ELECTRIC POWER STEERING ASSIST MECHANISM

Abstract: A gear isolator assembly includes a gearbox housing (101), a first gear (102) having a first axis of rotation about a first shaft (114) and the second gear (104) having a second axis of rotation about a second shaft (109). The first (102) and second gears (104) are rotatably mounted in a gearbox housing (101) so that the first (102) and second gears (104) mesh with each other such that the first axis of the first gear (102) is substantially perpendicular to the second axis of the second gear (104). The gear isolator assembly is preferably disposed within the gearbox housing (101) about the second gear (104) for reducing noise generated between the first (102) and second gears (104) during operation of the mechanism (100).

Description: ELECTRIC POWER STEERING ASSIST MECHANISM CROSS-REFERENCE The present application is related to copending United States Patent Application filed on October 7, 1999, and entitled "Electric Power Steering Assist Mechanism" in the name of Jammer, et al., attorney docket no. DE3-0042, the contents of which are incorporated by reference hereto. TECHNICAL FIELD This invention relates generally to a steering apparatus for a motor vehicle and, more particularly, to a worm isolator assembly utilized in a damped worm assist mechanism for reducing audible noise in an electric power steering assist mechanism. BACKGROUND OF THE INVENTION Presently, certain motor vehicles contain column-type power steering apparatus that employ an electric power steering assist mechanism. The electric power steering assist mechanism provides torque assist to the steering shaft of a vehicle via an electric motor and a worm/worm gear reduction mechanism. The worm/worm gear reduction mechanism is interposed between the output shaft and the motor to obtain an appropriate steering speed as well as sufficient steering assistance in the course of transmission of the rotational force from the motor to the output shaft. When the motor vehicle is operating, clearances between the worm and worm gear teeth and between other adjacent components in the mechanism commonly result in "rattle" noise. Road feedback forces and torques travel through the steering shaft to the worm gear. These vibration loads are transmitted through the worm gear to the worm. The oscillatory impact produced takes place where the worm and worm gear teeth mesh. This oscillatory impact translates into axial forces acting upon the worm. These axial forces react through the worm to the adjacent components and produce the resulting "rattle" noise. There exists a need for a worm/worm gear reduction mechanism with lower combined stiffness of the mechanical system and no free lash along the mechanism's worm axis. SUMMARY OF THE INVENTION This invention offers further advantages and alternatives over the prior art by providing a gear isolator assembly that reduces audible noise in a gear reduction mechanism. According to the present invention, a gear isolator assembly is provided. The gear isolator assembly is preferably used in a gear reduction mechanism for reducing noise generated by a first gear meshing with a second gear during operation of the mechanism. In an exemplary embodiment, the first gear has a first axis of rotation about a first shaft and the second gear has a second axis of rotation about a second shaft. The first and second gears are rotatably mounted in a gearbox housing so that the first and second gears mesh with each other such that the first axis of the first gear is substantially perpendicular to the second axis of the second gear. The gear isolator assembly is preferably disposed within the gearbox housing about the second gear for reducing noise generated between the first and second gears during operation of the mechanism. More specifically, the first and second gears rotate about their respective axis during operation of the mechanism. It is this interaction between the first and second gears, which generates noise, which travels through the gearbox housing. In an exemplary embodiment, the gear isolator assembly comprises an elastomeric member having first and second side surfaces. Each of the first and second side surfaces includes a plurality of ridges, which define a plurality of grooves there between. The elastomeric member is disposed between first and second washers. An exemplary washer has an annular planar section formed between opposing flared edges. Inner and outer planar surfaces of the elastomeric member seat within the planar sections of the first and second washers between the respective flared edges. The design of the first and second side surfaces of the elastomeric member permits the elastomeric member to compress under pressure. Advantageously, the present invention may also provide a gear isolator assembly that can maintain a highly nonlinear spring rate over an extended distance of travel. Advantageously, the present invention may also provide a gear isolator assembly that allows the first gear to float axially while controlling its axial movement. Advantageously, the present invention may also provide a gear isolator assembly that can maintain an indefinite load at a positive stop height without a metal-to-metal interface, which contributes to a reduction of the audible noise of a gear reduction mechanism during operation. Advantageously, the present invention may also provide a gear isolator assembly that can maintain an axial pre-load along the axis of the first gear and eliminate clearances between the components of a gear reduction mechanism, which contributes to a reduction