Impact absorbing steering column device for vehicle

In an impact absorbing type steering column apparatus for a vehicle in which two columns are fitted to each other in such a manner that an upper column, while being moved to the front part of the vehicle with respect to a lower column, absorbs the impact energy at a secondary collision of the vehicle, a bush is fixedly fitted to at least one of the outer peripheral side of the upper column and the inner peripheral side of the lower column in the fitting portion between the two columns.

TECHNICAL FIELD

The present invention relates to an impact absorbing type steering apparatus of a double tube scheme for a vehicle for absorbing, at a secondary collision, the impact energy thereof.

BACKGROUND ART

When an vehicle collides with other vehicle or a structure, etc., the driver might suffer a secondary collision with a steering wheel by its inertia. The vehicles manufactured over the recent years broadly employ an impact absorbing type steering column apparatus, together with a safety belt and an air bag, for the purpose of preventing the driver from being injured in that case. There are now various types of impact absorbing mechanisms to be employed in an impact absorbing type steering column apparatus. However, ones of a double tube scheme which get collapsed simultaneously with a steering shaft when the driver suffers a secondary collision so as to absorb the impact energy thereof on that occasion are generally employed.

In an impact absorbing type steering column apparatus of this scheme, at a secondary collision of the vehicle, when an upper column on the vehicle-rear-side moves to the front side of the vehicle with respect to a lower column which is fixed to the body of the vehicle, the upper column moves to the front side of the vehicle while resisting a frictional force (sliding resistance) which is caused by a sliding contact with the lower column.

However, when this upper column moves (slides) to the front side of the vehicle, the tip end edge thereof might be twisted with respect to a sliding surface of the lower column, so that the upper column can not always slide smoothly.

For these reasons, it is disclosed in Japanese Patent Application Laid-Open No. 8-142877 that a cylindrical spacer formed of synthetic resin is press-fitted into between the lower column and the upper column, whereby the tip end edge of the upper column smoothly slides on the outer peripheral surface of the cylindrical space. With this arrangement, “the twist” of the lower column can be prevented.

In Japanese Patent Application Laid-Open No. 2002-302048, disclosed is a thin film-like friction reducing material which is formed of polymer material and coats one of the sliding surfaces of the two columns in the thickness of 10 to 100 μm, whereby the frictional resistance between the sliding surfaces of the two columns are reduced, so as to prevent “the twist” of the upper column.

However, when the cylindrical space is formed of synthetic resin and the hardness of one of the columns which serves as the outer column is low, as disclosed in Japanese Patent Application Laid-Open No. 8-142877, both the columns might be deformed and difficult to slide or scoring or the like might be caused by deformation.

Also, even when one of the sliding surfaces of the outer and inner columns is coated with a thin film-like friction reducing material formed of polymer material, as disclosed in Japanese Patent Application Laid-Open No. 2002-302048, if the hardness of the outer column or the inner column is low, the column might not be smoothly collapsed.

Note that when an end of the spacer might be provided with a flange, this flange might be caught and might not be collapsed smoothly at a secondary collision. The same might happen even if the spacer had been divided.

DISCLOSURE OF THE INVENTION

It is an object of the present invention, which was devised under such circumstances, to provide an impact absorbing type steering column apparatus for a vehicle which can get an upper column collapsed smoothly without a hitch by certainly preventing “twist” of the upper column at a secondary collision.

In order to achieve the above object, according to the present invention, there is provided an impact absorbing type steering column apparatus for a vehicle in which an upper column and a lower column are fitted to each other in such a manner that the upper column, while moving to the front part of the vehicle with respect to the lower column, absorbs the impact energy thereof, characterized in that:

a bush is fixedly fitted to at least one of the outer peripheral side of the upper column and the inner peripheral side of the lower column in the fitting part between the two columns.

Also, in the impact absorbing type steering column apparatus for a vehicle of the present invention, it is preferable that the hardness of a sliding surface of said bush is set as higher than the hardness of one of the columns which slides on the sidling surface of the bush.

Further, in the impact absorbing type steering column apparatus for a vehicle of the present invention, it is preferable that a surface roughness of the sliding surface of the bush is set as smaller than the surface roughness of the column which slides on the sidling surface of the bush.

As described above, according to the present invention, it is possible to cause the upper column to collapse smoothly without a hitch by certainly preventing “twist” of the upper column at a secondary collision by the use of the bush.

