In the design of bolted joints, especially for explosion-proof enclosures, several critical factors must be considered to prevent failure. One common cause of bolt failure is when the selected bolt diameter and the number of bolts are insufficient to withstand the pressure generated during an explosion test. This leads to the bolt breaking at its weakest point. Another issue arises from the material properties of the cast aluminum housing used in the design. If the thread strength of the housing is inadequate, it may not support the forces applied during testing, resulting in thread damage—particularly at the most stressed areas.
Additionally, improper tightening torque can also lead to bolt failure. If the required torque is not clearly defined, assembly personnel might over-tighten the bolts, causing them to stretch or even break. This problem often occurs during assembly and can also affect the ability of inspection units to control the flat joint clearance of the sample, as the machining accuracy of the housing is not properly managed.
Anti-loosening measures are another important aspect of bolt design. In China, spring washers are commonly used, but there is no standardized regulation for the tightening force distance. This makes it difficult to ensure consistent tightening across multiple fasteners. In contrast, Germany typically relies on specified torque values rather than additional anti-loose mechanisms, leading to more standardized and reliable designs. This approach reduces human error during assembly and ensures better performance.
Steel has higher tensile and shear strength compared to cast aluminum, so the thread strength of the aluminum casing must be carefully considered during bolt design. The tightening torque should be determined based on the specific load requirements of the bolt.
Flameproof enclosures usually have two types of flameproof surfaces: flat and dead. For flat structures, bolts serve both as fasteners and as a means to maintain a gap between mating surfaces. During explosion tests, the axial load F is applied, increasing the bolt tension from F0 to F. This causes the bolt to elongate by ΔL1, while the cover experiences a reduction in preload, decreasing from F0 to F0 (residual), with a corresponding deformation ΔL2.
The stress distribution along the bolt thread is uneven. When tightened with a nut, about one-third of the force is concentrated on the first thread, while the eighth and subsequent threads experience little to no stress. However, when the bolt is engaged with the housing, the opposite occurs—the first thread of the bolt bears the majority of the load, around one-third. This is due to pitch and tooth shape errors, which cause the first thread to engage first, leaving the rest under less stress. This uneven stress distribution often leads to thread failure during testing, particularly on the first and second threads of the housing.
Bolt accuracy plays a significant role in ensuring proper performance. The accuracy of the bolt depends on the precision of individual threads and cumulative errors. Therefore, the design must account for both the length of the screw and the overall thread accuracy. Typically, for explosion-proof devices, the bolt tightening length is medium, and the thread accuracy is generally 6g. The internal thread on the housing must meet a minimum accuracy of 5H to ensure proper connection and stability.
When checking the thread strength, it's observed that the stress is not evenly distributed. If a bolt engages with ten threads, the last thread in the housing is the most stressed due to cumulative errors. During design, we can calculate how many threads will be engaged and ensure sufficient engagement length. If the housing’s thread accuracy is low, the number of effective threads decreases, increasing stress on the final thread and potentially weakening the connection, even if the pre-tightening torque is high.
To maintain good performance over time, the screwing length of the bolt should be medium or long, and the depth of the screw hole in the housing should be at least ten threads. This ensures proper engagement and prevents premature failure. Overall, attention to thread accuracy, bolt strength, and proper tightening procedures is essential for reliable and safe bolted joints in explosion-proof applications.
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Shenzhen Hongfa Automatic Door Co., Ltd. , https://www.hongfadoor.com