As a classic mechanical fastener, coach bolts are widely used in agricultural machinery, wooden structures and decoration projects. The round head diameter of this bolt is usually 1.5-2 times the thread diameter. For example, the corresponding head diameter of the M10 bolt is about 15-20mm, and the square neck section size is basically the same as the thread diameter. For example, the square neck of the M12 bolt is about 12mm×12mm. This design allows the square neck to be embedded in the prefabricated square hole of the connected part during installation, effectively preventing rotation problems during the fastening process.
The manufacturing standards of carriage bolts mostly refer to DIN 603 or ISO 8677 standards. Common specifications cover M6 to M20, and the length range is 20-200mm. In terms of material selection, Q235 carbon steel is used for ordinary working conditions, and the surface hot-dip galvanized layer thickness is 50-70μm, which can provide 5-8 years of protection in C2 corrosion environment. For special scenarios such as food processing equipment, 304 stainless steel is used, which has a nickel content of 8%-10.5%, a chromium content of 18%-20%, and a Vickers hardness of HV200-250, which can meet both hygiene requirements and have moderate strength. In recent years, aluminum alloy carriage bolts have also appeared, which are 60% lighter than steel, but the tensile strength is only 200-300MPa, which is suitable for light-load decoration.
Special attention should be paid to the preload control of carriage bolts. Taking M12 carbon steel bolts as an example, the recommended installation torque is 40-50N·m, corresponding to an axial preload of about 15-18kN. When connecting wooden structures, the pre-drilled hole diameter should be 1-2mm smaller than the major diameter of the thread. For example, M8 bolts are equipped with 6.5-7mm guide holes, which can not only ensure the thread bite strength, but also avoid wood cracking. For metal matrix connection, the square hole processing accuracy requirements are relatively high, usually wire cutting or electrospark machining is used, and the dimensional tolerance is controlled within ±0.1mm to ensure a close fit between the square neck and the hole wall.
The quality inspection process focuses on the transition area between the square neck and the head. The stress concentration coefficient of this part can reach 2.5-3.0, and magnetic particle inspection is required to ensure that there are no microcracks. In the salt spray test, the surface white rust area of the galvanized carriage bolt should be less than 5% after 96 hours of testing. With the development of modular buildings, new integrated carriage bolts with elastic washers have begun to be used. The preload attenuation rate is 40% lower than that of traditional structures, and it shows better seismic performance in prefabricated wooden structure wall connections. Although the cost of this improved product increases by 20%-25%, it can reduce the on-site adjustment time by 30%, and is gradually gaining market recognition.
Carriage bolts are a hot-selling product of Boltead Fastener, and we continue to provide reliable product solutions to customers around the world. The production line is equipped with precision cold heading machines and intelligent heat treatment equipment. Our inventory exceeds 4,000 tons, including wire, semi-finished products and spot goods, and regular specifications can be delivered within 24 hours. We support non-standard customization services and can produce special-shaped carriage bolts in the full range of sizes from M4 to M36. The surface treatment provides 12 process options such as Dacromet and hot-dip galvanizing. All products have passed ISO strength certification and SGS salt spray test.
Boltead fasteners are your first choice for economical fasteners. We operate on the philosophy of “Quality First, Customer Supreme,” committed to providing our clients with the highest quality fastener products and solutions.
