1 Introduction
In order to prevent the screw from falling off, solve the problem that is not easy to find after the screw is detached or avoid the adverse consequences caused by the falling off. The non-leaving screw is widely used in the installation of the panel (or cover) which is often required to be repaired and disassembled on the ship accessory products. ( Crews with waisted shank ), China has long formed the corresponding technical standards: such as GB/T 837-1988 slotted pan head does not pull out the screws, etc., and the structural dimensions of various non- leaving screws are given respectively. However, the standard does not form a corresponding technical specification for the joint structure without the screw.
Due to the structural speciality of the screw, the connection and the ordinary screw are different. However, designers often overlook the difference from ordinary screws. Such as:
Event 1 : The non-leaching screws installed on the panel of a product are difficult to install, difficult to disassemble, and the panel pressure is not equal. It is found that the screw is overturned to form a "sliding wire" illusion.
Event 2 : A control cabinet cover does not come out of the screw, and the anti-off circlip is largely detached at the site. After the test is completed, the screw is lost.
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2 analysis of structural characteristics of non-leaving screws
Take the commonly used slotted pan head as an example. Figure 1 shows the structure of the GB/T 837-1988 slotted pan head without the lead screw.
Figure 1
It can be seen from the figure that the screw that does not come out is composed of a screw head (outer diameter dk ) larger than the large diameter of the thread, a beam waist (diameter ds ) smaller than the thread diameter, and a thread (large diameter d ). When the pressure is connected to the connected member, the waist portion acts as an anti-offset, and the threaded portion acts as a coupling. Obviously, the screw structure itself does not have the function of preventing falling off, and the anti-ejection function of the screw is realized by the coupling structure with the connected member (ie, the cover or the panel), that is, the waist of the screw is formed by a certain structure. The waisted shank is stuck on the mounting hole of the connector to achieve the anti-out function.
It can be found from the standard size specification table of the screw, the length of the thread part b ≈ the thread diameter d*1.5 , the longer the thread size, the longer the thread length, the thread length is basically the minimum length to meet the joint strength, and the length of the screw engineering L It is not specified that the strict requirements of the screw connection structure can be seen from the thread length limitation.
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3 does not pull out the screw connection classification
In order to make the screws hang on the connected parts without going out, there are generally two ways to structure:
(1) The thread buckle is prevented from coming off (as shown in Figure 2 ): the thread on the mounting hole of the connector has the thread thread conforming to the screw thread specification; the screw is screwed into the connected piece and does not naturally come out. The above case 1 is the structure:
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Figure 2 Schematic diagram of the thread buckle anti-off structure
(2) Card type anti-off (as shown in Figure 3 ): the mounting hole of the connecting piece is a light hole larger than the outer diameter of the screw, and the screw is fixed on the connected piece by an elastic ferrule, a circlip, or a snap spring (Fig. In the middle, the circlip retaining structure is not shown. If the circlip is not removed, the screw cannot be removed from the hole of the connected member. The above case 2 is the structure.
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Figure 3 Schematic diagram of the anti-off structure of the elastic retaining ring
In Fig. 2 and Fig. 3 , h1 is the total thickness of the connected member and the circlip, h is the distance between the inner surface of the anti-exfoliation structure and the upper surface of the connecting screw hole, and H is the outer surface of the connected member (including the outer gasket) to The distance to the lower surface of the screw hole (ie the total thickness of the connecting member), b is the screw-in length.
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4 connection structure correct technical conditions
Although it is not stated in the standard, it can be found that the structure of the screw anti-off structure and the small thread length can be found, and the joint structure without the screw is strictly required.
If the connected piece is fixed with only one screw, the connected piece will rotate together with the screw during the process of tightening or loosening the screw. The anti-off screw and the ordinary screw may not have special requirements on the mounting structure except the anti-off structure. However, the connected member is often caught on the connected member, and the connected member is simultaneously restrained by a plurality of screws, and the connected member cannot rotate with the rotation of the single screw.
Just because the non-leaving screw has the characteristics of being stuck on the connecting piece, the thread size is limited, and usually a plurality of screws are simultaneously connected to a group of members, the joint structure without the screw must meet the following three technical conditions:
(1) The amount of screwing on the connected part should be equal to or slightly larger than the screwing amount of the screw to ensure that the screw can be screwed into the screw hole of the connecting piece. This is the basic condition for ensuring the connection is effective. Namely: L≥h1+2b
(2) The distance between the inner surface of the anti-off structure to the upper surface of the connecting screw hole is equal to or slightly larger than the screwing amount of the screw to ensure that the screw is screwed in and unscrewed during the disassembly. That h≥b
This is a necessary condition to ensure that the anti-drop structure on the connected parts does not interfere and ensure a smooth installation. Otherwise, when the screw is screwed in, the other screws will be caught, causing deformation of the connected piece, difficulty in screwing or damage of the thread. When the worker operates, multiple screws are screwed in at the same time, and the screw is withdrawn. When the threaded portion of the screw is completely removed from the connecting screw hole before being completely removed from the connecting screw hole, the screw is self-locking, and the disassembly is quite difficult. The forced disassembly may cause the screw to be damaged; the retaining structure of the retaining spring is when the screw is withdrawn. It will encounter the opposite side of the anti-separation structure of the connected part, forming the top pressure of the connected piece. When forcibly disassembled, the circlip will be screwed out along the thread, and finally the screw anti-off structure will be invalid.
(3) The length of the screw should not exceed the total thickness of the connecting member, so that the threaded portion of the screw is just inside the connecting screw hole, which is another basic condition for ensuring that the screw is not effectively connected. That is, L ≤ H look back, the reason why the thread length of the screw is specified as the minimum length to meet the joint strength in the standard specification is precisely to avoid the need to further complicate the joint structure, and this specification also determines the joint structure. The rigor. In addition, it should be noted that the non-leaching screw is not a loosening screw, and the two concepts of anti-out and anti-loose cannot be confused. The anti-loose structure should be considered separately.
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5 Analysis of the cause of the problem
Through the above analysis, the reasons for the problems in the aforementioned cases are clear:
In case 1, since the distance h between the inner surface of the retaining structure and the upper surface of the connecting screw hole is too small (may not be considered at all), the panel thread interferes with the thread of the connecting screw hole, so that the screw is screwed in and out. Difficult, even impossible to disassemble, panel mounting inequality problems, and because the screw length exceeds the total thickness of the connecting member, the threaded portion is screwed out of the screw hole to form a "sliding wire" illusion.
In case 2, the distance h between the inner surface of the anti-off structure to the upper surface of the connecting screw hole is insufficient. When the cover is installed, the four screws need to be screwed in at the same time; when the cover is removed, the four screws are simultaneously withdrawn, and the operator does not Knowing that there are structural problems, it is often necessary to disassemble the screws. When disassembling, the elastic retaining ring will continue to rotate after the cover plate is pressed against the cover, so that the retaining ring is withdrawn along the thread, causing the anti-off failure and the screw to fall off.
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6 Conclusion
In short, although the screws are small, different screws have different specifications, which should be paid enough attention by the designers. They should be familiar with the standard specifications and analyze their structural characteristics: during the installation operation, the operators should find the problems in time. And feedback problems, should not be confused, otherwise it will cause operational difficulties, functional failure, and even cause major weight accidents.
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