Energy absorbing steering system

In an energy absorbing steering system including a steering shaft to which a steering member is coupled, and a steering column that rotatably supports the steering shaft, at least one of the steering shaft and the steering column includes an impact absorbing portion that contracts when a load equal to or higher than a predetermined value is applied, and the impact absorbing portion includes a first impact absorbing portion having a first impact absorption load, and a second impact absorbing portion having a second impact absorption load different from the first impact absorption load.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-119319 filed on Apr. 27, 2007, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to energy absorbing steering systems.

2. Description of Related Art

An energy absorbing steering system is proposed which absorbs impact energy when an interior impact occurs upon a collision of the driver with a steering member, such as a steering wheel, following a collision of a vehicle with another vehicle or object. An example of the energy absorbing steering system is disclosed in Japanese Patent Application Publication No. 2000-159043 (JP-A-2000-159043).

In the energy absorbing steering system as disclosed in JP-A-2000-159043, a steering shaft, or the like, undergoes plastic deformation and contract upon occurrence of an interior impact, so as to absorb impact energy. In this system, it is preferable to deform the steering shaft, or the like, in a desired manner, so as to provide an intended impact absorbing effect.

SUMMARY OF THE INVENTION

The invention provides an energy absorbing steering system that absorbs impact energy in a desired manner.

An energy absorbing steering system according to a first aspect of the invention includes a steering shaft to which a steering member is coupled, and a steering column that rotatably supports the steering shaft, and at least one of the steering shaft and the steering column includes an impact absorbing portion that contracts when an interior impact occurs. In this system, the impact absorbing portion includes a first impact absorbing portion having a first impact absorption load, and a second impact absorbing portion having a second impact absorbing load.

According to the first aspect of the invention, one of the first and second impact absorbing portions, for which the impact absorption load is set to be relatively small, may contract before the other impact absorbing portion for which the impact absorption load is set to be relatively large contracts. Thus, contraction of the impact absorbing portion may be initiated from a desired location, whereby the impact absorption can be effected in a desired manner.

In the energy absorbing steering system according to the first aspect of the invention, the impact absorbing portion may include a first corrugated tube, and a second corrugated tube that covers a part of the first corrugated tube, and the first impact absorbing portion may consist of a part of the first corrugated tube which is not covered with the second corrugated tube, while the second impact absorbing portion may consist of the above-indicated part of the first corrugated tube which is covered with the second corrugated tube.

In this case, even if impact energy is applied from the driver to the steering shaft in a direction that is inclined relative to the axial direction of the steering shaft, whereby a bending load is applied to the impact absorbing portion, the first and second corrugated tubes that constitute the impact absorbing portion can contract while bending. Accordingly, the impact absorbing portion absorbs impact energy regardless of the direction of application of the impact energy. Furthermore, the impact absorbing portion may be bent at a boundary portion between the first and second impact absorbing portions. Thus, the impact absorbing portion may be bent at a desired location so that the impact absorption or energy absorption is effected in a more appropriate manner.

In the energy absorbing steering system according to the first aspect of the invention, the first and second impact absorbing portions may include corrugated tubes that extend continuously from each other and have different thicknesses. In this case, it is possible to set different impact absorption loads, using a single corrugated tube, while reducing the number of components.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Example embodiments of the invention will be described with reference to the accompanying drawings,FIG. 1is a schematic cross-sectional view of the construction of an electric power steering system1as an energy absorbing steering system according to one embodiment of the invention. Referring toFIG. 1, the electric power steering system1includes a steering shaft3coupled to a steering member2, such as a steering wheel, a steering column4that supports the steering shaft3such that the steering shaft3is freely rotatable, and an electric motor5that provides steering assist, which is coupled to the steering shaft3.

The steering shaft3includes a plurality of shafts. More specifically, the steering shaft3includes a first shaft6having one end to which the steering member2is fixed, a second shaft7coupled to the first shaft6and that is rotatable with the first shaft6, a third shaft9connected to the second shaft7via a torsion bar8that is rotatable relative to the second shaft7, and a fourth shaft10coupled to the third shaft9that is rotatable with the third shaft9. The steering column4restricts movements of the second shaft7in an axial direction S1(i.e., toward the lower end) of the steering shaft3. The second shaft7and the third shaft9are arranged to rotate together when the amount of rotation of these shafts7,9relative to each other exceeds a specified range. The fourth shaft10is connected to a steering mechanism11that includes a rack-and-pinion mechanism and so forth, via an intermediate shaft (not shown), etc.

