PRODUCTION OF A TAPER ROD

Abstract: A taper rod producing process by which a taper rod having a profile very near to an aimed profile can be produced with high accuracy. According to the process, during working of a wire diameter gradually decreasing portion of a taper rod in which the diameter of a stock gradually decreases in a feeding direction of the stock, the drawing length of the stock from starting to ending of acceleration of variable speed feeding means is made greater than an aimed length of the wire diameter gradually decreasing portion, and during working of a wire diameter gradually increasing portion in which the diameter of the stock gradually increases, the drawing length of the stock from starting to ending of deceleration of the variable speed feeding means is made smaller than an aimed length of the wire diameter gradually increasing portion of the taper rod.

Description: The invention relates to a process of producing a taper rod, for working a stock in the form of a straight wire or bar into a taper rod which is used as a taper coil spring for an automobile, a railway vehicle or the like. Various processes and equipments for producing a taper rod of the type mentioned are conventionally known. One of such conventional equipments is disclosed, for example, in Japanese Patent Laid-Open No. 60-56416 and Japanese Patent Laid-Open No. 60-56417. This equipment is illustratively shown in Fig. 1. Referring to Fig. 1, the equipment shown comprises two or more pairs of constant speed feed rollers 2 for holding and feeding a metal wire stock 1 at a constant speed, a heating device 3 located downstream of the constant speed feed rollers 2 in a direction of the metal wire stock 1 being fed, a cooling device 4 located further downstream of the heating device 3, two or more pairs of variable speed tension rollers 5 located further downstream of the cooling device 4 for holding and feeding the metal wire or bar stock 1 at a speed higher than the feeding speed of the constant speed feed rollers 2, and a speed controlling device 6 for controlling the speed of the variable speed tension rollers 5. In the equipment, the feeding speed Vi of the variable speed tension rollers 5 is gradually accelerated or decelerated while remaining higher than the feeding speed Vo of the constant speed feed rollers 2 in order to provide a portion of the metal wire stock 1 between the heating device 3 and the cooling device 4 with tensile plastic deformation to work the straight metal wire stock 1 into a taper rod 7. The feeding speed Vi of the variable speed tension rollers 5 for providing a straight metal wire stock 1 with tensile deformation to work the metal wire stock 1 into a tapered shape is controlled under a hypothesis as illustrated in Fig. 2. In particular, where the feeding speed of a metal wire stock 1 being supplied to the heating device 3 is denoted by Vo which is equal to the speed of the constant speed feed rollers 2, the sectional area of the metal wire stock 1 is denoted by Ao, the sectional area of the taper rod 7 after the metal wire stock 1 has been heated, plastically worked and then cooled by the cooling device 4 until it has no more plastic deformation is denoted by Ai, and the feeding speed of the taper rod 7 with the sectional area Ai is denoted by Vi, a following relation stands: Ao x Vo = Ai x Vi Accordingly, if the speed Vi of the variable speed tension rollers 5 is controlled in accordance with a relation Vi = Ao Ai · Vo then a taper rod 7 in which Ai gradually increases or decreases can be produced. When variable speed control of the variable speed tension rollers 5 is considered under such a premise, it can be understood that the variable speed control should be done by a combination of a straight line and simple curves in a V-t chart as shown in Fig. 3. It is to be noted that, in Fig. 3, V represents a feeding speed of a taper rod 7 by the variable speed tension rollers 5, and t a time. However, if a metal wire stock is actually worked to produce a taper rod under such a variable speed control as illustrated in Fig. 3, there is a problem that a product of an aimed shape cannot be obtained in that, relative to an aimed shape indicated by broken lines in Fig. 4 the taper rod becomes thicker within an L₁ tapered section (a section in which the diameter of the metal wire stock gradually decreases from the downstream side toward the upstream side in the feeding direction of the metal wire stock, the section being hereinafter referred to as "wire diameter gradually decreasing portion") so that a taper will be shorter with a substantially great gradient while the taper rod becomes thinner within another L₂ tapered section (a section in which the diameter of the metal wire stock gradually increases from the downstream side toward the upstream