EMBODIMENTS OF THE INVENTION

An impact absorbing type steering column apparatus for a vehicle according to an embodiment of the present invention will be described below with reference to the drawings.

First Embodiment

FIG. 1is a side view showing an impact absorbing type steering column apparatus for a vehicle according to the first embodiment of the present invention, containing a partially cut-away section at actual installation of the steering column apparatus in the vehicle.

FIG. 2Ais an enlarged schematic view showing a behavior of an upper column at a secondary collision according to the prior art, andFIG. 2Bis an enlarged schematic view showing a behavior of the upper column at a secondary collision.FIG. 3Ais a longitudinally sectional view of the steering column apparatus shown inFIG. 1,FIG. 3Bis a view of the steering column apparatus seen along the arrow shown inFIG. 3A, andFIG. 3Cis a front view of the bush shown inFIG. 3B.

As shown inFIG. 1, a steering column is comprised of a lower column1which is fixed to the body of the vehicle and an upper column2which is fitted to this lower column1to be slidable at a secondary collision. In the first embodiment, the lower column1serves as an outer column, while the upper column2as an inner column. The lower column1is attached to the body of the vehicle by means of a lower bracket3and an upper bracket4.

Inside both the columns1and2, a steering shaft is supported to be rotatable. The steering shaft is comprised of a lower shaft5and a cylindrical upper shaft6which is telescopically spline-fitted to the lower shaft5. The upper shaft6is supported on the upper column2through bearings7and8to be rotatable. A steering wheel9is attached to the rear end of the upper shaft6. The lower shaft5is supported at the front end of the lower column1through unrepresented bearings to be rotatable.

In the first embodiment, a bush10is fixedly fitted in a fitting portion between the two columns1and2on the inner peripheral side of the lower column1.

In the following embodiment, description is made on a case where the lower column serves as an outer column while the upper column as an inner column. However, the present invention is not limited to this. The upper column may be an outer column while the lower column may be an inner column.

A sliding surface which is the inner peripheral surface of the bush10is set to have high hardness and better (that is, small) surface roughness, compared with the hardness and the surface roughness of the respective materials of the lower column1(outer column) and the upper column2(inner column).

That is, the hardness of the sliding surface of the bush10is set as higher than the hardness of one of the columns which slides on the sliding surface of the bush10(in the illustrated example, the upper column2(inner column)). At the same time, or instead of this, the rough hardness of the sliding surface of the bush10is set smaller than the surface roughness of the column which slides on the sliding surface of the bush10(in the illustrated example, the upper column2(inner column)).

For example, a material of the lower column1serving as the outer column is aluminum (in this case, the material represents the hardness), and an aluminum die-cast surface is employed as it is as the surface of the lower column1(the surface processing substitutes for the surface roughness). Otherwise, a cutting work is added to the surface.

A material of the upper column2serving as the inner column is steel(also in this case, the material represents the hardness), a member which has been subjected to drawing may be used as it is as the surface of the upper column2(the surface processing substitutes for the surface roughness), or a cutting work is added.

A material of the bush10is SUS material (also in this case, the material represents the hardness), and for the processing of the surface thereof (substituting for the surface roughness), a member having a better (that is, smaller) surface roughness than at least the surface roughness of one of the columns which slides on the sliding surface of the bush10(in the illustrated first embodiment, the upper column2(inner column)).

Note that these materials are exemplary, and the present invention is not limited to these.

One or two bushes10are interposed between both the columns1and2. Further, the bush10may be set to have the same dimension as the length of the fitting portion between the two columns1and2, or may have the sufficient length for covering a point which serves as the fulcrum of the fitting portion between the two columns when a bending load is applied at a secondary collision.

Further, the fitting length between the bush10and the two columns may be set as smaller than the fitting length between the upper column2and the lower column. In this case, the thickness of the bush is arranged to be large so that an end of the upper column2(inner column) does not interfere with the lower column1(outer column) even if a bending load is applied thereon.

When the two columns1and2are formed of ferrous materials which have a small difference in hardness from the bush10, the bush10may be subjected to coating of Teflon, molybdenum, or the like. It is also effective if coating is performed on the outer peripheral surface of the upper column2or the inner peripheral surface of the lower column1(outer column) in combination with the coating on the bush10.

