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Why to say: the bolt does not loosen unceasingly, a looseness breaks?

The threaded fastener has the advantages of compact structure, convenient disassembly, large connection force and repeated use. It has great advantages in the use of the equipment, which ensures the reliability and stability of the mechanical system. Threaded fasteners have become one of the standardized components, which are widely used in aircraft, high-speed rail trains, engineering vehicles, ships and other fields of heavy or special mechanical equipment. They work under static ambient temperature. The traditional thread connection self-locking can meet the requirements without considering the anti loose structure design.

When the structure is impacted or vibrated, the preload of the threaded connection decreases gradually, especially under the action of the transverse ring dynamic load, the preload is more obvious, and finally leads to the screw nut falling off. In practical application, this kind of failure sometimes brings very serious disastrous consequences. With the rapid development of the industrial era, solving the problem of screw connection structure loosening has become one of the important problems in the work. It is very important to design more safe and reliable nuts. The commonly used anti loosing methods are spring washer, stop washer, nylon insert screw or self tapping screw glue, and their technical properties are different. This paper summarizes the problems existing in the design of the lock nut, and emphatically analyzes and introduces the structural design of a new type of lock nut.

20201111090321 49627 - Why to say: the bolt does not loosen unceasingly, a looseness breaks?

What is loose bolt?

Bolt looseness refers to the reduction of axial preload after nut tightening. The common looseness may be caused by repeated relative sliding wear or plastic deformation between bolt and connector. The pre tightening force of bolts plays a decisive role in all connections. The reduction of bolt preload will directly lead to the loss of connector function. There are two ways to produce looseness: rotating loosening and non rotating loosening. Both cases are caused by the relative rotation between the inner and outer screws. Bolt looseness will cause a series of problems, such as separation, falling off and sliding of connectors, which will lead to displacement and collision of connectors. With the increase of vibration amplitude and decrease of stiffness, fatigue damage of bolts and coupling may occur. It can be seen that the prevention of bolt loosening plays an important role in the connection of parts.

Causes of bolt fracture

Generally, we analyze bolt fracture from the following four aspects:

  1. The quality of bolts;
  2. The pre tightening torque of bolts;
  3. The strength of bolt;
  4. The fatigue strength of bolts.

In fact, most of the broken bolts are broken because of looseness. Because the situation of bolt loosening and breaking is basically the same as that of fatigue fracture. Finally, we can always find the reason from the fatigue strength. In fact, the fatigue strength is too high for us to imagine, and the bolt can not use the fatigue strength in the process of use.

The bolt fracture is not due to the tensile strength of the bolt

Taking an M20 × 80 grade 8.8 high strength bolt as an example, its weight is only 0.2kg, and its minimum tensile load is 20t, which is up to 100000 times of its own weight. Under normal circumstances, we only use it to fasten 20kg parts, and only use one thousandth of its maximum capacity. Even if it is the effect of other forces in the equipment, it is impossible to exceed a thousand times the weight of the parts. Therefore, the tensile strength of the threaded fasteners is sufficient, and it is impossible to damage the threaded fasteners because of the insufficient strength of the bolts.
20201111090618 90206 - Why to say: the bolt does not loosen unceasingly, a looseness breaks?

The fracture of the bolt is not due to the fatigue strength of the bolt

In the transverse vibration loosening test, the threaded fastener can be loosened only 100 times, while in the fatigue strength test, it needs repeated vibration for one million times. In other words, the threaded fastener is loose when it uses one tenth of its fatigue strength. We only use one tenth of its large capacity. Therefore, the looseness of the threaded fastener is not due to the fatigue strength of the bolt.

The real cause of damage to threaded fasteners is looseness

After the screw fastener is loose, huge kinetic energy MV2 will be produced. This huge kinetic energy directly acts on the fasteners and equipment, resulting in the fastener damage. After the fastener is damaged, the equipment can not work in the normal state, which further leads to the equipment damage.
For fasteners under axial force, the thread is damaged and the bolt is broken.
For fasteners subjected to radial force, the bolt is sheared and the bolt hole is made into ellipse.