side in the feeding direction, the section being hereinafter referred to as "wire diameter gradually increasing portion") so that a taper will be longer with a substantially small gradient as indicated by solid lines in Fig. 4. The inventor of the present patent application made various investigations through several experiments and found out that, in actual taper working, plastic deformation and its fixation of a metal wire stock do not occur suddenly as in the hypothesis of the model of Fig. 2 but deformation starts at a point further upstream as illustrated in Fig. 5 and that a so-called deforming zone 8 between the deformation starting point and a deformation ending point has an influence upon a profile of a taper of a product. In particular, if a metal wire stock being worked is instantaneously stopped to observe a profile of a heated portion of the stock in detail in order to examine a deforming condition of the stock in the deforming zone 8, the profile observed is such as illustrated in Fig. 6 and is thus quite different from that of the model of Fig. 2. As seen in Fig. 6, deformation of the heated stock starts at a rear half portion of the heating device 3 and continues to the inside of the cooling device 4. On the other hand, where a metal wire stock is, for example, made of steel, it is normally heated to a temperature higher than 750°C for working. Where such heating is made the most of to heat treat the steel wire stock for hardening, it is heated to a temperature higher than 900°C. However, it is known that, if a wire stock is worked at such a high temperature, fine cracks of depths of several microns or so will appear along crystal grain boundaries of a surface layer of a product worked from the wire stock. Then, if a product having such cracks is worked into a part such as, for example, a coil spring which will undergo a repetitive load, when the part is used, it is forecast that the cracks may accelerate fatigue of the product and consequently deteriorate the strength of the product. Accordingly, such a product is not preferable. It is an object of the present invention to provide a process by which a taper rod having a profile very near to an aimed profile can be produced with high accuracy. It is another object of the present invention to provide a process by which a taper rod having a profile very near to an aimed profile can be produced while preventing appearance of fine cracks in a surface layer of the taper rod produced. The present invention was made perceiving that there exists a deforming zone of a metal wire or bar stock in a location from the inside of a heating device to the inside of a cooling device in an equipment for working a metal wire or bar stock into a taper rod. It was also perceived that little or no fine cracks are caused to appear by preventing a wire or bar stock at a high temperature from contacting with oxygen when the wire or bar stock is being worked. In order to attain the objects, according to the present invention, a process of producing a taper rod is provided, using a taper rod producing equipment which includes constant speed feeding means for holding and feeding a metal wire of bar stock at a constant speed, a heating device located downstream of said constant speed feeding means in a feeding direction of the metal wire or bar stock, a cooling device located further downstream of said heating device, variable speed feeding means located further downstream of said cooling device for holding and feeding the metal wire or bar stock at a variable speed, whereby the feeding speed of said variable speed feeding means is accelerated to provide the metal wire or bar stock passing between said heating device and said cooling device with plastic deformation in order to form a wire diameter gradually decreasing portion of a predetermined length of a taper rod in which the diameter of the metal wire or bar stock gradually decreases, and the feeding speed of said variable speed feeding means is decelerated in order to form a wire diameter gradually increasing portion of a predetermined length of the taper rod in which the diameter of the metal wire or bar stock gradually increases, said process of producing a taper rod is characterized in that during working of the wire diameter gradually decreasing portion of the taper rod, the drawing length of the metal wire or bar stock from starting to ending of the acceleration of said variable speed feeding means is made greater than the predetermined length of the wire diameter gradually decreasing portion, and during working of the wire diameter gradually increasing portion, the drawing length of the metal wire or bar stock from starting to ending of the deceleration of said variable speed feeding means is made smaller than the predetermined length of the wire diameter