Further, it is possible to perform a collapsing operation smoothly by employing a bush which is formed of a material with high hardness such as stainless steel or spring steel to be interposed between the lower column1(outer column) of aluminum or magnesium system and the upper column2(inner column) of ferrous material system, thereby preventing deformation or depression of the soft lower column1(outer column) due to stress concentration by the use of a bending load of the upper column2(inner column).

From the above description, at a secondary collision, as shown inFIG. 1, when the crew P is moved to the front part of the vehicle by an impact energy (F) to collide with the steering wheel9, the upper column2(inner column) is collapsed to move toward the front part of the vehicle and, at the same time, as shown inFIG. 1, a bending load (M) and a collapse load (C) are generated.

As a result, the upper column2(inner column) is collapsing while receiving not only the collapse load (C), but also the bending load (M).

When the bending load (M) is applied on the upper column2(inner column), the strongest concentrated stress is applied on positions at the front end of the upper column2(inner column) and the rear end of the upper column2(inner column), which are reversed of each other by 180 degrees in the circumferential direction. Stress concentrated portions A and B are produced on the two columns1and2due to this bending load (M). Deformation, scoring and the like are generated due to the bending load (M) and the stress concentrated portions A and B. Such deformation or scoring is produced particularly when either one of the two columns1and2is of low hardness. In this respect, particularly when the upper column starts to move, since the fitting length between two columns1and2is short, the concentrated stress is applied most strongly.

That is, as shown inFIG. 2A, when there is no bush arranged, the tip end edge (the stress concentrated portion B) of the upper column2which serves as the inner column might be “twisted” with respect to the inner peripheral surface (that is, the sliding surface) of the lower column1serving as the outer column at a start of the collapse, whereby scoring is produced as indicated by a hatching part inFIG. 2A. As a result, a smooth collapsing performance can not be obtained.

Whereas, in the first embodiment, as shown inFIG. 2B, one (or two) bush10which has high hardness and better (small) surface roughness is interposed between the two columns1and2.

Note that the case shown inFIG. 2Bis different from the first embodiment shown inFIG. 1andFIG. 3and is similar to the second variation of the first embodiment shown inFIG. 4B, which will be described later.FIG. 3Bshows the case where the length of the bush10is set as small so that the front end edge of the bush10is positioned more backward than the front end of the upper column2(inner column), and the thickness of the bush is set as large so that the outer column and the inner column do not interface with each other in the course of collapse.

As a result, since the tip end of the upper column2slides smoothly on the inner peripheral surface (sliding surface) of the bush10, the tip end of the upper column2(inner column) is not brought into contact with the inner peripheral surface of the lower column1(outer column) or, even if the tip end is in contact with this inner peripheral surface, there is small resistance produced from the lower column1so that no “twist” is produced. Thus, deformation, scoring and the like are not generated. From the above description, it is possible to securely prevent “twist” of the upper column2(inner column) at a secondary collision, thereby performing a collapsing operation smoothly without a hitch.

Also, as shown inFIG. 1andFIG. 3A, there is only one bush10which is provided in the first embodiment, and the tip end edge of the bush10is extended forward from the front end of the upper column2(inner column). In this case, both the stress concentrated portions A and B are on the sliding surface of the bush10with high hardness so as to be smoothly collapsed.

Note that the bush10may be formed by rounding a single plate member, as shown inFIG. 3C, or may be formed to be in a cylindrical form completely closed.

(First Variation of First Embodiment)

FIG. 4Ais a longitudinally sectional view of an impact absorbing type steering column apparatus for a vehicle according to the first variation of the first embodiment of the present invention.

In the first variation, there are provided two bushes10and10a, and the tip end edges thereof are extended forward from the tip edge of the upper column2(inner column). The bush10ais fixedly fitted to the inner peripheral surface of the lower column1while the bush10is fixedly fitted to the outer peripheral surface of the upper column2, whereby a slip can be produced between these bushes10and10aat a collapse time of the upper column2due to the secondary collision. In the first variation shown inFIG. 4A, the tip end edge of the bush10is extended more forward than the tip end edge of the bush10a, while the rear edge end of the bush10is extended more backward than the rear end edge of the bush10a. The rear end edge of the bush10amay be bent outward in the radial direction, as shown inFIG. 4A, whereby it is possible to prevent a relative shift between the bush10aand the lower column1at a collapse. Though not shown in the drawings, the same effect can be obtained by bending the tip end edge of the bush10awhich is fixedly fitted to the inner column2inward in the radial direction.

