Speed reduction device for steering devices

Abstract: A speed reduction device for steering devices comprises a first rotary shaft (13) supported by a housing (1) and rotatable by an external force, a second rotary shaft (18) axially movably fitted to the first rotary shaft (13) and nonrotatable relative thereto, a movable member (21) attached to the housing (1) axially movably but nonrotatably and fitted to the second rotary shaft (18) axially movably and rotatably relative thereto, a first ball screw (21) connecting the second rotary shaft (18) to the housing (11), and a second ball screw (25) connecting the movable member (2) to the second rotary shaft (18).

Description: The present invention relates to a speed reduction device for steering devices according to the preamble of claim 1. The DE 39 10 814 A1 discloses a speed reduction device according to the preamble of claim 1, without particular reference to the use in connection with steering devices. The speed reduction device according to the reference comprises a first shaft entering non-rotatably but axially shiftably into a second hollow shaft. The second hollow shaft is in threaded engagement with the inner side of a housing of the first shaft. A movable member being used as an actuator for moving any object is inserted into the second rotary shaft via a second thread coupling. If the first shaft is rotated, the second shaft moves axially with respect to the housing while the movable member is moved together with the second rotary shaft on the one hand, and, on the other hand, moves relative to the second rotary shaft in axial direction. With electric motor-driven power steering devices of the rack-pinion type, the rotation of the motor needs to be transmitted to a rack shaft (movable member) upon speed reduction. With such a conventional power steering device having a d.c. motor incorporated therein, the rotation of the motor is delivered, for example, to a worm reduction gear mechanism for speed reduction and then to the rack shaft by a pinion and a rack. Another power steering device is also proposed which comprises a housing for supporting a rack shaft, a tubular rotary shaft disposed inside the housing and rotatable by an electric motor, and a ball nut (nut having recirculating balls) disposed inside the housing and rotatably supported, the rack shaft extending through the shaft and the nut. The rotation of the rotary shaft is transmitted through a planetary gear reduction mechanism to the ball nut, the rotation of which is transmitted to the rack shaft via a ball thread meachanism (see Unexamined Japanese Patent Publication SHO 60-154955). On the other hand, a hydraulic power steering device of the ball thread type is known which comprises a housing having a cylinder portion, a rotary shaft (output shaft) disposed in the cylinder portion and rotatable by the rotation of a steering wheel, and a piston member (movable member) disposed in the cylinder portion and partitioning the interior of this portion into front and rear oil chambers. The rotation of the rotary shaft is transmitted to the piston member through a ball thread mechanism to move the piston member forward or rearward and thereby rotate a pitman shaft. However, in the case of the conventional motor-driven power steering devices of the rack-pinion type stated, the characteristics of the d.c. motor involve a high-speed rotation and a low torque and therefore require a great reduction ratio, which necessitates a complex reduction mechanism such as those mentioned and exterts a great adverse effect on the performance involving return of the steering wheel. Furthermore, the use of the worm or planetary gear reduction mechanism for reducing the speed of rotation of the motor entails a great transmission loss, so that the motor needs to have an increased capacity. In the case where the ball thread mechanism is used for speed reduction, the reduction ratio is dependent on the lead of thread of the mechanism, whereas it is impossible to greatly change the lead of thread of the mechanism in view of the construction. This presents difficulty in greatly altering the reduction ratio, leading to reduced freedom of design. The hydraulic power steering device of the ball thread type similarly has the problem of reduced freedom of design since the ball thread mechanism is used for speed reduction. The same problem as above is also encountered with steering devices other than power steering devices and with other speed reduction devices. The main object of the present invention is to provide a steering device and speed reduction device which have a reduction mechanism which is simple, efficient and diminished in transmission loss so that the devices can be designed with greatly increased freedom. To comply with this object, the speed reduction device according to the present invention is characterized by the features of claim 1. The steering device and speed reduction device of the present invention comprise a first rotary tubular shaft supported by a housing and rotatable by an electric motor, a second rotary tubular shaft axially movably fitted into the first rotary shaft and nonrotatable relative thereto, a movable member attached to the housing axially movably but nonrotatably and fitted into the second tubular rotary shaft axially movably and rotatably relative