Abstract:
The present disclosure relates to a lifting assembly such as a jib crane or similar apparatus having an articulated arm assembly and a flexible means for connecting a load to an actuator. The lift arm assembly forms an articulated cantilever boom, which is extensible so as to traverse a suspended load within a work cell area. The lift arm assembly further includes a first and second member pivotally connected to one another whereby an inclined plane (e.g., a cam) at the pivot point of the articulating arms provides a vertical displacement of the secondary arms thereby moving the distal end of the secondary arm to counterbalance inherent forces acting thereon.

Description:
This application claims priority from U.S. Provisional Application 61/013,667 for a “LIFTING APPARATUS WITH COMPENSATION MEANS,” filed Dec. 14, 2007 by J. Alday, which is hereby incorporated by reference in its entirety. 
    
    
     The present invention is directed to a class of light duty hoists that are often used in manufacturing environments to move articles in and about a work cell or material handling within a shipping and receiving area. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     In manufacturing environments components and assemblies are often transported around the factory floor and between assembly stations using a crane having a hoist suspended from an overhead rail system. While numerous types of bridge cranes and hoists are currently available for lifting and moving objects and parts in and between work stations or cells, a particular problem inherent in these devices is the difficulty an operator encounters in accurately and controllably lifting and translating a load within a work station or between adjacent workstations; in other words enabling an operator to safely perform operations on the product without undo manual exertion, fatigue or injury. 
     The lifting apparatus of the present invention uniquely overcomes several drawbacks of articulated jib crane devices by providing a lifting apparatus that permits an object to be precisely lifted and moved in a ergonomic manner within or between work stations. For example, parts having an individual weight in excess of U.S. Occupational Safety and Health Administration (OSHA) lifting standards, require a lifting means to transfer the parts into and around the work station. Additionally the same lifting mechanics may be required to reach into machines or freely move the load around obstructions within the work space or a shipping/receiving dock. In order to overcome the limitations of a gantry hoist system or fixed length jib cranes, the current invention includes a free standing lifting device having an articulating jib arm that horizontally traverses or swings about the work space with only a nominal manual effort by the operator. Thus, the “double jointed” jib crane, having two arms attached to one another in one embodiment, provides for a reach zone that substantially encompasses the entire area of the work cell, while allowing movement of the load past obstacles (e.g., posts, columns, machinery) that would prevent rotation of a rigid jib crane with the same swept diameter. 
     Examples of such systems are found, in various publications by Gorbel, Inc., including its “EASY ARM™ Intelligent Lifting System” brochure (Copyright 2006, Gorbel, Inc.) depicting free-standing and under-hung embodiments; and “ARTICULATING WORK STATION JIB CRANES” (Copyright 2006, Gorbel, Inc.) depicting free-standing, wall-mounted and under-hung embodiments, the contents of both brochures being hereby incorporated by reference in their entirety. Various controls that may be employed in such a system, particularly those relating to the control of the load are disclosed in U.S. patents such as U.S. Pat. Nos. 5,865,426, 5,915,673, 6,299,139, 6,386,513, 6,575,317, 6,622,990, 6,796,447, 6,886,812, D477,901, 7,028,856 and 7,222,839, all of which are also hereby incorporated by reference in their entirety. 
     In actual practice, however, the implementation of a lifting beam consisting of articulated arms, having a load-bearing cable passing substantially within the rotational junction point of the articulated members, presents to the operator non-uniform, extraneous forces that are counterproductive to a free swinging arm(s) that will also remain in any desired stationary position, hereby referred to as a “put-n-stay” hoist operation. One solution, providing a uniform transitional force, in combination with a retained static position, would be a mechanism to provide a frictional force to resist movement of the jib arms that requires a substantial manual force to overcome the coefficient of friction before the arm(s) is able to be moved. While this approach achieves the objective of “put-n-stay” operation, the understandable limitation of the added work required by the operator to overcome the friction force prior to moving the arm makes such a solution impractical and/or requires means to actuate and release clutches, brakes or the like. 
     Another solution my include placing the cable/winch assembly at the distal end of the outer most arm, however the weight added to the arm assembly compounds the translation dynamics, and further reduces the load capacity of the arm. Therefore, objectives of the embodiments disclosed herein include, applying minimal uniform force to move, in a horizontal translation, a suspended load that is at rest, and once in motion the object may continue to be moved along a horizontal plane with a generally consistent force from the operator so as to facilitate positioning loads throughout area covered by the hoist or lift. Moreover, once the load is at rest the load should remain at the horizontal position, or “put-n-stay” as indicated above. 