Steering torque of the steering member2is transmitted to the steering mechanism11via the steering shaft3and others, so that steering or turning of steerable road wheels (not shown) is accomplished. The steering column4, which has a cylindrical shape, includes a first column tube12, a second column tube13that is connected to the first column tube12, and a third column tube14that is connected to the second column tube13.

The first column tube12is formed from a steel pipe, and surrounds the first shaft6. The first column tube12includes an upper column tube15, and a lower column tube16fitted in the upper column tube15such that the upper and lower column tubes15,16can slide relative to each other. The upper column tube15is a high-rigidity member having a relatively large thickness, and has relatively high flexural rigidity. An upper bracket17is attached to the upper column tube15. The upper bracket17serves to mount the upper column tube15on a vehicle body18, and is supported by a support portion19of the vehicle body18. The upper bracket17is supported on the support portion of the vehicle body18with a certain load-bearing capacity, and is arranged to be released from the support portion19when a specified amount of impact load or more is applied to the upper bracket17along one direction S1of an axis of the steering shaft3.

The upper column tube15supports the first shaft6via a bearing21such that the first shaft6is freely rotatable but cannot move relative to the upper column tube15in the axial directions S. The lower column tube16is a low-rigidity member having a relatively small thickness, and has relatively low flexural rigidity. The lower column tube16is fitted in one end portion of the upper column tube15. The lower column tube16is adapted to bend when a bending load equal to or greater than a specified value is applied to the first column tube12.

One end portion of the lower column tube16is fitted on one end portion of the second column tube13, such that movements of the low column tube16in one direction S1of the axis of the steering shaft3(toward the lower end) are restricted. The second column tube13surrounds the second shaft7, torsion bar8and a part of the third shaft9, and a torque sensor22is housed in the second column tube13. The torque sensor22detects the amount of relative rotation between the second shaft7and the third shaft9, so as to detect torque applied to the steering shaft3.

The second column tube13rotatably supports the second shaft7via a bearing23. One end of the third column tube14is fixed to the other end of the second column tube13. A resolver24that detects the rotational position of a rotor33(which will be described) of the electric motor5is housed in one end portion of the third column tube14. An intermediate portion of the third column tube14rotatably supports the third shaft9via a bearing25. The other end portion of the third column tube14rotatably supports the fourth shaft10via a bearing26. The fourth shaft10is rotatably supported by a lower movable bracket28via a bearing27. The lower movable bracket28is pivotably supported by a lower fixed bracket30via a pivot shaft29. The lower fixed bracket30is fixed to a securing portion31of the vehicle body18.

The electric motor5may be a so-called outer rotor type electric motor in which a rotor is placed outside a stator, and consists of a brushless motor disposed coaxially with the steering shaft3. The electric motor5includes a stator32fixed on the periphery of the third column tube14, and the above-mentioned rotor33that surrounds the stator32. The rotor33is shaped like a cup whose upper end is open, and includes a cylindrical sleeve34and an end wall35formed at one end of the sleeve34. A plurality of permanent magnets36are fixed to the inner circumferential surface of the sleeve34. The end wall35is rotatable with the sleeve34. The fourth shaft10is press-fitted and fixed in a circumferential wall of a through-hole35aof the end wall35. With this arrangement, the rotor33and the fourth shaft10are rotatable together as a unit.

The other end portion of the sleeve34is supported by the third column tube14via a bearing37, such that the sleeve34is freely rotatable. The sleeve34also serves as a housing of the electric motor5. An ECU (Electronic Control Unit) (not shown) controls the electric motor5via a drive circuit, based on the results of detection of the torque sensor22, a vehicle speed sensor (not shown) and other sensors. The torque of the electric motor5is given to the fourth shaft10of the steering shaft3as steering assist force.