(Second Variation of the First Embodiment)

FIG. 4Bis a longitudinally sectional view of an impact absorbing type steering column apparatus for a vehicle according to the second variation of the first embodiment of the present invention.

In the second variation, there is provided only one bush10and is set as short so that the tip end edge thereof is positioned more backward than the tip end edge of the upper column2(inner column) by a predetermined distance (Δ). In this case, it is arranged such that an end of the upper column2(inner column) does not interfere with the lower column1(outer column) even when the bending load (M) is applied thereon by increasing the thickness of the bush.

Also, in this case, as shown inFIG. 2B, at a secondary collision, since sliding on the inner peripheral surface (sliding surface) of the bush10smoothly, the tip end edge of the upper column2(inner column) is not brought into contact with the inner peripheral surface of the lower column1(outer column). As a result, there is no “twist” produced, and deformation, scoring occur and the like are not generated. Therefore, it is possible to perform a collapsing operation smoothly without a hitch by securely preventing “twist” of the upper column2(inner column) at a secondary collision. Other arrangements, effects, etc., are the same as those in the foregoing first embodiment.

Second Embodiment: Tilt Type

FIG. 5is a side view showing an impact absorbing type steering column apparatus for a vehicle according to the second embodiment of the present invention.

FIG. 6is a laterally sectional view of the impact absorbing type steering column apparatus shown inFIG. 5.

In the second embodiment, there is provided an electric power steering apparatus20. In the electric power steering apparatus20, since an on-board battery is used as a power source of an electric motor21, a direct drive loss of an engine is not generated. Since the electric motor21is started only at the time of steering assist, reduction of fuel efficiency of the vehicle (a drive loss of the engine related to an alternator) can be prevented. Other similar characteristics on which electron control can be easily effected are provided with the electric power steering apparatus20.

The electric power steering apparatus is fixed to a rotation transmitting mechanism housing101which is integrally formed with the lower column1as the outer column. An upper part at the front end of this housing101is supported by a pair of vehicle-side lower brackets102which are provided on the both sides seen from the left inFIG. 5, to be rotatable around pivots39. The vehicle body-side lower brackets102are mounted on a vehicle body-side reinforcing member by means of a vehicle-body mounting member102a which is extended horizontally inFIG. 5. Since the rotation transmitting mechanism itself for transmitting rotation from the electric motor21of the electric power steering apparatus20to the lower shaft5is not directly related to the present invention, description thereof is omitted here. However a generally known mechanism may be used as this transmitting mechanism.

In the second embodiment, the upper column2is formed as the inner column, and is fitted to the lower column1as the outer column along a predetermined length at the front end portion. In the fitting portion, the bush10is fixedly fitted on the inner peripheral surface of the lower column1as the outer column. The structure and function of the upper column2, the bush10and the lower column1are the same as those in the first embodiment.

Also in the second embodiment, there is provided a tilting type clamping mechanism, and a vehicle body-side upper bracket30is provided with a pair of vehicle-body mounting members31a,31bwhich are extended horizontally in the drawing. The vehicle-body mounting members31a,31bare mounted on the a vehicle body-side reinforcing member (not shown) through capsules132a,132bby means a bolt (not shown), or the like. At a secondary collision, the upper column2and the upper bracket30are released from the capsules131a,131bto move toward the front side of the vehicle due to an impact load. These paired vehicle-body mounting members31a,31bare integrally formed with a pair of right and left flat plate portions32a,32bwhich opposite to each other and are extended substantially in a vertical direction.

In the clamping mechanism, a clamping bolt34is inserted into a pair of tilt elongated holes33a,33bwhich are formed on the opposite flat plate portions32a,32b. This clamping bolt34is engaged with the tilt elongated hole33bat a part of a head34athereof, so as to be unrotational at all times and to be slidable inside the tilt elongated holes33a,33b.

A nut35is engaged with a threaded portion34bof the clamping bolt34. An operational lever36is fixed to this nut35by means of a mounting bolt37.

Accordingly, when a tilt adjustment is performed, if the operational lever36is rotated in one direction, the nut35is rotated to release the clamping by the clamping bolt34, whereby a space between the paired opposite flat plate portions32a,32bis extended and the pressure contact between the opposite flat plate portions32a,32band a distance bracket38is released. Thereupon, the upper column2is rotated around the tilting center39, together with the lower column1and the like, whereby the tilt adjustment can be performed.

