening methods
There are three common anti-loosening methods for bolts: friction anti-loosening, mechanical anti-loosening and permanent anti-loosening. Among them, mechanical anti-loosening and friction anti-loosening are called removable anti-loosening, while permanent anti-loosening is called non-removable anti-loosening.
1 Friction anti-loosening
1. Spring washer anti-loosening
The anti-loosening principle of spring washers is that after the spring washers are flattened, the spring washers will generate a continuous elastic force, so that the threaded connection pair of the nut and the bolt will continue to maintain a friction force, generating a resistance torque, thereby preventing the nut from loosening. At the same time, the sharp corners at the opening of the spring washer are embedded in the surface of the bolt and the connected part respectively, thereby preventing the bolt from rotating relative to the connected part.
2. Anti-loosening of top nuts (double nuts)
3. Anti-loosening of self-locking nuts
One end of the nut is made into a non-circular closing or radial closing after slit. When the nut is tightened, the closing expands, and the elastic force of the closing is used to tighten the screwing 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. Slotted nut and cotter pin anti-loosening
2. Stop gasket
After the nut is tightened, bend the single-ear or double-ear stop gasket to the side of the nut and the connected part respectively 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.
No.3 Permanent anti-loosening
Common permanent anti-loosening methods include: spot welding, riveting, bonding, etc. This method mostly destroys the threaded fasteners during disassembly and cannot be reused.
Spot welding, riveting, bonding, etc.
This method mostly destroys the threaded fasteners during disassembly and cannot be reused. In addition, there are other anti-loosening methods, such as: applying liquid adhesive between the screw threads, embedding nylon rings at the end of the nut, and riveting and punching to prevent loosening. Mechanical anti-loosening and friction anti-loosening are called removable anti-loosening, while permanent anti-loosening is called non-removable anti-loosening.
1. Anti-loosening by punching edge method
After tightening the nut, punch the end of the thread to destroy the thread.
2. Adhesive anti-loosening - nut anti-loosening liquid, apply the nut anti-loosening liquid to the tightening part of the bolt, then screw on the nut, and after self-curing, the anti-loosening effect is good.