     Analysis of the disclosed embodiments suggests that there may be at least three contributing factors, in various combinations, that compromise the convenience of an articulated arm having a remotely located actuator or winch (e.g., located on a central, free-standing column or ceiling structure where the articulated arm(s) are anchored as depicted in Gorbel&#39;s Easy Arm™ brochure noted above). One adverse force is due to the downward deflection of the arm(s) as a result of the combined bending moment of the articulated arms and the vertical mount of the hoist (e.g., column). This moment is a function of the linear distance of the load from the base mount and results in a force that has a tendency to cause a member to flex or bend In the embodiments disclosed, similar to the Easy Arm™ there are at least two moments. Combined with these moments is a mechanical deflection of the lifting rope or cable arising via connections of the horizontal members and the means by which the rope is passed over the pivot point of the articulated arms. 
     Ideally, a hoist system, absent any arm droop or deflection (i.e., vertical displacement) and forces that deviate from the centerline of the articulated arms, would only require an operator to overcome the inertia of the suspended load and nominal frictional resistance within the connecting joints between the arms. In reality, droop and other forces are intrinsic in an articulated arm when the winch or actuator is remotely located from the lifting position and where the lifting cable or rope must not constrain movement of the pivot points between the arms. For example, the early Gorbel EasyArm™ as well as the Donati (Model CBB-MBB), Scaglia/Indeva (Liftronic® Series) and Kahlman Produkter AB (QLA Series) are articulated arm lifts having designs that do not result in nearly effortless, uniform movement of the arms over the horizontal range of the lift. 
     Therefore, in accordance with the embodiments disclosed herein, there is provided a means to counteract, direct and otherwise control the intrinsic forces that prevent or diminish the ability of a conventional articulated arm lift to remain in a put-n-stay position where the forces promoting horizontal movement of a suspended load are essentially in equilibrium. 
     It is a further objective to facilitate movement of an articulated arm lift in response to a uniform transitional force that is consistently over the entire horizontal range of the lift once the load is in motion. It should be further appreciated that one source of forces tending to impact the uniform movement of the arms, or the put-n-stay operation, is that the lift cable passing over the length of the arms may result in forces tending to prevent movement or to cause movement when an operator releases a suspended load. To eliminate the impact of the cable, a design could be employed wherein the cable passes through the center of any pivot point (e.g., routed around very small pulleys, or actually through longitudinal holes in pivot pins), and thereby does not create the undesirable forces. However, such a design is believed to be impractical for safety, form and cost concerns. Referring briefly to  FIG. 1 , depicted in a perspective view therein is an exemplary articulated arm lift  20 , a Gorbel EasyArm™ lift, having a support column  20  and primary and secondary articulated arms  24  and  28 , respectively, connected thereto via pivots. The actuator or winching device  30  is located on the proximal end of primary arm  24 , and controls the line or wire rope  36  that is connected to a user control  40  and load  50  suspended from a hook or other end-effector associated with the control. 
     There are at least two forces that produce the aforementioned objectionable circumstances as a result of the use of a cable guide pulley  32  adjacent to the pivot point  34  between the arms. One such force is the result of “droop” that occurs when a load is applied to the end of the rope  36 , thereby creating a moment relative to the column and the position it is attached to the floor. This obviously varies with the location of the load relative to the column, and the height of the column. While moving the secondary (outermost) arm  28  from an in-line position relative to arm  24 , the cable moves toward and contacts the pulley, thereby producing a radial force having a tendency to move the secondary arm toward an in-line position with the primary (innermost) arm. Such forces may prevent the secondary arm from remaining in a stationary position. As the secondary arm is pivoted further relative to the primary arm, the cable  36  increasingly becomes wrapped about or engages a pulley  32 . Consequently, as the cable wraps about the circumference of the pulley, the moment force increases thereby producing a radial torque between the two arms. In addition, the load  50  is lifted slightly as direct function of the cable being wrapped about the pulley, thereby causing additional work as the suspended load is moved. The combination of the various forces results in a potentially imbalanced condition. 
