JPH0660652B2 - Thread forming screw - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement of a thread forming screw having a small screwing torque and a large holding torque, which is particularly suitable for fastening thin plates.

[Conventional technology]

Among the screws that can be tightened as is without tapping in the prepared hole drilled in the thin plate-like plate to be fastened, the thread forming screw is threaded plastically when forming the internal thread, so it fits with the internal thread. It is excellent in that it is difficult to loosen after tightening the screw and the strength of the female screw is improved by work hardening. However, this thread forming screw has a drawback in that the screwing torque is high, and a screw having a gradually decreasing screwing torque has been devised. This screw is based on Jitsuko Sho 57-36820.
As described in No. 3, it has a head having an engagement hole for engaging a torque transmission tool and a leg having a thread.
The leg portion has a substantially triangular cross-sectional shape with rounded corners, and has a bow shape that smoothly connects the rounded corner and the distance from the screw axis to the extreme of the rounded corner. The first part that is a substantially polygonal frustum that gradually increases the radius of curvature of the side of the leg toward the leg, and the radius of curvature of a rounded corner that is connected to this and has a constant distance from the screw axis to the extreme corner. Is gradually increased toward the head while the radius of curvature of the side piece is gradually decreased, and a third portion having a substantially circular cross section. Is provided.

With this thread forming screw, when the tip of the leg is applied to the prepared hole of the plate to be fastened and torque is applied to the screw,
While gradually increasing the rate of catching with the plate to be fastened with the substantially polygonal frustum of the portion, that is, by gradually screwing the side edges of the legs, female threads are gradually formed at each corner. At this time, since the cross-sectional shape of the first portion is substantially triangular, the contact area with the plate to be fastened is small, and the screw can be screwed in with low torque. When the first portion passes along the female thread of the plate to be fastened, the second portion subsequently enters, the side piece is pushed into the prepared hole of the plate to be fastened, and the female screw is completely screwed at each corner. Mold. At this time, the first portion is gradually changed so that the cross-sectional shape is changed from a triangular shape to a circular shape. Therefore, the contact area with the plate to be fastened is gradually increased, the screwing torque is also gradually increased, and it is suddenly increased. Torque does not increase.

Subsequently, the third part, which has a substantially circular cross-sectional shape, smoothly slides into it, and the thread provided therein firmly engages the internal thread to firmly hold the screw. Moreover, since the third portion has a substantially circular cross-sectional shape and the screw thread provided therein also has a substantially circular cross-sectional shape, even if a large thrust force is applied during screw tightening, the screw thread and the screw thread are Not only a large internal thread breaking torque is obtained by contacting over the circumference, but also a large internal thread load is obtained after completion of screwing.

[Problems to be Solved by the Invention]

In the thread forming screw described above, the first portion of the leg portion is tapered with a substantially polygonal truncated pyramid, but since the threads are formed on the entire surface of the first portion, the thread formed on the plate to be fastened is lower. When searching for holes, the diameter of the prepared hole must be sufficiently larger than the diameter of the tip. Therefore, the curl that occurs when expanding the prepared hole is reduced, and not only a sufficient screw holding force is not obtained, but also a part of the molding of the curl is performed with the screw thread. There are drawbacks such as damage and deterioration of biting of the plate to be fastened.

The present invention is intended to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a thread forming screw configured to facilitate the search for the prepared hole of the plate to be fastened and to increase the screw holding force. Is.

[Means for Solving the Problems]

In order to solve the above-mentioned problem, it is composed of a head having an engaging portion with a torque transmission tool and a leg having a screw thread. In the leg portion, the cross-sectional shape of the screw thread is a substantially polygonal shape having rounded corners, and the leg portion has a bow shape for smoothly connecting the distance from the screw axis to the extreme of the rounded corner and the adjacent corners. The first part that forms a substantially polygonal frustum that gradually increases the radius of curvature of the side edge toward the leg part, and the corner part that is connected to this and has a constant distance from the screw axis to the extreme of the angle toward the head side. It comprises a second portion for gradually increasing the radius of curvature while gradually reducing the radius of curvature of the side piece, and a third portion having a substantially circular cross-sectional shape.

