Steering system

A steering system for a vehicle including a fixed body secured to the body of the vehicle, a fixed shaft rotatably supported by the fixed bracket, a movable bracket connected for relative axial movement to the fixed bracket, a movable shaft rotatably supported by the movable bracket and connected to the shaft so as to be axially movable relative to the fixed shaft and rotatable as one unit with the fixed shaft, a driving mechanism disposed on the fixed bracket and connected to move the movable bracket and the movable shaft axially relative to both the fixed bracket and the fixed shaft, and a control mechanism for controlling the level of the frictional force acting between the fixed bracket and the movable bracket. The control mechanism normally maintains the frictional force at a level to ensure rigidity between the fixed and movable brackets, but is operable, upon actuation of the driving mechanism, to reduce the frictional force and thereby decrease resistance to relative axial movement of the fixed and movable brackets.

BACKGROUND OF THE INVENTION 
The present invention relates to a steering system for vehicles and, in 
particular, to such a steering system having a telescopic operation. 
A steering system of the type described above is disclosed in Japanese 
Patent Application No. 63-15299 (1988), filed by the applicant of the 
present invention. The prior art comprises a fixed bracket that is secured 
to a member on a vehicle body, a fixed shaft rotatably supported by the 
fixed bracket, a movable bracket connected for axial movement to the fixed 
bracket, a movable shaft connected to the fixed shaft in a manner to be 
axially movable relative to the axial shaft but rotatable with it as one 
unit, the movable shaft being rotatably supported by the movable bracket, 
and a driving mechanism disposed on the fixed bracket and connected to the 
movable bracket to move the movable bracket, together with the movable 
shaft, axially relative to both the fixed bracket and the fixed shaft. In 
this prior art, rigidity of the steering system is established by the 
frictional force that is generated at the joint of the fixed and movable 
brackets or the joint of the fixed and movable shafts. Accordingly, to 
enhance the rigidity of the steering system, it is necessary to increase 
the frictional force occurring between the fixed and movable brackets or 
between the fixed and movable shafts. However, if the frictional force is 
increased, the sliding resistance increases correspondingly, so that 
smooth sliding of the movable bracket and shaft is sacrificed. In 
addition, the size of the driving mechanism that causes the movable 
bracket and shaft to slide increases to be disadvantageous in terms of 
both space and cost. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a steering system which 
is designed so that system rigidity is enhanced without increasing the 
resistance to sliding of the movable bracket and shaft. 
To this end, a steering system for a vehicle is comprised of a fixed body 
secured to the vehicle body, a fixed shaft rotatably supported by the 
fixed bracket, a movable bracket connected for relative axial movement to 
the fixed bracket, a movable shaft rotatably supported by the movable 
bracket and connected to the shaft so as to be axially movable relative to 
the fixed shaft and rotatable as one unit with the fixed shaft, a driving 
mechanism disposed on the fixed bracket and connected to move the movable 
bracket and the movable shaft axially relative to both the fixed bracket 
and the fixed shaft, and a control mechanism for controlling the level of 
the frictional force acting between the fixed bracket and the movable 
bracket, the control mechanism normally maintaining the frictional force 
at a level to ensure rigidity between the fixed and movable brackets, but 
operable, upon actuation of the driving mechanism, to reduce the 
frictional force and thereby decrease resistance to relative axial 
movement of the fixed and movable brackets.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the present invention will be described below with 
reference to the accompanying drawings. 
In FIGS. 1 to 3, there is illustrated a breakaway bracket 1 having a column 
tube 3 secured thereto through a bush 2. The column tube 3 and the 
breakaway bracket 1 are secured as a fixed bracket to a member on a 
vehicle body through a bracket 4. A telescopic tube 6 is held for axially 
slidable movement by the breakaway bracket 1 through a bush 5 and the bush 
2. A holder 7 is secured to the telescopic tube 6 and together, the tube 6 
and the holder 7 establish a movable bracket. A tilt bracket 8 is 
pivotably supported from the holder 7 by bolts 9. A lower-main shaft 10 is 
rotatably supported by the breakaway bracket 1 and the column tube 3. 