     In order to enable a free-swinging, articulated arm lift, where the various interactive forces are reduced or otherwise controlled, the disclosed embodiment contemplate means to counteract or negate the forces that act to prevent movement of the load and/or preclude put-n-stay functionality. The various embodiment disclosed are intended to mitigate or reduce adverse forces that compromise a uniform and sustained effort by the operator to initiate the motion of arm/load, to sustain the arm in motion, and to assure the arm remains in a put-n-stay position when an operator-applied horizontal force to the load  50 , or controller  40 , is removed. As described below, the embodiments contemplate the use of mechanical cam or similar means to adjust or compensate for such forces. 
     It is further contemplated that alternative mechanical devices may be employed to achieve the desired operation and to compensate for the forces tending to effect movement of the load about the operating region of the articulated arm lift. Based on the recognition that work is a direct function of load displacement, maintaining a specific load/arm position to counteract reactive forces becomes one objective of the disclosed embodiments. In other words, effectively maintaining the vertical height of the load provides a generally constant potential energy (height) of the load to enable the arms to freely rotate relative to one another through the operating region and to stay in place once the operator stops moving the load; since the load will not be seeking a lowest-height position. 
     In accordance with one embodiment, a cam follower is situated to be responsive to the lobe of a cam that is positioned at least partly about, or near the pivot point of the lift arm assembly (between the primary and secondary arms), whereby the cam follower progressively modifies the vertical angular elevation of the secondary arm with respect to the primary arm, while the arm is being moved about the operating region of the articulated arms. The technique described herein is advantageous because it provides for an adjusting or compensating force that serves to counterbalance the previously identified inherent forces resulting from loading of the articulated arms and the lift cable or rope passing over pulleys adjacent the pivot points. The improved performance achieved using the combination of a cam and cam follower permits a more uniform manual force to induce and control the traversing of a suspended load attached to the arm. Furthermore, once the load is in motion the sustaining force is maintained by virtue of the varying cam radius associated with the relative radial position between the arms. 
     This technique is also advantageous because various cam profiles can be readily produced to accommodate a broad range of application specific variables. Therefore, a plurality of alternative cam lobes and configurations could be useful in providing the most appropriate and effective solution in any given situation. It will be further appreciated that the cam may be customized, not only for the typical loading of the articulated arms, but also for a particular workstation, where the operator may wish to have detents (e.g., localized changes such as bumps or divots) within the cam profile for specific arm orientations (relative to the primary arm) where the secondary arm “clicks” into a pre-established location, thereby eliminating overshoot and arm wandering, although an extra effort may required to initially dislocate the arms from such a position and to move a load to another position. 
     According to one embodiment, there is disclosed a lifting apparatus, comprising: a base; a primary arm pivotally coupled to said base; a secondary arm pivotally coupled to the distal end of said primary arm, thereby permitting free swinging movement of said arms within a generally horizontal plane defined by the primary and secondary arms; an actuator operatively associated with at least one of said arms, said actuator operatively engaging a cable so as to control the height of a load suspended from a free end of the cable beneath the distal end of the secondary arm; and a mechanical adjustment for displacing the second arm relative to the first arm and thereby counterbalance forces acting upon the secondary arm as a result of movement of the load suspended therefrom. 
     In accordance with a second embodiment, there is disclosed an articulating arm assembly for connecting a lifting device to a load, comprising: a first arm operatively attached to a support at a proximal end thereof and including a pivot pin at a distal end thereof; a second arm having a first member pivotally coupled by said pin to the distal end of said first arm and a second member adjustably coupled to the distal end of said first arm for permitting free swinging movement of said arms relative to one another; a cable between the lifting device and the load that mechanically constrained to pass lengthwise along said first and second arms; and a cam, positioned in proximity to and interposed between the pivot pin and the second member of the second arm, said cam angularly displacing the second member in a vertical direction relative to said first member. 
     In accordance with yet another embodiment, disclosed herein is a method for controlling the position of an articulated arm lift, comprising: operatively attaching a primary arm to a support at a proximal end thereof, said primary arm including a pivot pin at a distal end thereof; and pivotally coupling, using a pivot pin, a secondary arm to said primary arm to permit free swinging movement of said primary and secondary arms relative to one another, wherein the vertical angle between the primary arm and secondary arm is variably adjustable using a cam positioned in proximity to and interposed between the pivot pin and a portion of the secondary arm, said cam thereby angularly displacing the secondary arm in a vertical direction relative to said primary arm as said arms are horizontally pivoted relative to one another. 