The thread is circular over the first portion, the second portion, and the third portion, and the advancing flank angle of the thread is larger than the chasing flank angle. Further, the screw thread has a starting tip in the vicinity of a joint with the second portion of the substantially polygonal truncated pyramid forming the first portion, and is provided toward the head.

Furthermore, a serration-like ridge surrounding the screw axis may be formed on the flat portion of the third portion.

Further, the diameter of the prepared hole of the plate to be fastened may be slightly larger than the diameter of the substantially polygonal frustum tip of the second portion.

[Action]

In the thread forming screw, the first portion of the tip of the leg portion is a substantially polygonal truncated pyramid, and the thread is not formed in the tip portion. Therefore, even if the prepared hole formed in the plate to be fastened is small You can dig into. Then, when torque is applied to the screw to start screwing, the substantially polygonal frustum of the first portion of the leg portion expands the prepared hole without damaging the screw thread and forms a curl portion around the hole. To do. The screw thread of the second portion abuts on this curl portion, and a female screw starts to be formed on the curl portion. Subsequently, the screw thread of the second portion further expands the curl portion, and gradually the female screw is gradually opened. Enlarge.
At this time, since the pilot hole is formed small, the flat part between the screw threads of the second part is in contact with the curl part, but the corner with a substantially polygonal cross section is formed into a female thread in the curl part. Therefore, the torque for forming the corner female screw of the second portion can be small. In addition to this, the cross-sectional shape of the second portion changes from a substantially polygonal shape to a substantially circular shape toward the head, and the contact area between the female thread and the female thread gradually increases. Since the lead side flank angle of is large, the component force applied in the sliding direction of the lead side flank surface to the internal thread due to the thrust is large, and the screw thread slides easily during thread forming. A female screw can be formed on the curl portion.

As the third portion of the female thread is inserted into the female thread following the second portion, and the seat surface of the screw contacts the plate to be fastened, the cross-sectional shape of the neck is substantially circular. The thread is tightly attached to the entire circumference of the female screw and the thread is fitted to the female thread, so that the screw is firmly fastened. At this time, the contact area between the thread and the female screw is increased by the curl portion, and the curl portion contacts so as to hold the flat portion of the leg portion, and a sufficient holding force is obtained. Moreover, since the threads are in contact with the entire circumference of the female thread, the female thread breaking torque and the female thread shear load are distributed over the entire circumference of the female thread, the female thread breaking torque becomes large, and then a slightly large thrust force is applied to the screw. Even
Almost no female screw is destroyed.

〔Example〕

Embodiments will be described below with reference to the drawings. In FIGS. 1 and 2, reference numeral 1 denotes a head 2 having an engagement hole (not shown) which is an example of an engagement portion with a torque transmission tool (not shown).
And a leg portion 3 connected to the head portion 2, which is a thread forming screw. The leg portion 3 has a thread 4 which will be described later.
The first portion 3a which forms a guide portion to the prepared hole of the plate to be fastened having a plate thickness substantially equal to the pitch of and the second part which is connected to the first portion 3a and gradually forms a predetermined female screw on the plate to be fastened. It is composed of a portion 3b and a third portion 3c which is continuous with the second portion 3b and is in close contact with the plate to be fastened over the entire circumference. The first part 3a
Is an example of a substantially polygonal truncated pyramid, and its cross-sectional shape is a triangular shape, and each vertex is formed with a rounded corner, and this corner is the plate to be fastened. Five female threads 5a are formed. The radius of curvature of the corner is significantly smaller than the pitch diameter of the same cross-sectional shape,
Moreover, it is chosen to be less than half the distance from the screw axis to its extreme. Further, the adjacent corners are smoothly connected by bow-shaped side pieces, and the radius of curvature thereof is formed to be larger than half the pitch diameter of the cross-sectional shape. Moreover, the distance from the screw axis to the corner and the radius of curvature of the side piece are configured to gradually increase as they approach the second portion 3b located on the head 2 side. Further, the first portion 3a is provided with a screw thread 4 having a starting end in the vicinity of the joint with the second portion 3b so as to face the head 2, and the screw thread height is gradually increased at a predetermined pitch. The increasing incomplete thread portion 4a is configured to be located in the first portion 3a.

