High Strength Bolt Commonly Used in Railroad, Cranes and Bridges---ASTM A325 Bolts

The use of high strength bolt to connect large steel structure parts has become an important trend in fastening field. The main high strength bolts are ASTM A325 bolts, ASTM A193, A307 bolt, A490 bolts, etc. High strength bolts are made of high-strength steel and require a large pretension. High strength bolt has good mechanical and seismic properties, and resistance to fatigue. Its connection stiffness is high, construction is simple, more used for bridge, railroad, high pressure and ultra high pressure equipment connection. Due to the A325 bolt strength is high, it is especially suitable for heavy machinery structures with variable and dynamic loads. Therefore, it is widely used in the metal structure of the crane.

ASTM A325 Bolts

The most representative of high strength bolts are ASTM A325 bolts. The ASTM (ASTM International) is an International organization that sets International standards for a wide range of materials, products, systems, and services. ASTM A325 is its international standard for heavy hex head bolt.

Bolts conforming to ASTM A325 standard are conventionally called ASTM A325 Bolts. The A325 bolts have 120/105 ksi minimum tensile strength. The diameters of ASTM A325 bolts generally range from 1/2 to 1½ inches (13-38 mm). The A325 bolt lengths and other allowable sizes information are below.

The Difference between ASTM A325 Bolts and Ordinary Bolts

• Texture of material
  Ordinary bolts: Made of Q235(A3) steel
  ASTM A325 bolts: Made of high strength materials. The A325 nut, washer, bolt are made of high quality materials such as 35# or 45# steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, and other high-quality materials, and their strength is improved by heat treatment.

• Strength
  Ordinary bolts: The intensity level is low, generally 4.4, 4.8, 5.6 and 8.8.
  ASTM A325 bolts: Commonly used 8.8s and 10.9s strength grades.

• Stress characteristics
  Ordinary bolts: The connection depends on the shear resistance of the bolt and the pressure of the hole wall to transmit the shear force. When the nut is tightened, the pressure is very small, and its effect is negligible.
  ASTM A325 bolts: The A325 bolt applies pre-tension and transmits external force by friction. In addition to its high material strength, a large amount of preload is also applied to the bolts, which generates squeezing force between the connecting members, thereby causing a large friction force perpendicular to the direction of the screw. The bearing capacity of A325 bolt is directly affected by preload, slip resistance coefficient, and steel type. The specification range of high strength bolt is M12-M36, among which M16-M30 is commonly used.

Application of High Strength Bolt in Fastening of Railroad, Crane and Bridge

High strength bolts are sometimes used to fasten railroad rails. In the ordinary straight railroad line, the ordinary section of the rail is generally connected with ordinary bolts. But if in the mountain area with higher altitude or in the bend where the force is larger, then the use of high strength bolts for tightening. High strength bolts are also common on multiple sets of railway turnout frogs (use together with frog bolt) and heavy rail lines with heavy locomotive driving on.

Together with crane rail fasteners, high strength bolts can also be used on cranes. Due to structural reasons, the tower crane is in an unbalanced state regardless of whether it is empty or hoisted. In addition, the heavy weight of the goods that will be hoisted in operation has caused great pressure on the steel structure. The fastening performance of high strength bolt is very important at this time. The rigid connection of high strength bolts is adopted for the bridge of crane and the connection between main beams.

Large bridges are fixed with many steel structures. High strength bolt can play a good role in fastening the bridge structure.