I. Basic Definitions
Square head bolts are fasteners with a square head and an externally threaded shank, unlike hexagonal bolts. They rely on the four flat surfaces of the square head to engage with a wrench for tension, and are tightened with either a hexagonal or square nut.
In the early days of industry, square head bolts were the mainstream product; now, hexagonal bolts are suitable for automated equipment assembly, while square head bolts are preferred for heavy-duty, outdoor, and confined/dimly lit working conditions. The square head provides a larger clamping area, reducing wrench slippage, allowing for the application of higher tightening torque, and offering greater reliability for manual assembly and disassembly.
II. Adherence to International Standards
National Standards: GB/T8 (Ordinary Square Head Bolts), GB/T35 (Small Square Head Bolts)
German Standards: DIN478 (square head bolts with flange), DIN479 (square head bolts with short guide end), DIN 21346
Imperial: ASME ANSI B18.2.1, ASTM-A307, SAE J429 Grade5 / Grade8; Thread UNC, UNF specifications: 1/4″-1-1/2″.
III. Material and Strength Grade
1. Carbon steel
-Grade 4.8 (Q235 low carbon steel): General equipment, fences, supports; blackened, electro-galvanized;
-Grades 8.8 and 10.9 (45# and 35CrMo alloy steel): railway, engineering machinery, steel structure; mainstream for heavy-duty applications; Grades 10.9 and above must be baked at 190-230℃ after electroplating to remove hydrogen embrittlement.
2. Stainless steel material
-304 Stainless Steel: Suitable for freshwater environments and general outdoor use;
-316 stainless steel: contains molybdenum, used in coastal areas and chemical equipment;
IV. Product classification
1.Standard square head bolts DIN478 (with flange): Features a thickened flange below the head, providing a larger clamping area and preventing damage to casting surfaces; commonly used in mining equipment and cast iron frames.
2.DIN479 Dog-Point Guide Head Bolt: A short guide section at the front end of the bolt, which facilitates alignment with the through hole during assembly; commonly used in piping equipment and flange assemblies.
3.Full-thread square head bolts / Half-thread square head bolts: M6-M72, complete specifications, metric coarse and fine threads available; length 10-500mm.
4.Non-standard enlarged square head: The head is widened on both sides, making it more impact-resistant and earthquake-resistant. Customized for wind power supports and mining equipment.
V. Surface Finish
1.Electro‑plated Zinc: 72‑96‑hour salt‑spray resistance, ideal for general‑purpose indoor applications.
2.Hot‑Dip Galvanizing (HDG): Coating thickness ranges from 45‑85μm with 500‑800‑hour salt‑spray rating. It is widely adopted for outdoor steel structures and railway components.
3.Dacromet / Ruspert Coating: Zero risk of hydrogen embrittlement. It achieves 720‑1000‑hour salt‑spray performance and becomes a top‑choice 4.finish for European export orders.
5.Black Oxide: Low‑cost option only for dry indoor working conditions.
6.PTFE Coating: Applied for high‑temperature service to prevent thread galling and seizure.
6. Scenario application
1.Railway Transit (Core Application) Hot‑dipped galvanized square‑head bolts in Grade 8.8‑10.9 from M20‑M30 are mass‑ordered for rail clips, track‑joint components and locomotive chassis parts.
2.Construction Machinery & Mining Equipment They fit excavator frames, crushers and farm machinery. Square‑head design avoids wrench slipping under harsh outdoor site conditions.
3.Steel Structures, Guardrails and Heavy‑Duty Supports Commonly used on PV brackets, pipeline supports and bridge attachments.
4.Petro‑Chemical Equipment ASTM A193‑B7 alloy‑steel square‑head bolts fasten pipe flanges, while A4‑70 (316 stainless‑steel) versions serve chemical storage tanks.
5.Replacement Parts for Export Legacy‑Equipment These bolts act as spare components for old‑style European and American machinery maintenance, where original square‑head bolts were factory‑fitted.