Patent Publication Number: US-9885408-B2

Title: Differential lock actuator

Description:
TECHNICAL FIELD 
     This invention relates to a differential lock actuator for a differential locking mechanism, particularly for a differential locking mechanism for vehicles. 
     BACKGROUND 
     Vehicle drive axles typically include a pair of axle shafts for driving the vehicles wheels. Under normal conditions, when the vehicle is driven along a straight path, the wheels, and thereby both axle shafts, will be turning at approximately the same speed and the same amount of torque will apply to each wheel. However, once the vehicle starts to turn, the outer wheel covers a greater distance than the inner wheel in the same amount of time. Under these circumstances, the outer wheel will have to rotate faster than the inner wheel in order to avoid slip that causes substantial wear of the tires. For this purpose, the drive axle also includes a differential that distributes input speed and torque to the pair of axle shafts. Differentials allow the inner wheel to turn at a slower speed than the outer wheel as the vehicle turns. 
     In a commonly known differential, the engine power is transmitted from a vehicle drive-shaft to a pinion gear that engages a crown wheel. The crown wheel is rigidly attached to a gear housing that rotates together with the crown wheel. The gear housing includes four differential pinion gears that split the engine torque provided by the pinion gear between the two axle shafts, allowing the two axle shafts to spin at a different speed. 
     When the vehicle is driven in a straight path, the crown wheel, gear housing and the differential pinions all rotate together as one unit to distribute the power evenly across the two axle shafts. In this case, there is no relative movement between the differential pinion gears. However, when the vehicle turns, the differential pinion gears rotate on their respective shafts to speed up the rotation of one axle (outer wheel) whilst slowing down rotation of the other axle shaft (inner wheel). 
     In poor road conditions, i.e., slippery or rough road surfaces, the use of a differential can result in loss of control over the vehicle, since the differential mechanism always applies the same torque to both wheels. The maximum amount of torque that can be transferred by the wheels is limited to the greatest amount that will not make the wheels slip. Consequently, if one of the tires is on ice, all of the torque and speed will be transferred to the wheel on ice, leaving the tire spinning on the ice and stopping the vehicle from travelling forward. 
     As a solution to the aforementioned problem, differential locking mechanisms are known by the art that, when a wheel slips, allow some torque to be transferred to the non-slipping wheel. Such differential locking mechanisms essentially connect the two axle shafts together, such that the axle shafts rotate at the same speed even in poor road conditions. Differential locking mechanisms usually comprise an electric, pneumatic or hydraulic mechanism to lock the axle shafts together. This mechanism can either be activated manually by the vehicle driver or automatically by a control unit of the vehicle. 
     There are several types of differential lock mechanisms, such as the clutch type limited slip differential, viscous coupling, Torsen differentials and locking differentials. In case of a locking differential, a lock member, such as a shift collar, is provided that locks the gear housing to the axle shafts. The shift collar can be actuated by a differential lock actuator to engage or disengage with the gear housing, in response to a driver-controlled switch or automatic command. Once the shift collar engages with the gear housing, the differential is locked, thereby connecting the two axle shafts together. 
     Of course, it is desirable to unlock the differential as soon as normal road conditions are reached again. To this end, the shift collar is actuated in order to disengage from the gear housing, enabling the axle shafts to rotate at different speeds once again. It is a commonly known problem that the shift collar, which locks the gear housing to the axle shaft can get stuck to the gear housing and will not release the gear housing even if an automatic or driver controlled switch signal is present. For this reason, lock detection assemblies have been developed that sense the occurrence of a lock condition in a differential locking mechanism. These lock detection assemblies often facilitate a cam mechanism that indicates the position of a push rod of the actuator that is used to move the shift collar between its engaged/disengaged positions. The implementation of such cam assemblies is known to be time consuming and expensive, as very small tolerances need to be met. Furthermore, cam mechanisms are subject to wear and can, therefore, reduce the service life of the detector assembly. 
     SUMMARY 
     It is an object of the present invention to overcome the aforementioned problems and provide a differential lock actuator, which can sense the state of the differential locking mechanism, while being easily installed and exhibiting a longer service life. Moreover, it is another object of the present invention to provide a differential lock actuator that achieves the above stated objects, with no major re-design of the commonly known components. It is a more specific object of the present invention to provide a differential lock actuator that can be equipped with a lock detection assembly retrospectively. 
