Patent Publication Number: US-7896134-B2

Title: Low-rise vertical platform lift assembly with low-profile lifting mechanism

Description:
BACKGROUND 
     Persons with mobility impairments often depend on a wheelchair or walking aid to facilitate mobility. As a result, they are frequently subjected to physical barriers and obstacles such as stairs and curbs. The ADA legislation requires that these physical barriers be removed. Ramps provide some access; however, ramps can be very long and difficult to climb. Further, depending on the elevation change and available space, ramps may be impractical. One solution is a wheelchair lift. Wheelchair lifts for commercial buildings and private residences must be designed and tested to meet the requirements of the ASME Code: A18.1, SAFETY STANDARD FOR PLATFORM LIFTS AND STAIRWAY CHAIRLIFTS. 
     Low-rise platform lifts, or lifts that are limited to 24-inch maximum vertical travel, have been developed for use in courtrooms, church pulpits, meeting chamber podiums, and other similar environments. These types of installations not only provide a means for safe level changes, but must also be sensitive to decorum and surrounding architecture. 
     As low-rise lifts are being incorporated into new and remodel construction, obstacles are being encountered that require alternative lifting mechanisms to facilitate a simpler and cleaner interface with surrounding millwork finishes. For example, screw column type lifting mechanisms require the screw columns to be encased within the millwork walls, which directly influences and sometimes restricts the placement of screw columns and requires significant modifications to existing decorative finishes. 
     One suitable lifting assembly is a “scissor-type” lifting mechanism. Such lifting mechanisms typically have very high lifting ratios at the lower range of platform travel, and very low lifting ratios at the upper range of platform travel. This ratio differential causes the platform speed to vary throughout the range of vertical travel, which typically must be overcome with the use of hydraulics. Hydraulic systems are undesirable as they are known to bleed or leak over time. 
     Scissor mechanisms also tend to have a profile larger than desirable, as space is required within the scissor envelope for the actuator (or actuators). This space problem is often overcome with the use of a pit under the lift to house the actuator. However, retrofit applications often do not have sufficient pit space available. As a result, scissor lifts are undesirable in applications that require a pit to house actuator of the scissor lift. 
     Thus, a low-profile lift mechanism, that is discrete, achieves a suitable lifting power, and maintains a relatively constant lifting speed is desired. 
     SUMMARY 
     A wheelchair lift assembly is provided. The wheelchair lift assembly includes a frame, a platform, and a lift mechanism associated with the frame. The lift mechanism includes a stabilizing arm and at least one lift arm member having a first end connected to the platform. The first end of the lift arm member pivots and translates relative to the platform as the platform is reciprocated between raised and lowered positions. The wheelchair lift assembly also includes a drive assembly. The drive assembly selectively provides a driving force to the lift mechanism without directly providing the driving force to the stabilizing arm. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an isometric view of the platform lift in the raised position constructed in accordance with one embodiment of the present disclosure; 
         FIG. 2  is an exploded view of the platform lift of  FIG. 1 ; 
         FIG. 3  is a partial isometric view of the platform lift of  FIG. 1 ; 
         FIG. 4  is a partial cross-sectional view of the platform lift of  FIG. 1 , taken substantially through Section  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a partial cross-sectional view of the platform lift of  FIG. 1 , taken substantially through Section  6 - 6  of  FIG. 1  and showing the platform lift in the raised position; and 
         FIG. 6  is a partial cross-sectional view of the platform lift of  FIG. 1 , taken substantially through Section  6 - 6  of  FIG. 1  and showing the platform lift in the lowered position. 
     
    
    
     DETAILED DESCRIPTION 
     A low-profile, low-rise vertical platform lift  20  constructed in accordance with one embodiment of the present disclosure is best seen by referring to  FIG. 1 . The vertical platform lift  20  includes a frame  22  and a lift mechanism  24  slidably received therewithin. Lifting means are used to reciprocate the lift mechanism  24  between a lowered position (see  FIG. 6 ) and a raised position (see  FIG. 1 ). A lift platform  30  is coupled to the lift mechanism  24  so that the lift platform  30  translates between the lowered and raised positions along with the lift mechanism  24 . The vertical platform lift  20  is preferably constructed of steel or aluminum, yet any material of suitable strength and durability may be used. 