of the audible noise of a gear reduction mechanism during operation. Advantageously, the present invention may also provide a gear isolator assembly that produces a bearing preload, which delashes the gear reduction mechanism and contributes to a reduction of the audible noise of a gear reduction mechanism during operation. The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of an electric power steering assist mechanism including the worm isolator assembly embodying the present invention; Figure 2 is an enlarged cross-sectional view of a portion of the electric power steering assist mechanism including the worm isolator assembly of Figure 1; Figure 3 is a cross-sectional view of the worm isolator assembly of Figure 1; Figure 4 is an enlarged cross-sectional view of the worm isolator assembly in area 4 in Figure 3; Figure 5a is a cross-sectional view of the worm isolator assembly of Figure 1 at a free height position; Figure 5b is a cross-sectional view of the worm isolator assembly of Figure 1 at a working height position; and Figure 5c is a cross-sectional view of the worm isolator assembly of Figure 1 at a positive stop height position. DETAILED DESCRIPTION OF THE PRESENT INVENTION An electric power steering assist mechanism assembly utilizing the present invention is depicted in Figure 1. Those skilled in the art will appreciate that worm/worm gear reduction mechanisms are vehicle application specific. For each vehicle a worm/worm gear reduction mechanism is manufactured to the specifications of that particular vehicle. Those skilled in the art will further appreciate that the present invention may be modified for use with many different vehicle applications. Referring to Figure 1, it is seen that a damped worm assist mechanism, generally indicated at 100, includes a gearbox housing 101, a worm gear 102, a worm 104, a pair of bushings 106, a pair of ball bearing assemblies 108, a pair of retaining rings 110 and a pair of worm isolator assemblies 112. Worm gear 102 is disposed intermediate to an inner wall 103 of a gearbox housing 101 and steering shaft 114. Worm 104 is positioned within gearbox housing 101 by disposing a first end 105 and a second end 107 of worm 104 between bushings 106, ball bearing assemblies 108 and worm isolator assemblies 112. Second end 107 of worm 104 is coupled to an output shaft 109, which is connected to an electric motor that is not shown, so that the latter drives worm 104 when the electric motor is operating. In the illustrative embodiment, second end 107 is secured within an opening 111 of output shaft 109. Worm gear 102 is rotatably mounted in gearbox housing 101 and rigidly connected to steering shaft 114. In the illustrated embodiment, worm gear 102 comprises an annular shaped gear. It will be appreciated that worm gear 102 may include teeth (not shown) formed on a surface thereof for a coupling component of worm gear 102. The exemplary worm gear 102 comprises an inner ring 116 coupled to an outer ring 118. Inner ring 116 and outer ring 118 may be coupled together by many known techniques or methods as are understood by those skilled in the art. This is by example only, as not all worm gears are comprised of two components. Worm gear 102 is also rotatably supported within gearbox housing 101. Worm 104 is rotatably mounted in gearbox housing 101. The second end 107 of worm 104 being preferably connected directly to an output shaft 109. One exemplary worm 104 comprises an elongated shaft that crosses the length of gearbox housing and permits support on first and second ends 105, 107, respectively. Bushings 106 and ball bearing assemblies 108 also support worm 104 in gearbox housing 101. Bushings 106 and ball bearing assemblies 108 are held in gearbox housing 101 by annular retaining rings 110. Bushings 106 are preferably tubular shaped and are disposed about worm 104 to allow axial movement of the worm 104 with respect to ball bearing assemblies 108. Ball bearing assemblies 108 rotatably support worm 104 while remaining stationary against gearbox housing 101. Referring now to Figures 2-4, in which the worm isolator assembly 112 according to the present invention is illustrated. Worm isolator assembly 112 is disposed about worm 104 on both first end 105 and second end 107. More specifically, one worm isolator assembly 112 and another worm isolator assembly 112 both seat against a first surface 113 and a second surface 115 of worm 104. In addition, worm isolator assembly 112 is disposed intermediate to ball bearing assembly 108 and annular shoulder 117 formed on the first and second surface 113, 115, respectively, of worm 104. In other words, annular shoulder 117 serves to position and locate worm isolator assembly 112 relative to ball bearing assembly 108 and worm 104. Worm isolator assembly 112 comprises an elastomeric member 120 and a pair of washers 122 and 124. Elastomeric member 120 includes an inner surface 119 and an opposing outer surface 121. Formed on each of the surfaces 119, 121 is a plurality of ridges 123, which defines a plurality of grooves 129. The design of inner and outer surfaces 119, 121 in combination with the elastomeric nature of member 120 permits member 120 to compress under pressure. The washers 122, 124 each include an annular planar section 126 with opposing flared edges 125, 127. Inner and outer surfaces 119, 121 of elastomeric member 120 are received by and seat within annular planar sections 126 of washers 122, 124 and are secured between opposing flared edges 125, 127 of each washer 122, 124. More specifically, flared edge 125 comprises an inner annular