     The aforementioned objects of the present invention are accomplished by a differential lock actuator for a differential locking mechanism comprising a housing and a push rod configured to be moveable within the housing between a first position and a second position for activation/de-activation of a differential locking system, wherein the differential lock actuator further comprises a lock detection assembly adapted to detect a position of the push rod within the housing, wherein the lock detection assembly comprises a first detector component adapted to emit a magnetic field, said first detector component being attached to and movable together with the push rod. 
     The differential lock actuator of the present invention therefore utilizes a lock detection assembly, which is adapted to contactlessly monitor the position of the push rod within the housing. In contrast to the prior art cam solutions, this will significantly reduce wear of the detector assembly. As will be apparent from the descriptions below, the differential lock actuator of the present invention is also more compact and easier to manufacture than the commonly known technologies. 
     In another embodiment, the first detector component of the detection assembly is a magnet, preferably a permanent magnet. Although it is equally feasible to implement the first detector component as an induction coil, a magnet, particularly a permanent magnet is a small and energy efficient choice for the first detector component that can easily be attached to the push rod. 
     In another aspect of the present invention, the push rod and the first detector component of the detection assembly are translationally movable in a common direction, along a longitudinal axis of the housing. Accordingly, the movement of the push rod is not only used as a means to activate/deactivate the differential lock mechanism but, at the same time, facilitates the function of the lock detection assembly and greatly reduces the number of parts required. 
     In other embodiments of this invention, the push rod comprises a cavity on an outer surface, preferably a surface directed towards a removable part of the housing, said cavity being constructed to receive the first detector component of the detection assembly in such a manner that the first detector component is embedded within the push rod. In a particularly simple variant of this embodiment, the cavity may be provided as a blind hole on said outer surface of the push rod. The dimensions of the cavity are preferably such that the first detector component, for example a magnet, fits inside the cavity without protruding beyond said outer surface of the push rod. By embedding the first detector component within the cavity of the push rod, the first detector component is protected against wear and tear that might otherwise occur due to movement of the push rod within the housing. 
     Advantageously, the first detector component maybe removably attached within the aforementioned cavity, which facilitates replacement of the first component if servicing is required. 
     In another embodiment of the present differential lock actuator, the push rod is biased towards its first position, preferably by means of a compression spring. Accordingly, the push rod will remain in its first position until a force is applied in an opposite direction to the force of the compression spring. Once such a force is applied, preferably by means of compressed air, the push rod is moved from its first position into its second position, thereby actuating the differential lock. Once the actuating force has ceased the push rod will automatically be transferred back into its first position by means of the compression spring. 
     According to another embodiment, the housing comprises at least one air inlet for introduction of compressed air into the housing, said air inlet being arranged to enable displacement of the push rod from its first to its second position upon introduction of compressed air into the housing via said air inlet. It is, however, equivalently feasible to implement other ways of actuating the push rod, such as hydraulic fluids, for example. 
     In another embodiment, the push rod comprises opposite first and second end surface, the second end surface being oriented towards the air inlet, wherein the first detector component is attached to the second end surface of the push rod. 
     According to another aspect, the detection assembly further comprises a second detector component configured to detect the magnetic field of the first detector component. The second detector component may be attached to the housing of the differential lock actuator. Preferably, the second detector component is attached to a removable part of the housing, more preferable an air-inlet cap. In this regard, the air-inlet cap may comprise a cavity adapted to enclose the second detector component in such a way that the second detector component is separated from the push rod. According to this embodiment, the second detector component is placed inside the cap, thus will never be in direct contact with the push rod or the first detector component respectively. In contrast to the prior art cam solutions, this will significantly reduce wear of the lock detection assembly according to the present invention. Furthermore, cavity of the cap protects the first adaptor component from environmental influences, such as humidity. 
     In another disclosed embodiment, the second detector component comprises a reed switch connected to a circuit board and arranged to provide an electrical output signal when the push rod is in its first position. In this embodiment, the second detector component is constructed to detect a magnetic field that is emitted by first detector component that is attached to the push rod. In particular, the reed switch may be adjusted such that he magnetic field of the first detector component is only large enough to close the contacts of the reed switch when the push rod is in its first position. When the push rod is in its first position, the reed switch will recognize the magnetic field and close an electric circuit on the circuit board. This will generate an electric output signal that can be converted into a visual or acoustic feedback signal informing the driver about the state of the differential lock actuator/the differential lock. 
     According to another embodiment, the push rod is made of aluminum or polymer. Besides their light specific weight, these materials are generally non-magnetic, thus will not affect the magnetic field emitted by the first detector component. 
