Patent Publication Number: US-2020290853-A1

Title: Scissor lift with offset pins

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 62/819,197, filed Mar. 15, 2019, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Certain aerial work platforms, known as scissor lifts, include a frame assembly that supports a platform. The platform is coupled to the frame assembly using a system of linked supports arranged in a crossed pattern, forming a scissor assembly. As the supports rotate relative to one another, the scissor assembly extends or retracts, raising or lowering the platform relative to the frame. Accordingly, the platform moves primarily or entirely vertically relative to the frame assembly. Scissor lifts are commonly used where scaffolding or a ladder might be used, as they provide a relatively large platform from which to work that can be quickly and easily adjusted to a broad range of heights. Scissor lifts are commonly used for painting, construction projects, accessing high shelves, changing lights, and maintaining equipment located above the ground. 
     SUMMARY 
     One embodiment relates to a lift device including a base, a platform configured to support an operator, and a scissor assembly coupling the base to the platform. The scissor assembly includes a first scissor layer including a first inner arm pivotally coupled to a first outer arm. The first inner arm is configured rotate relative to the first outer arm about a first middle axis. The first scissor layer has a first end axis center point. An actuator is configured to move the platform between a fully raised position and a fully lowered position relative to the base. The first middle axis is offset vertically from the first end axis center point. 
     Another embodiment relates to a lift device including a base, a platform configured to support an operator, and a scissor assembly coupling the base to the platform. The scissor assembly includes a series of scissor layers and an actuator configured to extend and retract the scissor layers to raise and lower the platform relative to the base. Each scissor layer includes (a) an inner arm having an upper end defining a first end axis and a lower end defining a second end axis and (b) an outer arm having an upper end defining a third end axis and a lower end defining a fourth end axis. The inner arm is pivotally coupled to the outer arm such that the outer arm and the inner arm rotate relative to one another about a middle axis. The upper end of the inner arm, the lower end of the inner arm, the upper end of the outer arm, and the lower end of the outer arm are each pivotally coupled to at least one of the base, the platform, and another one of the scissor layers about the first end axis, the second end axis, the third end axis, and the fourth end axis, respectively. An end axis center point is defined for each scissor layer based on the first end axis, the second end axis, the third end axis, and the fourth end axis. A middle pin offset distance is defined for each scissor layer between the end axis center point and the middle axis. The middle pin offset distance is positive when the end axis center point is above the middle axis and negative when the end axis center point is below the middle axis. At least two of the scissor layers have middle pin offset distances that are not equal to zero. The sum of all of the middle pin offset distances is equal to zero. 
     Still another embodiment relates to a lift device including a base, a platform configured to support an operator, a series of scissor sections coupling the base to the platform, and an actuator coupled to at least one of the scissor sections. A first scissor section and a second scissor section of the scissor sections each include a first scissor arm, a second scissor arm, a first bearing member coupled to the first scissor arm and defining a first pin aperture, and a first pin coupled to the second scissor arm and extending into the first pin aperture. The first pin pivotally couples the first scissor arm and the second scissor arm. The first scissor arm has a top surface and a bottom surface. The first pin aperture is positioned one of (a) entirely above the top surface of the first scissor arm and (b) entirely below the bottom surface of the first scissor arm. The actuator is configured to extend and retract the scissor sections to move the platform between a fully raised position and a fully lowered position relative to the base. 
     The invention is capable of other embodiments and of being carried out in various ways. Alternative exemplary embodiments relate to other features and combinations of features as may be recited herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements, in which: 
         FIG. 1  is a perspective view of a lift device, according to an exemplary embodiment; 
         FIG. 2  is a front side view of the lift device of  FIG. 1 ; 
         FIG. 3  is a left side view of the lift device of  FIG. 1 ; 
         FIG. 4  is another left side view of the lift device of  FIG. 1 ; 
         FIG. 5  is a perspective view of a frame and a lift assembly of the lift device of  FIG. 1 , according to an exemplary embodiment; 
         FIG. 6  is another perspective view of the frame and the lift assembly of  FIG. 5 ; 
         FIG. 7  is a perspective view of a platform of the lift device of  FIG. 1  and the lift assembly of  FIG. 5 , according to an exemplary embodiment; 
         FIG. 8  is a side view of the lift assembly of  FIG. 5 ; 
         FIG. 9  is another side view of the lift assembly of  FIG. 5 ; 
         FIG. 10  is another side view of the lift assembly of  FIG. 5 ; 
         FIG. 11  is another side view of the lift assembly of  FIG. 5 ; 
         FIG. 12  is bottom perspective view of the lift assembly of  FIG. 5 ; 
         FIG. 13  is another side view of the lift assembly of  FIG. 5 ; 
         FIG. 14  is a side view of a middle scissor layer of the lift assembly of  FIG. 5  in a partially extended position, according to an exemplary embodiment; 
         FIG. 15  is a side view of the middle scissor layer of  FIG. 14  in a fully retracted position; 
         FIG. 16  is a side view of a bottom scissor layer of the lift assembly of  FIG. 5  in a partially extended position, according to an exemplary embodiment; 
         FIG. 17  is a side view of the bottom scissor layer of  FIG. 16  in a fully retracted position; 
         FIG. 18  is a side view of a top scissor layer of the lift assembly of  FIG. 5  in a partially extended position, according to an exemplary embodiment; 
         FIG. 19  is a side view of the top scissor layer of  FIG. 18  in a fully retracted position; 
         FIG. 20  is a side view of the lift assembly of  FIG. 5  in a fully retracted position; and 
         FIG. 21  is a side view of the lift assembly of  FIG. 5  in a fully extended position. 