flared edge and flared edge 127 comprises an outer annular flared edge. Worm isolator assembly 112, with the specific shapes of the plurality of ridges 123 and washers 122, 124 follows a prescribed load versus deflection relationship. Worm isolator assembly 112 maintains a desired preload along the worm axis under all operating conditions due to this relationship. This spring rate characteristic also minimizes the amplitude of the impact force between worm 104 and worm gear 102. Traditional springs do not achieve this variable spring rate throughout all operating conditions. As will be further illustrated in Figures 5 a, 5b and 5 c, the shape of elastomeric member 120 is important for attaining the necessary spring constant of worm isolator assembly 112. As illustrated in Figure 5a when worm isolator assembly 112 is at its free height, the diameter of a ridge 123 of elastomeric member 120 is less than the diameter of an inner surface 128 of washers 122, 124. Figure 5b depicts the worm isolator assembly at its working height. At the working height the elastomeric member 120 is in the state prior to hydrostatic compression. The ridges 123 are compressed together to form a column, which allows the assembly to maintain the desired spring rate. Further compression from the working height results in dominant hydrostatic compression. As shown in Figure 5c when elastomeric member 120 is hydrostatically compressed within washers 122, 124, elastomeric member 120 expands and scales in shape to the confines of the shape of washers 122, 124 until reaching the point commonly known as "bottoming out" or, in the present case, the positive stop height or shut height of worm isolator assembly 112. For example, a coil spring bottoms out when the coils are compressed to the point where each coil touches the other. In the present invention, the compression of elastomeric member 120 encapsulated within washers 122, 124 is hydrostatic. This hydrostatic compression takes place due to the encapsulation of elastomeric member 120 by washers 122, 124. The forces exerted upon elastomeric member 120 from every side are focused equally upon any single point in elastomeric member 120 so that elastomeric member 120 is not deformed or destroyed but rather scales in shape to washers 122, 124. The positive stop height of elastomeric member 120 prevents washers 122, 124 from making contact with each other while being compressed. A clearance 130, as defined between respective flared edges 125, 127 of washers 122, 124, is formed when the positive stop height of elastomeric member 120 is reached and member 120 has expanded to its limit. This clearance 130 prevents a metal-to-metal interface from occurring, which also contributes to a reduction in noise during operation of the worm/worm gear reduction mechanism. Referring again to Figure 2, which illustrates how worm gear 102 meshes with the worm 104. The axis of rotation 132 of worm 104 can be seen to be generally perpendicular to the axis of rotation 134 of worm gear 102. When in operation, road feedback forces and torques or vibrational loads travel through the steering shaft 114 to the worm gear 102. These vibrational loads are transmitted through worm gear 102 to worm 104. The oscillatory impact produced takes place where worm 104 and worm gear 102 mesh. This oscillatory impact translates into axial forces acting upon worm 104. These axial forces react through worm isolator assembly 112, bushings 106, ball bearing assemblies 108, and retaining rings 110 to gearbox housing 101. Worm 104 is allowed to float axially while worm isolator assembly 112 controls its axial movement. Worm isolator assembly 112 also produces a bearing preload that delashes ball bearing assembly 108. This interaction between worm 104, worm isolator assembly 112, and ball bearing assembly 108 lowers the combined stiffness of the mechanical system and delashes worm 104, bushings 106 and ball bearing assemblies 108. The end result is the reduction of audible noise by the worm/worm gear reduction mechanism. In operation worm isolator assembly 112 is under compression and maintains an axial pre-load along axis 132 of worm 104. Any clearances within ball bearing assembly 108 and between bushings 106 and retaining rings 110 are eliminated. The elimination of these clearances, in addition to the prevention of a metal-to-metal interface, reduces the noise or "rattle" of the worm/worm gear reduction mechanism. It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and

Claims: CLAIMS What is claimed is: 1. A mechanism (100) disposed within a gearbox housing (101), a first gear (102) having a first axis of rotation about a first shaft (114), a second gear (104) having a first end (105), a second end (107) and a second axis of rotation about a second shaft (109), the first (102) and second gears (104) being rotatably mounted in the gearbox housing (101) so that the first (102) and second gears (104) mesh with each other such that the first axis of the first gear (102) is substantially perpendicular to the second axis of the second gear (104), a plurality of ball bearing assemblies (108) disposed about the first gear (102), a plurality of bushings (106) disposed about the first gear (102), the ball bearing assemblies (108) and bushings (106) serving to support the first gear ( 102) within the gearbox housing ( 101 ), the mechanism ( 100) comprising: an elastomeric member (120); a plurality of washers (122, 124); the plurality of washers (122, 124) adapted to receive the elastomeric member (120); and the elastomeric member (120) received within the plurality of washers (122, 124) disposed about the second gear (104). 