JP2010018066A - Rack and pinion type steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rack and pinion type steering device hardly damaging a rack even when applied with impact or stress in using. <P>SOLUTION: The rack 21 for the rack and pinion type steering device is made of steel having a carbon content of ≥0.35 mass% and ≤0.55 mass%. That is, the rack is manufactured in a manner that it is molded by cold forging after rod-like or tubular material made of steel is soft-annealed, and is further subjected to induction hardening. The surface of the rack 21 is formed of a hardened layer by induction hardening, and surface hardness HV of a tooth of the rack is 550 or more. Regarding a part of the hardened layer having surface hardness HV of 300 or more and 550 or less, when relationship between hardness H and depth D (unit; mm) is shown by the expression, H=aD+b, a coefficient (a) of the expression is -750 or more and -120 or less. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a rack and pinion type steering device used for an automobile or the like.

  In passenger cars, a rack and pinion mechanism is mainly used as a mechanism for converting the rotation of the steering shaft into the motion of the left and right steered wheels because of its high rigidity and light weight. The rack of the rack and pinion type steering device is made of a medium carbon steel material (for example, a steel material corresponding to S35 to S55C), and is usually manufactured as follows. That is, after quenching and tempering a rod-shaped material obtained by rolling a medium carbon steel material, teeth that mesh with the teeth of the pinion are formed by cutting, and induction hardening is performed on the teeth. As described above, since the rack is formed by cutting, there is a problem in that the manufacturing requires a lot of labor and time and is expensive.

Therefore, there is a method of forming the rack by plastic working. According to the plastic working, the manufacturing cost is reduced because much labor and time are not required for the manufacturing as compared with the cutting. In recent years, it has been proposed to form a rack by plastic working using a tubular material such as a pipe instead of a rod-shaped material (see Patent Documents 1 to 6). By manufacturing the rack from a tubular material, the weight of the automobile can be reduced.
JP-A-10-58081 Japanese Patent Laid-Open No. 2001-79639 Japanese Patent No. 3442298 JP 2006-103644 A JP 2007-105751 A JP 2007-144433 A

In recent years, the usage conditions of automobiles have become stricter, and the performance required for rack and pinion type steering devices has been increasing year by year. For example, it is required that the rack has an excellent static strength that does not cause damage even when an impact or stress is input when the automobile rides on a curb or the like.
SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a rack-and-pinion type steering apparatus that solves the problems of the prior art as described above and is unlikely to cause damage to the rack even when an impact or stress is input during use.

  In order to solve the above problems, the present invention has the following configuration. That is, the rack and pinion type steering device according to claim 1 of the present invention includes a steering shaft that is rotated by a driver's steering, a pinion that is connected to the steering shaft and rotates as the steering shaft rotates, and the pinion The rack and pinion type steering apparatus includes a rack that meshes with the rack and that is coupled to the wheel, wherein the rack satisfies the following four conditions.

Condition A) It is made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less.
Condition B) A hardened layer is formed on the surface by induction hardening.
Condition C) The tooth surface hardness HV is 550 or more.
Condition D) When the relationship between the hardness H and the depth D (unit: mm) is expressed by the formula H = aD + b for the portion of the hardened layer having a hardness HV of 300 or more and 550 or less, The coefficient a is -750 or more and -120 or less.

Large impacts and stresses may be input to the rack of a rack and pinion type steering device, but if a hardened layer with the above configuration is formed, the static strength (especially bending strength) of the rack is excellent. Therefore, even if a large impact or stress is input, damage is unlikely to occur.
In other words, when induction hardening is performed, tensile residual stress is generated at the interface between the quenched portion (that is, the hardened layer) and the non-quenched portion (the portion that is not subjected to induction hardening inside the hardened layer). If it is large, when a large impact or stress is input to the rack, it tends to break not from the surface but from the inside (the interface), but the hardened layer having the above configuration is formed. Therefore, the occurrence of the above-described tensile residual stress is mitigated, so that it is difficult for the internal fracture to occur, and damage is not easily caused even if a large impact or stress is input to the rack (that is, the static load of the rack). Strength is high).

Here, the critical significance of each numerical value under each condition will be described.
[Condition A]
If the carbon content is less than 0.35% by mass, it may be difficult to make the surface hardness HV of the teeth formed on the rack 550 or more. On the other hand, if it exceeds 0.55 mass%, the workability of steel may be insufficient.

[Condition C]
Since stress is input from the pinion to the rack, the teeth formed on the rack are required to have sufficient strength. In order for the teeth to have sufficient strength, the surface hardness HV of the teeth needs to be 550 or more. In addition, in order for a tooth | gear to have higher intensity | strength, it is preferable that the surface hardness HV of a tooth | gear shall be 600 or more.

