Rear wheel steering apparatus for vehicle

Abstract

The utility model relates to a rear wheel steering device used for a vehicle, which comprises a combined lead screw, a first lead screw, a second lead screw, a second lead screw, a third lead screw, a fourth lead screw and a fifth lead screw, and the combined lead screw comprises a first lead screw shaft and a first lead screw nut; a second lead screw including a second lead screw shaft and a second lead screw nut and having a second transmission efficiency less than the first transmission efficiency and self-locking performance, the second lead screw shaft being coaxially connected to the first lead screw shaft to define a common axis and the second lead screw nut being coaxially connected to the first lead screw nut; and a load applying mechanism configured to apply a load to the first and second lead screw nuts such that the first and second lead screw nuts produce a synchronous rotational displacement about the common axis, whereby the first and second lead screw shafts produce a synchronous translation along the common axis for effecting rear wheel steering, the combined lead screw has transmission efficiency and self-locking performance between the first transmission efficiency and the second transmission efficiency, so that the rear wheel steering device has the self-locking performance but has higher transmission efficiency compared with the mode that the second lead screw is singly used.

Description

TECHNICAL FIELD

[0001] The utility model relates to the rear wheel steering technical field for vehicle more specifically, the utility model relates to the rear wheel steering device for vehicle. BACKGROUND

[0002] At present, the rear wheel steering device for vehicle mainly includes electronic control unit, a plurality of sensors, motor, load applying mechanism and screw rod to control the steering angle of rear wheel, wherein the plurality of sensors can monitor the information related to the current steering angle, speed, acceleration and the like of vehicle and transmit the information to the electronic control unit, the electronic control unit calculates the desired steering angle of rear wheel according to the information from the sensor and further controls the motor, load applying mechanism and screw rod to apply such desired steering angle (generally up to 8°) to the rear wheel to provide better maneuverability and stability for vehicle.

[0003] In the prior art, the screw rod is generally trapezoidal screw rod which includes trapezoidal screw rod shaft and trapezoidal screw rod nut, the load applying mechanism applies load to the trapezoidal screw rod nut by means of the power of motor to make the trapezoidal screw rod nut produce rotary displacement, the rotary displacement of trapezoidal screw rod nut can be further converted into the linear displacement of trapezoidal screw rod shaft, and the linear displacement of trapezoidal screw rod shaft can be further transmitted to the steering knuckle or steering link of rear wheel to realize the steering of rear wheel.

[0004] The trapezoidal screw rod has self-locking performance due to the geometric characteristics of trapezoidal thread and the effect of friction, which helps to provide better safety for vehicle, but the transmission efficiency of trapezoidal screw rod is low and the wear is fast, resulting in low working efficiency of the rear wheel steering device for vehicle. SUMMARY

[0005] An object of the utility model is to provide an improved rear wheel steering device for vehicle which can improve working efficiency while ensuring safety.

[0006] According to one aspect of the present application, a rear wheel steering device for a vehicle is provided, comprising: a combined lead screw, the combined lead screw comprising: a first lead screw comprising a first lead screw shaft and a first lead screw nut threadedly engaged on the first lead screw shaft, the first lead screw having a first transmission efficiency; a second lead screw comprising a second lead screw shaft and a second lead screw nut threadedly engaged on the second lead screw shaft, the second lead screw having a second transmission efficiency less than the first transmission efficiency and a self-locking property, wherein the second lead screw shaft is coaxially connected to the first lead screw shaft to define a common axis, and the second lead screw nut is coaxially connected to the first lead screw nut; and a load applying mechanism configured to apply a load to the first lead screw nut and the second lead screw nut to cause the first lead screw nut and the second lead screw nut to produce a synchronous rotational displacement about the common axis, whereby the first lead screw shaft and the second lead screw shaft produce a synchronous translational displacement along the common axis for effecting rear wheel steering, wherein the combined lead screw has a transmission efficiency and a self-locking property between the first transmission efficiency and the second transmission efficiency.

[0007] Optionally, the transmission efficiency of the combined lead screw is no more than but close to 50%.

[0008] Optionally, a portion of the load applying mechanism for a load output terminal is fixed around an outer peripheral surface of the first lead screw nut such that the portion of the load applying mechanism directly acts on the first lead screw nut.

[0009] Optionally, the first lead screw shaft is coaxially fixed to the second lead screw shaft in a form of shape fit and / or force fit; and / or, the first lead screw nut is coaxially fixed to the second lead screw nut in a form of shape fit and / or force fit.

