Loosening of bolts is common, but if not noticed, it will often cause equipment vibration, component damage, and even casualties. How to tighten a small nut has always been a perennial topic in mechanical design. Today, let's talk about the most basic method of fixing nuts at work.
Generally, we analyze bolt fracture from the following four aspects:
1. the quality of bolts
2.bolt pre-tightening torque
3. bolt strength
4. fatigue strength of bolts
In fact, most of the bolt breaks are due to looseness, which is broken due to looseness. Because the loosening and breaking of bolts are basically the same as the fatigue fracture. Finally, we can always find the reason from the fatigue strength. In fact, we can't imagine that the fatigue strength of bolts can not be used in the process of use.
The bolt fracture is not due to the tensile strength of the bolt
With one M20 × For example, Grade 8.8 high-strength bolt of 80 has a weight of only 0.2kg, and its minimum tensile load is 20 tons, up to 100000 times its own weight. Generally, we only use it to fasten parts of 20kg, and only use one thousandth of its maximum capacity. Even the effect of other forces in the equipment cannot exceed a thousand times the weight of the parts, so the tensile strength of threaded fasteners is sufficient, and it is not possible to be damaged due to insufficient strength of bolts.
The fracture of the bolt is not due to the fatigue strength of the bolt
Threaded fasteners can be loosened only one hundred times in the transverse vibration loosening test, and one million times in the fatigue strength test. In other words, threaded fasteners become loose when they use 1/10000 of their fatigue strength. We only use 1/10000 of their large capacity, so the loosening of threaded fasteners is not due to the fatigue strength of bolts.
The real cause of damage of threaded fasteners is looseness
After loosening of threaded fasteners, huge kinetic energy mv2 is generated. This huge kinetic energy directly acts on the fasteners and equipment, resulting in damage to the fasteners. After the fasteners are damaged, the equipment cannot work under normal conditions, further leading to equipment damage.
For fasteners subject to axial force, the thread is damaged and the bolt is pulled off.
For fasteners subject to radial force, the bolts are cut and the bolt holes are made into ellipses.
The key to solve the problem is to select the thread locking method with excellent locking effect
Take the hydraulic hammer as an example. The weight of GT80 hydraulic hammer is 1.663 tons, its side plate bolts are 7 sets of 10.9 M42 bolts, and the tensile resistance of each bolt is 110 tons. The preload is calculated as half of the tensile resistance, and the preload is up to 300 or 400 tons. But the bolts will also break. Now we are going to replace them with M48 bolts. The root cause is that the bolt locking cannot be solved
For bolt fracture, it is easy to draw the conclusion that the strength is not enough, so the method of increasing the strength grade of bolt diameter is mostly adopted. This method can increase the pretightening force of bolts, and its friction force can also be increased. Of course, the anti-loosening effect can also be improved. However, this method is actually a non-professional method, with too much investment and too little profit.
In short, the bolt is: "If it is not loose, it will break."
Cause analysis of bolt looseness
Threaded connection is designed according to self-locking conditions: ψ ≤ρ v. The friction pair produced in the threaded pair makes the bolt self-lock and thus fasten the bolt, so the connection will not loosen automatically under static load. However, under impact, vibration, variable load and large temperature changes, the friction force F of the screw pair will decrease or disappear instantaneously. If this phenomenon occurs repeatedly, the connecting bolts will gradually loosen. After the threaded fastener is loosened, the kinetic energy mv2 is generated. For the fastener subject to axial force, the thread is damaged and the bolt is pulled off. For fasteners subject to radial force, bolts are cut and bolt holes are damaged.
Bolt locking principle: limit the relative movement between threaded pairs, or increase the difficulty of relative movement.
Introduction to common anti-loosening methods
There are three commonly used methods of locking bolts: friction locking, mechanical locking and permanent locking. Among them, mechanical locking and friction locking are called removable locking, while permanent locking is called non-removable locking.
Friction lock
1. Spring washer anti-loose
The anti-loosening principle of the spring washer is that after the spring washer is pressed flat, the spring washer will produce a continuous elastic force, so that the thread connection pair of the nut and the bolt will maintain a constant friction force, producing a resistance torque, thus preventing the nut from loosening. At the same time, the sharp corners at the opening of the spring washer are embedded into the bolt and the surface of the connected part respectively, so as to prevent the bolt from rotating relative to the connected part.
2. Counter nut (double nut) anti-loose
3. Self-locking nut locking
One end of the nut is made into a non-circular closure or a radial closure after opening. When the nut is tightened, the necking will expand, and the elastic force of the necking will be used to compress the screwing threads.
4. Elastic ring nut locking
Insert fiber or nylon into the thread to increase friction. The elastic ring also plays a role in preventing liquid leakage.
Mechanical locking
1. Slotted nut and cotter pin locking
2. Stop gasket
After the nut is tightened, bend the single-lug or double-lug lock washer to the side of the nut and the connected part respectively to realize anti-loosening.
3. Series steel wire anti-loosening
Use mild steel wire to penetrate into the hole of each screw head, and connect the screws in series to brake each other.
Permanent locking
Commonly used permanent anti-loosening measures include spot welding, riveting, bonding, etc. This method will mostly destroy threaded fasteners during disassembly and cannot be reused.
In addition, there are other ways to prevent loosening, such as applying liquid adhesive between the screw-in threads, inlaying nylon rings at the end of nuts, riveting and punching to prevent loosening, etc. Mechanical locking and friction locking are called removable locking, while permanent locking is called non-removable locking.
1. Edge punching method to prevent loosening
After the nut is tightened, punch the end of the thread to destroy the thread.
2. Adhesive locking - nut locking fluid
Apply the nut locking liquid to the bolt tightening position, and then screw on the nut. After self-curing, the locking effect is good.