     
    
    
     DETAILED DESCRIPTION 
     Before turning to the figures, which illustrate the exemplary embodiments in detail, it should be understood that the present application is not limited to the details or methodology set forth in the description or illustrated in the figures. It should also be understood that the terminology is for the purpose of description only and should not be regarded as limiting. 
     According to an exemplary embodiment, a scissor lift includes a base, a platform configured to support at least one operator, and a lift assembly coupled to the base and the platform and configured to raise and lower the platform relative to the base. The lift assembly includes a series of scissor layers arranged on top of one another. Each scissor layer includes a pair of inner scissor arms pivotally coupled to a pair of outer scissor arms. The inner scissor arms of each scissor layer are pivotally coupled to the outer scissor arms of the adjacent scissor layers. The bottom scissor layer is coupled to the base, and the top scissor layer is coupled to the platform. One or more actuators rotate the scissor arms relative to one another such that the overall length of the scissor assembly changes, raising and lowering the platform. 
     Within each scissor layer, the inner arms are pivotally coupled to the outer arms about a middle axis that extends laterally. If this middle axis is placed in the center of the inner arms and the outer arms, the distance between the bottom ends of the inner and outer arms will be the same as the distance between the top ends of the inner and outer arms. However, placing a pin in this location can have a negative effect on the strength of the inner arms and outer arms. If the lateral axis is offset above or below the center of the inner arms and the outer arms, the distance between the bottom ends of the inner and outer arms will not be the same as the distance between the top ends of the inner and outer arms. This results in longitudinal movement of the platform. This longitudinal movement is undesirable, as it can cause the platform to contact other objects. By way of example, if the scissor lift is placed adjacent a wall, this movement can cause the platform to contact the wall, potentially damaging the wall or the scissor lift. However, offsetting the pin is advantageous, as the reduction in strength caused by placing a pin in the centers of the scissor arms can be avoided. 
     The scissor lift described herein utilizes multiple scissor layers having vertically offset pins. The pins are placed such that the net vertical offset of the pins is zero. By way of example, if two of the pins were each offset downward two inches, another pin would be offset upward four inches. This arrangement prevents the longitudinal movement of the platform while still permitting the pins to be offset, increasing the strength of the scissor arms. 
     According to the exemplary embodiment shown in  FIGS. 1 and 2 , a lift device (e.g., a scissor lift, an aerial work platform, etc.), shown as lift device  10 , includes a chassis or base, shown as frame assembly  12 . A lift device (e.g., a scissor assembly, etc.), shown as lift assembly  14 , couples the frame assembly  12  to a work platform, shown as platform  16 . The frame assembly  12  supports the lift assembly  14  and the platform  16 , both of which are disposed directly above the frame assembly  12 . In use, the lift assembly  14  extends and retracts to raise and lower the platform  16  relative to the frame assembly  12  between a fully lowered position and a fully raised position. The lift device  10  includes an access assembly, shown as an access assembly  20 , that is coupled to the frame assembly  12  and configured to facilitate access to the platform  16  from the ground by an operator when the platform  16  is in the fully lowered position. 
     Referring again to  FIGS. 1 and 2 , the frame assembly  12  defines a horizontal plane having a lateral axis  30  and a longitudinal axis  32 . In some embodiments, the frame assembly  12  is rectangular, defining sides extending parallel to the lateral axis  30  and sides extending parallel to the longitudinal axis  32 . In some embodiments, the frame assembly  12  is longer in a longitudinal direction than in a lateral direction. In some embodiments, the lift device  10  is configured to be stationary or semi-permanent (e.g., a system that is installed in one location at a work site for the duration of a construction project). In such embodiments, the frame assembly  12  may be configured to rest directly on the ground and/or the lift device  10  may not provide powered movement across the ground. In other embodiments, the lift device  10  is configured to be moved frequently (e.g., to work on different tasks, to continue the same task in multiple locations, to travel across a job site, etc.). Such embodiments may include systems that provide powered movement across the ground. 
     The lift device  10  is supported by a plurality of tractive assemblies  40 , each including a tractive element (e.g., a tire, a track, etc.), that are rotatably coupled to the frame assembly  12 . The tractive assemblies  40  may be powered or unpowered. As shown in  FIG. 1 , the tractive assemblies  40  are configured to provide powered motion in the direction of the longitudinal axis  32 . One or more of the tractive assemblies  40  may be turnable or steerable to steer the lift device  10 . In some embodiments, the lift device  10  includes a powertrain system  42 . In some embodiments, the powertrain system  42  includes a primary driver  44  (e.g., an engine, an electric motor, etc.). A transmission may receive mechanical energy from the primary driver and provide an output to one or more of the tractive assemblies  40 . In some embodiments, the powertrain system  42  includes a pump  46  configured to receive mechanical energy from the primary driver  44  and output a pressurized flow of hydraulic fluid. The pump  46  may supply mechanical energy (e.g., through a pressurized flow of hydraulic fluid) to individual motive drivers (e.g., hydraulic motors) configured to facilitate independently driving each of the tractive assemblies  40 . In other embodiments, the powertrain system  42  includes an energy storage device (e.g., a battery, capacitors, ultra-capacitors, etc.) and/or is electrically coupled to an outside source of electrical energy (e.g., a power outlet connected to a power grid). In some such embodiments, one or more of the tractive assemblies  40  include an individual motive driver (e.g., a motor that is electrically coupled to the energy storage device, a hydraulic motor fluidly coupled to the pump  46  etc.) configured to facilitate independently driving one or more of the tractive assemblies  40 . The outside source of electrical energy may charge the energy storage device or power the motive drivers directly. The powertrain system  42  may additionally or alternatively provide mechanical energy (e.g., using the pump  46 , by supplying electrical energy, etc.) to one or more actuators of the lift device  10  (e.g., a leveling actuator, the lift actuator  200 , etc.). One or more components of the powertrain system  42  may be housed in an enclosure, shown as housing  48 . The housing  48  is coupled to the frame assembly  12  and extends from a side of the lift device  10  (e.g., a left or right side). The housing  48  may include one or more doors to facilitate access to components of the powertrain system  42 . 
