Patent Publication Number: US-2020291722-A1

Title: Switchable gear device

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/816,947, filed on Mar. 12, 2019. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to gear devices, and more specifically to a switchable gear device. The present disclosure also relates to various arrangements and systems including the switchable gear device, such as a transfer gearbox for 2WD to 4WD conversion and a stop or lock device for a static position, such as a lock device for a folding multi-position ladder, as well as for different types of mechanisms employing articulated arms and related mechanical applications. 
     BACKGROUND 
     Switchable gear devices are used for many mechanical applications. However, existing switchable gear devices are inefficient in operation and typically require a large space when implemented in mechanical applications, which often increases the overall weight of the resulting device. Conventional switchable gear devices are also not adapted or configured for various types of mechanical applications. 
     Therefore, it is desirable to provide a switchable gear device which exhibits high operational efficiency, is adapted for various types of mechanical applications, and is configured to reduce the space and overall weight required for various types of mechanical applications. 
     SUMMARY 
     A switchable gear device is described. The switchable gear device includes an internal gear and an external case configured to receive the internal gear. The external case is configured to be drivingly connected to and disconnected from an external rotational shaft via the internal gear. When received within the external case, the internal gear is configured to be selectively switched between an “ON” state and an “OFF” state. In the “ON” state of the internal gear, the external case is connected to the external rotational shaft for undergoing rotation therewith (i.e., the external case transmits the rotation of the external rotation shaft). In the “OFF” state of the internal gear, the external case is disconnected from the external rotational shaft and does not undergo rotation therewith (i.e., the external case does not transmit the rotation of the external rotation shaft). 
     According to a feature of the present disclosure, the internal gear and external case are configured so as to allow the internal gear to slide within and relative to the external case to be selectively placed between the “ON” and “OFF” states. 
     The internal gear is provided with a plurality of external gear teeth and the external case is provided with a plurality of gear slots for meshing engagement with the respective external gear teeth of the internal gear. The meshing engagement between the external gear teeth and gear slots is such that the gear teeth are configured to slide along the respective gear slots to selectively place the internal gear between the “ON” and “OFF” states. 
     The internal gear can be readily switched to the “ON” state by sliding the internal gear relative to the external case to a first position in which the internal gear is brought into engagement with the external rotational shaft to transmit rotational movement of the external rotational shaft to the external case. The internal gear can be readily switched from the “ON” state to the “OFF” state by sliding the internal gear relative to the external case to a second position in which the internal gear is disengaged from the external rotational shaft so that the rotational movement of the external rotational shaft is not transmitted to the external case. 
     The internal gear has a tubular configuration and is provided with a plurality of internal gear teeth for meshing engagement with respective external gear teeth of the external rotational shaft when the internal gear is in the first position relative to the external case (i.e., when the internal gear is placed in the “ON” state). 
     In one embodiment, the internal gear is switched between the “ON” state and the “OFF” state by a hydraulic system (hydraulic pressure device) incorporated to the external case and the internal gear. Alternatively, the internal gear is switched between the “ON” state and the “OFF” state by an electro-magnetic system incorporated to the external case and the internal gear. 
     In one application according to the present disclosure, the switchable gear device is a transfer gearbox for 2WD to 4WD conversion. 
     In another application according to the present disclosure, the switchable gear device is used like a stop or lock device for a static position. In one implementation, the switchable gear is incorporated in a folding multi-position ladder. 
     In another application according to the present disclosure, the switchable gear device is applied to articulated components configured to undergo movement relative one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a switchable gear device according to an embodiment of the present disclosure. 
         FIG. 2  is a right-side exploded view showing the internal gear and external case of the switchable gear device along with an external rotational shaft to which the external case is configured to be connected to and disconnected from via the internal gear. 
         FIG. 3  is a perspective of the exploded view in  FIG. 2 . 
         FIG. 4  is another perspective of the exploded view in  FIG. 2 . 
         FIG. 5  is another perspective of the exploded view in  FIG. 2 . 
         FIG. 6  is a side perspective view showing the assembly of the internal gear relative to the external case and the external rotational shaft. 
         FIG. 7  is rear perspective view of the assembly in  FIG. 6 . 
