Platform selection principle of metal fasteners for auto

To reduce the production and management costs of metal fasteners for automobiles, minimize the variety of metal fasteners, and achieve generalization and serialization, from the structural elements of fasteners (wrench form, end form, nominal length, thread), material The platformization of bolts, studs, screws, nuts, and washers is discussed in terms of mechanical properties and surface treatment, providing a reference for the platformization of fasteners by various automobile manufacturers.

With the continuous development of the automobile industry, the platform work of fasteners is being used to curb the increase in production and management costs caused by the increasing number of metal fasteners (from now on referred to as fasteners) by more and more automobiles. Manufacturers pay attention.

The article discusses the platformization of fasteners from three aspects: structural elements, materials, mechanical properties, and surface treatment.

01

Platformization of Structural Elements

Structural elements mainly include wrench form, end form, nominal length, and 4 aspects.

1.1 Platformization in the form of wrenching

At present, the wrenching structures commonly used by automobile manufacturers include:

• Hexagonal flange surface structure.
• Hexagonal head structure.
• Outer hexagonal pattern structure.
• Cross groove structure.
• Inner hexagonal pattern structure.

1.1.1 Platformization in the form of bolt wrenching

The wrenching form of bolt products is mainly external wrenching structures, including hexagonal head, hexagonal flange surface, external hexagonal, etc. Hexagon head bolts are the primary type of bolt product, and most other bolt types are developed based on hexagon head bolts.

The support area of ​​hexagonal flange bolts is more significant than that of ordinary hexagonal head bolts, which can disperse the pressure on the supporting surface and avoid the deformation of the connection with the connecting piece. Under the same friction factor requires a more enormous assembly torque than hexagon head bolts. The anti-loosening effect of the head flange is also significantly better than that of hexagon head bolts.

In automobile design, it is recommended to use hexagon flange bolts, but sometimes hexagon head bolts are also selected to improve production efficiency. For example, a particular station in the production workshop needs to install bolts of M8 and M102 specifications. Face bolts (Q184) require 2 types of installation tools.

If the M8 bolts use hexagon head bolts (Q150B) and the M10 bolts use hexagon flange face bolts (Q184), the production workshop only needs one 13 mm socket to complete the assembly of two types of bolts, saving the replacement of installation tools time.

The outer hexagonal pattern bolt requires a small assembly space, and the installation method of the hexagonal socket is not suitable. Consider using this type of bolt in an environment limited by space. Table 1 shows the selection priority of bolt-type fasteners.

1.1.2 Platformization in the form of screwing

The wrenching form of the screw is an inner wrench structure, mainly including cross groove, internal hexagon, inner six flowers, and so on. In addition, cross recessed screws are available in pan head, countersunk head, semi-countersunk head, and other types according to the shape of the head. They are selected according to the assembly space and shape factors and are generally used for body, electrical, interior, and exterior parts.

Hexagon socket head cap screws and socket head cap screws have different head shapes, including cylindrical head, low cylindrical head, pan head, countersunk head, and semi-countersunk head. They are generally used for casing parts such as engines and transmissions and in automobile molds. Also widely used. It is suitable for occasions where the wrench space is small, and an extensive support surface is preferred.

The two types of screws are recommended to use the hexagon socket wrench form, and in the same product or the same production line, it is not suitable to use these two types of wrenching documents simultaneously. Table 2 shows the priority of screw fastener selection.

1.1.3 Platforming in the form of nut wrenching

Nut products are generally used with bolts, studs, and screws to form a fastening system. Hexagon flange face nuts have the same effect as hex flange face bolts. If the structure allows, the hexagon flange face nut is preferred.

Cap nuts are used on occasions with appearance or protection requirements, such as back doors and luggage racks, which are often used.

Hex flange face locknuts include all metal locknuts and non-metal insert locknuts. The all-metal lock nut produces a locking effect by squeezing the thread to deform the line. The all-metal lock nut has good temperature resistance, but it is not suitable for frequently disassembled parts.

