High strength bolts or structural bolts are intended for use with heavy hex nuts to connect structural members. For a connection to be considered structural, it must conform to specific ASTM standards, particularly ASTM A563 or ASTM A194.
Grades of Structural Bolts Structural bolts are categorized by various grades. The grading system takes into account the bolt’s material, size range, proof load, minimum yield strength, and minimum tensile strength.
Understanding ASTM A325 The American Society for Testing and Materials (ASTM) sets standards for various tools and industries. While these standards are voluntary, most fabricators adhere to them to build trust with their customers. In addition to ASTM A563 and A194, the A325 standard applies to heavy hex structural bolts, defining mechanical properties for bolts ranging from 0.5 to 1.5 inches in diameter. The metric equivalent is A325M, which covers bolts in sizes M12-36. In 2016, ASTM replaced the A325 designation with ASTM F3125, but the bolt head markings remain the same to avoid confusion.
Bolts vs. High Strength Bolts Bolts are generally made from steel, with the strength directly related to the materials used. Depending on the intended use, different grades of steel are used, each with varying tensile strengths. Higher tensile strength means tighter bolt joints and greater torque capacity. The Society of Automotive Engineers (SAE) assigns bolts seven grades, from two to eight, with additional grading systems for higher-strength structural bolts.
Grade 9 Bolts Grade 9 structural bolts, or grade 9 hex cap screws, are among the strongest structural bolts available. While grade 8 bolts have a tensile strength of 150,000 PSI, grade 9 bolts have a tensile strength of 180,000 PSI. These bolts are ideal for heavy-duty applications and are often used in heavy machinery like bulldozers. They are stronger and more resistant to corrosive elements than grade 8 bolts, thanks to yellow zinc chromate.
Grade 8 Bolts Grade 8 bolts are the highest grade SAE bolts, with a minimum tensile strength of 150,000 PSI. ASTM A325 bolts, roughly equivalent to SAE grade 5, are typically used for structural bolting projects. A grade 8 bolt meets ASTM A490 standards.
A325 Bolt Materials A325 bolts are made from three primary materials:
- Type 1: Medium carbon steel, alloy steel, or boron steel
- Type 2: Low carbon martensitic steel (removed from the standard in 1991)
- Type 3: Weathering steel
ASTM A325 vs. SAE Grade 5 Bolts Strength is crucial for structural bolting. ASTM A325 and SAE grade 5 bolts have similar properties and strengths, but differ in classification and recommended applications. Engineers recommend ASTM A325 bolts for structural steel connections in heavy construction, while SAE grade 5 bolts are common in OEM applications.
Key Differences:
- Grade 5 bolts typically have a finished hex bolt design; A325 bolts must have a heavy hex head.
- Nuts for grade 5 bolts are finished hex pattern; A325 bolts need heavy hex nuts like A192-2H or A563-DH.
- Grade 5 bolts range from 0.25 to 1.5 inches in diameter; A325 bolts cover 0.5 to 1.5 inches. For sizes over 1.5 inches, ASTM A449 is used.
- Thread lengths vary: Grade 5 fasteners have a standard thread length twice the bolt’s diameter or 0.25 inches for bolts ≤ 6 inches. For bolts > 6 inches, the length is twice the diameter plus 0.5 inches. A325 bolts have specific, generally shorter lengths.
Bolting Methods Using the correct high strength bolts is part of structural bolting; proper methods ensure correct fastening. Two standard methods are:
Turn of Nut Method This method involves turning the nut or bolt a specific angle based on the fastener’s length and diameter, ensuring the unturned element doesn’t rotate.
- Snug the joint to firm contact.
- Apply impacts with an impact wrench or full effort with a spud wrench.
- Inspect to verify “snug tight.”
- Match mark the nut and bolt end with a straight line.
- Apply the required turns systematically.
Calibrated Wrench Method Apply a calculated torque value based on daily testing of representative fastener samples, considering the exact configuration of power supplies and tools under similar conditions.
Custom High Strength Bolts When searching for the right bolts for structural applications, understanding tensile strength, material requirements, and other factors is crucial. Custom bolt manufacturers can help source or produce components tailored to project needs, from material selection to threading and bolt shapes.
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