         FIG. 8  is a right-side view showing the switchable gear device in which the internal gear switched to the “ON” state, and with the external case being depicted in semi-transparent form to allow observation of the internal gear. 
         FIG. 9  is a rear perspective view of  FIG. 8 . 
         FIG. 10  is a cross-sectional side view taken along a horizontal line cutting through the center of the device shown in  FIG. 8 . 
         FIG. 11  is a rear cross-sectional perspective view of  FIG. 10 . 
         FIG. 12  is a right-side view showing the switchable gear device in which the internal gear switched to the “OFF” state, and with the external case being depicted in semi-transparent form to allow observation of the internal gear. 
         FIG. 13  is a rear perspective view of  FIG. 12 . 
         FIG. 14  is a cross-sectional side view taken along a horizontal line cutting through the center of the device shown in  FIG. 12 . 
         FIG. 15  is a rear cross-sectional perspective view of  FIG. 14 . 
         FIG. 16  is a side perspective view illustrating an application of the switchable gear device as a transfer gear box for 2WD to 4WD conversion. 
         FIG. 17  is a side perspective view of  FIG. 16 . 
         FIG. 18  is an upper perspective view of  FIG. 16 . 
         FIG. 19  is a cross-sectional side view showing the switchable gear device activated by a hydraulic pressure device and the internal gear in the “ON” state. 
         FIG. 20  is a rear cross-sectional perspective view of  FIG. 19 . 
         FIG. 21  is a cross-sectional side view showing the switchable gear device deactivated by the hydraulic pressure device and the internal gear in the “OFF” state. 
         FIG. 22  is a rear cross-sectional perspective view of  FIG. 21 . 
         FIG. 23  is a diagrammatic view illustrating a conventional lock device in the closed state. 
         FIG. 24  is a diagrammatic view illustrating a conventional lock device in the open state. 
         FIG. 25  is a cross-sectional side view illustrating an application of the switchable gear device as a lock device with the internal gear shown in the “ON” (closed) state. 
         FIG. 26  is a rear cross-sectional perspective view of  FIG. 25 . 
         FIG. 27  is a cross-sectional side view similar to  FIG. 25 , but with the internal gear shown in the “OFF” (open) state. 
         FIG. 28  is a rear cross-sectional perspective view of  FIG. 27 . 
         FIG. 29  is a top perspective view illustrating the various angular positions which can be achieved by the lock device incorporating a switchable gear device according to the present disclosure. 
         FIG. 30  illustrates various exemplary locations for the locking device according to the present disclosure in a folding multi-position ladder. 
         FIG. 31  is a perspective view illustrating a switchable gear device according to the present invention applied to articulated arms. 
         FIG. 32  is a top view of the switchable gear device shown in  FIG. 31 . 
         FIG. 33  is a perspective view similar to  FIG. 31 , with the articulated arms of the switchable gear device in a disconnected state. 
         FIG. 34  is a top, partially exploded perspective view of the switchable gear device shown in  FIG. 32 . 
         FIG. 35  is a left perspective view of the switchable gear device shown in  FIG. 34 . 
         FIG. 36  a right perspective view of the switchable gear device shown in  FIG. 34 . 
         FIG. 37  is a side view in perspective of the switchable gear device shown in  FIG. 34 . 
         FIG. 38  is another exploded view in perspective illustrating a main shaft and switchable gear removed from one of the articulated arms of the switchable gear device. 
         FIG. 39  is a right perspective view of the switchable gear device shown in  FIG. 38 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary aspects and embodiments of the disclosure are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. 
       FIGS. 1-7  show various front, right side, exploded, and perspective views illustrating a switchable gear device, generally designated at  10 , according to an embodiment of the present disclosure. As shown in  FIG. 1 , switchable gear device  10  includes a gear connector or switchable gear  12  (hereinafter also “internal gear”) and an external gear or output shaft  14  (hereinafter also “external case”) configured to receive internal gear  12 . Internal gear  12  is provided with external gear teeth  12   a  and external case  14  is provided with gear slots  14   a  for meshing engagement with respective external gear teeth  12   a  of internal gear  12 . Internal gear  12  and external case  14  are configured so as to allow the internal gear  12  to slide within and relative to external case  14 . Internal gear  12  has a tubular configuration and is provided with internal gear teeth  12   a.    