The cross-sectional shape of the second portion 3b inherits that of the first portion 3a, but gradually changes from a substantially triangular shape to a substantially circular shape toward the third portion 3c, and in the vicinity of the joint with the third portion 3c. It is substantially circular. That is, the distance from the screw axis to the extreme corner is constant, but the radius of curvature of the corner gradually increases toward the head 2, while the radius of curvature of the side piece gradually decreases. Further, a thread 4 that inherits the screw thread 4 of the first portion 3a is screwed on the second portion 3b, and the cross-sectional shape of the screw thread 4 is the same as that of the second portion. It has a thread forming thread portion 4b that changes from a substantially triangular shape to a substantially circular shape. Since the corners of the female thread forming thread portion 4b are gradually formed by the corners of the incomplete thread portion 4a, it is possible to gradually form the thread into the female thread 5a having a predetermined shape while increasing the area to be fitted therein. Is configured.

Further, the third portion 3c has a substantially circular cross-sectional shape, and the thread 4 of the second portion is traversed under the neck of the inheritance head 2 in the third portion 3c. A screw thread 4 having a substantially circular surface shape is screwed. Further, a serration (not shown) extending parallel to the axis of the third portion 3c may be engraved in the flat portion between the 1-pitch thread ridges, and the serrated portion 5 is screwed into the plate 5 to be fastened. At times, sufficient frictional resistance can be obtained, sufficient holding force can be obtained, and higher screw holding force can be obtained.

The longitudinal cross-sectional shape of the thread 4 provided in the first portion 3a, the second portion 3b and the third portion 3c is an asymmetrical triangular shape, and the leading flank angle α thereof is the trailing flank angle. It is configured to be larger than β. In this screw thread 4, the sliding frictional force between the advancing flank surface and the internal thread 5a during screwing is larger than the sliding frictional force between the tracking side flank surface and the internal thread 5a during screw loosening, and the screw rotates in the screwing direction during screwing. It is easy to do, and it is hard to rotate in the loosening direction when the screw is loosened.

A torque transmission tool is engaged with the engagement hole of the thread forming screw to apply torque, and the first portion 3a of the leg portion 3 is provided with a substantially triangular truncated pyramid tip, which is slightly larger than the tip diameter. The hole of the prepared hole of the plate 5 to be fastened is searched. At this time, since the screw thread 4 is not screwed on the tip of the first portion 3a, even if the pilot hole is small, it is possible to easily and surely search for the hole with the tip of the substantially triangular frustum. You can
Thereafter, torque and thrust are applied to the thread forming screw 1, but the tip of the three-sided truncated pyramid does not damage the thread 4 as shown in FIG.
The pilot hole is gradually expanded to form a curl portion 5b around the hole. Since the prepared hole of the plate 5 to be fastened is formed small in the curled portion 5b, a sufficient length can be obtained.
The corners of the incomplete thread 4a of the first portion 3a bite into the curl portion 5b to start screwing the internal thread 5a. At this time, since the shape of the prepared hole is deformed into a substantially triangular shape, the biting with the starting end of the screw thread 4 becomes good, and the female screw forming is immediately started. Moreover, since the cross-sectional shape of the first truncated pyramid 3a is substantially triangular, its corners only contact the curled portion 5b of the plate 5 to be fastened and the contact area is small, and the fastened plate 5 is fastened. Since there is an escape portion where the screw thread 4 does not come into contact with, the female screw forming can be started with a low torque.

When the female thread forming thread portion 4b of the second portion 3b enters the screw 5a because it is screwed to the plate 5 to be fastened, its cross-sectional shape becomes the third.
Since it changes from a substantially triangular shape to a circular shape toward the portion 3c, the contact area with the plate 5 to be fastened is gradually increased while forming the screw 5a because it is formed by being plastically worked at a corner,
Since it is molded into the desired shape, it can be fitted to the screw,
No sudden torque is applied when screwing.

Further, the circular screw thread 4 of the cross-sectional shape of the third portion 3c is screwed onto the female screw 5a of the plate 5 to be fastened over the entire circumference thereof, and the screw thread 4 is screwed to the plate 5 to be fastened. It is firmly held by the screw 5a. Moreover, the thrust is transmitted to the internal thread 5a via the thread 4 until the thread forming screw 1 is completely screwed into the plate 5 to be fastened, but the advance flank angle α of the thread 4 is larger than the follow flank angle β. Since it is large, the sliding friction between the leading flank surface and the female screw 5a becomes a relatively large component as shown in FIG. 4A, the rotation in the screwing direction during screwing becomes smooth, and the screwing torque can be further reduced. . Moreover, since the female screw breaking load due to the thrust applied when screwing in extends over the entire circumference of the screw thread, a large female screw breaking torque can be obtained.
Even if the plate 5 to be fastened is a thin plate, the female screw 5 is never broken.