Similarly, an upper main shaft 12 is rotatably supported by the tilt 
bracket 8. A middle movable main shaft 11 is slidably connected to the 
lower main shaft 10. The middle main shaft 11 is pivotably connected to 
the upper main shaft 12 through a joint 13 that is located on the middle 
main shaft 11 in coaxial relation to the bolts 9. The middle main shaft 11 
is disposed inside the telescopic tube 6 and the holder 7. 
A telescopic motor 14 is secured to the breakaway bracket 1. A telescopic 
screw 16, which is rotatably supported by the breakaway bracket 1, is 
connected to a rotating shaft of the telescopic motor 14 through a 
reduction mechanism 15 comprising a worm 15a and a worm wheel 15b and 
which are mounted on the breakaway bracket 1. A telescopic nut member 17 
is rigidly secured to the telescopic tube 6 and brought into threaded 
engagement with the telescopic screw 16. The arrangement of the motor 14, 
the reduction mechanism 15, the nut member 17 and the screw 16 thus 
exemplify a driving mechanism for moving the movable bracket, represented 
by the tube 6 and holder 7, and the movable shaft 11 axially relative to 
the lower fixed main shaft 10 and the fixed bracket represented by the 
breakaway bracket 1 and the tube 3. In this arrangement, when the 
telescopic motor 14 is activated, the telescopic screw 16 rotates through 
the reduction mechanism 15. The rotation of the telescopic screw 16 causes 
the telescopic nut member 17 to move axially along the telescopic screw 16 
by virtue of the threaded engagement. Thus, the telescopic tube 6, the 
holder 7 and the tilt bracket 8 slide together as one unit relative to the 
breakaway bracket 1 and the column tube 3. At the same time, the middle 
main shaft 11 and the upper main shaft 12 slide together a one unit 
relative to the lower main shaft 10 (i.e., a telescopic operation is 
realized). 
A tilt motor 18 is secured to the breakaway bracket 1. A tilt screw 20, 
which is rotatably held by the holder 7, is connected to the tilt motor 18 
through a reduction mechanism 19 comprising a worm 19a and a worm wheel 
19b, which are disposed on the breakaway bracket 1. It should be noted 
that the reduction mechanism 19 and the tilt screw 20 are in splined 
connection with each other so that the tilt screw 20 is slidable axially 
while it is rotated by the reduction mechanism 19, in order to enable the 
above-described telescopic operation. 
A tilt nut member 21 is disposed on the holder 7 in such a manner that the 
nut member 21 is prevented from sliding axially relative to the holder 7. 
The tilt nut member 21 is in threaded engagement with the tilt screw 20 
and connected to the tilt bracket 8 by bolts 22. 
In this arrangement, as the tilt motor 18 is activated, the tilt screw 20 
rotates through the reduction mechanism 19. The rotation of the tilt screw 
20 causes the tilt nut member 21 to slide axially relative to the tilt 
screw 20 by virtue of the threaded engagement. Since the tilt nut member 
21 is prevented from sliding axially relative to the holder 7, the driving 
power from the nut member 21 is transmitted to the tilt bracket 8 through 
the bolts 22. Thus, the tilt bracket 8 pivots vertically about the bolts 9 
relative to the holder 7 due to the positional relationship between the 
bolts 9 and 22 and, at the same time, the upper main shaft 12 pivots 
vertically about the joint 13 relative to the middle main shaft 11 (i.e., 
a tilt operation is realized). 
