Abstract:
An electric actuator replaces the hydraulic reach actuator in a reach carriage for a reach truck. The electric actuator may be a linear actuator and may have a rotary motor and a combination ball screw and ball assembly for converting the rotary motion of the motor into linear motion to extend and retract a rod. A force limiter can be disposed between the motor and the ball screw, and may be coupled to the motor and the ball screw. The force limiter prevents a contact force on the rod from damaging the motor. The force limiter allows the reach carriage to operate normally until a contact force occurs, then disengages if a threshold level of force is exceeded. The force limiter may be a torque limiter that automatically reengages after the contact force has attenuated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION 
       [0003]    The invention relates to the field of vehicles having extending reach carriages, and more specifically to driving the extension arm of a reach carriage with an electric actuator that is protected against back-driving contact force. 
       BACKGROUND OF THE INVENTION 
       [0004]    Lift trucks, also known as forklifts, are commonly used to raise and position heavy loads on elevated surfaces. Some lift trucks include a reach carriage that extends horizontally away from the base or operating unit of the lift truck to “reach” a position that is at or within the reach carriage&#39;s limit of extension. Such lift trucks are referred to herein as “reach trucks.” A reach truck&#39;s reach carriage may be extended in various ways according to different models of a reach truck. In some models, the reach carriage is attached to the mast of the reach truck, and the mast extends away from and retracts toward the operating unit. The operator operates one or more hydraulic actuators to move the mast and reach carriage. In other modes, the reach carriage includes a pantographic extension arm connected between the mast and the fork assembly. Extension and retraction of the extension arm is conducted using a hydraulic ram cylinder attached between the mast and a pivot point of the extension arm. The extension arm extends as the cylinder fills with fluid, and retracts as the cylinder empties. 
         [0005]    In an effort to increase productivity, reach truck operators conduct their reach trucks in a very fluid manner, one operation seamlessly flowing into the next. An example of such a maneuver is the pallet scoop, in which an operator will approach a pallet with the reach carriage extended and, without stopping, engage and lift the pallet. The reach carriage essentially contacts with the pallet, imparting a contact force upon the extension arm and cylinder that can damage or reduce the life of the cylinder and other components. Typically, hydraulic cylinders are fitted with bypass valves that open when the pressure, such as excessive pressure imparted by the contact force, exceeds a certain threshold. This allows the extension arm to collapse to a retracted position without damaging the cylinder or the extension arm. 
         [0006]    Like all hydraulic actuators, hydraulic reach actuators are subject to inefficiencies. For example, hydraulic actuation requires a pump to pressurize and depressurize the cylinder, and fluid hoses to transfer hydraulic fluid between the pump and cylinder. The pump draws current and takes up space in the reach truck, which is desirably compact. The hoses must be routed between the components, and the route must consider that the reach carriage may have a vertical range of several meters. Hoses are typically routed over the reach truck&#39;s mast, adding hose length and orientation variations that may reduce hydraulic efficiency. 
         [0007]    Hydraulic efficiency advantages can be achieved by replacing the hydraulic actuators on a reach carriage with electric actuators. It would therefore be desirable to have an electric reach actuator that incorporates an electric actuator having a force limiter to operate the reach carriage. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention overcomes the drawbacks of the previous reach carriage actuators by eliminating the need for a hydraulic pump, cylinder and hydraulic hoses. 
         [0009]    A typical linear electric actuator is configured to resist back-driving of the actuator. That is, the motor rotates gears to extend the actuator, and the motor essentially locks into place so that it will not spin in the opposite direction to retract the actuator, such as in response to a contact force on the actuator. This design is favorable for use in a reach carriage under normal conditions, but in a maneuver such as the pallet scoop, the actuator or other components may be damaged due to the intrinsic resistance to back-driving. A linear actuator that includes a force limiter can be used. 
         [0010]    The invention provides an electric actuator for driving the extension arm of a reach carriage. The electric actuator is equipped with a force limiter that relieves back-driving force on the electric actuator before the actuator is damaged, allowing the extension arm to collapse in reaction to a contact. The force limiter may automatically re-engage the actuator after the excessive force has attenuated. 