[0010] Optionally, the first lead screw nut is coaxially fixed to the second lead screw nut by providing a locking sleeve around at least a portion of an outer peripheral surface of the first lead screw nut and at least a portion of an outer peripheral surface of the second lead screw nut, and a portion of the load applying mechanism for a load output terminal is fixed around an outer peripheral surface of the locking sleeve.

[0011] Optionally, the first lead screw shaft and the second lead screw shaft are formed integrally or separately, and the first lead screw nut and the second lead screw nut are formed integrally or separately.

[0012] Optionally, at least a portion of the outer circumference of the first screw shaft forms a first screw member, at least a portion of the inner circumference of the first screw nut forms a first mating screw member, the first mating screw member is helically engaged on the first screw member, at least a portion of the outer circumference of the second screw shaft forms a second screw member, at least a portion of the inner circumference of the second screw nut forms a second mating screw member, the second mating screw member is helically engaged on the second screw member, wherein at least a pitch of the first screw member is equal to a pitch of the second screw member, and at least a pitch of the first mating screw member is equal to a pitch of the second mating screw member.

[0013] Optionally, an inner diameter of the first screw member is greater than or equal to an outer diameter of the second screw member, or an outer diameter of the first screw member is less than or equal to an inner diameter of the second screw member.

[0014] Optionally, a stroke of the combined screw is determined based on an axial length of the first mating screw member.

[0015] Optionally, the first screw is configured as a ball screw, and the second screw is configured as a trapezoidal screw.

[0016] The rear wheel steering device for a vehicle according to the present application utilizes a combined screw to obtain a rear wheel steering device having self-locking performance but higher transmission efficiency (e.g., no more than but close to 50%) than a rear wheel steering device using only the second screw. For example, the first screw is configured as a ball screw, and the second screw is configured as a trapezoidal screw.

[0017] Other features of the present application, and their advantages, will become apparent from the following detailed description of exemplary embodiments of the present application with reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0018] The accompanying drawings, which form a part of the specification, illustrate embodiments of the present application and, together with the description, serve to explain the principles of the present application.

[0019] Figure 1 is a simplified sectional view of a rear wheel steering device for a vehicle according to an embodiment of the present application.

[0020] Figure 2 is Figure 1 is another simplified sectional view of the rear wheel steering device for a vehicle of

[0021] Figure 3 is yet another simplified sectional view of the rear wheel steering device for a vehicle of Figure 1

[0022] Figure 4 is a simplified sectional view of a rear wheel steering device for a vehicle according to another embodiment of the present application. ​

[0023] Figure 5 is a simplified sectional view of a rear wheel steering device for a vehicle according to still another embodiment of the present application. DETAILED DESCRIPTION

[0024] Various exemplary embodiments of the present application will now be described in detail with reference to the accompanying drawings. It should be noted that the relative arrangement of components and steps, numerical expressions, and numerical values set forth in these embodiments are not limitations on the scope of the present application unless otherwise specifically stated.

[0025] Techniques and equipment known to those of ordinary skill in the relevant art can not be discussed in detail, but should be considered part of the specification where appropriate.

[0026] In all of the examples shown and discussed herein, any specific values should be interpreted as merely exemplary and not limiting. Thus, other examples of the exemplary embodiments can have different values.

[0027] It should be noted that like numbers and letters refer to like items throughout the drawings, and once an item is defined in one drawing, that definition is applicable to subsequent drawings containing like items.

[0028] Referring to Figure 1 , the rear wheel steering device 10 for a vehicle according to the present application includes a combined lead screw including a first lead screw 14 including a first lead screw shaft 16 and a first lead screw nut 18 threadingly engaged on the first lead screw shaft 16, and having a first transmission efficiency, and a second lead screw 20 including a second lead screw shaft 22 and a second lead screw nut 24 threadingly engaged on the second lead screw shaft 22, and having a second transmission efficiency less than the first transmission efficiency and a self-locking property. Figure 1 In the present embodiment, the first lead screw 14 is configured as a ball screw, and the second lead screw 20 is configured as a trapezoidal screw, that is, the first lead screw shaft 16 is a ball screw shaft, the first lead screw nut 18 is a ball screw nut, the second lead screw shaft 22 is a trapezoidal screw shaft, and the second lead screw nut 24 is a trapezoidal screw nut.

