When manufacturing bolts, we need to establish a tolerance zone for bolts to ensure they are compatible with nuts from various standard fastener manufacturers. Therefore, standards specify the limit dimensions of bolts (e.g., maximum and minimum dimensions), and the range from the minimum to the maximum is the tolerance range. As long as a bolt's dimensions fall within this tolerance range, its universality during circulation is guaranteed, and there is no need to worry about incompatibility due to different manufacturers.
After bolts are manufactured, they need to undergo electroplating or hot-dip galvanizing, which may alter the original dimensions of the bolts. Therefore, to determine whether a bolt meets quality standards, its basic dimensions and performance must be inspected before electroplating; after galvanizing, the coating thickness and final dimensions must also be checked to ensure compliance. In addition, electroplating and hot-dip galvanizing have clear standards: electroplating follows GB/T 9799 Metallic Coatings - Zinc Coatings on Iron and Steel by Electrodeposition, while hot-dip galvanizing adheres to GB/T 13912 Metallic Coatings - Hot-Dip Zinc Coatings on Fabricated Iron and Steel Articles - Specifications and Test Methods (not GB/T 3934, which is the standard for Plain ISO Metric Screw Threads - Gauges and Gauge Blocks). These galvanizing standards stipulate that the dimensions of bolts after treatment must not exceed their original limit dimensions-even if thread dimensions cannot be measured entirely accurately with go-no-go gauges, their actual values must still be within the limit dimensions to ensure smooth screwing in for installation in practical applications.
In daily production, we often encounter bolt damage: some bolts have thread failure, while others break. Why do these seemingly hard metal bolts develop such issues? What are the real causes of bolt damage?
The primary cause of bolt damage is initial thread loosening. When bolt threads loosen, continuous vibration during equipment operation causes the bolt to sustain external forces several times greater than those during tightening. The kinetic energy generated can be expressed by the formula "MV²/2" (where M = mass, V = velocity). This enormous oscillating impact force from loosening causes the bolt to collide continuously with the equipment, damaging the threads; in severe cases, the bolt head may break off directly. If the equipment has moving parts such as adjustment wheels, the broken bolt may get stuck in the wheels, leading to equipment damage.
This process essentially occurs due to sudden changes in the axial force on the bolt, which ultimately pulls the bolt apart. Such failures are relatively common. To prevent such accidents, anti-loosening thread products are usually prioritized in vibrating environments-for example, using nylon lock nuts in conjunction with bolts. One of Wuxi Fan'ao's customers manufactures submersible pumps, which operate continuously in both vibrating conditions and underwater environments. Once bolts loosen, workers cannot detect the issue and shut down the pump immediately for bolt replacement. Therefore, the industry widely uses all-metal lock nuts paired with bolts; for large pump installations, double-stack self-locking washers are additionally used to further enhance the stability and reliability of the connection.
In summary, bolts must not be used if they exhibit the following issues:
Surface rust with abnormal yellow or black spots. Do not underestimate these rust spots: once corrosion points appear, full-scale corrosion will soon follow. If a rusted bolt is installed, the entire bolt will become completely rusted in a short period. Rusted bolts are prone to thread seizing, making subsequent disassembly and replacement more difficult.
Damaged threads. Bolts are likely to have thread damage due to collisions during transportation; bolts with damaged threads must not be used.
Eccentric threads. Threads that are not centered (eccentricity) are a serious quality defect, and such bolts must not be used.
Material grade inconsistent with design requirements. This is the most critical issue to address: many bolts break during use, but no obvious defects are found in the bolts themselves, and installation is performed in accordance with specifications. Upon further investigation, it is often discovered that the material properties of the bolts used do not match the design requirements.