         [0011]    In one embodiment, a reach actuator for a reach carriage having a carriage frame and a pantographic extension arm comprises: a housing attached to the carriage frame; a rod extending from the housing and attached to the extension arm; a rotary motor configured to drive the rod; and a force limiter disposed between the rotary motor and the rod and configured to disengage if a force applied to the rod exceeds a predetermined threshold. 
         [0012]    In another embodiment of a reach carriage having a carriage frame and a pantographic extension arm attached to the carriage frame, the improvement comprises an electric actuator attached to the carriage frame and the extension arm and configured to extend and retract the extension arm, the electric actuator comprising: a housing attached to the carriage frame; a ball screw disposed in the housing; a ball assembly movably attached to the ball screw; a rod attached to the ball assembly and the extension arm; a force limiter attached to the ball screw; and a rotary motor attached to the force limiter. 
         [0013]    To the accomplishment of the foregoing and related ends, the embodiments, then, comprise the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. However, these aspects are indicative of but a few of the various ways in which the principles of the invention can be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The embodiments will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0015]      FIG. 1  is a left side view of a reach truck in accordance with this disclosure; 
           [0016]      FIG. 2  is a top right rear perspective view of an extension arm, showing the connection between the carriage frame and the extension arm and further showing a hydraulic cylinder; 
           [0017]      FIG. 3  is a top left rear perspective view of an extension arm, showing the connection between the carriage frame and the extension arm and further showing an electric actuator in accordance with this disclosure; 
           [0018]      FIG. 4  is a partial cutaway perspective view of an embodiment of an electric actuator in accordance with this disclosure; 
           [0019]      FIG. 5  is an exploded view of another embodiment of an electric actuator in accordance with this disclosure; 
           [0020]      FIG. 6  is an exploded view of a force limiter in accordance with this disclosure; and 
           [0021]      FIG. 7  is a top left rear perspective view of an extension arm, showing the connection between the carriage frame and the extension arm and further showing the electric actuator of  FIG. 5 , in accordance with this disclosure. 
       
    
    
       [0022]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    The invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
         [0024]    It is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0025]    Unless specified or limited otherwise, the terms “connected” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. As used herein, unless expressly stated otherwise, “connected” means that one element/feature is directly or indirectly connected to another element/feature, and not necessarily electrically or mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/feature is directly or indirectly coupled to another element/feature, and not necessarily electrically or mechanically. Thus, although schematics shown in the figures depict example arrangements of processing elements, additional intervening elements, devices, features, or components may be present in an actual embodiment. 
         [0026]    The various embodiments of the invention will be described in connection with systems and methods for electrical actuation of a reach carriage for a reach truck. That is because the features and advantages of the invention are well suited for this purpose. Still, it should be appreciated that the various aspects of the invention can be applied to other vehicles and in other industries and processes capable of utilizing an extending arm, particularly a pantograph. 
         [0027]    Referring now to the Figures, and more particularly to  FIG. 1 , the general arrangement of a representative vehicle, such as a reach truck  20 , incorporating a reach carriage  24  is shown. For simplicity, the detailed description will describe the embodiments associated with the reach truck  20  incorporating the reach carriage  24 . It is to be appreciated that the details of the invention may also be beneficial and adapted for a wide variety of devices and vehicles, including a reach truck where the reach carriage is coupled to a mast  22 , and the mast is configured to extend and retract. Other known extension mechanisms are contemplated, including drive systems, rails, slides, glides, gears, cables, and the like. Although the reach truck  20 , by way of example, is shown as a standing configuration lift truck, it will be apparent to those of skill in the art that the features of the invention are not limited to vehicles of this type, and can also be provided in various other types of vehicles, including but not limited to, other material handling and lift vehicle configurations. 