     The present invention further relates to a differential mechanism comprising a differential gear assembly with a gear housing and a differential locking mechanism comprising a lock member, said lock member being moveable between a first position, in which the lock member is arranged distant from the gear housing, and a second position, in which the lock member engages with the gear housing, wherein the differential mechanism further comprises a differential lock actuator as mentioned before. 
     The push rod of the differential lock actuator is preferably operably connected to the lock member such that the lock member is moved from its first position to its second position upon movement of the push rod from this first position to its second position and vice-versa. Such a connection between the push rod and the lock member can be achieved by means of a shift fork, for example. 
     Finally, the present invention further relates to vehicle axle comprising the aforementioned differential mechanism. 
     These and other features of the present invention can be best understood from the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is a cross-sectional view of a first embodiment of the differential lock actuator according to the present invention with a push rod in its first position. 
         FIG. 1 b    is a cross-sectional view of the first embodiment with the push rod in its second position. 
         FIG. 2  is a cross-sectional view of the push rod and the removable part of the housing. 
         FIG. 3  is a schematic top view of the removable part of the housing in a disconnected state. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
       FIGS. 1 a  and 1 b    generally show a differential locking mechanism  3  in accordance with the first embodiment of the present invention. The differential locking mechanism  3  is moveable between an unlocked position (shown in  FIG. 1 a   ), in which the axle shafts  2  are able to rotate at different speeds and a locked position (shown in  FIG. 1 b   ) of the differential locking mechanism, in which the axle shafts  2  are locked together to rotate at a common speed. For this purpose, the differential locking mechanism  3  comprises a lock member  60 , which can be moved between a first position, shown in  FIG. 1 a   , and a second position, shown in  FIG. 1 b   . The lock member  60  is constructed as a shift collar, that is, a generally ring-shaped structure which is adapted to be mounted on the outer surface of one of the axle shafts  2 . To this end, the lock member  60  comprises a plurality of splines  61  constructed to cooperate with outer splines (not shown) of the axle shaft  2 . The splines  61  enable the lock member to rotate together with the axle shaft, when the lock member is in its first or second position. At the same time, the splines  61  enable the lock member  60  (here a shift collar) to shift between its first and second position. In its unlocked position, shown in  FIG. 1 a   , the lock member  60  is not engaged with the gear housing (not shown), thus the axle shaft  2  (and the lock member) can rotate at a speed different from the gear housing and the other axle shaft. If the lock member  60  is moved towards the gear housing, into its second position, the teeth  62  of the lock member  60  will engage with corresponding teeth (not shown) of the gear housing, thereby locking the axle shaft  2  and the lock member  60  with respect to the gear housing. In this second, locked position the axle shafts (only one shown at  2 ) will both rotate at the same speed as the gear housing. 
     As mentioned previously, the lock member  60  can be transferred from its first position into its second position by means of the differential lock actuator  1  that is also illustrated in  FIGS. 1 a  and 1 b   . To this end, the differential lock actuator  1  is connected to the lock member  60  by means of a shift fork  50  which is designed to engage an annular groove  63  of the lock member  60 . As will be explained in more detail below, the shift fork  50  further comprises an annular collar  52  which is attached to a circumferential surface of the push rod  20 . 
     The differential lock actuator  1  comprises a hollow inner space  13  that extends between a first opening  11  and an opposite second opening  12 . The generally cylindrical inner hollow space  13  is constructed to guide the push rod  20  along a longitudinal axis L of the substantially cylindrical housing  10 . The substantially cylindrical push rod  20  comprises a cylindrical main body part extending between a first annular end surface  21  and a second annular end surface  22 . The outer diameter of the generally cylindrical push rod  30  decreases in a step-wise fashion between the second annular end surface  22  and the first annular end surface  21 , thereby providing several flange sections  24 ,  28 ,  29 . 
     The push rod  20  can be moved between a first position, shown in  FIG. 1 a   , and a second position, shown in  FIG. 1 b   . In its first position, the larger second annular end surface  22  of the push rod is in alignment with the larger second opening  12  of the housing  10 . In its second position, the second end surface  22  of the push rod  20  is located distant from the second opening  12 , while a first annular flange  28  aligns with an annular stop surface  15  of the housing  10 . 
     According to the embodiment shown in  FIGS. 1 a  and 1 b   , the push rod  20  is biased towards the second opening  12  of the housing  10 , which is covered by a removable part  30  that functions as a stop surface when the push rod  20  is in its first position. In other words, in its first position, the push rod  20  is pressed against the removable part  30  of the housing  10 , which is constructed as a cap. The first position, thus, represents a rest position of the push rod  20 . In order to transfer the push rod  20  from its first position to its second position, it is necessary to overcome the spring force of the compression spring  40 , thereby compressing spring  40  and moving the push rod  20  towards the first opening  11  of the housing  10 , until a first flange  28  aligns with a stopping surface  15  of the housing  10 . 