     Referring to  FIGS. 2-6 , external case  14  is configured to be drivingly connnected to and disconnected from a main shaft  16  (hereinafter also “external rotational shaft”) via internal gear  12 . The driving connection or engagement is accomplished by bringing internal gear teeth  12   a  of internal gear  12  into meshing engagement with external gear teeth  16   a  of main shaft  16 . 
     When received within external case  14 , internal gear  12  is configured to be selectively switched between an “ON” state and an “OFF” state. In the “ON” state of internal gear  12 , external case  14  is connected to main shaft  16  for undergoing rotation therewith (i.e., external case  14  transmits the rotation of main shaft  16 ). In this “ON” state configuration, external gear teeth  12   a  of internal gear  12  engage gear slots  14   a  of external case  14 , and internal gear teeth  12   b  of internal gear  12  engage external gear teeth  16   a  of main shaft  16 . In the “OFF” state of internal gear  12 , external case  14  is disconnected from main shaft  16  and does not undergo rotation therewith (i.e., external case  14  does not transmit the rotation of main  16 ). In this “OFF” state configuration, external gear teeth  12   a  of internal gear  12  continue to engage gear slots  14   a  of external case  14 , however, internal gear teeth  12   b  of internal gear  12  are disengaged from external gear teeth  16   a  of main shaft  16 . Stated otherwise, in the “OFF” state, external case  14  can rotate freely. 
     The disengagement of internal gear teeth  12   b  from external gear teeth  16   a  described above is accomplished by sliding movement of internal gear  12  within and relative to external case  14 . Thus, according to a feature of the present disclosure, internal gear  12  and external case  14  are configured so as to allow internal gear  12  to slide within and relative to external case  14  to selectively place internal gear  12  between the “ON” and “OFF” states. 
     More specifically, internal gear  12  can be readily switched to the “ON” state by sliding internal gear  12  relative to external case  14  to a first position within external case  14  in which internal gear teeth  12   b  of internal gear  12  are brought into meshing engagement with external gear teeth  16   a  of external rotational shaft  16  to transmit rotational movement of external rotational shaft  16  to external case  14  via internal gear  12 . Internal gear  12  can be readily switched from the “ON” state to the “OFF” state by sliding internal gear  12  relative to external case  14  to a second position within external case  14  until internal gear teeth  12   b  of internal gear  12  disengage from external gear teeth  16   a  of external rotational shaft so that rotational movement of external rotational shaft  16  is not transmitted to external case  14  via internal gear  12 . 
       FIGS. 8-11  show various right side, rear, perspective and cross-sectional views of switchable gear device  10  in which internal gear  12  is in the “ON” state (i.e., internal gear  10  is connected to external rotational shaft  16 ).  FIGS. 12-15  show various right side, rear, perspective and cross-sectional views of switchable gear device  10  in which internal gear  12  is in the “OFF” state (i.e., internal gear  12  is disconnected from external rotational shaft  16 ), thereby enabling external case  14  to rotate freely. In  FIGS. 8, 9, 12 and 13 , external case  14  is depicted in semi-transparent form to allow observation of internal gear  12 . 
     It will be appreciated that switchable gear device  10  is readably switchable “ON” and “OFF” as described above to connect one gear (i.e., external case  14 ) to another (i.e., external rotational shaft  16 ). The configuration (e.g., shape) of switchable gear device  10 , including internal gear  12  positionable inside of external case  14  and adapted for sliding movement therein to achieve the “ON” and “OFF” states, effectively reduces the space required for this type of mechanical application, thus reducing the overall weight of the resulting mechanical system in which switchable gear device  10  is applied. Furthermore, since the gear connection between internal gear  12  and external rotational shaft  16  is internal (i.e., inside of external case  14 ), the resulting mechanical system is able to operate with high efficiency, particularly because it results in a coupling configured to operate as one single, integral piece. 
       FIGS. 16-18  are perspective views illustrating an application of the switchable gear device in an engine transfer gear box or transfer case for 2WD to 4WD conversion. In these figures, external case  14  is depicted in semi-transparent form to allow observation of internal gear  12 . 