     Now referring to  FIG. 2 , the frame  22  includes two opposing lateral brackets  88 . A rear bracket  90  disposed between the lateral brackets  88  at the distal end of the frame, and first and second cross supports  92  and  94  are disposed between the lateral brackets  88  at the proximal end of the frame  22 . The first and second cross supports  92  and  94  may be used for fixedly coupling various components within the frame. The lateral brackets  88  include a first pin aperture  96  disposed opposite one another for receiving the end of a first pin  54 . 
     The lateral brackets  88  further include upwardly projecting portions  102  disposed opposite one another, each having a second pin aperture  104  for receiving the ends of a second pin  160 . A first collar  56  and second collar  158  are coaxially aligned with first pin aperture  96  and second pin aperture  104 , respectively. The first and second collars  56  and  158  are fixedly coupled to lateral brackets  88  of the frame  22 . A plurality of gusset supports  60  are fixedly coupled to lateral brackets  88  and both first collar  56  and second collar  158 . 
     Still referring to  FIG. 2 , the lift mechanism  24  is pivotally and slidably received within the frame  22 . The lift mechanism  24  includes a main lift arm assembly  32  pivotally coupled to a stabilizing arm assembly  34 . The main lift arm assembly  32  includes two main lift arms  38 , each having an outer lift arm member  40  and an inner lift arm member  42 . The main lift arms  38  are rigidly coupled to one another with a first connecting plate  50  that extends therebetween; however, it should be appreciated that the main lift arms  38  may instead be independent of one another. As shown in  FIG. 6 , the first connecting plate  50  is tapered in cross-section, with the plate  50  decreasing in thickness as the plate  50  extends toward the midpoint of the main lift arms  38 . 
     The main lift arms  38  include bent portions at one end that are rigidly coupled to one another with a second connecting plate  52  to enhance strength and durability of lift arms  38 ; however, it should be appreciated that the bent portions of the main lift arms  38  may instead be independent of one another. The bent portions of the main lift arms  38  define a second end  46  having a lever pivot point  47 . A pushrod  61  is pivotally coupled to the end of each main lift arm  38 , or the second end  46 , for transmitting force from the crank assembly  28  (described in detail below) to the lift mechanism  24 . 
     A first hollow shaft  58  passes through the main lift arms  38  at the lever pivot point  47 . The first end of the first pin  54  passes through first collar  56  and first pin apertures  96  formed opposite one another in each lateral bracket  88 , wherein the first ends of first pins  54  protrude into first hollow shaft  58 . The second ends of first pins  54  are thereafter received within first collars  56 . A retaining pin or similar device passes transversely through the first collar  56  and the second end of first pin  54  to prevent the first pin  54  from rotating within the first collar  56 . Thus, hollow shaft  58  and main lift arms  38  are pivotally coupled to frame  22  with first pins  54 . 
     A bearing member may be disposed within the first hollow shaft  58  between the first hollow shaft  58  and the first pin  54 . Any suitable bearing member, such as a ball bearing assembly, roller bearing assembly, or bushing may be used. 
     It can be appreciated that the first end of first pin  54  could be fixedly attached to first hollow shaft  58 , and the second end of first pin  54  be allowed to rotate in first collar  56 . In such an embodiment, a bearing member may be disposed between the first collars  56  and the second end of first pins  54 . Accordingly, such embodiments and other variations are within the scope of the present disclosure. 
