1. Field of the Invention
The present invention relates to a rack assist type electric power steering apparatus, and more particularly, relates to a rack assist type electric power steering apparatus that can: eliminate a rattling noise and the resistance of a steering wheel, which are caused by vibrations that are due to the operation of a ball nut and a rack bar or which are caused by the impact load that is reversely input from the road surface; and improve the driving reliability.
2. Description of the Prior Art
A typical rack assist type electric power steering apparatus is configured to include a steering system including a structure from a steering wheel to wheels on both sides and an assist power mechanism for supplying steering assist power to the steering system.
FIG. 1 is a partially cross-sectional view schematically illustrating a typical rack assist type electric power steering apparatus, and FIG. 2 is a cross-sectional view schematically illustrating a portion of a typical rack assist type electric power steering apparatus.
A typical rack assist type electric power steering apparatus is configured to include: a steering system 100 that includes a structure from a steering wheel 105 to wheels 150 on both sides; and an assist power mechanism 160 for supplying steering assist power to the steering system.
The steering system 100 is configured to include a steering shaft 110 of which the upper end is coupled to the steering wheel 105 to then be rotated together with the steering wheel 105 and of which the lower end is coupled to a pinion shaft 120 through a pair of universal joints 115. In addition, the pinion shaft 120 is coupled to a rack bar 155 through a rack-pinion mechanism 135, and both ends of the rack bar 155 are coupled to vehicle wheels 150 through tie rods 140 and knuckle arms 145.
The rack-pinion mechanism 135 is formed by the engagement of a pinion gear 125 that is formed at the lower end of the pinion shaft 120 and a rack gear 130 that is formed on one side of the outer circumferential surface of the rack bar 155.
The assist power mechanism 160 is configured to include: a torque sensor 117 that detects a steering torque applied to the steering wheel 105 by the driver in order to thereby output an electric signal in proportion to the detected steering torque; an electronic control unit (ECU) that generates a control signal based on the electric signal that is received from torque sensor 117; a motor 165 that generates steering assist power based on the control signal that is received from the electronic control unit; and a belt-type driving device 170 that transfers the assist power that is generated in the motor 165 to the rack bar 155 through a belt 210.
The belt-type driving device 170 is comprised of a motor 165 that is controlled by the electronic control unit (ECU), a motor pulley 205a that is fixed to a shaft 165a of the motor 165, a belt 210 that is wound around the motor pulley 205a, a ball nut 220 that supports the rack bar 155 inside a rack housing 200 that encloses the rack bar 155, and a nut pulley 205b that is connected to the outer circumferential surface of the ball nut 220.
The ball nut 220 is coupled to the rack bar 155 to be rotatable by means of balls in order to thereby allow the rack bar 155 to slide in the rack housing 200. A bearing 340 for supporting the rotation of the ball nut 220 is mounted on the outer circumferential surface of the ball nut 220.
However, in the prior art described above, when the ball nut rotates to slide the rack bar, the resistance of the steering wheel as well as noise and vibration, which are caused by the rack bar, the ball nut, the nut pulley, the bearing, and the rack housing, occurs.
In addition, when the belt operates by the motor, the ball nut and the nut pulley may be deformed or damaged due to the vibration that is transferred to the nut pulley and the ball nut or due to the impact that is transferred from the road surface through the rack bar in order to thereby cause a serious safety problem whereby the steering wheel is locked while driving.