JP2007191091A - Rack and pinion type steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rack and pinion type steering device having high bending strength in a rack and high pitching strength in teeth formed in the rack. <P>SOLUTION: Nitriding is applied to the rack 21 of the rack and pinion type steering device, and high-frequency sintering is applied to the teeth formed in the rack 21. The surface of the tooth is formed of a nitride layer having a thickness of 3μm - 5μm, and a ratio HVs/HVc of the surface hardness HVs to the root hardness HVc of the tooth is set at 1.05 or more. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  Although the steering device is designated as an important safety part, the rack also has a skeletal role, so even if an excessive load is applied, such as when an automobile rides on a curb, it will break down significantly. It is hoped not to. Therefore, in addition to high bending strength required for the rack, high pitching strength is required for the teeth meshing with the pinion.

In order to satisfy such requirements, the rack is usually made of medium carbon steel (see Patent Document 1). That is, after quenching and tempering a steel bar obtained by rolling medium carbon steel, teeth are formed by cutting, and a rack is manufactured by subjecting the teeth to induction hardening.
JP 2000-153336 A

However, in the conventional rack, the bending strength and the pitching strength of the teeth may not be sufficient, and further strength improvement has been demanded.
SUMMARY OF THE INVENTION Accordingly, the present invention solves the problems of the prior art as described above, and provides a rack and pinion type steering device having high rack bending strength and high tooth pitching strength formed on the rack. And

  In order to solve the above problems, the present invention has the following configuration. In other words, the rack and pinion type steering apparatus according to the first aspect of the present invention includes a steering shaft that is rotated by a driver's steering, a pinion that is coupled to the steering shaft, a rack that meshes with the pinion and is coupled to a wheel. And a rack and pinion type steering apparatus, the rack is subjected to nitriding treatment, and the teeth formed in the rack are further subjected to induction hardening, and the thickness is 3 μm or more and 15 μm or less. A nitride layer is formed on the tooth surface, and a ratio HVs / HVc between the surface hardness HVs of the tooth and the hardness HVc of the root center portion is 1.05 or more.

  The rack and pinion type steering device of the present invention has high bending strength of the rack and high pitching strength of the teeth formed on the rack. Therefore, even if a large load is applied, the rack and pinion type steering device is not easily damaged.

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 with the lower portion inclined toward the front of the vehicle.

  The rack and pinion mechanism that converts the rotation of the steering shaft 11 into the movements 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. 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 for 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 movement.

  The rack 21 is subjected to the following heat treatment. First, a rod-shaped steel material is processed to form teeth and formed into a rack shape. Next, nitriding is performed to form a nitride layer having a thickness of 3 μm or more and 15 μm or less over the entire surface of the rack including the tooth surface, and nitrogen is diffused inside. Then, induction hardening is performed on the tooth surface. By the induction hardening process, the nitride in the nitride layer is decomposed to some extent and diffuses into the inside, but the induction hardening process is completed in a very short time, so that the nitride layer remains sufficiently. As a result of such heat treatment, the ratio HVs / HVc between the surface hardness HVs of the teeth and the hardness HVc of the root central portion is 1.05 or more.

The rack thus obtained has been subjected to nitriding treatment and has a high bending strength because nitrogen is more diffused inside by combining nitriding treatment and induction hardening treatment. . Further, since the nitride layer is formed on the teeth, it has high pitching strength.
If the thickness of the nitride layer is less than 3 μm, the thickness of the nitride layer is likely to be non-uniform, and there is a possibility that a portion where the nitride layer is hardly formed may occur, so that a good pitching strength may not be obtained. On the other hand, if the nitriding process is performed such that the thickness of the nitride layer exceeds 15 μm, the processing time becomes long, so a ζ phase is formed on the outermost surface of the nitride layer to become porous, and the pitching strength is significantly reduced. There is a fear. In order to make such inconvenience less likely to occur, the thickness of the nitride layer is more preferably 5 μm or more and 13 μm or less, and further preferably 5 μm or more and 10 μm or less.

  Further, if the above-mentioned hardness ratio HVs / HVc is 1.05 or more, it can be said that the diffusion of nitrogen is sufficiently advanced and the strength is improved, so that the rack has a high bending strength. When the above-mentioned hardness ratio HVs / HVc is less than 1.05, the diffusion of nitrogen is insufficient, so that the strength is hardly improved. In order to further increase the bending strength of the rack, the above-mentioned hardness ratio HVs / HVc is preferably 1.1 or more.

Furthermore, the surface hardness HVs of the rack teeth is preferably less than HV800 and more preferably HV770 or less so as not to damage the pinion. In order to improve the rack pitching life, the surface hardness HVs of the rack teeth is preferably HV650 or more.
Furthermore, the hardness HVc of the root central portion of the rack teeth is preferably HV620 or more, and more preferably HV650 or more in order to further increase the rack bending strength.

