JPH0256974B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a mainspring spring whose fatigue life is dramatically improved compared to conventional mainspring springs. A conventional example of manufacturing a mainspring spring will be explained based on FIG. A thin plate cold-rolled to a thickness of 0.13 mm using steel materials such as stainless steel or steel steel is slitted and green-printed (rounded).
The steel strip 1 that has undergone such a raw material process is sent to a bending member 4 through a feed roller 2 and a guide member 3 shown in FIG. 1, and the steel strip 1 is brought into contact with the member and bent. At this time, when bending is performed with a constant diameter, it becomes a constant torque spring spring, and when the diameter is changed to provide torque characteristics, it becomes a variable torque spring. Next, the bent spring is subjected to aging treatment (heat treatment) at 400°C for 2 hours, for example, to prevent variations in the finished diameter, or to prevent plastic deformation from occurring afterward. In order to
Then, it is wound around the setting drum 6 to perform presetting. This presetting is a process in which the spring after bending is reversely wound and its diameter is increased from, for example, φ12 to 13 during bending to φ14 to 16. Incidentally, there is a fatigue test as a test for measuring the fatigue life of the mainspring spring obtained in this manner. One method for this test is to wrap the set spring around the support drum 7, wrap it around the output drum 8, and then wind it back from the output drum 8 to the support drum 7, as shown in FIG. This winding and unwinding process is repeated to measure the number of times a crack occurs in the spring. The test may be based on a test box set or a unit test. Regarding the take-up spring used in the seat belt retractor of an automobile, when the reel (not shown) provided on the output shaft of the output drum 8 shown in Fig. 3 is pulled, the output drum rotates in the direction of the arrow. A mainspring spring is wound around the reel, so that when the reel is released, it is rewound onto the support drum 7 again. By the way, the fatigue life of the mainspring spring according to the conventional example described above is only about 30,000 to 70,000 cycles on average, and about 50,000 cycles on average when tested using a test box set. Therefore, the inventors of the present invention measured the residual stress of the mainspring spring according to this conventional example, and found that the residual stress on the tension side (generated stress) when using the spring during setting as shown in Fig. 2 was approximately +20 to 30 Kgf/ ^mm2 . Also, the stress generated by the support drum set is approximately
30Kgf/mm ² , stress generated during fatigue test is approximately 170Kg
f/mm ² , and the stress generated on the spring surface is approximately
It was found that it was about 220-230Kgf/ ^mm2 . Figures 4A and 4B are graphs showing the influence of residual stress on the tension side surface when using a spring on fatigue life.Figure 4A shows the tensile residual stress, fatigue test stress amplitude range, spring limit value, and The relationship with strength is shown in Figure B, which shows the S (stress) - N (number of times) curve. In FIG. 4, numeral 9 indicates a conventional example, and in this conventional example, the residual stress indicates the stress amplitude range on the + side.
In this way, when the stress amplitude range is based on the + side, the force spring limit value is exceeded, and only a mainspring spring with a short fatigue life can be obtained. However, in the case of mainspring springs, by applying a residual stress (compressive residual stress) of 1 as shown in 10 in Figure 4, the service life can be dramatically improved without exceeding the spring limit value as shown in the figure. can be achieved. That is, the present invention applies to the tension side surface when using a spring (the surface layer side where tensile stress acts when using a spring).
This invention relates to a mainspring spring with improved fatigue life, which is made by applying compressive residual stress to the spring. By the way, in the process of establishing the present invention, as a result of numerous basic experiments on a preferable method for obtaining a mainspring spring with a dramatically improved fatigue life by applying one residual stress to the tension side surface when the spring is used, the following results were discovered. We have obtained such knowledge and will explain it below. That is, when considering a method of imparting compressive residual stress to a mainspring spring in the manner shown in FIGS. We succeeded in imparting a residual stress of 1 by squeezing the steel strip. In Fig. 5, P indicates tension, W indicates the other tension, and 11
is a load, and 15 is a guide roll. Therefore, as a result of further intensive study, it was found that the best way to impart residual stress using such a drawing method is to iron the material with an ironing member such as a die, and then bend and stretch it. That is, in the embodiment shown in FIG. 5, the die R is made small and a large tension is applied. Figure 6 is a graph showing the relationship between die R and surface residual stress in the roller die method as shown in Figure 5B, and the residual stress in the conventional method (pushing method) is on the + side. On the other hand, it was found that the method of the present invention, in which tension is applied from two directions, shows a residual stress on one side, and that the smaller the die R is, the larger the residual stress on one side can be. FIG. 7 similarly shows the block die method shown in FIG. 5A, and the same thing can be seen. In light of the embodiment of the conventional method (pushing method) described above, the primary forming diameter is made smaller and the amount of reverse bending deformation due to presetting is increased. However, from the perspective of reducing the primary forming diameter, the present inventors have investigated reducing the radius during forming using the conventional method (pushing method), but in the conventional method such as the pressing method, It was found that if the radius was made smaller, cracks would occur in the steel strip, and that there was a limit to reducing the radius during forming. Furthermore, in the process of establishing the present invention, a spring smaller than a predetermined diameter is first formed, and then as a second form, the spring is bent in the opposite direction from the formed state to expand the formed diameter to a predetermined diameter, thereby reducing compressive residual stress. It has also been found that it is possible not only to simply impart this stress, but also to increase the absolute value of the compressive residual stress and obtain a mainspring spring with a further improved fatigue life. The method for manufacturing a mainspring spring of the present invention was completed based on the above knowledge, and when manufacturing a mainspring spring that is used by forming it into a spiral shape and then winding it backwards, a tension is applied to the steel strip for the mainspring spring. After first forming a spring smaller than a predetermined diameter by ironing, the spring is bent in the opposite direction to the direction of the first forming to enlarge the formed diameter to the predetermined diameter. Due to this, tensile stress is applied when the spring is used. A method for manufacturing a mainspring spring with excellent fatigue properties characterized by applying compressive residual stress to the surface layer side. The straining member used to obtain the mainspring of the present invention may be any material as long as it can be strained while applying tension in two directions, including roller dies,
Examples include R blocks and bearing dies (hereinafter simply referred to as R dies). The smaller the radius of the R die, the better, but it is usually best to use one with a radius of 5 mm or less. Also, the higher the tension, the better. The radius and tension of these R dies are appropriately selected depending on the type of steel material, etc. In the present invention, the absolute value of compressive residual stress is reduced by first forming a spring smaller than a predetermined diameter using the above method, and then bending the spring in the opposite direction from the formed state as a second forming to enlarge the formed diameter to a predetermined diameter. In order to obtain a mainspring spring that is larger and has a further improved fatigue life, this secondary forming can be performed continuously on the same production line as the primary forming,
Further, the aging treatment can be carried out separately before and after a suitable aging treatment after the primary molding. FIG. 8 schematically shows a method for manufacturing a mainspring spring of the present invention in which primary forming and secondary forming are carried out on a continuous line, in which a steel strip is rolled with rollers 14 while applying tension from two directions. The figure shows a method in which the material is firstly formed by squeezing, then bent in the opposite direction by rollers 16 and secondly formed, thereby increasing the absolute value of the compressive residual stress. In the secondary forming, as shown in FIG. 8, the material may be reversely bent by a uniaxial roller 16, or may be reversely bent by a biaxial roller. In Figures 9A to 9D, the rear tension 17 and the front tension 18 are applied and the rollers 14 are used to tighten [I in the same figure], and then the twin-shaft rollers 19 and 20 are used to bend in the opposite direction [in the same figure] C] The method is shown. In addition, springs 21 smaller than the specified diameter are shown in b and d of the same figure.
The spring 22 [D] in the same figure is shown in which the spring 22 is formed by secondary molding and then expanded to a predetermined diameter by bending in the opposite direction in the manner shown in C. According to the present invention, a constant torque mainspring spring can be created by squeezing with an R die of a constant diameter under a constant tension, or a variable torque mainspring spring can be created by applying a torque change to the spring, such as by changing the tension. can be achieved. The thus obtained variable torque mainspring spring can be suitably used as a winding spring for use in an automobile seat belt winding device, which has a dramatically improved fatigue life. Table 1 below summarizes the changes in residual stress (high torque area) during the forming process using the conventional method. A tensile residual stress of about 28 Kgf/mm ² was applied after presetting.