     Still referring to  FIG. 2 , the lift mechanism  24  includes main lift arms  38  and a stabilizing arm assembly  34 . The stabilizing arm assembly  34  includes two stabilizing arms  62  pivotally coupled to the interior of the main lift arms  38  by a pin or other suitable device at approximately the midpoint of the stabilizing arms  62  and the main lift arms  38  to form a pivot point  65 . 
     The stabilizing arms  62  may be, but not necessarily, tapered at each end with the widest portion in the middle. It is preferred, but not essential, that the stabilizing arms  62  are rigidly coupled together with a stabilizing arm connecting member  64  to enhance the strength and durability of the stabilizing arms assembly  34 . The stabilizing arm connecting member  64  is also tapered at each end. A translating rod  70  is transversely coupled to the lower end of the stabilizing arms  62 . The translating rod  70  includes rod pin ends  72  that are pivotally and slidably receivable within longitudinal slots  100  formed in the frame lateral brackets  88 . 
     The upper, or first, ends of the main lift arms  38  and the stabilizing arms  62  are coupled to a platform frame  36 . The platform frame  36  is generally rectangular in shape, and it includes two support arms  74  and a rear, transverse member  76  that extends between the rear ends of the supports arms  74 . The front ends of the support arms  74  are pivotally coupled to the ends  266  of a connecting rod  66  disposed between the upper ends of the stabilizing arms  62 . 
     The upper ends of the main lift arms  38  are pivotally and slidably received within longitudinal slots  78  formed within the support arms  74  of the platform frame  36 . A lift platform  30  is coupled to the top of the lift platform frame  36  with any suitable fastening means to define a lifting area for a passenger. As constructed, the upper ends of the main lift arms  38  pivot and translate within the slots  78  and relative to the lift platform  30  as the lift platform  30  is reciprocated between the raised and lowered positions. 
     Referring to  FIGS. 2 and 6 , the design of the lift mechanism  24  allows the main lift arms  38 , the stabilizing arms  62 , and the platform frame  36  to be collapsible and receivable within the frame  22  such that the lift  20  may be lowered in a low-profile configuration. Moreover, the stabilizing arms  62  and the stabilizing arm connecting member  64  are shown tapered such that they may be nestled within the frame against the first connecting plate  50 . 
     As may be seen best by referring to  FIG. 3 , lifting means for driving the lift mechanism  24  between a lowered and raised position includes a reversible drive assembly  26 , which includes an actuator  120 , and a crank assembly  28 . The lifting means are positioned relative to the lift mechanism  24  such that the lift mechanism  24  is collapsible within the frame  22 . Although illustrated as being adjacent the frame, other configurations may locate the lifting means remotely and independent from the frame. Accordingly, such configurations are within the scope of the present disclosure. 
     The reversible drive assembly  26  further includes a motor  106 . Coupled to the motor  106  is any suitable transmission, such as a gear assembly, a sprocket assembly, etc. Preferably, a gear reducer  109  is coupled to the motor  106  for translating the motor&#39;s energy into the rotation of the output shaft  108 . The output shaft  108  drives a tapered bushing  111  and an output sprocket  110  coaxially mounted thereon. A roller chain  112  extends between the output sprocket  110  and a drive sprocket  114  for transmitting torque to the drive sprocket  114 . 
     Referring to  FIGS. 3 and 4 , the actuator  120  is driven by the drive sprocket  114 . The drive sprocket  114  and a tapered bushing  116  are coaxially mounted to a screw drive shaft  118 . The screw drive shaft  118  forms the end of a lead screw  128 , which extends between the drive sprocket  114  and a lead screw end block  127 . In an alternate embodiment, the drive assembly  26  includes a motor  106 , which is directly coupled to the screw drive shaft  118 , either with or without an intermediate gear assembly, but without the use of chains or belts. 