Furthermore, in order to make the strength of the root center portion of the tooth after tempering sufficient, it is preferable to configure the rack with steel containing 0.4 mass% or more of carbon. However, in order to suppress the processing cost, the carbon content of the steel is preferably 0.6% by mass or less.
The type of nitriding treatment applied to the rack is not particularly limited, but for example, the following four nitriding treatments are suitable.

(1) A method of maintaining in a mixed gas of nitrogen gas, RX gas and ammonia gas at 400 to 600 ° C. for 2 to 3 hours (gas soft nitriding method)
(2) A method of performing a fluorination treatment at about 200 to 400 ° C. using a gas of a fluorine-based compound such as nitrogen trifluoride, and then holding it in ammonia gas at 400 to 600 ° C. for 1 to 3 hours (air water (NV nitriding process)
(3) Method of dipping in a salt bath and holding at 400 to 600 ° C. for 1 to 3 hours (tuftride method)
(4) A method in which glow discharge is performed at about 400 to 600 ° C. for about 10 hours in a mixed gas atmosphere of nitrogen and hydrogen using a cage as a cathode and an inner wall of a processing furnace as an anode (ion nitriding method)

〔Example〕
The present invention will be described more specifically with reference to the following examples. A tooth is formed on a round bar made of S45C specified in Japanese Industrial Standard JIS by cutting, and gas soft nitriding treatment (however, the treatment conditions are as shown in Table 1) is the same as (1) above. did. And after performing induction hardening processing on the tooth surface, tempering was performed at 170 ° C. for 2 hours to obtain a rack.

And about the obtained rack, tooth surface hardness HVs, tooth root central part hardness HVc, ratio HVs / HVc of both hardness, and bending strength were measured, respectively.
In addition, the surface hardness HVs of a tooth | gear is the value which measured the hardness of the part inside 0.05 mm from the surface. Further, the hardness HVc of the tooth root central portion is determined by the hardness of the central portion (see FIG. 2) of the boundary portion between the base portion of the rack and the tooth after the tooth is cut and the cut surface is polished and mirror finished. It is a measured value.

Furthermore, the bending strength of the rack was measured by a three-point bending test with a distance between supporting points of 100 mm. Installed in a bending tester with the side where the teeth are formed facing downward, and presses the opposite side of the side where the teeth are formed to create the maximum tensile stress in the valley between the teeth The test was conducted until breakage occurred (cracking), and the maximum load was defined as the bending strength of the rack.
Table 1 shows the measurement results. Further, the relationship between the ratio HVs / HVc between the surface hardness HVs of the teeth and the hardness HVc of the root center portion and the bending strength of the rack is shown in the graph of FIG. The bending strength described in the graphs of Table 1 and FIG. 3 is shown as a relative value when the bending strength of Comparative Example 1 is 1. As can be seen from the graphs in Table 1 and FIG. 3, when the ratio HVs / HVc between the surface hardness HVs of the teeth and the hardness HVc of the root center portion is 1.05 or more, the bending strength of the rack was high.

Next, in order to evaluate the pitching strength (tooth surface strength) of the teeth of the rack, the following durability test was performed. That is, the rack was assembled in a rack and pinion type steering device and driven under the following conditions, and the life of the rack was measured.
Pinion: Made of JIS SCr420 material, carburized and tempered.
Surface pressure: 2.5 GPa
Pinion input: 60 min ^-1
Lubricant: Molybdenum grease Test temperature: 80 ° C

At the same time, the thickness of the nitrided layer was measured to evaluate the relationship between the nitrided layer thickness and the rack life. The thickness of the nitride layer was measured by mirror-finishing the cut surface in the same manner as in the case of the hardness HVc at the center of the tooth, corroding with a nital, and observing with an optical microscope. At that time, the thickness of the nitride layer was measured in each of the five fields of view, and the average value thereof was defined as “the thickness of the nitride layer”.
The results are shown in Table 2 and the graph of FIG. In addition, the lifetime described in the graph of Table 2 and FIG. 4 is shown by the relative value when the lifetime of the comparative example 4 is set to 1. As can be seen from the graphs of Table 2 and FIG. 4, when the thickness of the nitride layer is 3 μm or more and 15 μm or less, the life of the rack is excellent.

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 root base part of a tooth. It is a graph which shows the relationship between ratio HVs / HVc of the surface hardness HVs of a tooth | gear, and hardness HVc of a root center part, and the bending strength of a rack. It is a graph which shows the relationship between the thickness of a nitride layer, and the lifetime of a rack.

Explanation of symbols

11 Steering shaft 21 Rack 22 Pinion

Claims (1)

In a rack and pinion type steering apparatus comprising: a steering shaft that is rotated by a driver's steering; a pinion that is coupled to the steering shaft; and a rack that meshes with the pinion and is coupled to wheels.
The rack is nitrided and the teeth formed on the rack are further induction hardened to form a nitrided layer having a thickness of 3 μm or more and 15 μm or less on the tooth surface. A rack-and-pinion steering device, wherein a ratio HVs / HVc between the surface hardness HVs of the teeth and the hardness HVc of the root center portion is 1.05 or more.

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