gradually increasing portion of the taper rod. The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings. Fig. 1 is a schematic representation illustrating a taper rod producing equipment for carrying out the process of producing a taper rod; Fig. 2 is a schematic representation illustrating a hypothesis of a conventional working theory; Fig. 3 is a speed - time chart of a variable speed tension roller in accordance with the conventional working theory; Fig. 4 is a schematic representation illustrating a profile of a taper rod produced in accordance with the conventional working theory; Fig. 5 is a schematic representation illustrating a working theory in accordance with the present process of producing a taper rod; Fig. 6 is a detailed view of part of Fig. 5; Fig. 7 is a front elevational view illustrating a profile of a taper rod produced in accordance with the process of producing a taper rod; Fig. 8 is a speed - time chart of a variable speed tension roller upon working of a wire diameter gradually decreasing portion of a metal wire stock; Fig. 9 is a diagram illustrating changes in profile of a wire diameter gradually decreasing portion of a metal wire stock in accordance with changes in overpull amount when the speed of a variable speed tension roller is changed in accordance with an equation of the first degree to work the wire diameter gradually decreasing portion of the metal wire stock; Fig. 10 is a diagram illustrating changes in profile of a wire diameter gradually decreasing portion of a metal wire stock in accordance with changes in overpull amount when the speed of a variable speed tension roller is changed in accordance with an equation of the second degree to work the wire diameter gradually decreasing portion of the metal wire stock; Fig. 11 is a speed - time chart of a variable speed tension roller upon working of a wire diameter gradually increasing portion of a metal wire stock; Fig. 12 is a digram illustrating changes in profile of a wire diameter gradually increasing portion in accordance with changes in underpull amount; Fig. 13 is a speed - time chart of a variable speed tension roller according to an embodiment of the present process of producing a taper rod; and Fig. 14 is a diagram illustrating a profile of a taper rod produced by the process of the embodiment illustrated in Fig. 13. At first, it was attempted to produce, using such a taper rod producing equipment as shown in Fig. 1, a taper rod having a profile as shown in Fig. 7 from a metal wire or bar stock, or from a straight round bar 1 of a stock diameter D₀ by varying the feeding speed V of tension rollers 5 serving as variable speed feeding means while keeping constant the feeding speed V O of constant speed rollers 2 serving as constant speed feeding means. Here, the predetermined profile of a taper rod to be produced has following dimensions: Larger diameter straight portion diameter D₁ = D₀ = 13.8 mm Smaller diameter straight portion diameter D₂ = 10.9 mm Larger diameter straight portion length L₄ = 550 mm Smaller diameter straight portion length D₃ = 270 mm Wire diameter gradually decreasing portion length Lth 1 = 344 mm Wire diameter gradually increasing portion length Lth 2= 344 mm In shaping the wire diameter gradually decreasing portion (L₁ taper) of the wire stock 1, following equations stand: V A = V O where V A is a feeding speed of the variable speed tension rollers 5 at a wire diameter gradual decrease starting point A in a section A-B on the V-t chart of the variable speed tension rollers shown in Fig. 8, and V B is a feeding speed of the variable speed tension rollers 5 at a wire diameter gradual decrease ending point B in the section A-B. It is to be noted that V B is equal to the feeding speed of the variable speed tension rollers 5 during shaping of the smaller diameter straight portion of the taper rod. If the feeding speed V of the variable speed tension rollers 5 in the section A-B is varied in accordance with an equation of the first degree as indicated by a curve 1 in Fig. 8, it is given by a following equation: V = V A + α · t provided here, Accordingly, the actual drawing length Lreal 1 in the section is given by Thus, taper rods were produced wherein wire diameter gradually decreasing portions thereof were shaped varying the difference between the Lreal 1 and the Lth 1 above, that is, the overpull amount ΔL₁ = Lreal 1 - Lth 1, to three stages of 0 mm, 170 mm and 300 mm, and profiles of the wire diameter gradually degreasing portions of the taper rods were examined. The results are illustrated in Fig. 9. It is to be noted that ΔL₁ can be varied by varying Δt = t B - t A . As can be seen from Fig. 9, in the case of the overpull amount ΔL₁ = 0, that is, where the drawing length of the variable speed tension rollers 5 is equal to the