     In the depicted embodiment, the push rod  20  is moved between its first and its second position by means of compressed air, which is introduced into the hollow inner space  13  via an air inlet  31  that forms part of the removable part  30 . Compressed air that enters the hollow space  13  will impact on the larger second annular end surface  22  of the push rod  20  and hence drive the latter towards the first opening  11  at the opposite end of the housing  10 , until the first annular flange  28  contacts the stop surface  15 . The push rod  20  further comprises a sealing ring  25  that is received within an annular groove  26  of the push rod  20 . The sealing ring  25  connects the push rod  20  with the inner wall of the hollow inner space  13  in an air tight manner, thereby facilitating build-up of air pressure between the air inlet  31  and the second end surface  22 . Of course, it is equally feasible to move the push rod  20  between its second and first position by means of alternative actuators, such as hydraulic fluids and electromagnetism. 
     As mentioned before, it is desirable to know the position of the push rod  20  within the housing  10  in order to evaluate whether the differential locking mechanism is activated or deactivated. The differential lock actuator of the present invention therefore comprises a lock detection assembly adapted to detect a position of the push rod  20  within the housing  10 , preferably at least the first position of the push rod.  FIG. 2  shows a schematic sectional view of an exemplary lock detection assembly comprising a first and a second detector component  70 ,  80 . The first detector component  80  is attached to the push rod  20 , whereas the second detector component  70  is attached to the removable part  30  of the housing  10 . In this particular example, the second detector component  70  of the lock detection assembly is constructed as a printed circuit board  72  that comprises a reed switch  71 . The depicted removable part  30  is constructed as a cap that is adapted to cover the second opening  12  of the housing  10  ( FIGS. 1 a  and 1 b   ). The second detector component  70  is embedded inside a cavity  73  of the cap-like removable part  30  and located such that a magnetic field emitted by the first detector component  80  is sensed by the second detector component  70  when the push rod is in its first position (depicted in  FIG. 2 ). 
       FIG. 2  shows that the cap-like removable part  30  not only houses the second detector component  70  but further comprises an air inlet  31  for introduction of compressed air into the hollow inner space  13  of the housing  10 . The air inlet  31  comprises an air channel  36 , which extends next to the cavity of the removable part  30  that houses the second detector component  70 . The second detector component  70  is spatially separated from the air channel  36  within the removable part  30 . 
     In the embodiment of  FIG. 2 , the first detector component  80  is attached to, and preferably embedded into, the push rod  20  such that the first detector component  80  is movable together with the push rod  20 . The second detector component emits a magnetic field that can be recognized by the reed switch of the first detector component. In this particular embodiment, the first detector component  80  is a permanent magnet. 
     As illustrated, the first detector component  80  is received within a cavity  23  of the push rod  20  along the second annular surface  22 . The first detector component  80  is attached within the cavity  23  such that it does not protrude the second annular surface  22 . Accordingly, when the push rod  20  is in its first position, the reed switch  71  of the second detector component  70  recognizes the presence of the magnetic first detector component  80  and thus creates an electric output signal via the printed circuit board  72 , which can be used to provide a visual or acoustic feedback to the driver. When the push rod  20  is moved from its first position (see  FIGS. 1 a    and  2 ) to its second position (see  FIG. 1 b   ), the magnetic field emitted by the first detector component  80  is not strong enough to trigger the reed switch any longer. Consequently, the reed switch will disconnect the electric circuit of the printed circuit board  72 , which can then be used as another output signal that will be fed back to the driver. 
     Due to the specific arrangement of the lock detection assembly shown in  FIG. 2 , the differential lock actuator  1  of the present invention exhibits a substantially longer service life than commonly known differential lock actuators with lock detection assemblies. This is due to the fact that the lock detection assembly does not require any direct contact between the first and the second detector components  70 ,  80 . Furthermore, the present solution provides for easy replacement of the lock detection assembly since the second detector component  70  is embedded within the cap-like removable part  30  which is also illustrated in  FIG. 3 . The removable part  30  comprises a plurality of attachment holes  32  which can be used to attach the removable part  30  to the housing  10  by means of fastening members  34 . The removable part  30  further comprises an electrical connector  35 , which connects the printed circuit board of the first detector component with a power supply and feedback signal lines of the vehicle. 