[Table] In contrast, Table 2 shows a stainless steel strip with a thickness of 0.13 mm and a width of 14 mm, and a φ9 spring with a rear tension of 22 kg.
f/mm ² , radius 0.7 and 0.8 mm dies, and then secondary molding to φ15. The side residual stress (X) is
Indicates compressive residual stress of 80Kg/mm2 or ^more .

[Table] As shown above, the mainspring spring to which compressive residual stress according to the present invention has been applied shows the fatigue test stress amplitude range based on the residual stress of 1, so it is understood that the mainspring spring has an improved fatigue life. , After the steel strip according to the present invention is squeezed under tension to form a spring smaller than a predetermined diameter, the steel strip is bent in a direction opposite to the direction of the primary forming to enlarge the formed diameter to a predetermined diameter. Due to the manufacturing method of molding,
We were able to not only simply apply compressive residual stress, but also increase the absolute value of compressive residual stress and obtain a mainspring spring with a further improved fatigue life. Incidentally, in the above example, in the case of invention example 1, 10
It has a fatigue life of over 10,000 cycles, and we have succeeded in making the fatigue life semi-permanent.

[Brief explanation of drawings]

Figure 1 is a side view explaining the conventional method, Figure 2 is a side view explaining presetting, Figure 3 is a side view explaining fatigue testing, and Figure 4A is residual stress and fatigue test stress amplitude range. , a graph showing the relationship between spring limit value and tensile strength, Fig. 4B is a graph showing the SN curve, Fig. 5A is an explanatory diagram of the block die method in the present invention, and Fig. 5B is a graph showing the same roller die method. Explanatory drawings, Fig. 6 is a graph showing the relationship between die R, formed diameter and surface residual stress by the roller die method, Fig. 7 is a graph showing a similar relationship by the block die method, and Fig. 8 is a spiral spring of the present invention. FIGS. 9A to 9D are side views showing other manufacturing examples for obtaining the mainspring spring of the present invention.

Claims (1)

[Claims] 1 When manufacturing a mainspring spring that is used by forming it into a spiral shape and then winding it backwards, the steel strip for the mainspring spring is squeezed under tension to obtain a spring smaller than the specified diameter. Excellent fatigue properties characterized by imparting compressive residual stress to the surface layer side, where tensile stress acts when the spring is used, through secondary forming in which the forming diameter is expanded to the predetermined diameter by bending in the opposite direction to the forming direction. How to manufacture a mainspring spring.