For explosion-proof equipment (explosion-proof positive pressure cabinet, explosion-proof analyzer house, explosion-proof distribution box), fasteners are usually an important part to maintain the integrity of its explosion-proof type, especially for flameproof "d" and some dust explosion-proof equipment, special fasteners must be used. They play a role in fastening the flameproof joint surface or maintaining the sealing of the enclosure, which is crucial to ensure the integrity of the explosion-proof type. If the screw pitch and tolerance fit of the fastener are not up to standard, it is easy to fall off and loosen, which will affect the explosion-proof performance of the equipment and form a potential safety hazard. Therefore, explosion-proof equipment should select fasteners that meet the requirements of explosion-proof standards and have passed the test verification, and can only be replaced with the same fasteners.
Fasteners play an important role in flameproof products. We all know that flameproof electrical equipment uses flameproof enclosures to achieve explosion-proof performance. If anything goes wrong with the fasteners, the flameproof enclosure and the joint surface will not match, but there will be gaps or other conditions, and the flameproof products will lose the significance of flameproof, so fasteners play an important role in explosion-proof products.
The design of fastening bolts for flameproof shell is mainly carried out from the bolt strength, the strength of cast aluminum shell thread, the calculation of bolt pre-tightening torque, the bolt anti-loosening and accuracy.
1. Strength: select fasteners with appropriate strength to ensure that the shell will not fall off when it bears the explosion pressure;
2. Blind hole: when the bolt is screwed into the blind hole of the shell wall without washer, at least one buckle of allowance shall be reserved at the bottom of the hole;
3. Protection: use elastic washer, flat washer, counterbore and other methods to protect fasteners;
4. Torque: the tightening torque shall be specified in the document, and the specified value shall be followed during installation;
5. Outer assembly:
(1) Hexagon socket: protruding from the screw hole after fastening
(2) Class I equipment: the head is provided with a protective cap or counterbore, or is protected by the inside of the equipment
(3) Class II equipment: the allowable thread depth h of fastener hole is at least equal to the outer diameter of fastener thread
6. Material requirements: plastic materials and light alloy materials are not allowed;
7. Other requirements:
(1) The door or cover fixed by thread shall be additionally fixed by means of hexagon socket set screw or equivalent method;
(2) Double-head screws shall be firmly fixed and permanently fixed on the shell by fusion welding or riveting or other equivalent methods;
(3) Fasteners shall not penetrate the flameproof enclosure wall unless they form a flameproof joint surface with the enclosure wall and the enclosure cannot be separated
(4) When the bolt without washer is completely screwed into the blind hole of the flameproof enclosure wall, at least one full thread allowance shall be kept at the bottom of the hole;
(5) For screw holes or studs that do not penetrate the wall of the flameproof enclosure, the remaining thickness of the wall of the flameproof enclosure shall be at least 1/3 of the diameter of the bolt or stud, and the minimum thickness shall be 3mm.
Flameproof enclosure generally has two types: plane and seam. For plane structure, bolts not only play a fastening role, but also ensure plane clearance. For the seam structure, when the flameproof surface only considers the cylinder part, the bolt only plays a fastening role, and at the same time, the clearance role of the plane part should be guaranteed. When tapping the thread directly on the shell, it should be noted that the screw hole of the fastener should not pass through the flameproof shell as far as possible. When passing through the shell, a margin of more than 3mm should be left at the bottom of the screw hole. When using aluminum alloy and other light alloy materials to make the flameproof shell, due to the low strength of aluminum alloy, when the flameproof shell that needs to be opened normally (such as changing the light source needs to be opened) is fastened with screws, the thread should not be tapped directly on the aluminum alloy shell, The strength of the screw hole can be increased by embedding the anti-loose internal and external threaded steel sleeve, and the failure caused by thread decay can be prevented. Do not open the bolts that have been installed before leaving the factory when the user changes the light source or repairs them. You can directly tap the fastening screw holes on the shell, but you cannot use the fine thread. Try to use the coarse thread, and there are enough meshing buckles to meet the fastening requirements.
In short, when designing enclosure fasteners, the role of flameproof enclosure should be clearly identified. Whether it only plays the role of fastening, or it also plays the role of ensuring the plane clearance, and then confirm that the maximum axial load of the bolt is appropriate.
General requirements for fasteners of explosion-proof equipment
1.1 For the fasteners necessary to ensure the special explosion-proof type or to prevent contact with exposed live parts, only tools (such as screwdriver, wrench, etc.) can be used to loosen or remove them.
1.2 If the material of the fastener is suitable for the shell material, the fastener for the shell containing light metal can be made of light metal or non-metallic material.
1.3 Only when the thread shape is suitable for the shell material, the screw hole can be tapped on the shell for installing the cover that may be opened in the work.
Requirements for special fasteners
Thread pitch, tolerance fit, bolt (nail) head or nut shall meet the requirements of relevant standards.
2.1 The pitch of thread shall meet the requirements of GB/T 9144 large-pitch metric thread. This series is selected from the common thread series specified in GB/T 193, and the nominal diameter range is 1mm~64mm; The tolerance fit of the thread shall comply with 6g/6H in GB/T 197 and GB/T 2516 (the number indicates the tolerance grade, the higher the accuracy, the smaller the value; the letter indicates the position of the tolerance zone, the lower case letter is used for external thread, and the upper case letter is used for internal thread).
2.2 Standard parts meeting standard requirements
2.2.1 Hexagon bolts shall meet the requirements of GB/T 5782 or GB/T 5783.
2.2.2 Hexagon socket head cap screws shall meet the requirements of GB/T 70.1.
2.2.3 Hexagon socket set screws shall meet the requirements of GB/T 77 (flat end), GB/T 78 (tapered end), GB/T 79 (cylindrical end) or GB/T 80 (concave end).
When other bolt (nail) head or nut fasteners are used, the explosion-proof equipment shall be marked with "X", and these fasteners shall be specified in detail in the product instructions, and it shall be explained that only the same fasteners can be replaced.
2.3 Requirements for special fastener holes
2.3.1 The allowable thread depth h of the special fastener hole shall be at least equal to the outer diameter of the fastener thread (as shown in Figure 1).
Figure 1: Allowable engagement thread depth h of special fastener hole
2.3.2 The thread tolerance of special fastener holes shall comply with 6H in GB/T 197 and GB/T 2516, and the following conditions shall be met:
a) The hole under the bolt head shall be in accordance with the size requirements of medium assembly not larger than H13 in GB/T 5277-1985, that is, the clearance of part c in Figure 2 shall not be greater than the maximum allowable clearance of H13 medium assembly in GB/T 5277-1985 (for example, the hole diameter of M6 bolt fitting shall not be greater than Ф 6.6)。
Figure 2: Fit clearance between the hole under the bolt head and the bolt
b) The hole under the thin rod bolt head (or nut) shall be tapped to ensure that the bolt does not fall off. The size of the screw hole shall ensure that the contact area with the connected part is at least equal to the contact area of the non-thin rod bolt in the light hole (the "X" ring contact part in Figure 3). That is, the contact dimension X between the thin rod fastening bolt and the connected part shall not be less than the contact dimension of the full thread fastening bolt (see Figure 2).
Figure 3: Contact surface between thin rod fastening bolt head and fastener
2.4 Requirements for hexagon socket set screws
For hexagon socket set screws (for example, for locking purposes) used for fastening threaded caps, the tolerance fit of the thread shall comply with 6g/6H in GB/T 197 and GB/T 2516, and shall not protrude from the screw hole after fastening.