         [0028]    As seen, one embodiment of the reach truck  20  includes an operating unit  21  that may serve as a weighted base for the reach truck  20  and also provides controls to an operator for moving reach truck  20  and operating the reach carriage  24 . The reach truck  20  includes a vertically oriented mast  22  mounted relative to the operating unit  21 . The reach carriage  24  includes an extension arm  25  that connects the mast  22  to a lift assembly  23 , which may comprise one or more forks  26 . The extension arm  25  is illustrated as a double pantographic arm but may be a single pantographic arm (see  FIG. 3 ), or more than a double pantographic arm. The extension arm  25  can extend and retract in other fashions, such as by telescopic operation. Manipulation of the controls by the operator may cause various motors, wheels, cables, pneumatic or hydraulic pistons, and other mechanical components to raise and lower the reach carriage  24  along the mast  22 , and to extend and retract the extension arm  25  with respect to the mast  22 . 
         [0029]    Referring to  FIGS. 2 and 3 , the extension arm  25  may include a plurality of pivotably interconnected rigid members, referred to herein as studs  30 . A stud  30  may connect to one or more adjacent studs  30  at a midpoint or an endpoint of each stud  30 , at one or both sides of the extension arm  25 , to create a pantograph as is known in the art. A pivot  35  is formed at each attachment point between studs  30 . The extension arm  25  pivotably attaches to a carriage frame  28  of the reach assembly  24 . In some embodiments, the carriage frame  28  can movably attach to and translate vertically along the mast  22 . The extension arm  25  attaches to the carriage frame  28  at a stationary pivot  39  at the top of the extension arm  25  and a translating pivot  40  at the bottom of the extension arm  25 . The stud or studs  30  attaching at the translating pivot  40  may be attached to a wheel  45  disposed within a vertical channel  50  on the carriage frame  28 . The wheel  45  moves vertically up the channel  50  to extend the extension arm  25  and down the channel  50  to retract the extension arm  25  according to pantographic motion. The top  51  of the channel defines the extension limit of the extension arm  25  and the bottom  52  of the channel defines the retraction limit, provided the reach truck  20  does not include a bumper or another structure for stopping extension and/or retraction. 
         [0030]    In existing systems, the motion of the extension arm  25  is mechanically controlled with at least one hydraulic ram cylinder  55 . The cylinder  55  attaches at a proximal end to the carriage frame  28  and at a distal end to an axle  34  attached between the studs  30  that are attached to the carriage frame  28  at the stationary pivot  39 . The cylinder  55  may be attached at one or both ends with a clevis fastener. The cylinder  55  may be attached to a fluid supply hose (not shown) connected to a hydraulic pump (not shown) that fills and empties the cylinder  55  to create the pressure differentials that operate the ram  56  of the cylinder  55  to extend or retract the extension arm  25 , as is known in the art. 
         [0031]    Referring to  FIG. 3 , in one embodiment the cylinder  55  may be directly replaced by an electric actuator  60 . That is, the electric actuator  60  may be attached between the attachment mechanisms that held the cylinder  55 , and attached to a power supply (not shown) and control unit (not shown). The electric actuator  60  may be a linear actuator having a rod  61  that attaches at its distal end to the axle  70 , such as with a clevis fastener. The rod  61  projects from a housing  62  that may be attached to the carriage frame  28  with a clevis fastener. In operation, the rod  61  is driven out of the housing  62  to extend the extension arm  25  in pantographic motion. The electric actuator  60  may be any electro-mechanical actuator capable of moving a load, and in some applications on the order of several tons, in a reach carriage. Suitable actuators include, without limitation, linear actuators with either a parallel-offset rotary motor, see  FIG. 4 , or an inline rotary motor. See  FIG. 5 . 
         [0032]      FIG. 4  illustrates a suitable electric actuator  60 , although other linear actuators are contemplated. The housing  62  contains a rotary motor  65  configured to drive a gear assembly  66  that, in turn, rotates a ball screw  75 . The ball screw  75  converts the rotary motion into linear motion by translating a ball assembly  76  linearly along the length of the ball screw  75  as is known in the art. The ball assembly  76  is attached to the distal end of the rod  61  and extends and retracts the rod  61  with respect to the housing  62 . Preferably, the electric actuator  60  is highly geared, which permits its use with large loads but prevents the rotary motor  65  from being back-driven by a contact force. 
