Electric power steering gear assembly with ball screw support shaft

Abstract

An electric power steering gear assembly having a ball screw support shaft is provided. An electric power steering assembly for a commercial vehicle includes a housing, a hollow ball screw, a ball nut, and an inner shaft. The housing includes a generally cylindrical portion extending in an axial direction. The hollow ball screw is disposed in the housing and defines a cavity extending in an axial direction. The ball nut is disposed in the housing, circumferentially surrounds the hollow ball screw, and is configured to axially translate relative to the housing. The inner shaft is disposed in the cavity of the hollow ball screw and rigidly attached to the hollow ball screw at a first end. The inner shaft is configured such that the hollow ball screw is deflectable relative to the axial direction in response to an external load applied by the ball nut.

Description

Technical Field

[0001] This disclosure relates to a steering gear assembly for a vehicle having a ball screw support shaft. Background Technology

[0002] Commercial vehicles use electric power steering (EPS). EPS is achieved using an electric power steering gear assembly. This assembly utilizes a rotating ball screw and a ball nut that translates linearly relative to the ball screw. This linear translational movement of the ball nut is transmitted via a rack to rotate a sector shaft and ultimately turn the vehicle's wheels. During normal operation, the vehicle's wheels, connected to the rack and sector shaft, are subjected to radial and tangential forces, as well as vibrations, transmitted back to the electric power steering gear assembly from the sector shaft and rack. These forces and vibrations applied to the sector shaft are transmitted to the rack, ball nut, and ball screw. In conventional electric power steering assemblies, these forces cause the ball nut to tilt relative to the ball screw, interfering with the optimal interaction between the ball nut and the ball screw. This results in suboptimal transmission of steering force and ultimately premature failure of the electric power steering gear assembly. Summary of the Invention

[0003] In a first aspect, embodiments of the present disclosure provide an electric power steering assembly for a commercial vehicle. The electric power steering assembly includes: a housing including a generally cylindrical portion extending in an axial direction; a hollow ball screw disposed within the housing and defining a cavity extending in the axial direction; a ball nut disposed within the housing, circumferentially surrounding the hollow ball screw and configured to translate axially relative to the housing; and an inner shaft disposed within the cavity of the hollow ball screw and rigidly attached to the hollow ball screw at a first end, the inner shaft being configured such that the hollow ball screw is capable of deflecting relative to the axial direction in response to an external load applied by the ball nut.

[0004] According to claim 1, the electric power steering assembly wherein the inner shaft is made of carbon steel that has been quenched and tempered to achieve a Rockwell hardness C (HRC) of about 28-34.

[0005] According to the first aspect of the implementation, the inner shaft is made of carbon steel, which is carburized to achieve surface hardness.

[0006] According to the first embodiment, the support bushing is made of steel.

[0007] According to an embodiment of the first aspect, a low-friction coating is applied to the outer surface of the support bushing.

[0008] According to the first aspect of the implementation, the inner shaft is threadedly connected to the hollow ball screw.

[0009] According to the first aspect of the implementation, the inner shaft is connected to the hollow ball screw using a locating pin to ensure that the screw thread does not loosen under torque.

[0010] According to an embodiment of the first aspect, the electric power steering assembly further includes: a first plurality of channels between the ball nut and the hollow ball screw; and a second plurality of circulating balls disposed in the channels, wherein the second plurality of circulating balls circulate in the first plurality of channels to transmit the rotational force from the hollow ball screw to cause axial translation of the ball nut.

[0011] According to an embodiment of the first aspect, the electric power steering assembly further includes a support bushing disposed on the outer circumference of the ball nut and configured to contact the inner surface of the housing when the hollow ball screw deflects.