     Referring to  FIG. 1 , the platform  16  includes a support surface, shown as deck  60 , defining a top surface configured to support operators and/or equipment and a bottom surface opposite the top surface. The bottom surface and/or the top surface extend in a substantially horizontal plane. A thickness of the deck  60  is defined between the top surface and the bottom surface. The bottom surface is coupled to a top end of the lift assembly  14 . In some embodiments, the deck  60  is rectangular. In some embodiments, the deck  60  has a footprint that is substantially similar to that of the frame assembly  12 . 
     A series of guards or railings, shown as guard rails  62 , extend upwards from the deck  60 . The guard rails  62  extend around an outer perimeter of the deck  60 , partially or fully enclosing a supported area on the top surface of the deck  60  that is configured to support operators and/or equipment. The guard rails  62  provide a stable support for the operators to hold and facilitate containing the operators and equipment within the supported area. The guard rails  62  define one or more openings  64  through which the operators can access the deck  60 . The opening  64  may be a space between two guard rails  62  along the perimeter of the deck  60 , such that the guard rails  62  do not extend over the opening  64 . Alternatively, the opening  64  may be defined in a guard rail  62  such that the guard rail  62  extends across the top of the opening  64 . In some embodiments, the platform  16  includes a door that selectively extends across the opening  64  to prevent movement through the opening  64 . The door may rotate (e.g., about a vertical axis, about a horizontal axis, etc.) or translate between a closed position and an open position. In the closed position, the door prevents movement through the opening  64 . In the open position, the door does not prevent movement through the opening  64 . 
     The access assembly  20  is coupled to a side of the frame assembly  12 . As shown in  FIG. 2 , the access assembly  20  is a ladder assembly. The access assembly  20  is aligned with the opening  64  such that, when the platform  16  is in the lowered position, the access assembly  20  facilitates access to the upper surface of the deck  60  through the opening  64 . 
     The lift assembly  14  is configured to extend and retract, raising and lowering the platform  16  relative to the frame assembly  12 . The lift assembly  14  is selectively repositionable between a fully retracted position and a fully extended position. The fully retracted position corresponds to a fully lowered position of the platform  16 . The fully lowered position may be used by an operator when entering or exiting the platform  16  (e.g., using the access assembly  20 ) or when transporting the lift device  10 . The fully extended position corresponds to a fully raised position of the platform  16 . The fully raised position and any positions between the fully raised position and the fully lowered position may be used by the operator when accessing an elevated area (e.g., to perform construction work, to visually inspect an elevated object, etc.). 
     Referring to  FIGS. 1-4 , the lift assembly  14  includes a series of subassemblies, shown as scissor layers. Specifically, the lift assembly  14  includes a first scissor section, shown as bottom scissor layer  100 , a pair of second scissor sections, shown as middle scissor layers  102  and  104 , and a third scissor section, shown as top scissor layer  106 . In other embodiments, the lift assembly  14  includes more or fewer middle scissor layers (e.g., zero, three, etc.). The bottom scissor layer  100  is directly coupled to the frame assembly  12  and to the middle scissor layer  102 . The middle scissor layer  102  is directly coupled to the bottom scissor layer  100  and the middle scissor layer  104 . The middle scissor layer  104  is directly coupled to the middle scissor layer  102  and the top scissor layer  106 . The top scissor layer  106  is directly coupled to the platform  16  and to the middle scissor layer  104 . 
     Each of the scissor layers includes a pair of first scissor arms or scissor members (e.g., tubular members, solid members, etc.), shown as inner arms, and a pair of second scissor arms or scissor members (e.g., tubular members, solid members, etc.), shown as outer arms. Each inner arm is coupled (e.g., fixedly) to the other inner arm within that scissor layer. Each outer arm is coupled (e.g., fixedly) to the other outer arm within that scissor layer. The inner arms of each scissor layer are pivotally coupled (e.g., by one or more pins or rods) to the corresponding outer arms of that scissor layer near the centers of both the inner arms and the outer arms. Accordingly, the inner arms of each layer pivot relative to the outer arms of that scissor layer about a lateral axis. Specifically, the bottom scissor layer  100  includes inner arms  110  and outer arms  112  that pivot relative to one another about a lateral axis, shown as middle axis  114 . The middle scissor layer  102  includes inner arms  120  and outer arms  122  that pivot relative to one another about a lateral axis, shown as middle axis  124 . The middle scissor layer  104  includes inner arms  130  and outer arms  132  that pivot relative to one another about a lateral axis, shown as middle axis  134 . The top scissor layer  106  includes inner arms  140  and outer arms  142  that pivot relative to one another about a lateral axis, shown as middle axis  144 . 
     The scissor layers are stacked atop one another to form the lift assembly  14 . Each pair of inner arms and each pair of outer arms has a top end and a bottom end. The ends of the inner arms and the outer arms are pivotally coupled (e.g., by one or more pins or rods) to the adjacent ends of the inner or outer arms of the adjacent scissor layers. Each set of inner arms is directly pivotally coupled to one or more sets of outer arms. This facilitates spacing each pair of inner arms a first distance apart from one another and spacing each pair of outer arms a second distance apart from one another, where the second distance is greater than the first distance. This facilitates ensuring that the fully lowered position is as low as possible, increasing the accessibility of the platform  16  and making the lift device  10  more compact. 