The non-metallic insert lock nut has good impact resistance, vibration resistance, and reusability but poor high-temperature resistance. Therefore, it is generally recommended that the temperature not exceed 100 °C.

Reed nuts are used for thin sheet metal and non-metallic plate parts and are widely used in car license plates, lamp brackets, body sheet metal parts, and other parts. Table 3 shows the selection priority of nut-type fasteners.

1.2 Platforming at the end

GB/T 2-2001 “Ends of External Threaded Parts of Fasteners” specifies 12 types of ends of fasteners. The primary function of the lots of fasteners is to facilitate fastener production, on-site assembly, and scraping. Table 4 shows the fastener end form selection priority.

1.3 Platformization of the nominal length

When selecting the length of bolts, studs, and screws, ensure that the end of the bolt or stud extends beyond the top surface of the nut by at least 2 thread pitches after matching with the nut. When screwing in blind holes, the length must not interfere with the blind spots.

When fasteners of the exact specification have the same nominal length, in principle, multiple thread lengths are not allowed. Table 5 shows the preferred sequence for nominal measurements.

1.4 Principle of thread platform

The common thread types commonly used by automobile manufacturers are common metric threads, including coarse thread and fine thread. Generally, the same thread pitch should be selected as far as possible for the duplicate nominal diameter threads in various series of vehicles developed by the same company to reduce the specifications.

Different thread precisions can be selected according to other surface treatments of fasteners. The recommended fit tolerance of the internal and external threads of the fastening system is 6H/6h. Table 7 shows the preferred sequence of thread precision.

 Note: If the bolt or nut whose surface treatment is zinc-aluminum coating cannot pass through the end of the stop gauge smoothly, it is allowed to use the matching nut or bolt for testing, and it can be screwed in smoothly.

1.5 Platforming of gaskets

In principle, gaskets are not allowed to exist alone. However, if gaskets are required for the structure of parts, it is recommended that gaskets, bolts and nuts, and screws be used in combination to improve production efficiency.

Application scope of gasket products:

• need to cover larger holes in the parts being fastened;

• It is necessary to increase the structure of the contact surface and reduce the stress;

• Need to protect the surface of the parts being connected to prevent scratches;

• Helps prevent loosening;

• Improve fatigue performance of joints;

• Compensate for uneven mounting surfaces.

02

Platformization of Fastener Materials and Mechanical Properties

2.1 Platformization of materials and mechanical properties of bolts, studs, and screws

There are 10 performance classes specified in GB/T 3098.1—2010 Mechanical Properties of Fasteners, Bolts, Screws, and Studs. However, it is not recommended to use all these 10 performance classes of fasteners in an enterprise.

Products below grade 8.8 do not require heat treatment. Generally, different performance grades are directly achieved by using other materials. Therefore, according to the default performance grade requirements in the Auto Standard Parts Manual—2012 Edition, 4.8, 5.8, 8.8, 10.9, and 12.9 are recommended.

According to the requirements of materials and performance grades in the GB/T 3098.1-2010 standard, combined with actual production, the related materials for each performance grade are recommended, as shown in Table 8.

2.2 Platform of self-tapping screw materials and mechanical properties

Self-tapping screw products do not have specific markings to indicate performance levels. GB/T 3098.5-2000 “Self-tapping screws with mechanical properties of fasteners” has made requirements on the mechanical properties of self-tapping screws.

Self-tapping screw products are made of cold heading and carburized steel by surface carburizing heat-treatment process. The recommended materials are SWRCH18A and SWRCH22A.

2.3 Platform of Nut Materials and Mechanical Properties

There are many mechanical performance coefficients of nuts specified in GB/T 3098.2-2015 “Mechanical Performance Nuts for Fasteners,” and 8 and 10 are recommended.

The selection of the performance grade of the nut mainly depends on the performance grade of the bolts, studs, and screws with which it is matched. Generally, grade 8 nuts are used in conjunction with grade 8.8 bolts, and grade 10 nuts are used in conjunction with grade 10.9 bolts. Nuts with high-performance qualities can replace the performance. Use lower-grade nuts. The recommended nut performance classes are shown in Table 9.