     The invention has been described in great detail in the foregoing specification, it is believed that various other alterations and modifications to the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such other alterations and modifications are included in the invention. In particular, it should be readily apparent to the person skilled in the art that the first and second detector components may equally be replaced by any other known contactless sensor arrangements, such as light sensors for example. 
     Further embodiments of the present disclosure are listed in the clauses below: 
     1. Differential lock actuator ( 1 ) for a differential locking mechanism, the Differential lock actuator ( 1 ) comprising: a housing ( 10 ); a push rod ( 20 ) configured to be movable within the housing ( 10 ) between a first position and a second position for activation/deactivation of a differential locking system; a lock detection assembly adapted to detect a position of the push rod ( 20 ) within the housing ( 10 ), wherein the lock detection assembly comprises a second detector component ( 70 ) connected to a removable part ( 30 ) of the housing ( 10 ). 
     2. The differential lock actuator ( 1 ) according to clause 1, wherein the removable part ( 30 ) of the housing ( 10 ) comprises at least one air inlet ( 31 ) for introduction of compressed air into the housing ( 10 ), said air inlet ( 31 ) being arranged to enable displacement of the push rod ( 20 ) from its first to its second position upon introduction of compressed air into the housing ( 10 ) via said air inlet. 
     3. The differential lock actuator ( 1 ) according to clause 1 or 2, wherein the housing ( 10 ) comprises a hollow inner space ( 13 ) adapted to guide the push rod ( 20 ) along a longitudinal axis of the housing ( 10 ), and wherein the removable part ( 30 ) of the housing ( 10 ) is a cap, adapted to cover an open side end of the housing ( 10 ). 
     4. The differential lock actuator ( 1 ) according to clause 3, wherein the cap comprises a cavity ( 37 ) adapted to enclose the second detector component ( 70 ) in such a way that the second detector component ( 70 ) is separated from the push rod ( 20 ). 
     5. The differential lock actuator ( 1 ) according to any one of clauses 1 to 4, wherein the removable part ( 30 ) of the housing ( 10 ) is attached to the housing ( 10 ) by means of a plurality of fastening members ( 34 ), preferably fastening screws. 
     6. The differential lock actuator ( 1 ) according to any one of clauses 1 to 5, wherein the second detector component ( 70 ) comprises a reed sensor ( 71 ) connected to a circuit board ( 72 ) and arranged to provide an electric output signal when the push rod ( 20 ) is in its first position. 
     7. The differential lock actuator ( 1 ) according to clause 6, wherein the circuit board is a printed circuit board ( 72 ). 
     8. The differential lock actuator ( 1 ) according to any one of clauses 1 to 7, wherein the lock detection assembly further comprises a first detector component ( 80 ) attached to the push rod ( 20 ), and wherein the second detector component ( 70 ) is adapted to detect the position of the first detector component ( 80 ) contactlessly. 
     9. The differential lock actuator ( 1 ) according to clause 8, wherein the first detector component ( 80 ) is adapted to emit a magnetic field, the first detector component ( 80 ) being attached to the push rod ( 20 ) such that the second detector component ( 70 ) detects the second detector component ( 80 ) when the push rod ( 20 ) is in its first position. 
     10. The differential lock actuator ( 1 ) according to clauses 9 or 10, wherein the push rod ( 20 ) comprises a cavity ( 23 ) on an outer surface, preferably a surface directed towards the removable part ( 30 ) of the housing ( 10 ), said cavity being constructed to receive the first detector component ( 80 ) of the detection assembly in such a manner that the first detector component ( 80 ) is embedded within the push rod ( 20 ). 
     11. The differential lock actuator ( 1 ) according to any one of clauses 1 to 10, wherein the push rod ( 20 ) is biased towards its first position, preferably by means of a compression spring ( 40 ). 
     12. Differential mechanism comprising: a differential gear assembly comprising a gear housing; a differential locking mechanism comprising a lock member ( 60 ), said lock member ( 60 ) being movable between a first position, in which the lock member is arranged distant from the gear housing, and a second position, in which the lock member engages with the gear housing, wherein the differential mechanism further comprises a differential lock actuator ( 1 ) according to any one of clauses 1 to 11. 
     13. Differential mechanism according to clause 12, wherein the push rod ( 20 ) of the differential lock actuator ( 1 ) is operably connected to the lock member such that the lock member is moved from its first position to its second position, upon movement of the push rod ( 20 ) from its first position to its second position and vice versa. 
     14. Differential mechanism according to clause 13, wherein the differential lock actuator ( 1 ) is connected to the lock member by means of a shift fork. 
     15. Vehicle axle comprising a differential mechanism according to any one of clauses 12 to 14. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.