The key to solve the problem is to select the thread anti loosing method with excellent anti loosing 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 grade M42 bolts. The tensile strength of each bolt is 110 tons. 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. The reason why the M48 bolt can’t be broken is that it can’t be replaced by a bolt.
The most easily concluded conclusion of bolt fracture is that the strength is not enough, so the method of increasing the bolt diameter strength grade is mostly adopted. This method can increase the pre tightening force of bolts, and the friction force is also increased. Of course, the anti loosing effect can be improved. However, this method is actually a non professional method, and its investment is too large and the income is too small.
In short, the bolt is: “do not loose, a loose will break.”

Analysis of bolt looseness

The thread connection is designed according to the self-locking condition: ψ ≤ ρ V, the friction pair generated in the thread pair makes the bolt self-locking, thus fastening the bolt, so the connection will not loose automatically under static load. However, the friction force F of the screw pair will decrease or disappear instantaneously under shock, vibration, variable load and large temperature change. If the bolt looses gradually, the connection will be repeated. When the threaded fastener is loose, the kinetic energy MV2 is generated. When the fastener is subjected to axial force, the thread is damaged and the bolt is broken. For the fastener subjected to radial force, the bolt is sheared and the bolt hole is damaged.

Bolt locking principle: limit the relative movement between screw pairs or increase the difficulty of relative movement.

Introduction of common anti loosing methods

There are three kinds of commonly used methods to prevent bolts from loosing: friction, mechanical and permanent. Mechanical anti loose and friction anti loose is detachable anti loose, and permanent anti loose is called non detachable anti loose.

Three ways to prevent nut from loosing

There are three commonly used methods to prevent nut from loosing: friction locking, mechanical loosening preventing and permanent loosening preventing.

Friction lock

Usage: counter nut, spring washer, self-locking nut, etc.
Spring gasket locking
The material of spring washer is spring steel. After assembly, the washer is flattened, and its reaction force can keep the compression force and friction force between threads, so as to prevent loosing
② Anti loosening of counter nut
The bolt type is subjected to additional tensile force and friction force by the action of nut against top. Due to the use of more than one nut, and the work is not very reliable, has been and less used.
③ Self locking nut locking
One end of the nut is made into non-circular closing or radial closing after slotting. When the nut is tightened, the necking is expanded, and the elastic force of the necking is used to compress the screw threads. The anti loose structure is simple and reliable, and can be disassembled and disassembled for many times without reducing the anti loosing performance.
④ Elastic ring nut locking
Insert fiber or nylon into the thread to increase friction. The elastic ring can also prevent liquid leakage.

Mechanical anti loosening

Usage: split pin and hexagon slotted nut, stop washer, series wire, etc.
Mechanical anti loose method is more reliable, for important connection to use mechanical anti loose method.
① Locking of slotted nut and cotter pin
After the slotted nut is tightened, the split pin is used to pass through the small hole at the tail of the bolt and the groove of the nut, or the ordinary nut can be used to drill the pin hole after tightening.
② Round nut and stop washer
Insert the inner tongue of the washer into the groove of the bolt (shaft), and fold one of the outer tongue of the washer into a groove of the nut after tightening the nut.
③ Stop gasket
After the nut is tightened, the single ear or double ear stop washer is bent to the side of the nut and the connected part respectively to achieve anti loosening. If two bolts need double interlocking, double locking gasket can be used.
④ Anti loosing of series steel wire
A low carbon steel wire is inserted into the hole of each screw head, and the screws are connected in series to brake each other. This kind of structure needs to pay attention to the direction of wire penetration.

Permanent locking

Usage: spot welding, riveting, bonding, etc.
This method is usually used to destroy threaded fasteners during disassembly, which can not be reused.
In addition, there are other ways to prevent loosing, such as coating liquid adhesive between screw threads, inserting nylon ring at the end of nut, riveting and punching to prevent loosing, etc. mechanical anti loosening and friction anti loosing are called detachable anti loosing, while permanent anti loosing is called non detachable anti loosing.
① Edge punching method to prevent loosing
After the nut is tightened, the thread is damaged by punching at the end of the thread
② Adhesion and anti loosing
Generally, anaerobic adhesive is applied to the screw thread surface. After tightening the nut, the adhesive can self solidify and has good anti loosing effect.

Source: China Bolts Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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