     The screw drive shaft  118  is at least partially encased within a screw bearing assembly  121 , as can best be seen by referring to  FIG. 4 . The bearing assembly  121  includes a first seal holder  122  coaxially disposed on the screw drive shaft  118 , which encases a first thrust spacer  182  and a grease seal  184 . The grease seal  184  is disposed within the gap defined by the thrust spacer  182  and the opening of the first seal holder  122 . A thrust bearing lock washer  180  is positioned adjacent the first thrust spacer  182  on the exterior of the first seal holder  122 , and a thrust bearing lock nut  178  is positioned adjacent the thrust bearing lock washer  180  on the screw drive shaft  118 . 
     Disposed coaxially on the screw drive shaft  118  adjacent the first seal holder  122  is a bearing support  124 . Encased within the bearing support  124  and partially within the first seal holder  122  (adjacent the first thrust spacer  182 ) is a thrust ball bearing  186 . Also disposed within the bearing support  124  is a radial ball bearing  190 . A plurality of lubrication fittings  188  are integrated within the bearing support  124  to provide the bearing assembly  121  with the proper lubrication. 
     Disposed coaxially on the screw drive shaft  118  adjacent the bearing support  124  is a second seal holder  125 . The second seal holder  125  encases a second thrust spacer  192  that abuts a portion of the radial ball bearing  190  and a third thrust spacer  196  that is positioned adjacent thereto. A grease seal  198  is disposed within the gap defined by the third thrust spacer  196  and the opening of the second seal holder  125 . A disc spring  194  is disposed between the second thrust spacer  192  and the third thrust spacer  196 . 
     The first seal holder  122 , the bearing support  124 , and the second seal holder  125  are coupled together with any suitable fasteners, such as screws, etc., to cooperatively form the bearing assembly  121 . The screw drive shaft  118  is freely rotatable within the bearing assembly  121 . 
     Still referring to  FIG. 4 , one screw plate  126  is positioned laterally along each side of the lead screw  128 , wherein each screw plate  126  is fixedly coupled at one end to the bearing support  124  and at the other end to the lead screw end block  127 . Each screw plate  126  includes a longitudinal slot  144  that extends substantially along the length of the plate  126 . 
     The actuator  120  further includes a pull-block assembly  134  coaxially and threadably disposed on the lead screw  128 , which may be threadably translated thereon. The pull-block-assembly  134  includes a first nut  130  and a second nut  132  threadably received on the lead screw  128  (see also  FIG. 3 ). First and second blocks  136  and  137  are coupled to each nut  130  and  132  and are coaxially disposed on the lead screw  128 . The blocks  136  and  137  are rigidly coupled together with two roller bearing assemblies  138 . 
     The roller bearing assemblies  138  are positioned laterally on each side of the pull-block assembly  134  and extend through the longitudinal slots  144 . The roller bearing assemblies  138  preferably include a plurality of roller bearings (not shown) disposed within a bearing frame. When the pull-block assembly  134  is threadably translated along the lead screw  128 , the roller bearing assemblies  138  translate within the longitudinal slots  144 . The roller bearing assemblies  138  translating within the slots  144  provide the reaction torque for first and second nuts  130  and  132 . 
     Referring to  FIG. 5 , first and second connecting links  140  and  141  are pivotally coupled at their first ends to the roller bearing assemblies  138 . Each connecting link  140  and  141  extends towards the lead screw end block  127 . The connecting links  140  and  141  are pivotally coupled at their second end to an inner yoke  154  of the crank assembly  28 . 
     Although the preferred embodiment of the actuator  120  is depicted using an acme lead screw  128 , it should be appreciated that other suitable actuators may be used to drive the crank assembly  28 . For instance, a ball screw assembly or a hydraulic actuator may similarly be used without departing from the spirit and scope of the present disclosure. 
     Referring back to  FIG. 3 , the crank assembly  28 , which is driven by the actuator  120 , includes a hollow crankshaft  152  that spans between the upwardly projecting portions  102  on the frame  22 . Disposed within the hollow crankshaft  152  is the first end of second pin  160  and a bearing assembly (not shown) therebetween, positioned inside the end of hollow crankshaft  152  such that the hollow crank shaft  152  may rotate freely about the first end of second pin  160 . Any suitable bearing assembly, such as a ball bearing assembly or a roller bearing assembly, may be used. 