     The arrangement of the transfer case in  FIGS. 16-18  illustrates the interconnection and relative positions between switchable gear device  10  (internal gear  12  and external case  14 ), external rotation shaft  16  (main shaft), first output shaft (4WD)  17 , second output shaft (2WD)  18  and corresponding gears  20  and  22 . In this configuration, gear device  10  is connected to an engine itself represented by external rotation shaft  16  (main shaft) as described above with reference to  FIGS. 2-7 . Alternatively, gear device  10  can be connected to a rotor or gear that brings the power of an engine. The function of gear device  10  is to transmit the movement of rotation of an engine when gear device  10  is in the “ON” state ( FIGS. 8-11 ) and to discontinue its rotation when gear device  10  is in the “OFF” state. 
     More specifically, the function of the arrangement shown in  FIGS. 16-18  is as a shift shaft. Internal gear  12  slides into external case  14  to achieve the “ON” state ( FIGS. 8-11 ) in which external case  14  is connected to first output shaft (4WD)  17  to transmit rotation for 2WD/4WD conversion. When internal gear  12  is to be placed in the “OFF” state, internal gear  12  is slid out from first output shaft (4WD)  17  and external case  14  is disconnected from first output shaft (4WD)  17  so that external case  14  can rotate freely ( FIGS. 12-15 ). 
     By the foregoing arrangement shown in  FIGS. 16-18 , gear device  10  functions as a switchable interior gear that is switchable “ON” and “OFF” to connect one gear (i.e., external case  14 ) to another (i.e., first output shaft (4WD)  17 ) in the “ON” state and disconnect these two gears from each other in the “OFF” state. The design and shape of this configuration, including the internal gear connection and reduction in space as described above, makes this arrangement more effective as compared to conventional transfer gearbox arrangements. Since the connection of the gear is internal, the overall efficiency of the mechanical system is increased because the resulting coupling operates integrally as one piece as internal gear  12  is contained outside by external case  14  and inside by first output shaft (4WD)  17 . 
       FIGS. 19-22  are cross-sectional side and perspective views illustrating a mechanism for activating switchable gear device  10 , according to an exemplary embodiment of the present disclosure. 
     In  FIGS. 19-22 , internal gear  12  of gear device  10  is activated (i.e., placed between the “ON” and “OFF” states as described above) by means of a hydraulic system including a hydraulic piston  24  operated by a pressure device  26  with pressure in  28  and pressure out  28  ports.  FIGS. 19 and 20  illustrate the configuration in which internal gear  12  is in the “ON” state, in which external case  14  is connected to first output shaft (4WD)  17  for transmission of rotation as described above.  FIGS. 21-22  illustrate the configuration in which internal gear  12  is in the “OFF” state, in which external case  14  is not connected to first output shaft (4WD)  17  and external case  14  is able to rotate freely as described above. 
     It will be appreciated that the mechanism for activating switchable gear device  10  is not limited to a hydraulic system. For example, switchable gear device  10  can be activated by an electro-magnetic system or other suitable activation means without departing from the spirit and scope of the invention. 
     Another application of the switchable gear according to the present disclosure is described below with reference to  FIGS. 23-29 . In this application, the switchable gear device of the present disclosure is used like a stop or lock device for a static position. 
       FIGS. 23 and 24  are diagrammatic view illustrating a conventional stop or lock device for a folding multi-position ladder, with the lock device shown in the closed ( FIG. 23 ) and open ( FIG. 24 ) states. These conventional folding ladders can be placed in various adjustable angle positions with push knobs locking a hinge and also adjusted in height using spring loaded “J” locks, as well as use 3-position multi-locking hinges. However, these conventional stop or lock arrangements for folding multi-position ladders suffer from the problems that the adjustments cannot be accomplished with high efficiency and also the degree of adjustment (e.g., the number of angles in which the ladder can be locked) is rather limited. 
       FIGS. 25-28  are rear and side cross-sectional perspective views illustrating an application of the switchable gear device according to the present disclosure as a stop or lock device for a static position, such as for a folding multi-position ladder, which overcomes the foregoing drawbacks in the conventional art. 