2. A mechanism (100) recited in claim 1, wherein the mechanism (100) comprises a pair of worm isolator assemblies (112), a pair of worm isolator assemblies (112) comprise a first worm isolator assembly and a second worm isolator assembly. 3. A mechanism (100) recited in claim 2, wherein the first worm isolator assembly being disposed on the first end (105) of second gear (104) and the second worm isolator assembly being disposed on the second end (107) of the second gear (104). 4. A mechanism (100) recited in claim 1, wherein the elastomeric member (120) includes an outer planar surface (121), an inner planar surface (119), a first side surface and a second side surface, the first and second side surfaces permit compression of the elastomeric member (120), the outer (121) and inner planar surfaces (119) include a plurality of ridges (123). 5. A mechanism (100) recited in claim 1 , wherein the plurality of washers (122, 124) is a pair of washers. 6. A mechanism (100) recited in claim 1, wherein the plurality of washers (122, 124) includes an inner surface (119) having a first diameter , the elastomeric member (120) includes a second diameter of a ridge (123), the first diameter being larger than the second diameter. 7. A mechanism (100) recited in claim 1, wherein the plurality of washers (122, 124) include an inner annular flared edge (125) and an outer annular flared edge (127). 8. A mechanism (100) recited in claim 3, wherein the second gear (104) experiences a vibrational load when the first gear (102) meshes with second gear (104) along the perpendicular axis formed by the rotational axis of first gear (102) and rotational axis of second gear (104). 9. A mechanism (100) recited in claim 8, wherein the second gear (104) experiences an axial force. 10. A mechanism (100) recited in claim 9, wherein the worm isolator assembly (112), bushings (106), ball bearing assemblies (108), and retaining rings (110) experience the axial force experienced by the second gear (104). 11. A mechanism (100) recited in claim 10, wherein the bushings (106), the ball bearing assemblies (108) and the retaining rings (110) disposed in gearbox housing (101) define a plurality of clearings (130). 12. A mechanism (100) recited in claim 11, wherein the second gear (104) is a worm. 13. A mechanism (100) recited in claim 12, wherein the worm floats axially and the worm isolator assemblies (112) control the axial movement of the worm. 14. A mechanism (100) recited in claim 13, wherein the worm isolator assembly (112) experiences a compressive force exerted by the axial movement of the worm and maintains an axial pre-load along the rotational axis (132) of the worm. 15. A mechanism (100) recited in claim 14, wherein the axial force translates into a compressive force, the compressive force translates into a hydrostatic force. 16. A mechanism (100) recited in claim 15, wherein when experiencing the compressive force the worm isolator assembly (112) maintains a first position before experiencing a hydrostatic compression, the first position is a working height position. 17. A mechanism (100) recited in claim 16, wherein when experiencing the hydrostatic compression the worm isolator assembly (112) maintains a second position, the second position is a positive stop height position. 18. A mechanism (100) recited in claim 17, wherein the worm isolator assembly (112) reduces the plurality of clearances (130) between the bushings (106), the ball bearings (108) and the retaining rings (110) disposed in the gearbox housing (101) when the worm isolator assembly (112) the experiences the compressive force and the hydrostatic force. 19. A mechanism (100) recited in claim 18, wherein the reduction of the plurality of clearances (130) includes a reduction of an audible noise produced when experiencing the compressive force and the hydrostatic compression. 20. A mechanism (100) recited in claim 19, wherein the worm isolator assembly (112) produces a bearing preload when experiencing the compressive force and the hydrostatic compression, wherein the worm isolator assembly (112) delashes the ball bearing assemblies (108), wherein de-lashing the ball bearing assemblies (108) includes a reduction of an audible noise produced. 21. A mechanism (100) recited in claim 20, wherein the worm isolator assembly (112) includes a spring rate characteristic, the spring rate characteristic minimizes the amplitude of the axial force exerted by the first gear (102) and experienced by the worm isolator assembly (112). 22. A mechanism (100) recited in claim 21, wherein when worm isolator assembly (112) experiences the compressive force at a first position the elastomeric member (120) forms a column when the plurality of ridges (123) are compressed together, the first position is a working height position. 23. A mechanism (100) recited in claim 22, wherein the pair of washers (122, 124) adapted to receive the elastomeric member (120) focus the hydrostatic compression onto a single point in the elastomeric member (120). 24. A mechanism (100) recited in claim 23, wherein the elastomeric member (120) expands within the pair of washers (122, 124) when experiencing the compressive force at the first position and the hydrostatic compression at a second position, the second position is a positive stop height position.

Inventors: OZSOYLU SUAT, STRIETER TROY P, OZSOYLU, SUAT, STRIETER, TROY, P.
Applicants: DELPHI TECH INC, DELPHI TECHNOLOGIES, INC.
Application year: 2000
Main classification: B62D3
IPC codes: B62D3, B62D5, F16F15, F16H1, F16H57