[Condition D]
When the coefficient a in the above formula is less than −750, the static strength of the rack becomes insufficient, and the rack may be easily damaged when a large impact or stress is input. On the other hand, if the coefficient a is more than −120, it may take a long time for induction hardening, deformation may occur due to induction hardening, and crystal grains may become coarse.
A rack and pinion type steering apparatus according to claim 2 of the present invention is the rack and pinion type steering apparatus according to claim 1, wherein the rack is made of the steel and is subjected to soft annealing. It is characterized by being obtained by performing the induction hardening after being formed into a predetermined shape by cold forging.

  In forming a material into a rack shape by cold forging, sufficient plastic workability is required. Therefore, it is necessary to first soften and anneal the material. By performing this soft annealing, the carbide in the material is spheroidized. When induction hardening is performed on this spheroidized structure, the structure is cured so as to satisfy the above conditions C and D due to systematic factors. Layers can be easily formed. In addition, since hot forging, cutting, drawing, and the like are unnecessary, the rack and pinion type steering device can be manufactured easily and at low cost.

  Since the rack and pinion type steering device of the present invention has excellent static strength of the rack, the rack is not easily damaged even if an impact or stress is input during use.

An embodiment of a rack and pinion type steering apparatus according to the present invention will be described in detail with reference to FIG.
A steering shaft 11 having a steering wheel 10 fixed to the upper end portion is supported inside a steering shaft housing 12 so as to be rotatable about an axis. Further, the steering shaft housing 12 is fixed at a predetermined position in the vehicle interior in a posture in which the lower portion is inclined toward the front of the vehicle.

  The rack and pinion mechanism that converts the rotation of the steering shaft 11 into the movement of the left and right steered wheels 15 and 15 is supported by the rack 21 that is movable in the axial direction and the rack 21 teeth that are supported obliquely with respect to the axis of the rack 21. And a pinion 22 having teeth meshing with each other, and a rack 21 and a cylindrical rack housing 23 that supports the pinion 22. The rack and pinion mechanism is arranged substantially horizontally in the engine room at the front of the vehicle so that the longitudinal direction thereof is along the width direction of the vehicle.

Further, the upper end portion of the pinion 22 and the lower end portion of the steering shaft 11 are connected by two universal joints 25 and 26. Further, steered wheels 15 and 15 are connected to both ends of the rack 21.
When a steering torque (rotational force) is applied to the steering wheel 10 by the driver, the steering shaft 11 rotates, and the pinion 22 rotates as the steering shaft 11 rotates. Then, the rotation of the pinion 22 is converted into a slide motion in the left-right direction of the rack 21 by the rack and pinion mechanism, and the steered wheels 15 and 15 are driven to steer the automobile.

  The rack and pinion type steering device of the present embodiment may be provided with a so-called power steering mechanism. That is, the steering torque is detected by a torsion bar (not shown) attached to the steering shaft 11, and the output of the electric motor 13 (rotational force assisting steering) is controlled based on the detected steering torque. The output of the electric motor 13 is supplied to an intermediate portion of the steering shaft 11 (may be supplied to the pinion 22), and is combined with the steering torque to drive the steered wheels 15 and 15 by a rack and pinion mechanism. Converted into motion.

  In this rack and pinion type steering apparatus, the rack 21 is made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less. Then, the rack 21 is formed into a predetermined shape by cold forging after softening and annealing the rod-like material or tubular material (hereinafter sometimes referred to as a material) made of steel as described above, and further induction-hardened. It is manufactured by giving.

  Since a hardened layer (not shown) is formed on the surface of the rack 21 thus manufactured by induction hardening, the tooth surface hardness HV of the rack 21 is 550 or more. Furthermore, this hardened layer has the following configuration. That is, when the relationship between the Vickers hardness H and the depth D (unit: mm) is expressed by an equation H = aD + b for a portion of the cured layer having a hardness HV of 300 to 550, the coefficient of the equation a is -750 or more and -120 or less. Since the carbides in the steel are spheroidized by the soft annealing, a hardened layer having the above-described configuration is formed by subsequent induction hardening.

  In such a rack-and-pinion steering device, the rack 21 has excellent static strength, so that it is difficult to cause damage even if a large impact or stress is input. Further, since the plastic workability of the material is excellent due to the soft annealing, damage such as cracks is hardly generated when the rack 21 is formed by cold forging. Furthermore, since this rack and pinion type steering device can be manufactured without performing hot forging, cutting, drawing, etc., it can be manufactured easily and at low cost.