[0012] According to an embodiment of the first aspect, the electric power steering assembly further includes: a rack including a set of teeth, the rack being rigidly attached to the outer circumference of the ball nut and configured to translate together with the ball nut when the ball nut is axially translated relative to the housing; and a sector shaft including a set of gear teeth that engage with the set of teeth of the rack and is configured to rotate based on the translation of the rack. Attached Figure Description

[0013] Embodiments of this disclosure will now be described in more detail with reference to the exemplary accompanying drawings. This disclosure is not limited to exemplary embodiments. In embodiments of this disclosure, all features described and / or shown herein may be used individually or in combination in different ways. The features and advantages of various embodiments of this disclosure will become apparent from the following detailed description, which is illustrated in the accompanying drawings:

[0014] Figure 1 A portion of a gear assembly of a vehicle according to one or more examples of this disclosure is shown. Detailed Implementation

[0015] Examples of this application will now be described more fully below with reference to the accompanying drawings, which illustrate some, but not all, examples of this application. In fact, this application may be exemplified in different forms and should not be construed as limited to the examples set forth herein; rather, these examples are provided so that this application will meet the requirements of applicable law. Furthermore, when something is said herein to be “based on” something else, it may also be based on one or more other things. In other words, unless otherwise expressly indicated, as used herein, “based on” means “at least partially based on” or “at least partially based on”.

[0016] Commercial vehicles use electric power steering (EPS) for steering. The electric power steering system utilizes an electric power steering gear assembly. This assembly includes a ball screw that is substantially horizontal and oriented transversely to the vehicle's origin. The ball screw is housed within a gear housing and circumferentially surrounded by a ball nut, also housed within the housing, to prevent rotational movement of the ball nut relative to the housing. Upon receiving a steering command from the driver, the electric power steering gear assembly of the vehicle, including the ball nut and ball screw, is configured to transmit the steering command from the driver to the vehicle's wheels. For example, upon receiving a steering command from the drive unit, the ball screw rotates and the ball nut is configured to translate axially above the ball screw. Due to the threads of the ball screw and ball nut, the rotation of the ball screw causes the ball nut to translate to the right or left, depending on the direction of rotation of the ball screw. The ball nut is rigidly attached to a rack that translates linearly together with the ball nut. The sector shaft has gear teeth that engage with the teeth of the rack, causing the sector shaft to rotate in response to the translational movement of the rack. This, in turn, causes the vehicle's wheels to rotate, thus steering the vehicle. During normal vehicle operation, radial and tangential loads and vibrations are applied to the rack from the wheels via the sector shaft. Forces transmitted from the sector shaft to the rack and the ball nut cause the ball nut to tilt or wobble relative to its ideal position. In conventional electric power steering gear assemblies, the solid shaft of the ball screw has relatively high stiffness, and the tilting of the ball nut relative to this rigid ball screw adversely affects the force transmission between the threads of the ball nut and the threads of the ball screw. This results in a suboptimal wheel response to steering commands and ultimately leads to premature failure of the electric power steering gear assembly.

[0017] One way to mitigate this problem is to use a hollow threaded ball screw (also known as a hollow ball screw) mounted on an inner shaft that transmits the rotational force of the steering command to the hollow threaded ball screw. This inner shaft is capable of bending along its axis under radial loads applied by the tilted ball nut. This inner shaft allows the hollow threaded ball screw to deflect axially under radial loads applied by the ball nut, which allows the hollow threaded ball screw to maintain a more coaxial relationship with the tilted ball nut. This allows for improved force transmission between the hollow ball screw and the ball nut and reduces unwanted stresses on both. A bushing is mounted between the ball nut and the housing on the outer circumference of the ball nut and limits the permissible tilt of the ball nut relative to the housing.

[0018] Figure 1 A portion of a gear assembly of a vehicle according to one or more examples of this disclosure is shown. In particular, Figure 1A side section view 100 shows a portion of a complete gear assembly of a vehicle. Side section view 100 depicts a housing 118, which is substantially cylindrical and extends horizontally and transversely to the driving direction of the vehicle. A ball nut and a hollow ball screw are disposed within this housing. An input shaft 116 receives steering commands from a user operating the vehicle's steering wheel, causing it to rotate in one direction or the other. This input shaft can be rigidly connected to an inner shaft 108, such that the inner shaft 108 rotates together with the input shaft 116. Based on the steering commands received from the driver at the input shaft 116, the inner shaft 108 can be rigidly attached at one end 110 to a hollow threaded ball screw 106, thereby causing the hollow threaded ball screw 106 to rotate together with the inner shaft 108. The threads on the outer circumferential surface of the hollow ball screw 106 interact with the corresponding threads on the inner surface of the ball nut 102. To prevent the ball nut from rotating relative to the housing, the rotational movement of the hollow ball screw 106 causes the ball nut 102 to translate laterally in the direction of the ball's axis, which depends on the rotational direction of the hollow ball screw 106. A rack 120 is rigidly connected to the ball nut and translates together with the hollow ball screw 106. A sector shaft 112 has gear teeth that mesh with the gear teeth of the rack, causing the sector shaft 112 to rotate in response to the translational movement of the ball nut 102 and the rack 120. This rotational movement of the sector shaft is ultimately transmitted to the vehicle's wheels, causing them to rotate.