     Other and further objects, features and advantages will be apparent from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein the examples of the disclosed embodiments are given for the purposes of disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an articulated arm lift device as briefly described above; 
         FIG. 2  is a partial top-view of the articulated arm pivot embodiment as described below; 
         FIGS. 3-5  are also partial top-views of the articulated arm pivot of  FIG. 2 , positioned at about a 45, 90 and 120 degree angles; 
         FIG. 6  is a planar overview of an articulated arm lifting device in accordance with the embodiments disclosed below; 
         FIG. 7  is a perspective cut-away view of the pulley assembly in an embodiment employing an offset of the secondary arm; 
         FIG. 8  is a perspective view of the pivot connection of the articulated arms including a cam; 
         FIG. 9  is an illustration of an exemplary cam; 
         FIG. 10  is a generalized graphical representation of an exemplary cam profile (radius) relative to the angle at the pivot point between the primary and secondary arms; 
         FIG. 11  is a bottom perspective view of the pivot junction between the primary and secondary arms; and 
         FIGS. 12 and 13  are perspective bottom views of the junction between the primary and secondary arms showing a bearing slot and cam follower at two different angular orientations of the arms. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the disclosed embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. 
     Considering the figures, controlled and uniform motion of the articulated arms is compromised by a combination of forces. One force results from the cable  110  consistently pulling the secondary arm  104  towards an equilibrium position, this force results from the tension on the lifting cable, particularly when a load is suspended therefrom, and the offset of the cable from the centerline of the pivot point. Referring for example to  FIGS. 2 and 3  the clevis or pivot pin  106  defines the center of the pivot and any offset of the cable from such a position, as shown in  FIG. 3 , produces a moment between a cable tangent point on pulley  108  and pin  106 . Another force is the result of the load being vertically displaced as a function of arm pivoting and wrapping of the cable about the pulley when the arms are moved relative to one another. Yet another force is derived from the bending moment of the articulated arms and an associated column or support structure. 
     The offset, denoted as distance “r” in both  FIGS. 3 and 4  represents the distance measured between the cable and the pivot point. This radial moment, in conjunction with wrapping of the cable  110  about pulley  108  tends to resist pivoting of the secondary arm relative to the primary arm, as well as, potentially causing the secondary arm to move after being released by the operator. 
     Reference is now made to  FIGS. 2-4 , where various positions of primary and secondary arms  102  and  104  are illustrated according to the following stages of movement: 
     Stage 1—(aligned) In  FIG. 2  cable  110  passes substantially over the centerline of arms  104  and  102 , therefore there is no offset and accordingly no force moment or cable wrapping about pulley  108  is evident. However, a moment caused by the load  116  suspended at the distal end of secondary arm  104  is at a maximum, therefore the arms are deflected downward. In some embodiments, due to the cable tension caused by the load, the arms are not inclined to freely articulate from the generally aligned configuration of  FIG. 2  without a considerable sidewise force to “snap” them off their respective centerline. 
     Stage 2—(arms @ ˜45 degrees) As seen in  FIG. 3 , a minor moment force develops as the cable moves off-center and begins to make contact with the outer circumference of pulley  108 . Additionally, as the angle between the arms decreased (from a maximum of 180° when aligned), the load is moved closer to the column or support, thereby reducing the bending moment. 
     Stage 3—(arms @ ˜90 degrees) At this point, shown in  FIG. 4 , cable  110  has now completely moved across the open space between arm  104  centerline and the circumferential surface of the pulley  108 . Notably cable  110  is increasingly wrapped about the radius of pulley  108 , thereby progressively raising the vertical position of the load. 
     Stage 4—(arms @ ˜120 degrees) As represented in  FIG. 5 , cable  110  continues to cause a larger moment (deflection from the center of the arms) as it wraps about the circumference of the pulley. Additionally, the wrapping about pulley  108  further causes cable  110  to shorten thereby further raising load  116  —resulting in additional work=(force)(distance). 
     Stage 5—(fully retracted) The cable is now substantially wrapped about approximately one-half of the pulley circumference and the maximum radial torque is present, as well as the maximum amount of work expended to attain the load position. However, the moment caused by the load is at a minimum, therefore the arm deflection or droop is also near a minimum. 