[0029] A ball screw shaft has a helical ball rolling groove formed on an outer peripheral surface thereof, and a ball screw nut has a helical mating ball rolling groove formed on an inner peripheral surface thereof. When the ball screw nut is threadedly engaged on the ball screw shaft, the mating ball rolling groove of the ball screw nut is opposed to the ball rolling groove of the ball screw shaft to form a ball channel. The ball screw shaft further has a circulation assembly to communicate one end of the ball channel to the other end to form a circulation loop, and a plurality of balls accommodated in the circulation loop. When the ball screw nut is subjected to a rotational displacement relative to the ball screw shaft, the plurality of balls are able to circulate in the circulation loop to convert the rotational displacement of the ball screw nut into a linear displacement of the ball screw shaft by rolling friction. The ball screw has a high conversion efficiency (also referred to as a transmission efficiency) of more than 90%. It is understood that the ball screw can have other known fittings, such as a sealing baffle, etc., as allowed.

[0030] A trapezoidal screw shaft has a trapezoidal thread formed on an outer peripheral surface thereof, and a trapezoidal screw nut has a mating trapezoidal thread formed on an inner peripheral surface thereof. When the trapezoidal screw nut is threadedly engaged on the trapezoidal screw shaft, the mating trapezoidal thread of the trapezoidal screw nut is in contact with the trapezoidal thread of the trapezoidal screw shaft, so that when the trapezoidal screw nut is subjected to a rotational displacement relative to the trapezoidal screw shaft, the rotational displacement of the trapezoidal screw nut can be converted into a linear displacement of the trapezoidal screw shaft by sliding friction. As known, the trapezoidal screw has a low transmission efficiency, for example, between 26% and 40%, but the trapezoidal screw has a self-locking performance due to the geometric characteristics of the trapezoidal thread and the effect of friction, where tan a < f < 1, tan a is the tangent value of the helix angle a of the trapezoidal thread, the helix angle a determines the pitch and / or lead of the trapezoidal thread (i.e. the distance moved per turn of the thread), and f is the friction coefficient and is usually less than 1. When a vehicle is driven with the rear wheels steered at a steering angle, if the steering angle is changed due to a load from the outside (e.g. a collision), the subsequent driving of the vehicle will be adversely affected. The self-locking performance of the trapezoidal screw can irreversibly transmit motion, i.e. the rotational displacement from the trapezoidal screw nut can be converted into a linear displacement of the trapezoidal screw shaft, but a load from the outside cannot or is difficult to drive the trapezoidal screw shaft to produce a linear displacement. It is understood that the trapezoidal screw can have other known fittings, such as lubricating oil, etc.

[0031] The first screw 14 can also be configured as a screw other than a ball screw, and likewise, the second screw 20 can also be configured as a screw other than a trapezoidal screw, as long as the first transmission efficiency is greater, especially significantly greater, than the second transmission efficiency, and the second screw 20 has a self-locking performance.

[0032] Here, the second lead screw shaft 22 is coaxially connected to the first lead screw shaft 16 to define a common axis L, and likewise, the second lead screw nut 24 is coaxially connected to the first lead screw nut 18, that is, the common axis L coincides with the axes of the first and second lead screw shafts 16, 22 and the axes of the first and second lead screw nuts 18, 24.

[0033] The rear wheel steering device 10 for a vehicle further comprises a motor 26 and a load applying mechanism 28. The load applying mechanism 28 is configured to apply a load to the first and second lead screw nuts 18, 24 and comprises a first pulley 30 fixed coaxially around at least one of the first lead screw nut 18 and the second lead screw nut 24, a second pulley 32 coaxially connected to the motor 26 to be driven to rotate by the motor 26, and a belt 34 adapted to be tensioned around at least a portion of the radially outer surfaces of the first and second pulleys 30, 32, when the motor 26 drives the second pulley 32 to rotate, the second pulley 32 will drive the first pulley 30 to produce a rotational displacement about the common axis L by means of the belt 34, thereby further driving the first and second lead screw nuts 18, 24 fixed to the first pulley 30 to produce a synchronous rotational displacement about the common axis L, that is, the rotational displacement of the second pulley 32 is converted into the rotational displacement of the first pulley 30 and the first and second lead screw nuts 18, 24, whereby the first and second lead screw shafts 16, 22 produce a synchronous translation along the common axis L for realizing the rear wheel steering. In addition, the radially outer surface of the first pulley 30 can be provided with sawtooth shape to increase the friction when the belt 34 moves on the radially outer surface of the first pulley 30.