FIG. 2is an enlarged view of a principal part of the electric power steering system ofFIG. 1. Referring toFIG. 2, this embodiment is characterized in that the steering shaft3is provided with an impact absorbing portion40that contracts in response to an interior impact, and that the impact absorbing portion40includes first and second impact absorbing portions41,42for which different impact absorption loads are set. More specifically, the first shaft6of the steering shaft3is formed of a material, such as metal, having appropriate elasticity, and includes a cylindrical portion43on which the bearing21is fitted, a first corrugated tube44that extends from the cylindrical portion43in one direction S1of the axis of the steering shaft3, a second corrugated tube45that covers a part of the first corrugated tube44, and a connecting portion46that extends from the first corrugated tube44in one S1of the axial directions S and is connected to the second shaft7.

For example, the cylindrical portion43, first corrugated tube44and the connecting portion46may be formed integrally from a single member. The first corrugated tube44has a bellows-like portion47, that contracts in the axial directions S. The second corrugated tube45is shaped in accordance with the shape of the first corrugated tube44, and has a bellows-like portion49.

The second corrugated tube45covers the entire circumference of a part of the first corrugated tube44located on the S1side (lower side) as viewed in the axial directions S, such that the inner circumferential surface of the second corrugated tube45contacts the outer circumferential surface of the first corrugated tube44. One end of the second corrugated tube45abuts the one end of the second shaft7so that movement of the second corrugated tube45in axial directions S1is restricted.

The connecting portion46is received in a hole51formed in one end portion of the second shaft7, and an end face of the connecting portion46abuts an annular stepped portion52of the second shaft7, so that movement of the connecting portion46in axial direction S1is restricted. The first impact absorbing portion41consists of a non-superimposed region53of the first corrugated tube44, where the second corrugated tube45is not superimposed over the first corrugated tube44. The second impact absorbing portion42consists of a superimposed region54, where the second corrugated tube45is superimposed over the first corrugated tube44.

In the superimposed region54, the first and second corrugated tubes44,45contract as a unit in the axial direction S. The impact absorption load of the first impact absorbing portion41is set to be relatively low, and the impact absorption load of the second impact absorbing portion42is set to be relatively high. Thus, the impact absorption load of the second impact absorbing portion42is higher than that of the first impact absorbing portion41. The “impact absorption load” means a load under which the impact absorbing portion41,42undergoes plastic deformation, for example, contracts in the axial directions S.

The first and second impact absorbing portions41,42have a boundary portion55, at which these impact absorbing portions41,42are connected. Referring toFIG. 1, if an exterior impact arises from a collision of a vehicle in which the electric power steering system1is installed, with another vehicle or object, and an interior impact then arises from a collision of the driver with the steering member2, following the exterior impact, an impact load received from the steering member2is transmitted to the upper bracket17via the first shaft6, bearing21and the upper column tube15.

If the impact load transmitted to the upper bracket17is equal to or greater than a predetermined value, the upper bracket17is released from the support portion19of the vehicle body18. As a result, the steering column4becomes pivotable about the pivot shaft29. For example, if an impact load is applied to the steering member2in a direction that is inclined relative to the axial directions S, in a condition as shown inFIG. 1in which no impact load is applied to the steering shaft3, a bending load is applied to the first shaft6. As a result, the first shaft6bends at the boundary portion55, in other words, the first impact absorbing portion41pivots about the boundary portion55relative to the second impact absorbing portion42, as shown inFIG. 3. At the same time, a bending load is applied to the first column tube12, whereby the lower column tube16is deformed and bent by the upper column tube15.

According to this embodiment, the first impact absorbing portion41for which the impact absorption load is set to be relatively small contracts before the second impact absorbing portion42for which the impact absorption load is set to be relatively large contacts. Thus, contraction of the impact absorbing portion40may be started from a desired portion (the first impact absorbing portion41). Consequently, impact absorption or energy absorption may be effected in a desired manner.

Also, even if a bending load is applied to the impact absorbing portion40, for example, if impact energy is applied from the driver to the steering shaft3in a direction that is inclined relative to the axial directions S, the first and second corrugated tubes44,45that constitute the impact absorbing portion40are able to contract while bending. Thus, the impact absorbing portion40absorbs impact energy, regardless of the direction of application of the impact energy.

Furthermore, bending may be induced in the impact absorbing portion40at the boundary portion55between the first and second impact absorbing portions41,42. Because the impact absorbing portion40may be bent at a desired location in this manner, the impart absorption may be effected in a further desired manner. Moreover, the first impact absorbing portion41is located closer to the steering member2, so that the impact absorbing portion40is mainly bent at a location far from the pivot shaft29. Consequently, the impact absorbing portion40is prevented from bending by a large degree, and the bent portion is prevented from largely protruding toward the driver.

