Knowledge combing and interpretation of automobile nuts

Knowledge combing and interpretation of automobile nuts

The nut is also called a nut, which is screwed together with a bolt or screw for fastening.
There are many kinds of nuts, and there are also nuts of different materials. Each type of nut has different specifications, each specification of the thread has its own mechanical properties and role. Under complicated conditions, the selection of suitable nuts needs to follow certain rules to meet the design requirements.

Today, based on the mechanical performance standards of fasteners GB/T3098.2, GB/T3098.4 and GB/T3098.4, the types of automotive nuts and related selection rules are combed.

01 Classification standards and types of automobile nuts

Nut shape, all hexagonal cylindrical:

a. Tooth pitch is different: coarse common thread and fine common thread;

B. Different heights: thin nuts, type 1 nuts, type 2 nuts and thick nuts;

c. Flange face: hexagon flange nut, hexagon flange self-chip nut, hexagon flange bearing face with toothed nut;

d. Different locking forms:

Type 1 all-metal lock nut, Type 1 non-metallic insert lock nut;

Type 2 all-metal lock nut, type 2 non-metallic insert lock nut;

All metal flange face lock nuts, non-metal insert flange face lock nuts.

02 How to correctly represent a nut

xx is the identification mark of the manufacturer.

The performance grade code "10" represents 1/100 of the minimum guaranteed stress Sp that the nut can bear, I .e. the guaranteed stress Sp = 1000MPa.

This number "10" is the first part of the performance level mark of the 10.9 grade bolt. In the bolt mark, the first number is the nominal tensile strength σb of the bolt.

This bolt shall be of the highest performance rating of any bolt that matches this nut. That is, a nut with a high performance level can replace a nut with a lower performance level.

It is feasible that the stress of the bolt-nut assembly is higher than the yield strength or guaranteed stress of the bolt.

 

03 Basic principles of nut and bolt coordination

a. Sufficient internal thread strength of the nut, so that the bolt tightening failure in the form of fracture rather than sliding.

B. the strength of nuts and bolts should be reasonable and meet the standard recommendation.

C. good thread fit accuracy, ensure the thread accuracy after electrophoresis or coating.

D. do not easily change the ISO and GB recommended pitch, nut chamfer size, nut height, side width and other dimensions.

e. The friction coefficient of the connection pair should be controlled within the design range, such as m = 0.10~0.16, otherwise all calculations and checks will lose their foundation.
Due to over-tightening, the failure forms of bolt and nut combination are screw fracture and thread tripping. The fracture of the screw is sudden and easy to find; while the tripping occurs gradually, it is difficult to find and increase the risk of accidents caused by fastener failure.

Therefore, for the design of threaded connections, it is always hoped that the failure mode is screw fracture. However, because the material strength of nuts and bolts, thread clearance and the impact on the width of the trip strength occurs, it can not be guaranteed in all cases to obtain this failure form.

According to the thickness and locking form of the nut, and in accordance with the provisions in Table II of GB/T3098.2, GB/T3098.4 and GB/T3098.4, the nut with appropriate performance shall be selected for the bolt. When the bolt is tightened to guarantee the load, the bolt assembly will not be tripping. This is guaranteed by the marking system of the nut.

However, the tightening exceeding the guaranteed load of the bolt will occur due to the use of the yield tightening method of the corner method. Therefore, the design of the nut should ensure at least 10% of the over-tightening (the guaranteed load is about 90% of the yield load), and the failure of the thread combination is the bolt breaking, so as to warn the user that the assembly operation is improper.

The nut and bolt coordination must follow the basic principles.

 

04

The connection and difference between type 1 hexagon nut and type 2 hexagon nut

 

The different nut heights are to increase the thread trip strength of the bolt-nut assembly.

There are two types of nuts, type 2 is about 10% higher than type 1.

Type 1 height is mainly suitable for three situations:

a. Nut of performance class 4, size M³ 16;

B. Nuts of performance classes 5, 6, 8, 10, size M £ 39;

c. Performance class 12, size M16 nuts.

Type 2 height is mainly also suitable for three situations:

a. Performance Class 8, 16 £ M £ 39 nuts;

B. Nut of performance class 9, size M £ 16;

c. Performance Class 12, 16 £ M £ 39 nut.

Type 2 nuts mainly provide an economical cold-formed nut for grade 9.8 bolts and screws, and also provide a good toughness heat-treated nut for grade 12.9 bolts and screws.

There are only two cases of overlap between Type 1 and Type 2 nuts.

For Type 1: Grade 8 less than or equal to M16, non-quenching and tempering (cold forming, low carbon) are allowed;

Grade 8 nuts larger than M16 must be quenched and tempered. At this time, it is possible to use a thicker type 2 nut instead of a type 1 nut without quenching and tempering, which is an economic problem for comprehensive analysis. For Class 12: Type 1 nuts are not suitable for use with sizes larger than M16. The hardness of the nut needs to be increased due to the requirement of the guaranteed load, with the result that toughness is impaired. From a practical point of view, toughness is needed. Therefore, in this case, it is necessary to use a thicker, quenched and tempered type 2 nut. If necessary and possible, limit the use of nuts larger than M16, and do not overlap Type 1 and Type 2 for Grade 12 nuts.

In the actual automobile development, in order to control the size of the fastener library, usually only 8 and 10 grade nuts are used, and at the same time, only 10 grade nuts with specifications greater than or equal to M12 are used. It can be seen that type 1 nuts are generally used.

Type 1 full metal hex lock nut Q334

 

05 All metal lock nuts and non-metal insert lock nuts

A. all-metal lock nut end made of non-circular closing or radial closing after slotting (the latter is also called groove beam type lock nut), so that the thread local deformation. When the nut is tightened, the closing part of the nut is opened outward, and the elastic force of the closing is used to compress the screw thread, and the local deformation of the thread is used to increase the friction force, eliminate the bolt gap, and make the bolt and the nut firmly shrink together.

The advantages of all-metal lock nuts are simple structure, simple manufacturing process, good temperature resistance, and controllable volume and quality. The disadvantage is that it is not suitable for the connection parts that need to be disassembled frequently, and it is easy to scrape off the coating of the bolt, causing corrosion.

B. non-metallic insert lock nut is a nylon block or a nylon ring whose inner diameter is slightly smaller than the middle diameter of the thread in the threaded hole. When the bolt is screwed in, the inner thread is squeezed out of the nylon ring. The nylon material with excellent elasticity forms a large and stable friction force with the bolt, which prevents the fastener from loosening and achieves a reliable locking. It can also play a role in preventing liquid leakage.

The advantages of non-metallic insert lock nuts are impact resistance, vibration resistance, and repeated use of the all-metal type, but the temperature resistance is poor, and avoid use in high temperature areas.

06 Bearing face with toothed nut

The bearing surface of the toothed nut is serrated on the bearing surface of the nut. When the nut is tightened, friction is generated between the supporting surface and the connection, and when the serrations are embedded in the surface of the connection, the locking is very firm. A stable torque-tension relationship and a sufficiently high clamping force are prerequisites for maintaining the tightening capacity of the toothed nut.

Generally speaking, the hardness of the connected piece should be lower than the hardness of the fastener to ensure the embedding of the teeth. Serrations are prone to damage to connected surfaces.

Therefore, toothed nuts cannot be used in threaded connections that cannot withstand high clamping forces, nor can they be used in applications where the surface of the connected part is sensitive to scratches and corrosion.