     The upper ends of the outer arms  112  are pivotally coupled to the lower ends of the inner arms  120  such that they rotate relative to one another about a lateral axis, shown as end axis  150 . The upper ends of the inner arms  110  are pivotally coupled to the lower ends of the outer arms  122  such that they rotate relative to one another about another end axis  150 . The upper ends of the outer arms  122  are pivotally coupled to the lower ends of the inner arms  130  such that they rotate relative to one another about a lateral axis, shown as end axis  152 . The upper ends of the inner arms  120  are pivotally coupled to the lower ends of the outer arms  132  such that they rotate relative to one another about another end axis  152 . The upper ends of the outer arms  132  are pivotally coupled to the lower ends of the inner arms  140  such that they rotate relative to one another about a lateral axis, shown as end axis  154 . The upper ends of the inner arms  130  are pivotally coupled to the lower ends of the outer arms  142  such that they rotate relative to one another about another end axis  154 . 
     Referring to  FIG. 5 , the lower ends of the inner arms  110  are pivotally coupled to the frame assembly  12  such that the inner arms  110  rotate relative to the frame assembly  12  about a lateral axis, shown as end axis  160 . The end axis  160  is fixed to the frame assembly  12  such that the lower ends of the inner arms  110  are translationally fixed relative to the frame assembly  12 . A pair of bosses, shown as bearing blocks  162 , are coupled (e.g., welded, fastened, etc.) to the frame assembly  12 . The bearing blocks  162  are each configured to receive a rod or pin, shown as pin  164 . The bearing blocks  162  and the pins  164  may be configured to facilitate rotation of the pins  164  about the end axis  160 . The pins  164  each extend along the end axis  160  through one of the bearing blocks  162  and the corresponding inner arms  110 . The pins  164  and the bearing blocks  162  pivotally couple the inner arms  110  to the frame assembly  12 . 
     Referring to  FIG. 6 , the lower ends of the outer arms  112  are pivotally and slidably coupled to the frame assembly  12  such that the outer arms  112  rotate relative to the frame assembly  12  about a lateral axis, shown as end axis  170 . The end axis  170  is translatable longitudinally relative to the frame assembly  12  such that the lower ends of the outer arms  112  are slidable longitudinally relative to the frame assembly  12 . A tubular member, shown as rod  172 , extends laterally between both of the outer arms  112 . The rod  172  is coupled (e.g., welded, fastened, etc.) to the outer arms  112 . The rod  172  further extends laterally outside of the outer arms  112 . Each end of the rod  172  is received within an aperture defined by a block, shown as sliding block  174 . The sliding blocks  174  are accordingly pivotally coupled to the rod  172 . A pair of frame members, shown as channels  176  are coupled to (e.g., fastened to, welded to, integrally formed with, etc.) the frame assembly  12 . The channels  176  extend longitudinally along the frame assembly  12 . The channels  176  each define a recess  178  that receives the sliding block  174 . Each of the recesses  178  face toward a longitudinal centerline of the lift device  10  such that the sliding blocks  174  are captured laterally by the channels  176 . The sliding blocks  174  are free to translate longitudinally along the channels  176  to permit pivoting of the outer arms  112  relative to the inner arms  110 . 
     Referring to  FIG. 3 , the upper ends of the outer arms  142  are pivotally coupled to the deck  60  of the platform  16  such that the outer arms  142  rotate relative to the deck  60  about a lateral axis, shown as end axis  180 . The end axis  180  is fixed to the platform  16  such that the upper ends of the outer arms  142  are translationally fixed relative to the platform  16 . In one embodiment, a pair of pins couple the outer arms  142  to the platform  16 . The pins may each extend along the end axis  180  through one of the outer arms  142  and a portion of the deck  60 . 
     Referring to  FIG. 7 , the upper ends of the inner arms  140  are pivotally and slidably coupled to the deck  60  of the platform  16  such that the inner arms  140  rotate relative to the deck  60  about a lateral axis, shown as end axis  190 . The end axis  190  is translatable longitudinally relative to the platform  16  such that the upper ends of the inner arms  140  are slidable longitudinally relative to the platform  16 . A tubular member, shown as rod  192 , extends laterally between both of the inner arms  140 . The rod  192  is coupled (e.g., welded, fastened, etc.) to the inner arms  140 . The rod  192  further extends laterally outside of the inner arms  140 . Each end of the rod  192  is received within an aperture defined by a block, shown as sliding block  194 . The sliding blocks  194  are accordingly pivotally coupled to the rod  192 . A pair of frame members, shown as channels  196  are coupled (e.g., fastened, welded, integrally formed with, etc.) to the frame assembly  12 . The channels  196  extend longitudinally along the platform  16 . The channels  196  each define a recess  198  that receives the sliding block  194 . Each of the recesses  198  face toward a longitudinal centerline of the lift device  10  such that the sliding blocks  194  are captured laterally by the channels  196 . The sliding blocks  194  are free to translate longitudinally along the channels  196  to permit pivoting of the inner arms  140  relative to the outer arms  142 . 