On the other hand, when a clamping is effected at a tilt adjustment position, the operational lever36is rotated in a reverse direction, so that the tilt nut35is also rotated in the reverse direction. Thus, the clamping bolt34is clamped and the space between the paired opposite flat plate portions32a,32bis narrowed, so that the opposite flat plate portions32a,32bare urged against the distance bracket38.

FIG. 7is a side view showing an impact absorbing type steering column apparatus for a vehicle according to the third embodiment of the present invention.

FIG. 8is a sectional view of the impact absorbing type steering column apparatus shown inFIG. 7, taken along the line X-X.

The third embodiment shows a steering column of a double-collapse type, in which the steering column apparatus is comprised of the upper column2, an intermediate column201which is fitted on the front end of the upper column2, and a lower column40which is fitted on the front end of the intermediate column201and is fixed to the body of the vehicle at a lower part of the front end.

The structures of the intermediate column201and the upper column2in the fitting portion are the same as those of the lower column1and the upper column2in the fitting portion in the second embodiment shown inFIG. 5. The bush10is fixedly fitted on the inner peripheral surface of the intermediate column201in a front part of the fitting portion between the intermediate column201and the upper column2, that is, in a portion in a predetermined length at the front end of the upper column2.

On the other hand, the front end of the intermediate column201is fitted in a rear end of the lower column40along a predetermined length. In these fitting portions, the bush10b is fixedly fitted on the inner peripheral surface of the lower column40which serves as the outer column. The structure of these fitting portions including the bush10band the structure between the lower column40and the body of the vehicle and the function thereof are the same as the structure between the lower column1and the upper column2through the bush10and the structure between the lower column1and the body of the vehicle and the function thereof shown inFIG. 5.

The steering column apparatus of the third embodiment is provided with a clamping mechanism of a tilt/telescopic scheme, and a vehicle body-side upper bracket50is provided with a pair of vehicle-body mounting members51a,51bwith the upper column2therebetween, on the side of the vehicle more forward than an intermediate portion in the longitudinal direction of the upper column2. Parts extended downward from these paired vehicle-body mounting members51a,51bare provided with a pair of right and left flat plate portions which are opposite to each other and are extended substantially in a vertical direction. The vehicle-body mounting members51a,51bare secured on the vehicle body-side reinforcing member respectively through capsules132a,132bby means of fixing means such as a bolt (not shown).

Further, the intermediate column201is integrally provided, in the rear part of the vehicle, with clamp portions53a,53bwith a predetermined distance in the axial direction therebetween, which are extended toward the opposite flat plate portions52a,52band which are respectively brought into engagement contact with said opposite flat plate portions52a,52b. (InFIG. 8, out of two pairs of clamp portions53a,53b, only the clamp portions53a,53bin the front part of the vehicle are illustrated). Over a length portion of the intermediate column201on which these clamp portions53a,53bare formed, there are provided upper and lower slits S, S which are extended in the axial direction.

With this arrangement, when a space between the paired opposite flat plate portions52a,52bof the vehicle body-side upper bracket50is reduced and the clamp portions53a,53bare pressed toward each other, the intermediate column201is contracted in diameter at the portions of the clamp portions53a,53bso as to clamp the upper column2.

A substantially annular tension member54which surrounds the upper column2with a gap in the radial direction is provided in a space between the two pairs of the clamp portions53a,53bin the axial direction. The tension member54is composed of two half bodies54b,54cwhich are clamped and fixed to each other by upper and lower two nuts54a.

An adjusting bolt58is thread-engaged with one of the half bodies54bof the tension member54through a cam mechanism which consists of a pair of cam members55and56and an adjusting bolt58and through the operational lever57.

The cam mechanism is comprised of a first cam member55which rotates together with the operational lever57and has a convex part and a concave part, and a second cam member56which is unrotational and has a convex part and a concave part to be engaged with the convex part and the concave part of the first cam member55.

An adjusting bolt59is thread-engaged with the other half body54cof the tension member54. The tip ends of the two adjusting bolts58and59are respectively engaged with elongated holes71and72for telescopic adjustment, which are respectively formed on both the columns1and2. Intermediate large-diameter portions of the two adjusting bolts58and59are respectively inserted into the elongated holes73a,73bfor tilt adjustment formed on the opposite flat plate portions52a,52b.