[0034] Here, "synchronous" means that the rotational directions and speeds of the first and second lead screw nuts 18, 24 are consistent as if the first and second lead screw nuts 18, 24 are an integral lead screw nut, and the translational directions and speeds of the first and second lead screw shafts 16, 22 are consistent as if the first and second lead screw shafts 16, 22 are an integral lead screw shaft.

[0035] The rear wheel steering device 10 can further comprise an electronic control unit 36 and a plurality of sensors, for example, the electronic control unit 36 is configured as a RWS (Rear Wheel Steering) control ECU (Electronic Control Unit), which can be installed to the motor 26 or other suitable positions, and is configured to receive information from the plurality of sensors, including information related to the current steering angle, vehicle speed, acceleration, etc. of the vehicle, to determine control instructions based on these information, the control instructions are used to control the energization state of the motor 26, that is, to control the assistance of the motor 26.

[0036] At least a portion of the outer peripheral surface of the first lead screw shaft 16 forms a first helical component 38 (e.g., a ball groove in the helix of a ball screw shaft), and at least a portion of the inner peripheral surface of the first lead screw nut 18 forms a first mating helical component 40 (e.g., a ball groove in the helix of a ball screw nut). The first mating helical component 40 is helically engaged with the first helical component 38. At least a portion of the outer peripheral surface of the second lead screw shaft 22 forms a second helical component 42 (e.g., a trapezoidal thread in a trapezoidal lead screw shaft), and at least a portion of the inner peripheral surface of the second lead screw nut 24 forms a second mating helical component 44 (e.g., a trapezoidal thread in a trapezoidal lead screw nut). The second mating helical component 44 is helically engaged with the second helical component 42. Here, the pitch of the first helical component 38 should be substantially equal to the pitch of the second helical component 42, and correspondingly, the pitch of the first mating helical component 40 should be substantially equal to the pitch of the second mating helical component 44, so that the first and second lead screw nuts 18 and 24 can generate synchronous rotational displacement.

[0037] like Figure 1 As shown, the inner diameter of the first helical component 38 is larger than the outer diameter of the second helical component 42. In this case, the helix angle of the first helical component 38 is not equal to the helix angle of the second helical component 42. Alternatively, the outer diameter of the first helical component 38 may be smaller than the inner diameter of the second helical component 42. In this case, the helix angle of the first helical component 38 is not equal to the helix angle of the second helical component 42. Alternatively, the outer diameter of the first helical component 38 may be equal to the inner diameter of the second helical component 42, or the inner diameter of the first helical component 38 may be equal to the outer diameter of the second helical component 42. In this case, the helix angle of the first helical component 38 may be equal to the helix angle of the second helical component 42. In all the above cases, the first and second helical components 38 and 42 will actually only engage (also called screw-in) with their respective first and first mating helical components 40 and 44. Therefore, the second helical component 42 will not interact with the first mating helical component 40.

[0038] As described above, when the load applying mechanism 28 applies a load F to the first and second lead screw nuts 18, 24 A At this time, the first and second lead screw nuts 18 and 24 will generate synchronous rotational displacement around the common axis L. Since multiple first mating spiral coils are formed between the first spiral component 38 and the first mating spiral component 40, the circumferential torque generated by the rotation of the first mating spiral component 40 will be converted into a first axial force T acting on the first spiral component 38. N1 and the first frictional force f N1 (For example, rolling friction), the first axial force T N1 and the first frictional force f N1The number of the plurality of first engagement helical turns can be added turn by turn. Also, since the plurality of second engagement helical turns are formed between the second helical member 42 and the second engagement helical member 44, the circumferential direction torque generated by the rotation of the second engagement helical member 44 will be converted into the second axial force T N2 and the second friction force f N2 (e.g., sliding friction), the second axial force T N2 and the second friction force f N2 The number of the plurality of second engagement helical turns can be added turn by turn. Therefore, when estimating the transmission efficiency of the combined lead screw, the sum of the load F A , the first axial force T N1 and the second axial force T N2 , and the sum of the first friction force f N1 and the second friction force f N2 may be considered, and in this case, the combined lead screw has a transmission efficiency between the first transmission efficiency and the second transmission efficiency, for example, the transmission efficiency of the combined lead screw is not more than but close to 50%, because the combined lead screw will have a self-locking performance under the condition that the transmission efficiency of the combined lead screw is not more than 50%.