  Below, an example of the manufacturing method of the rack 21 is demonstrated. First, soft annealing is performed on a rod-like material or a tubular material made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less. An example of the conditions of softening annealing is shown. The material is held at 740 to 860 ° C. above the A1 transformation point for 0.1 hour or more, then cooled to 680 to 720 ° C. at a cooling rate of 20 to 70 ° C./h, and held for 1 to 5 hours. Then, it cools to 620-680 degreeC with the cooling rate of 10-100 degreeC / h, and also cools to 500-560 degreeC with the cooling rate of 10-150 degreeC / h. By such soft annealing, the steel becomes a structure containing ferrite, spheroidized cementite, and acicular cementite.

Next, the material subjected to soft annealing is subjected to cold forging as shown in FIGS. 2 and 3 and formed into a predetermined shape. First, an example in which a solid bar material having a circular cross section is formed by cold forging will be described with reference to FIG.
First, the solid rod-shaped material 31 is placed on the die 32 in which the groove 32a having an arcuate cross section is formed. The curvature radius of the groove 32a is set larger than the radius of the solid rod-shaped material 31, and the solid rod-shaped material 31 is disposed in the groove 32a (see FIG. 2A). Then, by pressing the punch 33 against the solid rod-shaped material 31 from above and pressing the punch 33 downward, the solid rod-shaped material 31 is brought into close contact with the groove 32a and deformed. Since the contact surface of the punch 33 with the solid rod-shaped material 31 is flat, a flat portion 31a is formed on the solid rod-shaped material 31 (see FIG. 2B). Thereby, the solid rod-shaped material 31 becomes a shape having the flat surface portion 31a and the cylindrical surface portion 31b. In addition, the contact surface with the solid rod-shaped material 31 of the punch 33 is not limited to a planar shape, and may have a taper.

  Next, a die 34 having a groove 34a deeper than the above-described groove 32a is prepared, and the solid bar-shaped material 31 is disposed in the groove 34a with the cylindrical surface portion 31b facing downward (bottom side of the groove 34a). . At this time, since the width of the groove 34a is designed to be slightly smaller than the diameter of the solid rod-shaped material 31, the solid rod-shaped material 31 is not completely accommodated in the groove 34a ((c) in FIG. 2). See). Then, when the punch 35 having the tooth groove 35 a is pressed against the flat portion 31 a of the solid bar-shaped material 31 and the punch 35 is pressed downward, the solid bar-shaped material 31 is pushed into the groove 34 a of the die 34. At that time, both sides of the solid bar-shaped material 31 (the portions contacting the side walls of the grooves 34a of the die 34) are squeezed and deformed to become parallel planes, and the teeth 31c corresponding to the tooth grooves 35a are formed. It is formed on the flat portion 31a of the solid rod-shaped material 31 (see (d) of FIG. 2). And the meat for the squeezed portions on both sides of the solid rod-shaped material 31 is supplied to the teeth 31c, and the shape of the teeth 31c becomes larger.

  Next, a die 36 in which a groove 36a having a substantially rectangular cross section is formed and a punch 37 in which a groove 37a having a circular arc cross section is prepared. On the bottom surface of the groove 36a formed in the die 36, a tooth groove 36b corresponding to the tooth 31c formed in the flat portion 31a of the solid rod-shaped material 31 is formed. On the other hand, the groove 37 a formed in the punch 37 has a shape corresponding to the cylindrical surface portion 31 b of the solid rod-shaped material 31.

  When the solid rod-shaped material 31 is disposed in the groove 36a formed in the die 36, the width of the groove 36a is slightly smaller than the diameter of the solid rod-shaped material 31, so that the solid rod-shaped material 31 is completely in the groove 36a. Is not accommodated (see FIG. 2E), but when the punch 37 is pressed against the solid bar material 31 from above and pressed downward, the solid bar material 31 is pushed into the groove 36a of the die 36, The shape of the actual bar-shaped material 31 is adjusted. Since both sides of the solid bar-shaped material 31 (parts contacting the side wall of the groove 36a of the die 36) are flat, no surplus is produced when the solid bar-shaped material 31 is pushed into the groove 36a of the die 36. (See (f) in FIG. 2).

  Next, an example in which a tubular material 41 having a circular cross section is formed by cold forging will be described with reference to FIG. In FIG. 3, the same or corresponding parts as those in FIG. 2 are denoted by the same reference numerals as those in FIG. Since the forming method is exactly the same as that of the solid rod-shaped material 31, detailed description is omitted. However, when the tubular material 41 is formed by cold forging, the diameter of the hollow hole 42 is set in the hollow hole 42. It is preferable to put another member 43 having substantially the same diameter. At the stage of first deformation (see FIG. 3B), the hollow member 42 is filled with the separate member 43, and the tubular material 41 and the separate member 43 are integrated.