thereto, first thread coupling means connecting the second rotary shaft to the housing, and second thread coupling means connecting the movable member to the second rotary shaft, whereby the rotation of the first rotary shaft is transmitted to the movable member as an axial movement thereof upon speed reduction. The movable member may be a rack shaft. The rotor of the motor is attached to the first rotary shaft, and the stator of the motor to the housing. The electric motor may be a brushless electric motor, and a sensor for detecting the rotated position of the motor rotor may be attached to the motor directly or indirectly via a coupling mechanism. The rotation of the first rotary shaft is transmitted to the second rotary shaft, rotating the second rotary shaft with the first rotary shaft and causing the first thread coupling means to axially move the second rotary shaft at the same time. The rotation and axial movement of the second rotary shaft are converted by the second thread coupling means to an axial movement of the movable member. Consequently, the rotation of the first rotary shaft is transmitted to the movable member upon speed reduction. Since the rotational speed of the first rotary shaft is reduced by the thread coupling means, the transmission loss involved is small. Furthermore, the amount of movement of the movable member per unit amount of rotation of the first and second rotary shafts, i.e., the reduction ratio (transmission ratio), can be optionally determined in accordance with the direction (hand) of threads of the two thread coupling means and the lead of threads thereof. Thus, the steering device and the speed reduction device of the present invention are diminished in the overall transmission loss and permit use of a motor of reduced capacity. Furthermore, the reduction ratio can be determined as desired in accordance with the hand and lead of the threads of the two thread coupling means. The devices can therefore be designed with greatly increased freedom. Use of ball thread means as at least one of the first and second thread coupling means results in a greatly reduced transmission loss. However, other thread coupling means such as slide screws are alternatively usable. When the electric motor has a rotor attached to the first rotary shaft and a stator attached to the housing so as to rotate the first rotary shaft directly by the motor, the transmission loss occurs only at the locations of the first and second thread coupling means. Since the transmission through the thread coupling means involves only a small loss, the overall transmission loss is very small. However, even if the rotation of the motor is transmitted to the first rotary shaft upon speed reduction by one reduction gear mechanism, the portion where a transmission loss occurs is almost limited only to this mechanism, with the result that the overall transmission loss is small. Because the brushless motor rotates at a low speed with a high torque, the reduction ratio required can be lower, permitting use of a simple reduction mechanism. The low-speed rotation and high-torque motor characteristics can be fully utilized by using the brushless motor in combination with a reduction mechanism including ball thread means or like thread coupling means, and the reduced motor inertia achieves great improvements in the return of steering wheel and like performance. Nevertheless, an electric motor other than the brushless motor can of course be used. FIG. 1 is a rear view partly broken away and showing an electric motor-driven power steering device of the rack-pinion type embodying the invention; FIG. 2 is an enlarged rear view partly broken away and showing a speed reduction device included in the power steering device of FIG. 1; An embodiment of the invention will be described below with reference to the drawings. FIGS. 1 and 2 show an electric motor-driven power steering device of the rack-pinion type for the front wheels of motor vehicles. FIG. 1 shows the main portion of the steering device, and FIG. 2 shows a speed reduction device of the steering device in detail. In the following description, the right-hand side and the left-hand side of theme drawings will be referred to as "right" and "left," respectively. A housing 1 elongated in the right-left direction is fixed to the frame of the motor vehicle. A rack shaft (movable member) 2 extending in the right-left direction is supported by the housing 1 inside thereof movably in the same direction (axial direction) although not rotatable relative thereto. The housing 1 is formed toward the left end thereof with a gear box 3 having an input shaft 4 and an output shaft 5 which are rotatably supported and interconnected by a torsion bar (not shown). The output shaft 5 is integral with a pinion 6 meshing with a rack (not shown) on the rack shaft 2. The input shaft 4 is connected to an unillustrated steering wheel. The gear box 3 is provided with a torque sensor 7 for detecting the steering torque of the steering wheel by detecting the torsion of the torsion bar between the input shaft 4 and the output shaft 5. The right and left ends of the rack shaft 2 projecting from the housing 1 have tie rods 10, 11 connected thereto by ball joints 8, 9, respectively. Disposed within an intermediate portion of the housing 1 is a tubular outer rotary shaft (first