     As shown in  FIGS. 25-28 , the lock device, generally designated with reference numeral  100 , includes a locking gear  12  and gear case  14  corresponding to the internal gear and external case described above for switchable gear device  10 , a main shaft  15 , a push knob  32  and spring  34 . As best shown in  FIG. 29 , main shaft  15  is provided with external gear teeth  15   a.    
       FIGS. 25 and 26  show lock device  100  in a closed state in which gear teeth  12   b  of locking gear  12  are in meshing engagement with gear teeth  15   a  of main shaft  15  under the bias of spring  34  (in the left direction of  FIG. 27 ). In this closed state, locking gear  12  is connected to main shaft  15  and gear case  14  is locked with main shaft  15 , thereby preventing gear case  14  from undergoing rotation relative to main shaft  15 . 
       FIGS. 27 and 28  show lock device  100  in an open state in which gear teeth  12   b  of locking gear  12  are not in meshing engagement with gear teeth  15   a  of main shaft  15 . In this open state, locking gear  12  is not connected to main shaft  15  and gear case  14  is not locked with main shaft  15 , thereby enabling gear case  14  to freely rotate relative to main shaft  15 . 
     During operation, lock device  100  is activated by manually pushing and releasing push knob  32 . When not activated, the lock device  100  is in the closed state as described above ( FIGS. 25-26 ). When activated, the lock device  100  is in the open state as described above ( FIGS. 27-28 ). To activate lock device  100 , push knob  32  is manually pushed so that locking gear  12  slides into gear case  14  (in the right direction of  FIG. 27 ) against the bias of spring  34  until locking gear  12  is disconnected from main shaft  15  (i.e., by releasing the meshing engagement between gear teeth  12   b  of locking gear  12  and gear teeth  15   a  of main shaft  15 ), thereby placing locking device  100  in the open state. While in the open state, gear case  14  is able to freely rotate relative to main shaft  15  to place it in any of a multitude of selected angular positions relative to main shaft  15  as illustrated in  FIG. 29 , for example. When the selected angular position is achieved, push knob  32  is released and locking gear  12  is connected back to main shaft  15  by the action of spring  34  and the lock device  100  is again placed in the closed state as described above. 
     In an exemplary embodiment, locking device  100  according to the present disclosure can be applied at multiple locations of a folding multi-position ladder as illustrated in  FIG. 30 . Locking device  100  allows one to lock the ladder in a multitude of angular positions (e.g., as shown in  FIG. 29 ) and with high efficiency as compared to conventional ladder lock mechanisms, such as described above with reference to  FIGS. 23 and 24 . 
       FIGS. 31-39  show another exemplary application of the switchable gear device according to the present invention. In  FIGS. 31-39 , the switchable gear device, generally designated at  200 , is applied to two articulated components in the general form of an articulated arm  220  (first component) and articulated arm  224  (second component) configured to undergo angular movement relative to one another. 
       FIG. 31  is a perspective view and  FIG. 32  is a top view of switchable gear device  200  shown in an assembled state. Articulated arms  220 ,  224  include respective arm portions  204 ,  202  configured to be connected to various elements or objects requiring articulation at various angles relative one another. In this embodiment, arm portions  202 ,  204  are provided with an opening configured to receive such elements or objects. For example, the openings are configured to receive suitable wooden or metal beams which are integrally connected to arm portions  202 ,  204  by friction fit, welding, clamping and/or using suitable fasteners such as nails, bolts and/or screws to form a support structure for a scaffold, for example. It will be appreciated that articulated arms  220 ,  224  can be used for structures other than scaffolds, such as platforms, support beams, etc. By this construction and arrangement, it is appreciated that switchable gear device  200  according to the present invention can hold, support and/or extend any elements or objects attached to articulated arms  220 ,  224  giving it the ability to position the elements or objects at different angles relative one another. 
       FIGS. 33-39  illustrate various disassembled states of switchable gear device  200  in partial perspective and exploded views showing the various components and corresponding structural and positional relationships thereof, as further described below. 
     Articulated arm  220  has a gear case  222  extending from arm portion  204 . Gear case  222  has the same internal construction as external case  14  described above with reference to  FIGS. 1-15 . In this embodiment, gear case  222  is formed in one piece with arm portion  204 . Alternatively, gear case  222  and articulated arm  220 , including arm portion  204 , may be separate components which are integrally connected together using suitable connecting means. As shown in  FIGS. 33, 37 and 38 , arm portion  204  is provided with opening  221  for receiving any element/object as described above. 