Further, the thread forming screw 1 maintains a predetermined tightening torque after the screwing is completed, but a thread shearing load is applied from the thread 4 of the thread forming screw 1 to the thread 5a. Since this internal thread shear load is also distributed over the entire circumference of the internal thread 5a, a large internal thread shear load can be obtained. Further, even if a torque is applied in the direction in which the thread forming screw 1 is relaxed, the flank angle β on the trailing side is small, so that FIG.
As shown in FIG. 5, the component force acting as a sliding frictional force between the tracking flank surface and the female screw 5a is small, the thread forming screw 1 is less likely to rotate in the loosening direction, and the thread 4 of the leg portion 3 and the plate 5 to be fastened. The holding force of the thread forming screw can be increased in combination with the increase in the contact area of the internal thread 5a.

〔The invention's effect〕

As described above, according to the present invention, the tip of the leg portion of the thread forming screw is formed in a polygonal frustum, and the tip of the polygonal frustum is not provided with a screw thread, and the screw thread provided in the leg portion is provided. Since the longitudinal cross-sectional shape of the is made an asymmetrical triangle and the flank angle on the advancing side is larger than the flank angle on the trailing side, even if the prepared hole of the plate to be fastened is made small, this can be surely searched for. A curl part with sufficient contact area can be molded around the hole to increase the holding force of the screw, and the component force in the sliding direction of the advancing flank surface against the female screw is large due to the thrust during screwing. Even if the torque is small and the screw receives rotational torque in the loosening direction, the flank angle on the trailing side is small, so the component force in the sliding direction applied to the flank on the trailing side is small and the screw loosens. Rotation of direction is hardly, there are advantages such as it is possible to increase the holding power of the screw.

In addition, in the embodiment, the cross-sectional shape of the leg portion is substantially triangular, but the same effect can be obtained even if the leg portion has a polygonal shape other than this. Further, the present invention is not limited to the embodiments and can be applied within a range not departing from the spirit of the present invention.

[Brief description of drawings]

FIG. 1 is a front view of a thread forming screw of the present invention,
2 is a bottom view of the leg portion of the thread forming screw of FIG. 1, FIG. 3 is an operation explanatory view showing the start of screwing according to the present invention, and FIGS. 4A and 4B show the action of the screw thread according to the present invention. It is an explanatory view explaining. 1 ... Thread forming screw, 2 ... head part, 3 ... leg part, 3a ... first part, 3b ... second part, 3c ... third part, 4 ... thread, 4a …… Incomplete thread, 4b …… Female thread forming thread, 5 …… Fastened plate, 5a …… Female thread, 5b …… Curl section,

Claims (3)

[Claims] 1. A head portion having an engaging portion for engaging a torque transmission tool and a leg portion having a screw thread, the leg portion having a substantially polygonal cross-sectional shape with rounded corners and a screw axis. To the extreme of a rounded corner and a first part consisting of a polygonal frustum that gradually increases the radius of curvature of the side of the bow that smoothly connects adjacent rounded corners toward the leg. And a second portion which is connected thereto and which gradually increases the radius of curvature of the rounded corner while heading toward the head with a constant distance from the screw axis to the corner extreme, while gradually decreasing the radius of curvature of the side piece, A thread forming screw having a third portion whose cross section is substantially circular, each portion being provided with a crotch thread, formed over the first portion, the second portion and the third portion. The leading flank angle of the thread While making the flank angle larger than the side flank angle,
A thread forming screw, which is provided with a screw thread starting from the vicinity of the joint with the head portion and heading toward the head. 2. The serration-like ridge is engraved on a part of the flat portion under the neck of the third portion of the leg so as to surround the axis of the screw. Thread forming screw. 3. The thread forming screw according to claim 1, wherein the diameter of the prepared hole of the plate to be fastened is slightly larger than the diameter of the substantially polygonal frustum tip of the second portion.