     An actuator (e.g., a hydraulic cylinder, a pneumatic cylinder, a motor-driven leadscrew, etc.), shown as lift actuator  200 , is configured to extend and retract the lift assembly  14 . As shown in  FIG. 1 , the lift assembly  14  includes one lift actuator  200 , and the lift actuator  200  is a hydraulic cylinder fluidly coupled to the pump  46 . The lift actuator  200  is pivotally coupled to the inner arms  110  at one end (e.g., a cap end) and pivotally coupled to the inner arms  130  at the opposite end (e.g., a rod end). In other embodiments, the lift assembly  14  includes more or fewer lift actuators  200  and/or the lift actuator  200  is otherwise arranged. The lift actuator  200  is configured to selectively reposition the lift assembly  14  between the fully extended and fully retracted positions. In some embodiments, extension of the lift actuator  200  moves the platform  16  vertically upward (extending the lift assembly  14 ), and retraction of the lift actuator  200  moves the platform  16  vertically downward (retracting the lift assembly  14 ). In other embodiments, extension of the lift actuator  200  retracts the lift assembly  14 , and retraction of the lift actuator  200  extends the lift assembly  14 . The lift device  10  may include various components configured to drive the lift actuator  200  (e.g., pumps, valves, compressors, motors, batteries, voltage regulators, etc.). 
     Referring to  FIGS. 8-13 , the scissor arms are coupled to one another by a series of pins. Each of the pins extends through a laterally extending aperture. The laterally extending apertures are centered about and extend parallel to the end and middle axes described herein (e.g., the end axes  150 , the middle axis  114 , etc.). As shown in  FIG. 8 , a bearing member, shown as middle bushing  210 , extends through and is coupled to the outer arm  132 . The middle bushing  210  defines an aperture, shown as middle pin aperture  212 . The inner arm  130  utilizes a similar middle bushing  210 . The middle pin aperture  212  receives a rod or pin, shown as middle pin  214 . The middle pin  214  also extends through the middle pin aperture  212  corresponding to the inner arm  130 , pivotally coupling the inner arm  130  and the outer arm  132 . One or more retraining members (e.g., retaining rings, machined shoulders, clamping collars, fasteners, etc.), shown as snap rings  216 , limit the lateral movement of the middle pin  214  relative to the inner arm  130  and the outer arm  132 . The middle bushing  210 , the middle pin aperture  212 , and the middle pin  214  are centered about and extend parallel to (e.g., are aligned with) the middle axis  134 . The outer arm  132  has a height Hi defined between a top surface  218  and a bottom surface  219  of the outer arm  132 . The middle axis  134  is offset a distance D 1  below the top surface  218  of the outer arm  132 . The distance D 1  is approximately half of the height H 1  such that the middle axis  134  is substantially vertically centered on the outer arm  132 . The middle axis  134  is similarly centered on the inner arm  130 . The other outer arm  132  and inner arm  130  may utilize a similar bushing and pin arrangement. The scissor arms of each middle scissor layer (e.g., the middle scissor layer  102 , the middle scissor layer  104 ) utilize middle bushings  210  and middle pins  214  positioned in this way to pivotally couple the outer and inner arms. 
     As shown in  FIGS. 9 and 10 , a bearing member (e.g., a roller bearing, a ball bearing, a bushing, etc.), shown as upper bushing  220 , extends through and is coupled to an upper end portion of the outer arm  132 . The upper bushing  220  defines an aperture, shown as upper pin aperture  222 . The upper end portion of the inner arm  120  includes a similar upper bushing  220 . A bearing member, shown as lower bushing  224 , extends through and is coupled to a lower end portion of the outer arm  132 . The lower bushing  224  defines an aperture, shown as lower pin aperture  226 . The lower end portion of the inner arm  140  includes a similar lower bushing  224 . The upper pin aperture  222  and the lower pin aperture  226  are each configured to receive a rod or pin, shown as end pin  228 . An end pin  228  extends through both the upper bushing  220  of the outer arm  132  and the lower bushing  224  of the inner arm  140 , pivotally coupling the outer arm  132  and the inner arm  140 . Another end pin  228  extends through both the lower bushing  224  of the outer arm  132  and the upper bushing  220  of the inner arm  120 , pivotally coupling the outer arm  132  and the inner arm  120 . Additional snap rings  216  limit the lateral movement of the end pins  228  relative to the outer arm  132 , the inner arm  120 , and the inner arm  140 . 
     The upper bushing  220 , the upper pin aperture  222 , and the corresponding end pin  228  are centered about and extend parallel to (e.g., are aligned with) the end axis  154 . The lower bushing  224 , the lower pin aperture  226 , and the corresponding end pin  228  are centered about and extend parallel to (e.g., are aligned with) the end axis  152 . The end axis  154  is offset a distance D 2  below the top surface  218  of the outer arm  132 . The distance D 2  is less than the distance D 1  such that the end axis  154  is positioned above the center of the outer arm  132 . The end axis  152  is offset a distance D 3  below the top surface  218  of the outer arm  132 . The distance D 3  is greater than the distance Di such that the end axis  154  is positioned below the center of the outer arm  132 . In some embodiments, the end axis  154  and the end axis  152  are approximately equidistant from the middle axis  134  (e.g., D 3 −D 1 =D 1 −D 2 ). In some embodiments, the middle bushing  210 , the middle pin aperture  212 , the middle  214 , the upper bushing  220 , the upper pin aperture  222 , the lower bushing  224 , the lower pin aperture  226 , and/or the end pins  228  are positioned entirely between the top surface  218  and the bottom surface  219  of the outer arm  132 . The upper and lower ends of each of the inner arms  120 , the outer arms  122 , the inner arms  130 , and the outer arms  132  each utilize this pivotal coupling arrangement. The lower ends of the inner arms  140  and the outer arms  142  utilize this pivotal coupling arrangement. The upper ends of the inner arms  110  and the outer arms  112  utilize this pivotal coupling arrangement. Offsetting the end pins  228  of the upper ends upward and offsetting the end pins  228  of the lower ends downward facilitates positioning the scissor arms closer to a horizontal orientation when in the fully retracted position, reducing the height of the lift assembly  14  in the fully retracted position. 