[0019] Cross-sectional view 100 depicts the cross-section of ball nut 102 and hollow ball screw 106. A channel 114 exists between the hollow ball screw and the ball nut. Recirculated balls pass through the channel 114 and, depending on the direction of rotation of the input shaft 116 in response to a steering command, facilitate the transmission of rotational motion of the hollow ball screw 106 to a leftward or rightward translational motion of the ball nut 102. A rack 120, rigidly connected to the outer portion of the ball nut 102, has teeth that engage with corresponding teeth of the sector shaft 112. Translation of the ball nut 102 in the left or right direction causes the rack 120 to translate along with it, which in turn causes the sector shaft 112 to rotate, resulting in the vehicle's wheels turning left or right.

[0020] In some embodiments, the sector shaft 112 may be connected to the corresponding wheel of the vehicle via a steering arm, tie rod, steering knuckle, and wheel bearing. The interaction between the vehicle's wheels and the road subsequently exerts forces and vibrations on the sector shaft 112 and the rack 120 of the electric power steering gear assembly. The lateral force exerted by the sector shaft 112 on the rack 120 is mostly parallel to the axis of the ball nut, but at a distance from it. Therefore, applying forces asymmetrically to the ball nut 102 may tend to cause the ball nut 102 to tilt (or oscillate) relative to its initial position and to deviate the axis of rotation of the hollow ball screw 106. The tilting of the ball nut 102 applies a radial load to the hollow ball screw 106 via the circulating balls. Tilting of the ball nut 102 can cause uneven loading of the recirculated balls in the channel 114 between the ball nut 102 and the hollow ball screw 106, and result in suboptimal force transmission between the hollow ball screw 106 and the ball nut 102. Additionally, tilting or wobbling of the ball nut 102 can also impose undesirable radial loads on the hollow ball screw 106.

[0021] To mitigate these undesirable effects caused by the tilting of the ball nut, the hollow ball screw 106 can be mounted on the inner shaft 108 at one end. The hollow ball screw 106 defines a cavity extending along the length of the hollow ball screw 106. The diameter of the inner shaft 108 is smaller than the diameter of the hollow ball screw 106, such that the inner shaft 108 is fitted into the cavity of the hollow ball screw 106. The inner shaft 108 can be rigidly attached to the hollow ball screw 110 at a first end, such that the outer portion of the hollow ball screw rotates together with the inner shaft, as... Figure 1 As shown. In some embodiments, the hollow ball screw 106 can be threadedly connected to the inner shaft 108. This thread can be used to counteract high axial forces from the hollow ball screw 106. The threads in this thread are designed with slight interference to minimize axial end clearance in the threaded connection. Additionally and / or alternatively, a locating pin can be used in this connection. This locating pin can be installed after multiple parts are threaded together. When torque is applied to the connection between the hollow ball screw 106 and the inner shaft 108, the locating pin ensures that the screw thread will not loosen.

[0022] Due to the external force applied to the rack from the sector shaft 112, the inner shaft 108 is capable of axial bending under the load of the ball nut 102. This property of the inner shaft 108 allows the hollow ball screw 106 to deflect relative to the axial direction based on the load applied by the ball nut 102 and the recirculating balls, such that the hollow ball screw 106 remains substantially coaxial with the tilted ball nut 102. In some embodiments, the inner shaft 108 is made of medium carbon steel and is quenched and tempered (through hardening) to achieve a Rockwell hardness C (HRC) of approximately 28-34. Additionally and / or alternatively, the inner shaft 108 may also be made of low carbon steel and carburized to a much higher surface hardness.