rotary shaft) 13 rotatable by a brushless electric motor 12 and supported by bearings 14, 15 so as not to move axially. The motor 12 comprises a stator 16 fixedly provided inside the housing 1 and a rotor 17 mounted directly on the outer periphery of the outer rotary shaft 13. A position sensor 35 comprising a rotary encoder is mounted on a suitable portion of the housing 1 around the rack shaft 2. A gear 36 fixed to the input shaft of the sensor 35 is in mesh with a gear 37 fixedly provided around the right end of the outer rotary shaft 13. The rotation of the outer rotary shaft 13 is transmitted to the sensor 35 upon speed reduction by the gears 37, 36 for the sensor 35 to detect the position of the outer rotary shaft 13, i.e., of the rotor 17, as rotated. A tubular inner rotary shaft (second rotary shaft) 18 is disposed inside of and attached to the outer rotary shaft 13 so as to be movable axially but not rotatably relative thereto as by splining as indicated at 19. The rack shaft 2 extends through the inner rotary shaft 18 with a bush 20 interposed therebetween and is rotatable and axially movable relative to the shaft 18. The inner rotary shaft 18 is always positioned as partially projected leftward beyond the outer rotary shaft 13 even when moved relative to the outer rotary shaft 13. The projected portion is connected to the housing 1 by first ball thread means 21 constituting first thread coupling means. More specifically, the inner rotary shaft 18 is externally threaded as at 22, and this threaded portion 22 is screwed in a ball nut 23 secured to the housing 1, with a multiplicity of recirculating balls 24 provided between the portion 22 and the nut 23. The rack shaft 2 is connected to the left end of the inner rotary shaft 18 by second ball thread means 25 constituting second thread coupling means in the following manner. More specifically, the rack shaft 2 is externally threaded as at 26, and this threaded portion 26 is screwed in a ball nut 27 fixed to the left end of the inner rotary shaft 18, with a multiplicity of recirculating balls 28 provided between the portion 26 and the nut 27. According to the present embodiment, the first ball thread means 21 has a right-handed thread, and the second ball thread means 25 has a left-handed thread. Thus, the threads of the two ball thread means 21, 25 are opposite to each other in hand. The lead of thread of the second ball thread means 25 is greater than that of the first ball thread means 21. When the driver rotates the steering wheel, the rotation is transmitted to the pinion 6 through the input shaft 4, torsion bar and output shaft 5. The rotation of the pinion 6 moves the rack shaft 2 rightward or leftward to steer the wheels coupled thereto. At this time, the motor 12 is driven in accordance with the direction and magnitude of the steering torque detected by the torque sensor 7, whereby the rack shaft 2 is moved in the same direction as the movement thereof effected by the pinion 5. For example when the rack shaft 2 is moved leftward by manipulating the steering wheel, the rotor 17 of the motor 12 is rotated rightward (clockwise) when seen from the right. This rotates the outer rotary shaft 13 and the inner rotary shaft 18 in the same direction by the same amount. The clockwise rotation of the inner rotary shaft 18 causes the first ball thread means 21 to move the shaft 18 leftward relative to the housing 1, also causing the second ball thread means 25 to move the rack shaft 2 leftward relative to the inner rotary shaft 18. As a result, the rack shaft 2 moves leftward relative to the housing 1. The amount of this movement is equal to the amount of movement of the inner rotary shaft 18 relative to the housing 1 plus the amount of movement of the rack shaft 2 relative to the inner rotary shaft 18. The amount of movement of the inner rotary shaft 18 relative to the housing 1 is in proportion to the amount of rotation of the shaft 18 and to the lead of thread of the first ball thread means 21. The amount of movement of the rack shaft 2 relative to the inner rotary shaft 18 is in proportion to the amount of rotation of the shaft 18 and to the lead of thread of the second ball thread means 25. Accordingly, the amount of movement of the rack shaft 2 relative to the housing 1 is in proportion to the sum of leads of threads of the two ball thread means 21, 25 and to the amount of rotation of the inner rotary shaft 18. When the rack shaft 2 is moved rightward by manipulating the steering wheel, the rack shaft 2 moves rightward by an amount which is similarly in proportion to the sum of leads of threads of the two ball thread means 21, 25. If the threads of the two ball thread means 21, 25 are the same in hand, the direction of movement of the inner rotary shaft 18 relative to the housing 1 is opposite to the direction of movement of the rack shaft 2 relative to the inner rotary shaft 18. Consequently, the direction of movement of the rack shaft 2 relative to the housing 1 is dependent on the direction of rotation of the inner rotary shaft 18 and on the relation between the leads of threads of the two ball thread means 21, 25, and the amount of movement of the rack shaft 2 is in proportion to the difference between the leads of these threads and to the amount of rotation of the inner rotary shaft 18. For example in the case