[0039] Here, "close to" means that the transmission efficiency of the combined lead screw is 0.5% to 5% less than 50%.

[0040] Alternatively, a part (e.g., the first pulley 30) of the load applying mechanism 28 for the load output terminal is fixed around the outer circumferential surface of the first lead screw nut 18 to act directly on the first lead screw nut 18. When the first lead screw 14 is configured as a ball screw and the second lead screw 20 is configured as a trapezoidal screw, the load applying mechanism 28 directly acting on the ball screw nut makes the ball screw shaft able to receive the load more uniformly by means of the plurality of balls 14a, compared to directly acting on the trapezoidal screw nut.

[0041] The axial length of the first helical member 38 / first engagement helical member 40 can be substantially equal to the axial length of the second helical member 42 / second engagement helical member 44, or the axial length of the first helical member 38 / first engagement helical member 40 can be different (e.g., greater or smaller) from the axial length of the second helical member 42 / second engagement helical member 44 to achieve a desired transmission efficiency and self-locking performance. It can be understood that, since the desired steering angle applied to the rear wheel by the rear wheel steering device 10 is generally at most 8°, the axial length of the first helical member 38 and the axial length of the second helical member 42 can be significantly smaller than the lead screw used for the front wheel steering device of a vehicle, i.e., the stroke of the combined lead screw can be significantly smaller.

[0042] Reference will now be madeFigures 1 to 3 Describing the stroke of the combined lead screw, in the various figures, the first lead screw 14 is a ball screw and the second lead screw 20 is a trapezoidal lead screw. The stroke of the combined lead screw involves: a first segment 38a of the first helical component 38 connected to the second helical component 42, a second segment 38b connected to the first segment 38a, and a third segment 38c connected to the second segment 38b; a first section 40a of the first mating helical component 40 connected to the second mating helical component 44, a second section 40b connected to the first section 40a, and a third section 40c connected to the second section 40b. Figure 1 In the process, when the wheel is in the return position, that is, when the combined lead screw is in the neutral position, a portion of the second helical component 42 is located within the first section 40a of the first mating helical component 40 but does not contact the first section 40a of the first mating helical component 40; the first segment 38a of the first helical component 38 is helically engaged with the second section 40b of the first mating helical component 40, and the plurality of balls 14a are therebetween; the second segment 38b of the first helical component 38 is helically engaged with the third section 40c of the first mating helical component 40, but there are no balls therebetween; and the third segment 38c of the first helical component 38 is located outside the first lead screw nut 18.

[0043] exist Figure 2 In this configuration, when the wheel, for example, turns left to its maximum position, i.e., when the combined screw is at its first extreme position, a greater portion of the second helical component 42 is located within the first and second sections 40b of the first mating helical component 40 but does not contact the first and second sections 40b of the first mating helical component 40; the first segment 38a of the first helical component 38 is helically engaged with the third section 40c of the first mating helical component 40, and the plurality of balls 14a are located therebetween; and the second and third segments 38b, 38c of the first helical component 38 are located outside the first screw nut 18.

[0044] exist Figure 3 In the process, when the wheel turns right to its maximum position, that is, when the combined lead screw is at its second limit position, the first segment 38a of the first helical component 38 helically engages with the first section 40a of the first mating helical component 40, and the plurality of balls 14a are therebetween; the second and third segments 38b and 38c of the first helical component 38 helically engage with the second and third sections 40b and 40c of the first mating helical component 40, but there are no balls therebetween.

[0045] Therefore, the stroke of the combined lead screw can be determined based on the axial length of the first mating helical component 40.