predetermined length of a product, the length of the wire diameter gradually decreasing portion is significantly shorter than the predetermined length Lth 1 = 344 mm. To the contrary, in the case of ΔL₁ = 170 mm and 330 mm, the length and profile of the wire diameter gradually decreasing portion are very near to or substantially coincident with the predetermined length and profile. On the other hand, if the feeding speed of the variable speed tension rollers 5 in the section A-B is varied in accordance with an equation of the second degree as indicated by a curve 2 in Fig. 8, it is given by a following equation: V = V A + α · t + β · t² and similarly as in the equation of the first degree, ΔL₁ = Lreal 1 - Lth 1 Thus, profiles of taper rod wire diameter gradually decreasing portions when ΔL₁ = 0 mm, 170 mm and 300 mm are illustrated in Fig. 10. Also in the case of Fig. 10, similar tendencies to those of Fig. 9 can be seen, but the profiles of the wire diameter gradually decreasing portions exhibit a more linear variation. It is to be noted that, in Fig. 9 and 10, a tendency can be observed that the greater the overpull amount ΔL₁, the greater the length of the taper portion, and there is an optimum value in ΔL₁. As apparent from the foregoing description, in order to shape a wire diameter gradually decreasing portion of the metal wire stock 1 into a profile of predetermined dimensions while passing the metal wire stock 1 from a heating device 3 to a cooling device 4, it is necessary to set the drawing length Lreal 1 of the wire stock from starting to ending of acceleration of the variable speed tension rollers 5 to a value greater than the predetermined length Lth 1 of the wire diameter gradually decreasing portion, that is, to take a suitable overpull amount ΔL₁. It is to be noted that while in Figs. 9 and 10 the speed variations in the section A-B depend upon equations of the first and second degrees, respectively, if it is intended to make the profile of the wire diameter gradually decreasing portion further closer to a straight one, a more accurate approximate expression may be experimentally produced suitably. Further, in order to make shapes at starting and ending points of the wire diameter gradually decreasing portion of the metal wire stock 1 more accurate, it is desirable to temporarily increase the acceleration around the starting point of the wire diameter gradually decreasing portion and temporarily decrease the acceleration around the ending point of the wire diameter gradually decreasing portion as indicated by a curve 3 in Fig. 8. In particular, when a drawing force is caused to act upon the metal wire stock 1 to provide the metal wire stock 1 with plastic deformation, the metal wire stock 1 yields deformation not immediately but after some elastic deformation. Accordingly, the metal wire stock 1 is not deformed just when acceleration of the variable speed tension rollers 5 is started, but its deformation begins after some time lag. In order to minimize the time lag, the acceleration should be increased instantaneously. Meanwhile, the reason why the acceleration is decreased around the ending point of the wire diameter gradually decreasing portion is that otherwise if the high speed is maintained, it is sometimes difficult to fix the speed to V B instantaneously at the point of time t B . Now, the wire diameter gradually increasing portion (L₂ taper) will be described. In shaping the L₂ taper portion, the feeding speed of the variable speed tension rollers 5 at a wire diameter gradual increase starting point C in a section C-D on the V-t chart of the variable speed tension rollers 5 shown in Fig. 11 is denoted by V C (= V B ), and the feeding speed at a wire diameter gradual increase ending point D in the section C-D is denoted by V D . If the speed of the variable speed tension rollers 5 in the section C-D is varied in accordance with an equation of the second degree as indicated by a curve 4 in Fig. 11, the speed in the section is represented by a following expression: V = V D - γ · t + δ · t² Accordingly, the actual drawing length Lreal 2 in the section is given by Thus, taper rods were produced wherein wire diameter gradually increasing portions were shaped varying the difference between Lth 2 and Lreal 2, that is, the underpull amount ΔL₂ given by ΔL₂ = Lth 2 - Lreal 2 to 3 stages of ΔL₂ = 0 mm, 100 mm and 170 mm, and profiles of the thus produced taper rods were examined. The results are illustrated in Fig. 12. As can be seen from Fig. 12, in the case of the underpull amount ΔL₂ = 0, that is, where the drawing length of the variable speed tension rollers 5 is equal to the predetermined length of a product, the length of the wire diameter gradually increasing portion is significantly longer than the predetermined length of 344 mm. To the