     Referring to  FIGS. 11 and 12 , a pair of supports, shown as side plates  240  are each coupled (e.g., welded, fastened, etc.) to opposite sides of the outer arm  112 . The side plates  240  extend below the outer arm  112 . A bearing member, shown as bottom middle bushing  242 , extends through and is coupled to the side plates  240 . The bottom middle bushing  242  defines an aperture, shown as bottom middle pin aperture  244 . The inner arm  110  utilizes a similar set of side plates  240  and a similar bottom middle bushing  242 . The bottom middle pin aperture  244  receives a rod or pin, shown as bottom middle pin  246 . The bottom middle pin  246  also extends through the bottom middle pin aperture  244  of the corresponding bottom middle bushing  242  of the inner arm  110 , pivotally coupling the inner arm  110  and the outer arm  112 . One or more retraining members (e.g., retaining rings, machined shoulders, clamping collars, fasteners, etc.), may be coupled to the bottom middle pin  246  to limit the lateral movement of the bottom middle pin  246  relative to the inner arm  110  and the outer arm  112 . The bottom middle bushing  242 , the bottom middle pin aperture  244 , and the bottom middle pin  246  are centered about and extend parallel to (e.g., are aligned with) the middle axis  114 . The outer arm  112  has a height H 2  defined between a top surface  250  and a bottom surface  252  of the outer arm  112 . The middle axis  114  is offset a distance D 4  below the top surface  250  of the outer arm  112 . The distance D 4  is greater than the height H 1  such that the middle axis  114  is vertically below the bottom surface  252 . The bottom middle bushing  242 , the bottom middle pin aperture  244 , and/or the bottom middle pin  246  are positioned entirely below the bottom surface  252 . Accordingly, the bottom middle bushing  242 , the bottom middle pin aperture  244 , and/or the bottom middle pin  246  do not extend through the outer arm  112 . This pivotal coupling arrangement may increase the strength of the outer arm  112  (e.g., relative to the outer arm  122 ), because no holes are required through the outer arm  112 . The bottom middle bushing  242  is similarly positioned on the inner arm  110 . The other outer arm  112  and inner arm  110  may utilize a similar bushing and pin arrangement. 
     Referring to  FIG. 13 , a pair of supports, shown as side plates  260  are each coupled (e.g., welded, fastened, etc.) to opposite sides of the outer arm  142 . The side plates  260  extend above the outer arm  142 . A bearing member, shown as top middle bushing  262 , extends through and is coupled to the side plates  260 . The top middle bushing  262  defines an aperture, shown as top middle pin aperture  264 . The inner arm  140  includes similar set of side plates  260  and a similar top middle bushing  262 . The top middle pin aperture  264  receives a rod or pin, shown as top middle pin  266 . The top middle pin  266  also extends through the top middle pin aperture  264  of the corresponding top middle bushing  262  of the inner arm  140 , pivotally coupling the inner arm  140  and the outer arm  142 . One or more retraining members (e.g., retaining rings, machined shoulders, clamping collars, fasteners, etc.), may be coupled to the top middle pin  266  to limit the lateral movement of the top middle pin  266  relative to the inner arm  140  and the outer arm  142 . The top middle bushing  262 , the top middle pin aperture  264 , and the top middle pin  266  are centered about and extend parallel to (e.g., are aligned with) the middle axis  144 . The outer arm  142  has a height H 3  defined between a top surface  270  and a bottom surface  272  of the outer arm  142 . The middle axis  144  is offset a distance D 5  above the top surface  270  of the outer arm  142 . The top middle bushing  262 , the top middle pin aperture  264 , and/or the top middle pin  266  are positioned entirely above the top surface  270 . Accordingly, the top middle bushing  262 , the top middle pin aperture  264 , and/or the top middle pin  266  do not extend through the outer arm  142 . This pivotal coupling arrangement may increase the strength of the outer arm  142  (e.g., relative to the outer arm  122 ), because no holes are required through the outer arm  142 . The top middle bushing  262  is similarly positioned on the inner arm  140 . The other outer arm  142  and inner arm  140  may utilize a similar bushing and pin arrangement. 
     A point, referred to herein as an end axis center point, is defined for each of the scissor layers. The end axis center point is a point centered between each of the end axes corresponding to that scissor layer. The end axis center point of a scissor layer is defined by (a) within a plane perpendicular to the lateral axis  30 , defining (e.g., drawing) a first straight line between the end axes of the inner arms of that scissor layer and (b) within the plane, defining a second straight line between the end axes of the outer arms of that scissor layer. The point at which these two lines intersect is the end axis center point. By way of example, the end axis center point for the middle scissor layer  102  is shown in  FIG. 14 . To locate the end axis center point, a first straight line is drawn between the end axis  150  and the end axis  152  of the inner arms  120 . A second straight line is drawn between the end axis  150  and the end axis  152  of the outer arms  122 . The end axis center point for the middle scissor layer  102 , shown as point C 2 , is the point where these two lines intersect. Using a similar process, the end axis center points of the bottom scissor layer  100 , the middle scissor layer  104 , and the top scissor layer  106  can be located. The end axis center points of the bottom scissor layer  100 , the middle scissor layer  104 , and the top scissor layer  106  are shown in  FIGS. 14-21  as point C 1 , point C 3 , and point C 4 , respectively. 