[0046] Optionally, the first and second lead screw shafts 16, 22 are integrally formed or separately formed, and the first and second lead screw nuts 18, 24 are integrally formed or separately formed. For example, as shown in FIG. 1, when the first and second lead screw shafts 16, 22 are integrally formed and the first and second lead screw nuts 18, 24 are integrally formed, the first and second lead screw nuts 18, 24 can be directly screwed onto the first and second lead screw shafts 16, 22 from one end of one of the first and second lead screw shafts 16, 22 having a smaller outer diameter. For example, as shown in FIG. 2, when the first and second lead screw shafts 16, 22 are integrally formed and the first and second lead screw nuts 18, 24 are integrally formed, the first and second lead screw nuts 18, 24 can be directly screwed onto the first and second lead screw shafts 16, 22 from one end of one of the first and second lead screw shafts 16, 22 having a smaller outer diameter. Figures 1 to 3 Optionally, the first and second lead screw shafts 16, 22 are integrally formed or separately formed, and the first and second lead screw nuts 18, 24 are integrally formed or separately formed. For example, as shown in FIG. 1, when the first and second lead screw shafts 16, 22 are integrally formed and the first and second lead screw nuts 18, 24 are integrally formed, the first and second lead screw nuts 18, 24 can be directly screwed onto the first and second lead screw shafts 16, 22 from one end of one of the first and second lead screw shafts 16, 22 having a smaller outer diameter. For example, as shown in FIG. 2, when the first and second lead screw shafts 16, 22 are integrally formed and the first and second lead screw nuts 18, 24 are integrally formed, the first and second lead screw nuts 18, 24 can be directly screwed onto the first and second lead screw shafts 16, 22 from one end of one of the first and second lead screw shafts 16, 22 having a smaller outer diameter. Figure 4 Optionally, the first and second lead screw shafts 16, 22 are integrally formed or separately formed, and the first and second lead screw nuts 18, 24 are integrally formed or separately formed. For example, as shown in FIG. 1, when the first and second lead screw shafts 16, 22 are integrally formed and the first and second lead screw nuts 18, 24 are integrally formed, the first and second lead screw nuts 18, 24 can be directly screwed onto the first and second lead screw shafts 16, 22 from one end of one of the first and second lead screw shafts 16, 22 having a smaller outer diameter. For example, as shown in FIG. 2, when the first and second lead screw shafts 16, 22 are integrally formed and the first and second lead screw nuts 18, 24 are integrally formed, the first and second lead screw nuts 18, 24 can be directly screwed onto the first and second lead screw shafts 16, 22 from one end of one of the first and second lead screw shafts 16, 22 having a smaller outer diameter.

[0047] Optionally, the first lead screw shaft 16 can be coaxially fixed to the second lead screw shaft 22 in a form-fit and / or force-fit manner, as long as the first and second lead screw shafts 16, 22 can produce synchronous translation. For example, an axial end of the first lead screw shaft 16 can be threadedly engaged with an axial end of the second lead screw shaft 22. Alternatively, as shown in FIG. 3, an axial end of the first lead screw shaft 16 can be engaged with an axial end of the second lead screw shaft 22 by means of one or more snap fittings 23. Figure 4 Optionally, the first lead screw shaft 16 can be coaxially fixed to the second lead screw shaft 22 in a form-fit and / or force-fit manner, as long as the first and second lead screw shafts 16, 22 can produce synchronous translation. For example, an axial end of the first lead screw shaft 16 can be threadedly engaged with an axial end of the second lead screw shaft 22. Alternatively, as shown in FIG. 3, an axial end of the first lead screw shaft 16 can be engaged with an axial end of the second lead screw shaft 22 by means of one or more snap fittings 23.

[0048] Optionally, the first lead screw nut 18 can be coaxially fixed to the second lead screw nut 24 in a form-fit and / or force-fit manner, as long as the first and second lead screw nuts 18, 24 can produce synchronous rotational displacement. Alternatively, as shown in FIG. 4, the first lead screw nut 18 can be engaged with the second lead screw nut 24 by means of one or more snap fittings 25. Figure 4 Optionally, the first lead screw nut 18 can be coaxially fixed to the second lead screw nut 24 in a form-fit and / or force-fit manner, as long as the first and second lead screw nuts 18, 24 can produce synchronous rotational displacement. Alternatively, as shown in FIG. 4, the first lead screw nut 18 can be engaged with the second lead screw nut 24 by means of one or more snap fittings 25.