contrary, in the case of ΔL₂ = 100 mm, the length and profile of the wire diameter gradually increasing portion are substantially coincident with the predetermined length and profile. However, in the case of ΔL₂ = 170 mm, the length of the wire diameter gradually increasing portion is too small. Accordingly, it will be appreciated that there is an optimum value in ΔL₂. As apparent from the foregoing description, in order to shape the wire diameter gradually decreasing portion of the metal wire stock 1 into a profile of predetermined dimensions, it is necessary to set the drawings length Lreal 2 of the wire stock from starting to ending of deceleration of the variable speed tension rollers 5 to a value smaller than the predetermined length Lth 2 of the wire diameter gradually increasing portion, that is, to take a suitable underpull amount ΔL₂. Also in working for the wire diameter gradually increasing portion, in order to approximate the taper shape to a linear one and further make shapes at starting and ending points of the wire diameter gradually increasing portion of the metal wire stock 1 more accurate, it is desirable to temporarily increase the deceleration around the starting point of the wire diameter gradually increasing portion and temporarily decrease the deceleration (or acceleration) around the ending point of the wire diameter gradually increasing portion (refer to a curve 5 shown in Fig. 11). Here, prevention of appearance of fine cracks in a surface layer of a taper rod will be described with reference to Fig. 1. The taper rod producing equipment shown further includes an oxidation preventing housing 8 located at the same location with the work coil or heating device 3 along the feeding direction of the metal wire stock 1. Thus, the oxidation prevention housing 8 is constructed such that the metal wire stock 1 may pass the inside thereof and includes a cylindrical member 9 of a ceramics material or the like fitted in the work coil 3, and a ring-formed gas supply member 10 securely mounted at a forward end of the cylindrical member 9 along the feeding direction of the metal wire stock 1. It is to be noted that a ring-formed seal member may be provided at the opposite rear end of the cylindrical member 9 in order to promote a sealing effect. The gas supply member 10 is supplied with non-oxidizing gas such as N₂ gas, Ar gas or denatured gas from an external gas supply apparatus (not shown) to isolate the metal wire stock 1 at a high temperature (700 to 1000°C where the metal wire stock 1 is made of steel) within the work coil 3 from oxygen. In this manner, the metal wire stock 1 is heated to a predetermined temperature by the heating device 3 within such a non-oxidizing atmosphere as described above and then cooled within the cooling device 4 which is filled with coolant. After all, during working of the metal wire stock 1, while the metal wire stock 1 is maintained at a high temperature (for example, a temperature higher than 500 °C), it is prevented from contacting with oxygen. As a result of working of the metal wire stock 1 under such a circumstance, little or no fine cracks will appear in a surface layer of the metal wire stock 1. Accordingly, a product having a good surface quality can be obtained. It is to be noted that it is possible to provide, in place of the oxidation preventing housing 9 as shown in Fig. 1, a film forming device such as, for example, a nickel plating device or an oxidation preventing coating device for forming a film on an outer periphery of a wire stock for preventing the metal wire stock 1 from contacting with oxygen, between the constant speed feeding means (constant speed rollers) 2 and the heating device (work coil) 3. Now, an embodiment of the present process of producing a taper rod will be described. Taper rods were produced using the equipment as shown in Fig. 1. Details of a sample stock used and a taper rod to be produced were as follows: Sample Stock: low alloy steel wire 14.12 mm ⊘ (C 0.56 %, Si 1.44 %, Mn 0.72 %, Cr 0.72 %, the rest being Fe and impurities) Profile of taper rod (predetermined) Larger diameter straight portion: Diameter 14.0 mm, Length 595 mm Smaller diameter straight portion: Diameter 10.7 mm, Length 256 mm Tapered portion (wire diameter gradually decreasing portion, wire diameter gradually increasing portion): Length 790 mm Meanwhile, working conditions were as follows: Heating: high-frequency heating device (with oxidation preventing housing and supply of N₂ gas) Highest heating temperature 950°C Cooling: ring nozzle cooling device water soluble quenching liquid (concentration 20 %) Working speed, overpull amount, underpull amount Working of large diameter straight portion: drawing speed = 1.5 m/min Working of small diameter straight portion: drawing speed = 2.567 m/min Feeding