2.4 Flat washer material and mechanical properties platform

Flat washers are generally used with bolts or screws, and two levels of hardness, 200HV and 300HV, are recommended. Different car manufacturers can formulate the outer diameter and thickness specifications of the company’s gaskets according to actual needs. Table 10 shows a recommended table of plain washer materials and mechanical properties.

Table 10 Flat Washer Material and Mechanical Properties Recommendations

 03

Platformization of Fastener Surface Treatment

Common fastener surface treatments include oiling, phosphating, electro-galvanizing, zinc-nickel alloy, zinc-aluminum coating, oxidation, copper plating, and chrome plating. Each surface treatment is different in appearance, rust prevention, conductivity, etc. Therefore, surface treatment selection needs to be selected according to the actual working environment and installation requirements, as shown in Table 11.

 3.1 Oiling

Oiling is to apply a layer of anti-rust oil on the surface of the fastener base. Oiled fasteners require that no rust will occur within half a year under normal storage conditions.

 3.2 Phosphating

Phosphating is a fasteners’ chemical and electrochemical reaction with phosphoric acid or phosphate to form a water-insoluble phosphate chemical conversion film. Commonly used in automobiles are zinc-based phosphating and manganese-based phosphating.

Zinc-based phosphating hardness is low, and there is a phenomenon of powder falling, so it cannot be used in environments where cleanliness is required in the casing of engines, transmissions, etc. Therefore, manganese-based phosphating is generally used in interior cases such as engines and transmissions.

 3.3 Electro-galvanizing

Electro galvanizing is depositing metallic zinc onto the surface of fasteners using an electrolytic process. To reduce the chemical activity of zinc and improve the anti-rust performance and appearance of fasteners, generally galvanized fasteners need to be passivated by chromate to form a chromate conversion film on the surface of fasteners. The process is called galvanizing passivation.

The composition of chromate solution is different, and fasteners of different colors can be obtained, such as black, white, blush, and military green. They are used in car cabs.

 3.4 Zinc-nickel alloys

The zinc-nickel alloy surface treatment production process is similar to that of electro-galvanizing. A zinc-nickel alloy coating is formed on the surface of the fastener. The metal nickel content in the zinc-nickel alloy coating is 12%~15%, and its cost and corrosion resistance are higher than those of electro-galvanizing. Therefore, they are generally used in environments with high rust resistance and appearance requirements.

3.5 Zinc-aluminum coating

Zinc-aluminum coating is a thin film composed of overlapping multi-layer metal flakes and special inorganic adhesives formed by immersing the fasteners in zinc-aluminum coating solution after cleaning and shot blasting and then baking at high temperature.

The zinc-aluminum coating has good corrosion resistance and no risk of hydrogen embrittlement. It cannot be used in environments with electrical conductivity and cleanliness requirements. It is not recommended for fasteners with thread specifications smaller than M8. It is generally used in engine rooms, chassis, and other areas. The zinc-aluminum coating can be manufactured in various colors as required, and silver-gray zinc-aluminum coating and black zinc-aluminum coating are recommended.

3.6 Oxidation

Oxidation treatment is to immerse the fastener in a solid oxidant to form a thick oxide film on the surface of the fastener. It is often referred to as bluing or blackening; its corrosion resistance is lower than phosphating and electro galvanizing. As a result, it is less used in automobiles and is not recommended.

04

Summarize

The article introduces some experiences summed up in practical work and provides a reference for those engaged in this work.

The platformization of fasteners is a long-term and systematic process. It needs to be carried out as an essential task in the early stage of vehicle design, and the synchronous method of fasteners and other parts must be realized to avoid freezing due to other factors. Causes the problem of difficulty in the selection of fastener platforms.

Each car manufacturer can formulate the company’s platform-based selection principles for fasteners based on its actual situation, comprehensively considering all aspects of design, production, assembly, and after-sales.