       FIG. 14  illustrates the middle scissor layer  102  in a partially extended position, and  FIG. 15  illustrates the middle scissor layer  102  in the fully retracted position. The end axis center point C 2  is positioned along the middle axis  124  such that there is no offset between the end axis center point C 2  and the middle axis  124  (i.e., OffsetMP 2 =0). A longitudinal distance L 1  is shown between the end axes  150 , and a longitudinal distance L 2  is shown between the end axes  152 . Due to the relative positioning of the end axis center point C 2  and the middle axis  124 , as the lift assembly  14  moves from the fully retracted position to the fully extended position, the distance L 1  and the distance L 2  decrease at an equal rate. Accordingly, the distance L 1  and the distance L 2  are equal in all positions of the middle scissor layer  102 . Similarly, within the middle scissor layer  104 , the end axis center point C 3  is positioned along the middle axis  134  (i.e., OffsetMP 3 =0). 
       FIG. 16  illustrates the bottom scissor layer  100  in a partially extended position, and  FIG. 17  illustrates the bottom scissor layer  100  in the fully retracted position. The end axis center point C 1  is offset a distance OffsetMP 1  vertically above the middle axis  114  (i.e., OffsetMP 1 &gt;0). A longitudinal distance L 1  is shown between the end axis  160  and the end axis  170 , and a longitudinal distance L 2  is shown between the end axes  150 . As the lift assembly  14  moves from the fully retracted position toward the fully extended position, the distance L 1  and the distance L 2  decrease. Due to the relative positioning of the end axis center point C 1  and the middle axis  114 , the distance L 2  decreases more rapidly than the distance L 1 . Accordingly, while the distance L 1  and the distance L 2  may be equal in the fully retracted position, the distance L 1  is greater than the distance L 2  in the partially extended position. 
       FIG. 18  illustrates the top scissor layer  106  in a partially extended position, and  FIG. 19  illustrates the top scissor layer  106  in the fully retracted position. The end axis center point C 4  is offset a distance OffsetMP 4  vertically below the middle axis  144  (i.e., OffsetMP 4 &lt;0). A longitudinal distance L 1  is shown between the end axes  154 , and a longitudinal distance L 2  is shown between the end axis  180  and the end axis  190 . As the lift assembly  14  moves from the fully retracted position toward the fully extended position, the distance L 1  and the distance L 2  decrease. Due to the relative positioning of the end axis center point C 4  and the middle axis  144 , the distance L 1  decreases more rapidly than the distance L 2 . Accordingly, while the distance L 1  and the distance L 2  may be equal in the fully retracted position, the distance L 1  is less than the distance L 2  in the partially extended position. 
     Referring to  FIG. 20 , the distances between the end axes of each inner arm and each outer arm are substantially equal. By way of example, (a) the distance between the end axis  180  and the end axis  154  of the outer arm  142 , (b) the distance between the end axis  152  and the end axis  150  of the outer arm  122 , and (c) the distance between the end axis  160  and the end axis  150  of the inner arm  110  are all substantially equal. Because these distances are all equal, the magnitude of each middle pin offset distance (i.e., |OffsetMP|) determines the angle between the corresponding inner arms and outer arms of that scissor layer. As shown in  FIGS. 14, 16, 18 , and  21 , an angle θ is defined between the straight lines used to define the end axis center point. Specifically, the bottom scissor layer  100  has an angle θ 1 , the middle scissor layer  102  has an angle θ 2 , the middle scissor layer  104  has an angle θ 3 , and the top scissor layer  106  has an angle θ 14 . In the embodiment shown in  FIG. 21 , the middle pin offset distances of the middle scissor layer  102  and the middle scissor layer  104  are both zero (i.e., OffsetMP 2 =OffsetMP 3 =0). Accordingly, the angles of the middle scissor layer  102  and the middle scissor layer  104  are equal (i.e., θ 2 =θ 3 ). The middle pin offset distances of the bottom scissor layer  100  and the top scissor layer  106  have equal magnitudes (i.e., |OffsetMP 1 |=|OffsetMP 4 |). Accordingly, the angles of the bottom scissor layer  100  and the top scissor layer  106  are equal (i.e., θ 1 =θ 4 ). 
     The lift assembly  14  is shown in the fully retracted position in  FIG. 20 . In this embodiment, the end axes are vertically aligned with one another in the fully retracted position. Specifically, a first vertical line can be drawn through the middle axis  114 , the middle axis  124 , the middle axis  134 , the middle axis  144 , and the each of the end axis center points. In this embodiment, the end axes are vertically aligned with one another in the fully retracted position. Specifically, a second vertical line can be drawn through the end axis  180 , the end axis  154 , the end axis  152 , the end axis  150 , and the end axis  160  on one side of the lift assembly  14 , and a third vertical line can be drawn through the end axis  190 , the end axis  154 , the end axis  152 , the end axis  150 , and the end axis  170  on the other side of the lift assembly  14 . 