Also, the electric motor5, which is in the form of the outer rotor type motor, provides a large area over which the permanent magnets36of the rotor33are opposed to the stator32. Accordingly, the electric motor5is able to produce high torque. Furthermore, the bearing27is firmly retained by the lower movable bracket28and the lower fixed bracket30, whereby the axis of the rotor33is prevented from being displaced or shifted when the electric motor5is driven.

It is to be understood that the invention is not limited to details of the illustrated embodiment, but may be embodied with various changes or modifications. For example, a first column tube12A may include first and second corrugated tubes44A,45A, which constitute an impact absorbing portion40A, as shown inFIG. 4. In the following, the modified embodiment ofFIG. 4will be described mainly in terms of its features different from those of the embodiment as shown inFIG. 1-FIG.3, and the same reference numerals as used inFIG. 1-FIG.3are used for identifying the same or corresponding members or elements, of which no further explanation will be provided.

One end (the upper end) of the second corrugated tube45A may be coextensive with one end (the upper end) of the second corrugated tube45in the axial directions S. With this arrangement, when an interior impact occurs, and impact energy equal to or greater than a specified value is applied from the driver to the steering shaft3in a direction inclined relative to the axial directions S, a first impact absorbing portion41A of the first column tube12A bends, and the first impact absorbing portion41of the first shaft6bends, as shown inFIG. 5. A boundary portion55A at which the first column tube12A bends is substantially aligned with the boundary portion55at which the first shaft6bends, in the axial directions S.

In the arrangement as described above, the second corrugated tube45may be eliminated. Also, in the arrangement as shown inFIG. 4, the first and second corrugated tubes44,45may be replaced with first and second cylindrical portions56,57, as shown inFIG. 6. The first cylindrical portion56is a high-rigidity member that is relatively thick in comparison to the second cylindrical portion57, and has relatively high flexural rigidity. The second cylindrical portion57is a low-rigidity member that is relatively thin in comparison to the first cylindrical portion56, and has relatively low flexural rigidity. The inner circumferential surface of one end portion of the first cylindrical portion56is fitted on the outer circumferential surface of one end portion of the second cylindrical portion57by, for example, spline fitting, such that the first and second cylindrical portions56,57can slide relative to each other.

With this arrangement, if an interior impact occurs, and impact energy equal to or greater than a specified value is applied from the driver to the steering shaft3in a direction inclined relative to the axial directions S of the steering shaft3, the first impact absorbing portion41A bends, and the second cylindrical portion57bends, as shown inFIG. 7. It is also possible to superimpose three or more corrugated tubes on each other to form the second impact absorbing portion42or42A.

Also, the first and second corrugated tubes44,45of the steering shaft3may be replaced with a composite corrugated tube61as shown inFIG. 8. The composite corrugated tube61is formed as a single unit made of a single material, and includes a thin corrugated tube62having a relatively small thickness and a thick corrugated tube63having a relatively large thickness. The thin corrugated tube62has substantially the same shape as the non-superimposed region53(seeFIG. 2). The thick corrugated tube63has substantially the same shape as the superimposed region54(seeFIG. 2).

The thin corrugated tube62and thick corrugated tube63extend continuously from each other, and have different thicknesses. The thin corrugated tube62provides a first impact absorbing portion41B, and the thick corrugated tube63provides a second impact absorbing portion42B. The first impact absorbing portion41B and second impact absorbing portion42B are connected to each other at a boundary portion55B.

In this case, it is possible to set different impact absorption loads using a single corrugated tube, while reducing the number of components. The first and second corrugated tubes44A,45A of the first column tube12A may also be replaced with a composite corrugated tube similar to the composite corrugated tube61as described above.

Also, the first impact absorbing portion and the second impact absorbing portion may be formed of different materials, such that the first impact absorbing portion is formed of a material having lower rigidity than that of the second impact absorbing portion.

The present invention may also be applied to a tilt steering system for adjusting the position of the steering column about a pivot or tilt center. The invention may also be applied to other types of electric power steering systems, such as a column-type electric power steering system in which an electric motor that provides steering assist is not disposed coaxially with the steering shaft, or may also be applied to manual steering systems, where no steering assist device is provided.