[0023] Due to the flexible nature of the inner shaft 108, the combination of the ball nut 102 and the hollow ball screw 106 swings or tilts together within the housing 118. The tilt of the ball nut 102 relative to the housing 118 is limited by the distance between the outer circumference of the ball nut 102 and the inner wall of the housing 118. A bushing 104 may be mounted on the outer circumference of the ball nut to help protect the ball nut 102 from wear and tear caused by the general contact between the outer circumference of the ball nut 102 and the inner wall of the housing 118. In some embodiments, the bushing may be made of steel and have a low-friction coating applied to the outer diameter.

[0024] Any statements made herein characterizing the invention should not be considered limiting, as the invention is defined by the claims. It should be understood that changes and modifications can be made by those skilled in the art within the scope of the appended claims, which may include any combination of features from the different embodiments described above.

[0025] The terms used in the claims should be interpreted as having the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the articles “a” or “the” when introducing an element should not be interpreted as excluding multiple elements. Similarly, the expression “or” should be interpreted as inclusive, such that the expression “A or B” does not exclude “A and B” unless it is clear from the context or the foregoing description that only one of A and B is expected. Furthermore, the expression “at least one of A, B, and C” should be interpreted as one or more of a group of elements consisting of A, B, and C, and should not be interpreted as requiring at least one of each of the listed elements A, B, and C, regardless of whether A, B, and C are related as a category or otherwise. Moreover, the expressions “A, B, and / or C” or “at least one of A, B, or C” should be interpreted as including any singular entity from the listed elements, such as A, any subset from the listed elements, such as A and B, or the entire list of elements A, B, and C.

Claims

1. An electric power steering assembly for commercial vehicles, comprising: A housing, the housing comprising a generally cylindrical portion extending in the axial direction; A hollow ball screw, wherein the hollow ball screw is disposed in the housing and defines a cavity extending in the axial direction; A ball nut, which is disposed in the housing, circumferentially surrounds the hollow ball screw, and is configured to translate axially relative to the housing; and An inner shaft is disposed in the cavity of the hollow ball screw and rigidly attached to the hollow ball screw at a first end. The inner shaft is configured to deflect the hollow ball screw relative to the axial direction in response to an external load applied by the ball nut.

2. The electric power steering assembly according to claim 1, wherein, The inner shaft is made of carbon steel, which is quenched and tempered to achieve a Rockwell hardness of approximately 28-34 C (HRC).

3. The electric power steering assembly according to claim 1, wherein, The inner shaft is made of carbon steel, which is carburized to achieve surface hardness.

4. The electric power steering assembly according to claim 1, wherein, The support bushing is made of steel.

5. The electric power steering assembly according to claim 4, wherein, A low-friction coating is applied to the outer surface of the support bushing.

6. The electric power steering assembly according to claim 1, wherein, The inner shaft is threadedly connected to the hollow ball screw.

7. The electric power steering assembly according to claim 6, wherein, The inner shaft is connected to the hollow ball screw using a locating pin to ensure that the screw thread will not loosen under torque.

8. The electric power steering assembly according to claim 1, further comprising: The first plurality of channels are located between the ball nut and the hollow ball screw; as well as A second plurality of circulating balls are disposed in the channel. The second plurality of circulating balls circulate in the first plurality of channels, transmitting the rotational force from the hollow ball screw to cause axial translation of the ball nut.

9. The electric power steering assembly of claim 1, further comprising a support bushing disposed on the outer circumference of the ball nut and configured to contact the inner surface of the housing when the hollow ball screw deflects.

10. The electric power steering assembly according to claim 1, further comprising: A rack, the rack including a set of teeth, the rack being rigidly attached to the outer circumference of the ball nut and configured to translate together with the ball nut when the ball nut is axially translated relative to the housing; as well as A sector shaft comprising a set of gear teeth that engage with a set of teeth on a rack and is configured to rotate based on the translation of the rack.

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