     Referring to  FIGS. 31-36 , articulated arm  224  is provided with two housing portions  225  extending from arm portion  202 . In this embodiment, housing portions  225  are formed in one piece with arm portion  202 . Alternatively, housing portions  225  and articulated arm  224 , including arm portion  202 , may be separate components which are integrally connected together using suitable connecting means. Housing portions  225  are spaced-apart from one another to form a space  203  therebetween, as shown in  FIGS. 31 and 34-36 . Space  203  is configured to receive and accommodate gear case  222  of articulated arm  222 , in the manner shown in  FIGS. 31, 32, 38 and 39 , so as to permit articulated arms  220 ,  224  to be moved and placed at various angles relative one another. 
     Referring to  FIGS. 33-39 , gear case  222  is configured to receive and accommodate switchable gears  210  (hereinafter also “internal gears”) each having the same construction (e.g., internal gear teeth) as internal gear  12  described above with reference to  FIGS. 1-15 . As described above for internal gear  12  and external case  14  in  FIGS. 1-15 , internal gears  210  and gear case  222  are configured so as to allow internal gears  210  to slide within and relative to gear case  222 . 
     Each housing portion  225  of articulated arm  224  is configured to receive and support therein a main shaft  208  which has the same general construction (e.g., external gear teeth) as main shaft  16  described above with reference to  FIGS. 1-15 . Each main shaft  208  is supported by and retained in corresponding housing portion  225  by a washer  206  so that the external gear teeth of each main shaft  208  extends into space  203  of articulated arm  224  as shown in  FIG. 34 , for example. Each washer  206  has internal gear teeth configured to mesh with the external gear teeth of the corresponding main shaft  208 . In the assembled state of switchable gear device  200 , an outer surface of each washer  206  engages a lip of gear case  222  while an inner surface of each washer  206  engages an outer side of the corresponding main shaft  208 . By this construction, relative sliding movement between articulated arms  220 ,  224  is prevented. 
     In the assembled state of switchable gear device  200 , a portion of the external gear teeth of each main shaft  208  is configured to extend into gear case  222  from opposite ends thereof for driving engagement with the internal gear teeth of the corresponding internal gear  210 , as shown for example in  FIGS. 33-37 , when internal gear teeth of internal gears  210  are slid into engagement with the external gear teeth of respective main shafts  208  as further described below. In  FIGS. 33-37 , main shafts  208  and internal gears  210  are disposed outside of gear case  222  for illustration purposes only to show how these components are configured for driving engagement with one another while positioned inside of gear case  222 . 
     Internal gears  210  are configured for undergoing sliding movement within and relative to gear case  222  to selectively place them into engagement with and disengagement from main shafts  208 . In a first state of switchable gear  200 , internal gear teeth of internal gears  210  are disposed in meshing engagement with the external gear teeth of respective main shafts  208 . In this first state, articulated arms  220 ,  224  are placed in a locked state and prevented from undergoing movement relative to one another. In a second state of switchable gear  200 , internal gear teeth of internal gears  210  are completely disengaged from the external gear teeth of respective main shafts  208 . In this second state, articulated arms  220 ,  224  are placed in an unlocked state and are permitted to undergo movement relative one another to position the corresponding elements or objects attached to articulated arms  220 ,  224  at different angles relative one another. 
     Switchable gear  200  is provided with switching means for switching between engagement and disengagement of internal gears  210  and main shafts  208  in the first and second states of switchable gear  200 . Referring to  FIGS. 34-39 , the switching means comprises, for each internal gear  210 , a push knob  212 , a push rod  213  having one end connected to push knob  212  and another (opposite) end integrally connected to internal gear  210 . For each internal gear  210 , movement of push knob  212  in a direction towards (first direction) the corresponding housing portion  225  of articulated arm  224  causes internal gear  210  to slide inside gear case  222  in a direction which results in the disengagement of internal gear teeth of internal gear  210  from the external gears of the corresponding main shaft  208  to achieve the second state of switchable gear  200  as describe above. Likewise, for each internal gear  210 , movement of push knob  212  in a direction away (second direction) from the corresponding housing portion  225  of articulated arm  224  causes internal gear  210  to slide inside gear case  222  in a direction which results in the internal gear teeth of internal gear  210  coming into meshing engagement with the external gears of the corresponding main shaft  208  to achieve the first state of switchable gear  200  as describe above. 