FIGS. 3-6 of the drawings illustrate a control mechanism for developing and 
controlling variable levels of frictional force acting between the 
bracket, represented by the greakaway bracket 1 and column tube 3, and the 
movable bracket established by the telescopic tube 6 and holder 7. As 
shown in FIGS. 3 to 6, the control mechanism includes a lock bolt 25 
connected to a lock motor 23 through a reduction mechanism 24 comprising a 
worm 24a, a worm wheel 24b and gears 24c and 24d. The lock bolt 25 is 
slidably held through a fitting hole 26 with a non-circular or flat 
configuration, which is formed in one side of the lower part of the 
breakaway bracket 1, so that the rotation of the lock bolt 25 is prevented 
by the fitting hole 26. The lock motor 23 and the reduction mechanism 
members 24a, 24b and 24c are accommodated in casing 27 which is secured to 
the side of the lower part of the breakaway bracket 1. The reduction 
mechanism member 24d and the joint of the member 24d and the lock bolt 25 
are accommodated in between the breakaway bracket 1 and the casing 27 
while being positioned by a bush which serves as a bearing for the member 
24d 
A space 28 is formed at the lower side of a retaining hole 1a that is 
provided in the breakaway bracket 1 for retaining the telescopic tube 6, 
the space 28 having a slant bottom surface 28a A pressing member 29 is 
disposed within the space 28. The pressing member 29 has a slant surface 
29a which is in sliding contact with the slant surface 28a. The pressing 
member 29 is in contact with the lock bolt 25. Between the pressing member 
29 and the telescopic tube 6 is disposed a pressure contact member 30 
which is capable of pressing against the telescopic tube 6. In this 
arrangement, when the lock motor 23 is activated, the lock bolt 25 slides 
through the reduction mechanism 24 while being guided by the breakaway 
bracket 1. The sliding movement of the lock bolt 25 forces the pressing 
member 29 to slide upwardly by virtue of the sliding contact between the 
slant surfaces 28a and 29a. The sliding movement of the pressing member 29 
forces the pressure contact member 30 to slide and firmly press against 
the telescopic tube 6. Thus, the frictional force occurring between the 
column tube 6 and the pressure contact member 30 increases sufficiently to 
ensure the required rigidity of the steering system. 
When the lock motor 23 is reversed, the lock bolt 25 slides in the opposite 
direction to the above, so that the column tube 6 is released from the 
pressure that is applied from the pressure contact member 30. Thus, the 
frictional force acting between the column tube 6 and the pressure contact 
member 30 decreases, and the column tube 6 becomes easy to slide. 
The above-described operation may be conducted as shown in the time chart 
of FIG. 7. More specifically, the pressure contact member 30 is normally 
pressed against the telescopic tube 6 to obtain sufficient frictional 
force to ensure the required rigidity. If the telescopic operation switch 
(not shown) is actuated, the telescopic motor 14 is activated to start a 
telescopic operation and, at the same time, the lock motor 23 is activated 
to release the telescopic tube 6 from the pressure applied from the 
pressure contact member 30, thus the frictional force decreasing. In 
consequence, the resistance to sliding of the column tube 6 decreases and 
a smooth telescopic operation is ensured. In response to a telescopic 
operation completion signal operation of the telescopic motor 14 is 
suspended, while the lock motor 23 is activated to press the pressure 
contact member 30 against the telescopic tube 6 again. 
Thus, it is possible according to the present invention to increase the 
rigidity of the steering system and, at the same time, to minimize the 
sliding resistance. It is therefore possible to realize a smooth 
telescopic operation while ensuring the required rigidity of the steering 
system. 
In FIGS. 8 and 9, there is illustrated another mode of the control 
mechanism. In this mode, a time duration during which the press contact 
member 30 is pressed against the telescopic tube 6 is set to be longer 
than a time duration during which the press contact member 30 is released 
away from the telescopic tube 6. This ensures the complete frictional 
engagement between the press contact member 30 and the telescopic tube 6 
even through a clearance therebetween is generated due to the usage with 
age or the inaccurate structure of the telescopic tube 6. Such operations 
which are different in the moving directions of the press contact member 
30 can be established by a microprocessor 31 which is set to activate the 
telescopic motor 14 and the lock motor 23 via a driver circuit 33 and a 
driver circuit 34, respectively, upon closure of a switch 32 at a 
dashboard (not shown). 
Although the present invention has been described through specific terms, 
it should be noted here that the described embodiments are not necessarily 
exclusive and that various change and modifications may be imparted 
thereto without departing from the scope of the invention which is limited 
solely by the appended claims.