     The second ends of the second pins  160  are received within the second pin apertures  104  of the projecting portions  102 . The second ends of second pins  160  are thereafter received within second collars  158 , which are fixedly coupled to the projecting portions  102  of lateral brackets  88 . Retaining pins or similar devices pass transversely through the second ends of the second pins  160  and the second collars  158  to prevent the rotation of the second pins  160  within the second collars  158 . 
     The crank assembly  28  further includes a pair of inner yokes  154  and a pair of outer yokes  156  fixedly coupled to the crankshaft  152 . The inner yokes  154  are pivotally coupled to the second ends of the connecting links  140  and  141  (see  FIG. 5 ). The outer yokes  156  are pivotally coupled to pushrods  61 , which are in turn pivotally coupled to the bent portions of the main lift arms  38 , or the second end  46  (see  FIG. 2 ). 
     In an alternate embodiment, only one inner yoke  154  and one outer yoke  156  is used. In this embodiment, the pull-block assembly  134  would include only one connecting link  140  that would drive the crank assembly  28 , and the lift mechanism  24  would include only one pushrod  61  for translating the main lift arms  38 . In yet another embodiment, yokes  154  and  156  could be combined into a pair of compound yokes, such that pushrods  61  and connecting links  140  and  141  act on the same pair of yokes. In still yet another embodiment, a drive assembly  26  could be directly connected between levers  46  and frame  22  and not use pushrods  61  or connecting links  140  and  141 . 
     The crank assembly  28  may be further supported by a bearing assembly  148  coupled to the screw plates  126 . The bearing assembly  148  includes two crutch bearings  150  that engage the exposed portion of the crankshaft  152  within the inner yokes  154  to provide transverse support to the crankshaft  152  when it is being actuated. However, it should be appreciated that the crank assembly  28  is fully operable without the support of the crutch bearings  150 . 
     Referring still to  FIG. 3 , the vertical platform lift  20  further includes a control system for detecting and maintaining the positions of the lift mechanism  24  and for preventing extreme mechanical travel of the lift  20 . The control system includes a controller (not shown) and a plurality, such as four, proximity switches  164 ,  166 ,  168 , and  170 . The proximity switches are suitably mounted on a switch bracket  172  received within the frame  22 . Preferably, inductive proximity switches or optical sensors are used; however, it can be appreciated that other types of sensors, such as mechanical sensors, may also be used without departing from the spirit and scope of the present disclosure. 
     The first connecting link  140 , positioned adjacent the switch bracket  172 , includes first and second knobs  142  and  143 . The first and second knobs  142  and  143  are suitably manufactured from a conductive material, such as steel, brass, aluminum, etc., which is detectable by the proximity switches  164 - 170 . The proximity switches are positioned within the switch bracket  172  such that at least the first or second knob  142  and  143  is detectable by a proximity switch  164 ,  166 ,  168 , or  170  when the connecting link  140  is translated by the pull-block assembly  134 . 
     In use, the drive assembly  26  and at least a portion of the frame  22  are positioned beneath a landing, platform, stair, or other building element (not shown). Thus, the lift  20  can be easily incorporated into new and remodel construction without encroaching on the surrounding walls. The lift mechanism  24  and lift platform  30  extend outwardly from the landing, platform, stair, or building element such that they may be reciprocated between lowered and raised positions. Moreover, the lift mechanism  24  is fully receivable within the frame  22  such that the lift  20  is low-profile when in the lowered position. 
     In operation, the drive assembly  26  reciprocates the lift mechanism  24  and lift platform  30  between at least the lowered and raised positions. As shown in  FIG. 6 , the lift platform  30  and lift mechanism  24  are lowered within the frame  22 . To reciprocate the lift  20  into a raised position, the motor  106  is activated to rotate the output sprocket  110  and the drive sprocket  114  in the clockwise direction. The rotation of the drive sprocket  114  causes the screw drive shaft  118  and lead screw  128  to rotate about its longitudinal center axis in the clockwise direction within the screw bearing assembly  121 . 
