High-strength bolt connections typically consist of one bolt, one nut, and one washer. Unlike standard bolts, they feature a distinctive plum-shaped head at the end. Historically, high-strength bolts were primarily made from 45 steel and 40B. However, as the diameter of the bolts increases, these materials often struggle with heat treatment hardenability, which limits their performance in larger sizes.
The working principle of shear-type high-strength bolts relies on the generation of a strong pre-tightening force when tightened. This force creates a clamping pressure on the contact surface between connected components, and the friction between those surfaces is used to resist shear forces. In contrast, ordinary bolts rely entirely on the screw’s engagement for both shear and compressive resistance. The shear capacity of high-strength bolts depends significantly on the friction coefficient of the contact surface and the magnitude of the pre-tightening force applied to the steel plates.
When using high-strength bolts, it's essential to treat the steel surfaces to enhance the friction coefficient between the connected parts. The Roots water jet unit helps optimize the performance of these bolts. Surface treatment is critical, and several methods are commonly used in construction:
(1) The grinding wheel method is labor-intensive and less consistent in quality, so it is usually reserved for small batches or when other methods are not feasible. The direction of grinding should be perpendicular to the direction of the applied force to ensure uniformity and effectiveness.
(2) Sandblasting or shot peening is another popular method. The friction coefficient (L value) depends on the flatness, cleanliness, and roughness of the contact surface. These processes remove rust and scale, revealing a clean metallic surface. While the surface may appear smooth, microscopic irregularities allow the bolt heads to interlock under pressure, creating a high friction coefficient. Shot peening is generally more reliable and environmentally friendly compared to sandblasting. Since it's not always practical to assemble the steel immediately after treatment, protective measures like inorganic zinc-rich paint are often applied to prevent re-rusting. Our factory has been using this method for several years, and it has proven very effective. For on-site installations, it's important to protect the contact area with adhesive tape near the bolt holes to avoid contamination that could reduce the friction coefficient.
(3) The red rust method involves blasting, pickling, or grinding the steel surface to create a thin layer of red rust. This enhances the surface roughness and improves the friction characteristics. The optimal time for rust formation should be determined through experimentation, taking into account local temperature and humidity conditions. This method is particularly useful when other surface treatments are not suitable or available.
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