In mechanical manufacturing, steel structures, automotive and wind power industries, fasteners may be small but they directly determine connection reliability and service life. A complete fastener assembly typically covers five categories: bolts, nuts, screws, washers and pins. Procurement professionals need to systematically select components based on load conditions, environmental corrosion levels and assembly processes. This guide outlines selection criteria and assembly logic across all fastener categories.
Bolts are the primary load-bearing elements in a fastening system. Common types include hex head bolts (GB/T 5783, DIN 933), socket head cap screws (GB/T 70.1, DIN 912) and studs (GB/T 901). For material selection, standard applications use Grade 4.8 carbon steel (Q235), medium-load applications require Grade 8.8 medium carbon steel (35K/45 steel), while high-load vibrating environments demand Grade 10.9 or 12.9 alloy steel (SCM435, 40Cr). Nuts must match bolt grades: Grade 8.8 bolts pair with Grade 8 nuts (GB/T 6170), Grade 10.9 with Grade 10 nuts, and Grade 12.9 with Grade 12 nuts, with thread tolerances typically at 6H/6g.
Screws are commonly used for thin-sheet connections or precision assembly, including self-tapping screws (GB/T 845, DIN 7981), machine screws (GB/T 818) and set screws (GB/T 70.3). Washers are divided into flat washers (GB/T 97.1, DIN 125) and spring lock washers (GB/T 93, DIN 127). Flat washers increase bearing area and protect connected surfaces, while spring washers provide anti-loosening torque. For high-strength connections, high-hardness flat washers (HV300, GB/T 97.2) are recommended to prevent washer crushing and preload loss.
Surface treatment directly affects corrosion resistance and assembly torque. Standard environments use electro-galvanizing (blue-white or color zinc, 48-96 hours salt spray), while outdoor or marine environments recommend hot-dip galvanizing (500+ hours salt spray) or Dacromet coating (hydrogen-embrittlement-free). Stainless steel fasteners (A2-70, A4-80) suit highly corrosive scenarios but carry galling risk — anti-seize compound is advised. For procurement, ordering bolt-nut-washer kits with unified standards and surface treatments ensures batch consistency and full traceability.