     With the lift mechanism  24  in the lowered position, the pull-block assembly  134  is positioned coaxially on the lead screw  128  such that the roller bearing assemblies  138  abut the rear end of the longitudinal slots  144 , as shown in  FIG. 6 . As stated above, the lead screw  128  is threadably received within the first and second nuts  130  and  132  of the pull-block assembly  134 . Thus, the rotation of the lead screw  128  within the first and second nuts  130  and  132  of the pull-block assembly  134  threadably translates the pull-block assembly  134  along the lead screw  128 . 
     The second nut  132  is fixed to pull-block assembly  134  at second block  137  by nut flange  200  and screws  300 . The first nut  130  is rotatably fixed to pull-block assembly  134  at first block  136  by nut flange  320  and shoulder screws  310 . First nut  130  is translatably free and a gap between nut flange  320  and first block  136  is maintained by the helix of screw  128 . As second nut  132  wears, the gap closes. When the gap is fully closed, the first nut  130  begins to bear the load of the screw. A switch (not shown) then actuates as the gap closes, and the control system actuates an alarm and/or shuts down the lift, i.e., removes power. Thus, the first nut  130  acts as a safety device. 
     To move the lift mechanism  24  from the lowered position toward the raised position, the lead screw  128  rotates clockwise to translate the pull-block assembly  134  linearly along the lead screw  128  toward the drive sprocket  114 . The roller bearing assemblies  138  and connecting links  140  and  141  are translated along with the pull-block assembly  134 , thereby pulling the inner yokes  154  in a counterclockwise direction and torquing the crankshaft  152  in a counterclockwise direction. The counterclockwise rotation of the crankshaft  152  drives the outer yokes  156  in a counterclockwise direction, thus pulling the pushrod  61  and rotating the second end  46  of the main lift arms  38  clockwise about the lever pivot point  47 . 
     Referring to  FIG. 5 , the clockwise rotation of the second end  46  causes the main lift arms  38  to rotate in a clockwise direction about the lever pivot point  47  and translate in a generally upward direction. The rotation of the main lift arms  38  about the lever pivot point  47  drives the stabilizing arms  62  in an upward direction as pivot  65  is moved upward. As the main lift arms  38  and stabilizing arms  62  are translated upward, the upper ends of the main lift arms  38  pivot and slide forward within the longitudinal slots  78  of the platform frame  36 . Thus, the rod pin ends  72  of the stabilizing arms  62  pivot and slide forward within the longitudinal slots  100  of the frame  22 . Furthermore, the upper ends of the stabilizing arms  62  pivot about the axis defined by the connecting rod  66  and pivots  266  with respect to the platform frame  36 . The simultaneous movement of the arms  38  and  62  drives the platform  30  into the elevated, raised position, while maintaining the platform  30  substantially level. 
     As configured, the drive assembly  26  provides a driving force to the main lift arms  38  without providing such a force directly to the stabilizing arms  62 . Instead, when the main lift arms  38  are reciprocated into the raised or lowered position by the application of the driving force, the stabilizing arms  62  merely travel with the main lift arms  38  as they are connected. The stabilizing arms  62  assist in maintaining the platform  30  in a level position without being driven by the drive assembly  26 . 
     Now referring to  FIG. 6 , the drive assembly  26  may be similarly actuated to reciprocate the lift  20  into a lowered position. The drive assembly  26  is reversed. The lead screw  128  rotates in a counterclockwise direction, thereby translating the pull-block assembly  134  and the connecting links  140  and  141  along the screw  128  towards the lead screw end block  127 . The translation of the connecting links  140  and  141  drives the inner yokes  154  in a clockwise direction, thereby causing the crankshaft  152  to rotate in a clockwise direction. 