speed by constant speed feed rollers = 1.475 m/min Working of wire diameter gradually decreasing portion Working speed: as illustrated in Fig. 13 Overpull amount (ΔL₁): 327 mm Working of wire diameter gradually increasing portion Working speed: as illustrated in Fig. 13 Underpull amount (ΔL₂): 93 mm A profile of a taper rod actually produced is illustrated in Fig. 14. In Fig. 14, the profile of the actually produced taper rod is indicated by a solid line, and it can be seen, from comparison with an predetermined profile which is indicated in a broken line in Fig. 14, that the predetermined profile is almost attained with predetermined dimensions. Besides, the taper rod has a good surface quality with little or no fine cracks appearing in a surface layer thereof. As apparent from the foregoing description, the process according to the present invention is advantageous in that, in producing a taper rod from a straight metal wire stock 1 using a so-called heating drawing shaping method, a taper rod of which tapered portions (a wire diameter gradually decreasing portion and a wire diameter gradually increasing portion) have almost predetermined lengths and profiles can be produced readily, and accordingly a taper rod of a very high quality as a stock for a taper coil spring for use with an automobile, a railway vehicle and so on can be obtained efficiently.

Claims: A process of producing a taper rod using a taper rod producing equipment which includes constant speed feeding means (2) for holding and feeding a metal wire or bar stock (1) at a constant speed, a heating device (3) located downstream of said constant speed feeding means (2) in a feeding direction of the metal wire or bar stock (1), a cooling device (4) located further downstream of said heating device (3), variable speed feeding means (5) located further downstream of said cooling device (4) for holding and feeding the metal wire or bar stock (1) at a variable speed, whereby the feeding speed of said variable speed feeding means (5) is accelerated to provide the metal wire or bar stock (1) passing between said heating device (3) and said cooling device (4) with plastic deformation in order to form a wire diameter gradually decreasing portion of a predetermined length of a taper rod in which the diameter of the metal wire or bar stock (1) gradually decreases, and the feeding speed of said variable speed feeding means is decelerated in order to form a wire diameter gradually increasing portion of a predetermined length of the taper rod in which the diameter of the metal wire or bar stock (1) gradually increases, characterized in that during working of the wire diameter gradually decreasing portion of the taper rod, the drawing length of the metal wire or bar stock (1) from starting to ending of the acceleration of said variable speed feeding means (5) is made greater than the predetermined length of the wire diameter gradually decreasing portion, and during working of the wire diameter gradually increasing portion, the drawing length of the metal wire or bar stock (1) from starting to ending of the deceleration of said variable speed feeding means (5) is made smaller than the predetermined length of the wire diameter gradually increasing portion of the taper rod. A process of producing a taper rod according to claim 1, characterized in that during working of the wire diameter gradually decreasing portion of the taper rod, the acceleration is temporarily increased upon starting of the working and temporarily decreased upon ending of the working. A process of producing a taper rod according to claim 1, characterized in that during working of the wire diameter gradually increasing portion of the taper rod, the deceleration is temporarily increased upon starting of the working, and the feeding speed of said variable speed feeding means (5) is temporarily accelerated upon ending of the working. A process of producing a taper rod according to any one of the claims 1 to 3, characterized in that during working of the metal wire or bar stock (1), the metal wire or bar stock (1) at a high temperature is prevented from contacting with oxygen. A process of producing a taper rod according to claim 4, characterized in that the metal wire or bar stock (1) at a high temperature is surrounded by a non-oxidizing atmosphere. A process of producing a taper rod according to claim 4, characterized in that a film for preventing the metal wire or bar stock (1) from contacting with oxygen is formed on an outer periphery of the metal wire or bar stock (1) before the metal wire or bar stock is heated.

Inventors: KATSUBE, KOZO, KAWAGUCHI, YASUNOBU, KAWAKAMI, HEIJIRO, MURAHASHI, MAMORU, TAKADA, SUSUMU
Applicants: KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1987
Main classification: B21C37
IPC codes: C21D8, B21C37