Further, a pair of multi-plate type frictional engagement mechanisms61a,61bare employed in this steering column apparatus. That is, two sets each including two frictional plates62a,62a;62b,62bwhich are extended in a direction in which the tilt elongated holes73a,73bare extended are secured to the outer side surfaces of the paired opposite flat plate portions52a,52bby means of two sets each including upper and lower rivets63a,63a;63b,63b. Tilt elongated holes62c,62c;62d,62dare formed on the frictional plates62a,62a;62b,62b, respectively. The adjusting bolts58and59are inserted through these tilt elongated holes62c,62c;62d,62d, respectively. Frictional plates64a,64bare interposed between these frictional plates62a,62a;62b,62band between the frictional plates62a,62band the opposite flat plate portions52a,52bto be extended in the axial direction. Telescopic position adjusting elongated holes75(only the elongated hole75of the frictional plate64ais illustrated inFIG. 7) which are extended in the axial direction are formed on the frictional plate64a,64b. The frictional plates64a,64bare mounted on the outer column2by bolts64e(seeFIG. 7).

With such an arrangement, in order to perform tilt/telescopic adjustment, the driver first rotates the operational lever57clockwise. Then, the first cam member55engaged with the operational lever57is rotated relatively to the second cam member56, whereby the width of the cam mechanism is reduced.

With the above operation, a pressure contact force between the frictional plate62aand the frictional plate64ain the multi-plate frictional engagement mechanism61abecomes extinct. A pressure contact force between the frictional plate62band the frictional plate64bin the other multi-plate frictional engagement mechanism61balso becomes extinct through the tension member54. As a result, the columns2,201and40become capable of tilting movement.

When the width of the cam mechanism is reduced, the tension acting between the paired opposite flat plate portions52a,52bthrough the tension member54becomes extinct, whereby a pressing force of the inner side surfaces of the paired opposite flat plate portions52a,52bagainst the clamp portions53a,53bbecomes extinct.

Thereupon, the size of a part with the rear slits S of the intermediate column201formed therein is increased in diameter by the elasticity thereof, so that a tight clamping force with respect to the upper column2is lost. As a result, the upper column2is enabled to move telescopically.

The driver, when completing positional adjustment of the steering wheel by effecting tilting or telescopic adjustment, rotates the operational lever57counter-clockwise. Thus, the width of the cam mechanism is increased, so the pressure contact force between the frictional plates62a,62a;62b,62band the frictional plates64a;64bis generated through the tension member54, whereby the two columns1and2are fixed in the tilting direction by means of the multi-plate frictional engagement mechanisms61a,61b.

Simultaneously, the width of the paired opposite flat plate portions52a,52bis reduced through the tension member54so that, when the clamp portions53a,53bare pressed, the lower column1is contracted in diameter. Thereupon, the upper column2is clamped by the contracted intermediate column201, to be fixed in the telescopic direction.

At a secondary collision, the upper column2slides with respect to the intermediate column201to be collapsed. When the upper column2reaches the collapse end, the capsules132a,132band the vehicle body-side upper bracket50are removed from the vehicle body-side reinforcing member. Subsequently, a part between the intermediate column201and the lower column50starts to collapse.

At a secondary collision, the collapsing order is not limited to that described above. For example, it may be arranged such that the capsules132a,132band the vehicle body-side upper bracket50are first removed from the vehicle body-side reinforcing member, then a part between the intermediate column201and the lower column40is collapsed, and finally the part between the intermediate column201and the upper column2is collapsed. Or, it may be arranged such that either two of these collapses happen simultaneously.

Note that the present invention is not limited to the embodiments described above, but may be changed in various manners. For example, an impact absorbing type steering apparatus for a vehicle according to the present invention can be applied to either of a non-tilting scheme, a non-telescopic scheme, a tilting scheme, a telescopic scheme, and a tilt/telescopic scheme.

As described above, according to the embodiments described above, the hardness of the sliding surface of the bush is set higher than the hardness of one of the columns which slides on the sliding surface of the bush. At the same time, or instead of this, the surface roughness of the sliding surface of the bush is set smaller than the surface roughness of the column which slides on the sliding surface of the bush. As a result, it is possible to securely prevent a “twist” of the upper column at a secondary collision, so as to collapse the upper column smoothly and without a hitch.