     In an exemplary embodiment, push rods  213  are in the form of threaded rods extending through a through hole of the corresponding main shaft  208  and securely threaded at one end to a threaded hole of the corresponding push knob  212 . Each threaded rod  213  also engages the corresponding internal gear  210  and is securely retained thereto by a suitable fastener, (e.g., a threaded nut) denoted with reference numeral  211  in  FIG. 34 , that is securely engaged with the opposite end of threaded rod  213 . 
     Each push knob  212  is associated with a biasing member  214  configured to bias push knob  212  in the second direction described above (i.e., in a direction away from the corresponding housing portion  225  of articulated arm  224 ) to maintain switchable gear  200  in the first state. By this arrangement, pressing push knobs  212  in the first direction described above (i.e., against the bias of springs  214 ) cause internal gear teeth of internal gears  210  to disengage from external gear teeth of respective main shafts  208  to achieve the second state of switchable gear  200  (i.e., permitting movement of articulated arms  220 ,  224  relative one another). In this embodiment, each biasing member  214  is in the form of a coil spring. It is understood, however, that other types of biasing members may be selected to perform the functions described without departing from the spirit and scope of the invention. 
     Referring to  FIGS. 34-39 , a stopper element  216  is securely mounted to an outer sidewall of each housing portion  225  for retaining the corresponding main shaft  208  within housing portion  225 . Stopper element  216  is mounted to housing portion  225  using any suitable connecting means, such as by screws using holes  217  formed in stopper element  216 . 
     Referring to  FIGS. 31-29 , removable knob covers  218  are provided for covering the respective push knobs  212 . For each push knob  212 , knob cover  218  has an opening providing access to a pressing surface of push knob  212 , as shown in  FIGS. 31, 33 and 37 . Knob covers  218  also serve to prevent push knobs  212  from moving side to side and guide them during movement in the first and second directions as described above (i.e., to keep knob covers  218  aligned straight). 
     As best shown in  FIGS. 34, 35 and 38 , each knob cover  218  is provided with tab elements  219  configured for engagement with corresponding recessed portions  223  formed in stopper element  216  to removably mount knob cover  218  to stopper element  216 . 
       FIGS. 31 and 32  show switchable gear device  200  in the first state described above, in which articulated arms  220 ,  224  are placed in a locked state and prevented from undergoing movement relative to one another. This is because in this first state the internal gear teeth of internal gears  210  are disposed in meshing engagement with the external gear teeth of respective main shafts  208  as described above. To switch switchable gear device  200  from the first state to the second state in which articulated arms  220 ,  224  are permitted to undergo movement relative one another, both push knobs  212  are pressed in the first direction (i.e., against the bias of biasing members  214 ) to cause internal gear teeth of internal gears  210  to slide inside gear case  222  until they are completely disengaged from the external gear teeth of respective main shafts  208 . While push knobs  220 ,  224  are maintained in this pressed state, articulated arms  220 ,  224  can be moved relative one another to position the corresponding elements or objects attached to articulated arms  220 ,  224  at different preselected angles relative one another. After the desired relative angular position between elements or objects of the corresponding articulated arms  220 ,  224  is achieved, push knobs  212  are released by the biasing action of biasing members  214  (i.e., are caused to move in the second direction) to place switchable gear device  200  in the first state (i.e., to place articulated arms  220 ,  224  in the locked state). It will be appreciated that in the first and second states of switchable gear device  200 , the engaging and disengaging actions between the main shafts  208  and corresponding internal gears take place inside gear case  222 . 
     Without departing from the spirit and scope of the invention, it will be understood that the various components of the devices and systems according to the present disclosure described herein, including the switchable gear devices, transfer gear box and locking device, can be fabricated of any material required to achieve the intended purposes and functions. These materials include, for example, suitable metals such as steel and aluminum and as well as suitable plastic materials. 
     The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein but are to be accorded the widest scope consistent with the principles and novel features disclosed herein.