     Referring to  FIG. 21 , the lift assembly  14  is shown in the fully extended position. In this embodiment, the middle axes are all vertically aligned with one another. However, the end axes are not all vertically aligned with one another. The end axis  160  and the end axis  180  are aligned with one another. The end axis  150 , the end axis  152 , and the end axis  154  are also vertically aligned with one another. However, the end axis  150 , the end axis  152 , and the end axis  154  are offset longitudinally inward from the end axis  180  and the end axis  190 . This variation in vertical alignment is due to the variation in middle pin offset distances (i.e., OffsetMP) between each scissor layer. In the bottom scissor layer  100 , the end axis center point C 1  is offset above the middle axis  114  (i.e., OffsetMP 1 &gt;0), so the end axis  150  is offset longitudinally inward from the end axis  160 . In the middle scissor layer  102  and the middle scissor layer  104 , the end axis center point C 2  and the end axis center point C 3  are vertically aligned with the middle axis  124  and the middle axis  134 , respectively (i.e., OffsetMP 2 =OffsetMP 3 =0). Accordingly, the end axis  150 , the end axis  152 , and the end axis  154  are all in the same longitudinal position. In the top scissor layer  106 , the end axis center point C 4  is offset below the middle axis  144  (i.e., OffsetMP 4 &lt;0), so the end axis  180  is offset longitudinally inward from the end axis  154 . As shown in  FIG. 21 , the middle pin offset distances of the top scissor layer  106  and the bottom scissor layer  100  have equal magnitudes (i.e., |OffsetMP 1 |=|OffsetMP 4 |). Specifically, the middle pin offset distances of the top scissor layer  106  and the bottom scissor layer  100  have equal magnitudes but are offset in opposite directions (i.e., OffsetMP 1 +OffsetMP 4 =0). Accordingly, the longitudinal offsets caused by the top scissor layer  106  and the bottom scissor layer  100  cancel one another out, keeping the end axis  160  and the end axis  180  vertically aligned. 
     When using a scissor lift, a purely vertical movement of the platform is desired by the user. This type of movement is typically what a user expects when using a scissor lift, and the user will typically set the scissor lift up in a location according to this assumption. Accordingly, any longitudinal movement of the platform may be considered undesirable by the user. By way of example, the user may place the scissor lift up against a wall of a structure. If the platform were to move longitudinally toward the wall, the platform could contact the wall, causing damage to the wall and/or the lift device. 
     The lift assembly  14  is configured to eliminate any longitudinal movement of the platform  16 . The frame assembly  12  is longitudinally fixed to the end axis  160 , and the platform  16  is longitudinally fixed to the end axis  180 . Accordingly, if the end axis  180  were to move longitudinally relative to the end axis  160 , the platform  16  would also move longitudinally the same distance. However, because the middle pin offset distances of the top scissor layer  106  and the bottom scissor layer  100  are equal, the platform  16  moves purely vertically. This arrangement permits the increased strength from offsetting the middle pins without introducing longitudinal movement to the platform  16 . 
     In other embodiments, the middle pin offset distances of the top scissor layer  106  and the bottom scissor layer  100  are not equal and opposite. Additionally or alternatively, one or more of the middle scissor layers may include offset middle pins. The lift assembly  14  may additionally or alternatively include more or fewer middle sections. In such embodiments, the middle pins of each scissor layer are arranged such that the sum of all of the middle pin offset distances is equal to zero. This may be relationship may be represented by the following expression: 
       Offset MP   1 +Offset MP   2 + . . . +Offset MP   n =0  (1)
 
     where n is equal to the total number of scissor layers within the lift assembly  14  (e.g., n=(the number of middle scissor layers)+2). In this arrangement, if the distances between the end axes of all of the inner arms and the outer arms are substantially equal, any offset in longitudinal position of the platform  16  caused by offsetting the middle pin of one of the scissor layers is nullified by the offsets introduced by one or more other layers. 
     In some embodiments, the middle pin offset distances of the top scissor layer  106  and the bottom scissor layer  100  are equal to zero, and middle pin offset distances of the middle scissor layer  102  and the middle scissor layer  104  have equal magnitudes but are offset in opposite directions (i.e., OffsetMP 2 =OffsetMP 3 ; OffsetMP 1 =OffsetMP 4 =0). In other embodiments, the middle pin offset distances of each of the scissor layers are not equal to zero (e.g., OffsetMP 1 =−3 in; OffsetMP 2 =5 in; OffsetMP 3 =2 in; OffsetMP 4 =−4 in). In yet other embodiments, the middle pin offset distances are otherwise configured such that the sum of the middle pin offset distances is equal to zero (e.g., OffsetMP 1 =−5 in; OffsetMP 2 =5 in; OffsetMP 3 =0 in; OffsetMP 4 =−2 in; OffsetMP 5 =2 in; OffsetMP 6 =0 in). 
     In other embodiments, different parts of the lift assembly  14  are translationally fixed relative to the frame assembly  12  and/or the platform  16 . By way of example, the end axis  160  may be free to translate relative to the frame assembly  12 , and the end axis  170  may be fixed relative to the frame assembly  12 . By way of another example, the end axis  180  may be free to translate relative to the platform  16 , and the end axis  190  may be fixed relative to the platform  16 . In such embodiments, the platform  16  will not move longitudinally if the lift assembly  14  satisfies Equation 1. 
     As utilized herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims. 
     It should be noted that the terms “exemplary” and “example” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     The terms “coupled,” “connected,” and the like, as used herein, mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent, etc.) or moveable (e.g., removable, releasable, etc.). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. 
     References herein to the positions of elements (e.g., “top,” “bottom,” “above,” “below,” “between,” etc.) are merely used to describe the orientation of various elements in the figures. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure. 
     Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, Z, X and Y, X and Z, Y and Z, or X, Y, and Z (i.e., any combination of X, Y, and Z). Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y, and at least one of Z to each be present, unless otherwise indicated. 
     It is important to note that the construction and arrangement of the systems as shown in the exemplary embodiments is illustrative only. Although only a few embodiments of the present disclosure have been described in detail, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements. It should be noted that the elements and/or assemblies of the components described herein may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be included within the scope of the present inventions. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the preferred and other exemplary embodiments without departing from scope of the present disclosure or from the spirit of the appended claims.