where the two ball thread means 21, 25 are right-handed, the outer and inner rotary shafts 13, 18, when rotated rightward as seen from the right side, cause the first ball thread means 21 to move the inner rotary shaft 18 leftward relative to the outer rotary shaft 13, also causing the second ball thread means 25 to move the rack shaft 2 rightward relative to the inner rotary shaft 18. If the lead of thread of the first ball thread means 21 is then greater than that of the second ball thread means 25, the rack shaft 2 moves leftward by an amount in proportion to the difference between the leads of threads of these ball thread means 21, 25. If the outer rotary shaft 13, as seen from the right side, is rotated leftward, the rack shaft 2 moves rightward similarly by an amount in proportion to the difference. In the case where the two ball thread means 21, 25 are left-handed, the outer and inner rotary shafts 13, 18, if rotated rightward when seen from the right side, cause the first ball thread means 21 to move the inner rotary shaft 18 rightward relative to the outer rotary shaft 13, also causing the second ball thread means 25 to move the rack shaft 2 leftward relative to the inner rotary shaft 18. If the lead of thread of the first ball thread means 21 is then smaller than that of the second ball thread means 25, the rack shaft 2 moves leftward by an amount in proportion to the difference between the leads of threads of these ball thread means 21, 25. When the outer rotary shaft 13 as seen from the right side is rotated leftward, the rack shaft 2 moves rightward similarly by an amount in proportion to the difference. Thus, the amount of movement of the rack shaft 2 relative to the housing 1 per unit amount of rotation of the inner rotary shaft 18 effected by the motor 12, i.e., the reduction ratio, is dependent on the leads of threads of the two ball thread means 21, 25, so that the reduction ratio can be determined as desired by determining the leads suitably. The rotation of the inner rotary shaft 18 effected by the motor 1 is transmitted to the rack shaft 2 by the ball thread means 21, 25 which are very small in transmission loss. This ensures a greatly reduced overall transmission loss and therefore makes it possible to use a motor 12 of reduced capacity. For example in the case where the rotation of the motor is transmitted to the rack shaft through a worm and rack-pinion assembly as in the prior art, the overall transmission efficiency is about 0.6, whereas when the rotation of the motor is transmitted to the rack shaft via the two ball thread means as in the present embodiment, the overall transmission efficiency is about 0.9. The present embodiment therefore provides the same drive force as afforded by the conventional arrangement with use of a motor having about 2/3 of the capacity of the motor conventionally used. The brushless motor 12 rotates at a low speed with a high torque, so that the reduction ratio can be lower, permitting use of a simple reduction mechanism. The low-speed rotation high-torque motor characteristics can be fully utilized by the combination of brushless motor 12 and reduction mechanism including the two ball thread means 21, 25. The reduced inertia of the motor leads to improvements in performance including return of the steering wheel. Although the brushless motor 12 used requires the position sensor 35, this sensor 35 can be provided at a portion around the rack shaft 2 with great freedom. The speed reduction device of the invention is not limited to the above embodiments in construction but can be modified suitably within the scope of the invention as defined in the appended claims.

Claims: Speed reduction device for steering devices, the speed reduction device comprising: a first rotary shaft (13) supported by a housing (1) and rotatable by an external force, a second rotary shaft (18) axially movable with respect to the first rotary shaft (13) and non-rotatable relative thereto, the second rotary shaft being tubular, a movable member (2) attached to the housing (1) axially movable but non-rotatably relative thereto and being fitted into the second tubular rotary shaft (18) axially movable and rotatably relative thereto, first thread coupling means (21) connecting the second rotary shaft (18) and the housing (1), second thread coupling means (25) connecting the movable member (2), an electric motor (12) being provided to rotate the first rotary shaft (13), characterized in that the first rotary shaft (13) is a tubular shaft, the second rotary shaft (18) being fitted into the first rotary shaft (13), a stator (16) of the electric motor (12) is arranged inside the housing (1) around the first rotary shaft (13), a rotor (17) is attached to the first rotary shaft (13). Speed reduction device as claimed in claim 1, characterized in that the movable member is a rack shaft (2). Speed reduction device as claimed in claim 1, characterized in that the electric motor (12) is a brushless electric motor, and a sensor (35) for detecting the rotated position of the motor is attached to the motor directly or indirectly with a coupling mechanism (36,37) therebetween.

Inventors: DAIDO TOSHIHIKO, SANO OSAMU, DAIDO, TOSHIHIKO, SANO, OSAMU
Applicants: KOYO SEIKO CO, KOYO SEIKO CO., LTD.
Application year: 1992
Main classification: B62D5
IPC codes: F16H25, B62D5