U-bolts, also known as horse-collar bolts, are non-standard fasteners. They are called U-bolts because of their U-shaped form. Both ends of the U-bolt have threads that can be combined with nuts. They are mainly used to fix tubular objects such as water pipes or flat objects such as leaf springs in automobiles.
Because the way they fix objects is like a person riding a horse, they are called horse-collar bolts. Now, let’s see what the specification representation method of U-bolts is and how to select the bolt grade.
Specification Representation Method of U-bolts
First, the thread specification needs to be marked, and then the outer diameter of the pipe to be clamped.
Examples of standard marking for U-bolts
Marking of U-bolts for pipes with a fixed outer diameter D0 = 25mm;
U-bolt 25 JB/ZQ 4321-97
Marking for surface galvanized U-bolts for fixed outer diameter D0 = 25mm pipes;
U-bolt 25-Zn JB/ZQ 4321-97
Explanation of the Characteristics of U-bolts
U-shaped bolts are generally used in cargo trucks to stabilize the chassis and frame of the vehicle. For instance, leaf springs are connected by U-bolts.
U-bolts have a wide range of applications. Their main uses include building installation, mechanical parts connection, vehicles and ships, bridges, tunnels, railways, etc. The main shapes are semi-circular, square right-angled, triangular, and oblique triangular, etc.
1. Material properties: density, flexural strength, impact toughness, compressive strength, elastic modulus, tensile strength, temperature resistance, and color are determined based on the usage environment.
The commonly used materials include carbon steel Q235A, Q345B, alloy steel, and stainless steel, etc. Among the stainless steel materials, there are 201, 304, 321, 304L, 316, and 316L.
National standard for U-bolts.
How to select the grade of bolts
The grades of bolts are mainly divided into high-strength bolts and ordinary bolts.
High-strength bolts are mainly used in some applications such as outdoor engineering, wind power or dynamic machinery, with relatively high tensile strength. Generally, bolts of lower strength grades can be used in small-scale machinery, such as turbines, depending on the application environment and operation mode.
High-strength bolts are increasingly widely used. The two common strength grades are 8.8s and 10.9s, with 10.9 grade being more prevalent. The strength grades of ordinary bolts are lower, typically 4.4 grade, 4.8 grade, 5.6 grade and 8.8 grade.
1. From the perspective of raw materials: High-strength bolts are made of high-strength materials. The bolts, nuts and washers of high-strength bolts are all made of high-strength steel, commonly using 45# steel, 40 boron steel and 20 manganese titanium boron steel. Ordinary bolts are usually made of Q235 steel.
2. From the perspective of strength grades: High-strength bolts are increasingly widely used. The commonly used strength grades are 8.8s and 10.9s, with 10.9 grade being the most common. The strength grades of ordinary bolts are lower, generally being 4.4 grade, 4.8 grade, 5.6 grade and 8.8 grade.
3. From the perspective of force application characteristics: High-strength bolts apply preload and transfer external forces through friction. Ordinary bolt connections rely on the shear resistance of the bolt shank and the bearing capacity of the hole wall to transfer shear forces.
When tightening the nut, the preload generated is very small and can be ignored. However, in addition to having a very high material strength, high-strength bolts also apply a large preload to the bolts, generating compressive force between the connected components, thereby creating a significant friction force in the direction perpendicular to the bolt shank.
Moreover, the preload, slip resistance coefficient, and type of steel all directly affect the bearing capacity of high-strength bolts.
4. In terms of usage: For the main components of building structures, bolt connections typically employ high-strength bolts. Ordinary bolts can be reused, but high-strength bolts cannot. High-strength bolts are generally used for permanent connections.
High-strength bolts are prestressed bolts. For friction type, a torque wrench is used to apply the specified prestress, while for bearing type, the hexagonal head is removed. Ordinary bolts have poor shear resistance and can be used in secondary structural parts. Ordinary bolts only need to be tightened.
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