     The clockwise rotation of the crankshaft  152  drives the outer yokes  156  in a clockwise direction, and the clockwise rotation of the outer yokes  156  urge the pushrod  61  outwardly toward the lift mechanism  24 , thereby rotating the second end  46 , and the main lift arms  38 , in a counterclockwise direction about the lever pivot point  47 . The rotation of the main lift arms  38  about the lever pivot point  47  drives the stabilizing arms  62  in a downward direction, thereby collapsing the lift mechanism into the lowered position. 
     Referring back to  FIG. 3 , when the lift  20  is reciprocated between the lowered and raised positions, the proximity switches  164 - 170  detect the position of the lift  20 . The proximity switches are suitably electrically connected to the controller (not shown), which regulates power supplied to the drive assembly  26  to maintain the lift mechanism  24  in one of at least multiple positions, such as four positions. 
     When the lift  20  is in the lowered position (as shown in  FIG. 6 ), the pull-block assembly  134  and connecting links  140  and  141  are positioned on the lead screw  128  such that the upper knob  142  of the first connecting link  140  is in substantial alignment with the first proximity switch  164 . The first proximity switch  164  senses the knob  142  and signals the controller that the lift  20  is in the lowered position. 
     When the lift  20  is reciprocated toward the raised position, the actuator  120  translates the pull-block assembly  134  and connecting links  140  and  141  along the lead screw  128  until the upper knob  142  of the first connecting link  140  comes into substantial alignment with the second proximity switch  166 . The second proximity switch  166  senses the upper knob  142  and signals the controller to maintain the lift  20  within a partially raised position. 
     When the lift  20  is translated further into the fully raised position (as shown in  FIG. 5 ), the upper knob  142  comes into substantial alignment with the fourth proximity switch  170 , which signals the controller to maintain the lift  20  in the fully raised position. 
     When drive assembly  26  is activated to lower the lift  20 , the actuator  120  translates the pull-block assembly  134  and connecting links  140  and  141  until the lower knob  143  of the first connecting link  140  substantially aligns the third proximity switch  168  and signals the controller to maintain the lift  20  in a partially lowered position. It should be appreciated that fewer or more proximity switches may be used to control fewer or more positions of the lift mechanism  24 . 
     While the proximity switches  164 - 170  are used to help maintain the position of the lift  20 , a limit switch assembly is used to limit mechanical travel of the lift  20 . Referring to  FIGS. 5 and 6 , the limit switch assembly includes a lever arm  176  and a limit switch  174 . The limit switch  174  is activated and shuts down the lift when the lever arm  176  travels a predetermined distance in the clockwise direction. Thus, when the inner yoke  154   a  is rotated in a counterclockwise direction by the first and second connecting links  140  and  141 , the inner yoke  154   a  engages the lever arm  176  and urges the lever arm  176  in a clockwise direction. 
     If the connecting link  140  translates the inner yoke  154   a  past a predetermined position, the lever arm  176  actuates the switching mechanism within the limit switch  174  to shut down the lift  20 . Thus, the limit switch  174  minimizes the risk of extreme mechanical travel of inner yokes  154 , thereby preventing the crank assembly  28  from reciprocating the lift  20  beyond an extreme raised position. 
     While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. As a non-limiting example, the lift mechanism  24  may be actuated by a reversible rotary drive, a hydraulic actuator, or the like attached to the first pin  54  to reciprocate the platform  30  between the raised and lowered positions. Such an embodiment results in main lift arms  38  that do not include the bent second end  46 . Thus, although in certain embodiments it is desirable that the lift arms  38  act like a lever to reciprocate the platform, other configurations are also within the scope of the present disclosure. As such, it is intended that the claims be construed to include such embodiments. Further, it should be apparent that directional terms, such as clockwise, counterclockwise, upper, lower, inner, outer, etc., are used throughout as a matter of convenience and are not intended to be limiting.