  The material formed into the shape of the rack in this way was subjected to induction hardening and tempering to form a hardened layer on its surface, and then subjected to grinding and superfinishing to complete the rack 21. The conditions for induction hardening are not particularly limited, and examples include conditions such as an output current of 220 to 270 A, a frequency of 100 kHz, a heating time of 3 to 5 seconds, and a cooling time of 10 to 15 seconds.

〔Example〕
Hereinafter, the present invention will be described more specifically with reference to examples. The round bar material having a diameter of 25 mm made of S35C, S45C, or S55C defined in Japanese Industrial Standards JIS was softened and annealed under the same conditions as in the above example. Such a material was cold forged as shown in FIG. 2 and formed into a rack shape. And after performing induction hardening and forming the hardened layer on the surface, tempering was performed at 170 degreeC for 2 hours, and the rack of the Example was manufactured.

In the case of the rack of the comparative example, it was manufactured in substantially the same manner as in the case of the rack of the example, but in order to form a hardened layer having the same configuration as the conventional product, a vacuum was formed between the cold forging and the induction hardening. A process of applying a heat treatment of holding at 1150 ° C. for 30 minutes and then air cooling was incorporated.
Each of the racks of Examples and Comparative Examples has a hardened layer formed on the surface by induction hardening, and the surface hardness HV of the rack teeth is 550 or more. Moreover, when the relationship between the Vickers hardness H and the depth D (unit: mm) is expressed by an equation H = aD + b for a portion of the cured layer having a Vickers hardness HV of 300 or more and 550 or less, The coefficient a is from −750 to −120. The conditions for induction hardening are an output current of 200 to 270 A, a heating time of 3 to 5 seconds, and a cooling time of 10 to 20 seconds. By changing these conditions, a rack having hardened layers having different coefficients a can be used. Manufactured.

For these racks, Vickers hardness HV at various depth positions of the cured layer was measured. The Vickers hardness HV was measured with a Vickers hardness meter (load load 9.8 N).
An example of the relationship between Vickers hardness and depth is shown in the graph of FIG. As can be seen from this graph, the configuration of the hardened layer is different between the example and the comparative example. Then, from this graph, the coefficient a can be obtained when the relationship between the Vickers hardness H and the depth D (mm) is expressed by the equation H = aD + b. FIG. 5 is a graph showing a method for obtaining the coefficient a by the least square method with respect to the plot of Example 1 of the graph of FIG. The results of obtaining the coefficient a of each rack as shown in the graph of FIG. Note that the method for obtaining the coefficient a is not limited to the least square method.

These racks were mounted on a test apparatus (see FIG. 6) having a structure similar to a rack and pinion type steering apparatus, and the static strength was measured by inputting a load until the rack broke. The results are summarized in Table 1 and the graph of FIG. In addition, the numerical value of the static strength of the graph of Table 1 and FIG. 7 is shown by the relative value when the static strength of the comparative example 1 is set to 1 about Examples 1-4. The static strength of Comparative Example 2 is shown for Examples 5 to 8, and the static strength of Comparative Example 3 is shown as 1 for Examples 9 to 12, respectively.
From the graph of FIG. 7, it can be seen that the static strength of the rack is high when the coefficient a is −750 or more.

It is a figure explaining the composition of the rack and pinion type steering device concerning the present invention. It is a figure explaining the process of giving cold forging to a rod-shaped raw material, and shape | molding in the shape of a rack. It is a figure explaining the process of giving cold forging to a tubular raw material and shape | molding in the shape of a rack. It is a graph which shows the relationship between the Vickers hardness and the depth of the hardened layer of a rack. It is a figure explaining the method of calculating | requiring the coefficient a. It is a figure which shows the structure of a static strength test apparatus. It is a graph which shows the relationship between the static strength of a rack, and the coefficient a.

Explanation of symbols

11 Steering shaft 21 Rack 22 Pinion 31 Bar material 41 Tubular material

Claims (2)

A rack-and-pinion type steering including: a steering shaft that is rotated by a driver's steering; a pinion that is connected to the steering shaft and that rotates as the steering shaft rotates; and a rack that meshes with the pinion and is connected to a wheel. A rack and pinion type steering device, wherein the rack satisfies the following four conditions:
Condition A) It is made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less.
Condition B) A hardened layer is formed on the surface by induction hardening.
Condition C) The tooth surface hardness HV is 550 or more.
Condition D) When the relationship between the hardness H and the depth D (unit: mm) is expressed by the formula H = aD + b for the portion of the hardened layer having a hardness HV of 300 or more and 550 or less, The coefficient a is -750 or more and -120 or less.   The rack is obtained by forming the material made of the steel and subjected to soft annealing into a predetermined shape by cold forging and then performing the induction hardening. The rack and pinion type steering device according to claim 1.

Images