         [0033]    Referring to  FIG. 5 , in some embodiments, an electric actuator  78  may include a force limiter  80  that prevents damage to the electric actuator  78  by the contact force by disengaging, or “slipping,” when a threshold force is exceeded. The force limiter  80  may be disposed between the rotary motor  65  and the ball screw  75  or other component for converting rotary motion into linear motion. The force limiter  80  may be connected in series with the rotary motor  65  and the ball screw  75 , and preferably couples the rotary motor  65  to the ball screw  75 . The force limiter  80  may be coaxial with both the rotary motor  65  and the ball screw  75 , and may further be coaxial with one or both of the rod  61  and the housing  62 . When the force limiter  80  disengages, the extension arm  25  may collapse toward its retracted position. Once the contact force has been reduced to below the threshold, the force limiter  80  may be reengaged and normal operation of the reach carriage  24  may resume. The force limiter  80  may be manually reengaged by the operator, but preferably the force limiter  80  automatically reengages. 
         [0034]    The force limiter  80  may be any limiter device that responds to an excessive force by automatically disengaging when the force exceeds a threshold. Suitable force limiters  80  include a torque limiter, a friction clutch, a permanent magnet or electromagnet disconnect, a shear pin, or a passive or semi-active disengaging shock absorber. Referring to  FIG. 6 , in one embodiment the force limiter  80  may be a ball-detent torque limiter. The torque limiter has a first gear  85  that cooperates with the rotary motor  65 . A detent ring  90  may be affixed to or integral with the first gear  85 . The detent ring  90  comprises a series of detents of substantially uniform width and height. Each detent has a peak, such that a valley is formed between adjacent detents. A torque plate  95  may be disposed adjacent the detent ring  90 , and may touch the peaks of the detents when the torque limiter is assembled. One or more bearing holes  100  are disposed through the torque plate  95 , each to retain a ball bearing  105 . The torque plate  95  may further comprise a spindle  110  upon which a spring plate  115  is positioned. A spring  120  may be biased between the spring plate  115  and a second gear  125 , which is attached to the end of the spindle  110 . The second gear cooperates with the ball screw  75 . The first gear  85 , torque plate  95 , spindle  110 , spring plate  115 , spring  120 , and second gear  125  can all be coaxial with the rotary motor  65  or its gear assembly  66 , and with the ball screw  75 . 
         [0035]    The spring  120  holds the spring plate  115  under tension against the torque plate  95 , forcing each ball bearing  105  into a valley between detents in the detent ring  90 . With the ball bearings  105  thus secured, the entire torque limiter rotates with the rotary motor  65 , in turn rotating the ball screw  75  to actuate the rod  61 . When the rod  61  is in an extended position, a contact force that drives the rod  61  back into the housing will attempt to rotate the ball screw  75 , force limiter  80 , and rotary motor  65  in the opposite direction, but the rotary motor  65  will resist. This resistance causes torque between the first gear  85  and the torque plate  95 , which pushes the ball bearings  105  against the spring plate  115  as the ball bearings  105  attempt to roll over the peak of the adjacent detent. If the torque is high enough, the ball bearings  105  will overcome the biasing force of the spring  120  and slip over the detent, disengaging the torque limiter. Specifically, this allows the torque plate  95  and second gear  125  to rotate in the opposite direction as the rod  61  is pushed back into the housing  62 , and the extension arm  25  collapses toward its retracted position. As the contact force attenuates, the spring  120  forces the ball bearings  105  back into a valley in the detent ring  90  and the torque limiter automatically reengages and normal operation of the reach carriage may resume. 
         [0036]    Referring to  FIG. 7 , the hydraulic cylinder  55  may be replaced by the electric actuator  78  of  FIG. 5 , which includes a force limiter  80  and a rotary motor  65  that are each coaxial with the rod  61 . The electric actuator  78  may be attached to the reach carriage  28  and the axle  70  of the extension arm  25  as described with respect to  FIG. 3 . In still other embodiments, the electric actuator  78  may be attached between the operating unit  21  and the mast  22 , or between other structures of the reach truck  20  such that actuation of the electric actuator  78  extends and retracts the reach carriage  24 . 
         [0037]    Preferred embodiments have been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiments described. 
         [0038]    Finally, it is expressly contemplated that any of the processes or steps described herein may be combined, eliminated, or reordered. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.