[0049] Optionally, as shown in FIG. 5, the first lead screw nut 18 can be coaxially fixed to the second lead screw nut 24 by providing a locking sleeve 27 around at least a portion of an outer peripheral surface of the first lead screw nut 18 and at least a portion of an outer peripheral surface of the second lead screw nut 24, and a portion of the load application mechanism 28 for a load output terminal is to be fixed around an outer peripheral surface of the locking sleeve 27. Figure 5

[0050] While some specific embodiments of the present application have been described in detail by way of example with reference to the drawings, it is to be understood that various modifications and enhancements can be made to the above-described embodiments without departing from the scope and spirit of the present application. The scope of the present application is defined by the appended claims.​

Claims

1. A rear wheel steering device (10) for a vehicle, characterized by, Comprise: A combined lead screw comprising: A first lead screw (14) including a first lead screw shaft (16) and a first lead screw nut (18) threadingly engaged on the first lead screw shaft (16), and having a first transmission efficiency; A second lead screw (20) including a second lead screw shaft (22) and a second lead screw nut (24) threadingly engaged on the second lead screw shaft (22), and having a second transmission efficiency less than the first transmission efficiency and a self-locking property, wherein the second lead screw shaft (22) is coaxially connected to the first lead screw shaft (16) to define a common axis, and the second lead screw nut (24) is coaxially connected to the first lead screw nut (18); and A load applying mechanism (28) configured to apply a load to the first lead screw nut (18) and the second lead screw nut (24) to cause the first lead screw nut (18) and the second lead screw nut (24) to produce a synchronous rotational displacement about the common axis, whereby the first lead screw shaft (16) and the second lead screw shaft (22) produce a synchronous translation along the common axis for effecting rear wheel steering, Wherein the combined lead screw has a transmission efficiency and a self-locking property between the first transmission efficiency and the second transmission efficiency.

2. The rear wheel steering device (10) for a vehicle according to claim 1, characterized by, The transmission efficiency of the combined lead screw is no more than but close to 50%.

3. The rear wheel steering device (10) for a vehicle according to claim 1, characterized by, A portion of the load applying mechanism (28) for a load output terminal is fixed around an outer peripheral surface of the first lead screw nut (18) to cause the portion of the load applying mechanism (28) to act directly on the first lead screw nut (18).

4. The rear wheel steering device (10) for a vehicle according to claim 1, characterized by, The first lead screw shaft (16) is coaxially fixed to the second lead screw shaft (22) in a form of a shape fit and / or a force fit; and / or, the first lead screw nut (18) is coaxially fixed to the second lead screw nut (24) in a form of a shape fit and / or a force fit.

5. The rear wheel steering device (10) for a vehicle according to claim 1, characterized by, The first lead screw nut (18) is coaxially fixed to the second lead screw nut (24) by providing a locking sleeve (27) around at least a portion of an outer peripheral surface of the first lead screw nut (18) and at least a portion of an outer peripheral surface of the second lead screw nut (24), and a portion of the load applying mechanism (28) for a load output terminal is fixed around an outer peripheral surface of the locking sleeve (27).

6. The rear wheel steering device (10) for a vehicle according to claim 1, characterized by, The first lead screw shaft (16) is integrally formed with or separately formed from the second lead screw shaft (22), and the first lead screw nut (18) is integrally formed with or separately formed from the second lead screw nut (24). The first lead screw shaft (16) is integrally formed with or separately formed from the second lead screw shaft (22), and the first lead screw nut (18) is integrally formed with or separately formed from the second lead screw nut (24).

7. The rear wheel steering apparatus (10) for a vehicle according to claim 1, characterized by At least a portion of an outer circumference of the first lead screw shaft (16) forms a first helical member (38), at least a portion of an inner circumference of the first lead screw nut (18) forms a first mating helical member (40), the first mating helical member (40) helically engages the first helical member (38), at least a portion of an outer circumference of the second lead screw shaft (22) forms a second helical member (42), at least a portion of an inner circumference of the second lead screw nut (24) forms a second mating helical member (44), the second mating helical member (44) helically engages the second helical member (42), wherein at least a pitch of the first helical member (38) is equal to a pitch of the second helical member (42), and at least a pitch of the first mating helical member (40) is equal to a pitch of the second mating helical member (44).

8. The rear wheel steering apparatus (10) for a vehicle according to claim 7, characterized by An inner diameter of the first helical member (38) is greater than or equal to an outer diameter of the second helical member (42); or, an outer diameter of the first helical member (38) is less than or equal to an inner diameter of the second helical member (42).

9. The rear wheel steering apparatus (10) for a vehicle according to claim 7, characterized by A stroke of the combined lead screw is determined based on an axial length of the first mating helical member (40).

10. The rear wheel steering device (10) for a vehicle according to any one of claims 1 to 9, characterized by, The first lead screw (14) is configured as a ball lead screw, and the second lead screw (20) is configured as a trapezoidal lead screw.

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