     Referring to  FIGS. 3 and 4 , secondary arm  104  is illustrated as having been translated into a position where cable  110  is now displaced from the centerline of secondary arm  104 . Accordingly the associated radial force derived from moment r and r′ is a function of the included angle between the cable and the x and y force components, or more specifically as depicted in  FIGS. 3-4 , the tangential force from the point of contact of cable  110  onto pulley  108  applies an off-center force F r  to the pivot point by way of moment “r”. Accordingly, moment “r” is equal to the F y  component divided by the sine of the deviation angle of cable  110  from the centerline, expressed as θ, or simply:
 
 F   r   =F   y /sin θ in triangle  xyz ,
 
     where F y  is a component of F r . 
     In other words, as angle θ increases so does F r  and the corresponding radial torque (τ) derived from F r  acting upon secondary arm  104 , also increases where τ=r(F r ). 
     Turning next to the second force, the figures demonstrate that as secondary arm  104  continues to pivot about the pivot at pin  106 , cable  110  increasingly engages the perimeter of pulley assembly  108 , which in turn further raises the load. For example, as observed in  FIG. 3 , the curvilinear cable contact distance at 90-degrees is equal to about one-quarter the circumference. This second force can be generally characterized as the fundamental work required to raise/lower the load as the secondary arm is rotated about the pivot and relative to the primary arm. 
     Referring also to  FIG. 6  another force is present, which is attributed to the secondary arm length  142 , primary arm length  144  and further relative to distance  140 . In the depicted embodiment deflection or droop and includes at least two components, a bending and a torsion moment. In regard to the bending moment, this force component is directly proportional to distances between the base and the load, therefore is at a maximum force at Stage 1 above and near a minimum at Stage 5. Conversely, a torque or twisting moment on primary arm  102  is a minimum at Stage 1 and approaches a maximum at Stage 3. Therefore the negative deflection angle of the distal end of secondary arm is anticipated to be the greatest between Stage 1 and Stage 2. 
     The significance of the combined bending and torsion forces is the tendency for the distal end of arm  104  to droop as a result of the perpendicular load applied to the end of secondary arm  104 . As with the previously characterized forces, this force is also variable as the secondary arm is pivoted through Stages 1-5. Nevertheless, it is a force component, that must be contemplated within the overall transitional force equation and therefore equalized accordingly. 
     Referring, once again, to  FIG. 4 , there is depicted an overall embodiment of a lifting assembly  103  having pedestal or base  101  which may include a post or column (as shown), or a wall or under-hung support structure. Pivotally connected to the base is a lift arm assembly  100  having cable  110  connecting load  116  to actuator  118 . Lift arm assembly  100  forms an articulated cantilever boom extending from lift pedestal  101 , and is pivotable so as to move load  116  in a work area or region of operation within reach of the arm assembly  100 . Lift arm assembly  100  further includes primary arm  102  pivotally connected by means of clevis or pivot pin  106  and clevis members to a secondary arm  104 . Actuator  118  and its associated end effector are designed to grip and control the position of a load  116 . In one embodiment, the actuator and end effector may be as described, for example, in detail in U.S. Pat. No. 7,222,839 or US Publication 2007/0205405, both hereby incorporated by reference, where the actuator and end effector are designed to lift load  116  when gripped by a human hand, particularly in response to a measured human-applied force to the load or end effector (i.e., the force applied by the human operator against the load or handle, etc). 
     Lift arm assembly  100 , as seen in  FIGS. 7-8  and  11 - 13 , includes a pivot coupling at approximately its midpoint, at the respective ends of the primary and secondary arms depicted in the figure, and constrained to move the secondary arm relative to the primary arm in a generally radial manner by clevis members  107   a ,  107   b  on the end of the secondary arm. More specifically, the clevis members  107   a, b  engage, preferably via bearings (top and bottom), clevis pin  106  at the junction of the distal end of primary arm  102 . In operation, the bearing affixed to the top clevis member  107   b , while permitting the arm  104  to pivot relative to arm  102 , also permits movement of the distal end of arm  104  in a vertical orientation. Thus, the bearing in the top clevis  107   b , permits “spherical” movement as the lower bearing about pin  106  slides within the slot  126  in clevis member  107   a  as described below in more detail. 
     Primary and secondary arms  102  and  104 , working in unison, allow transporting of a suspended load from the free end of the secondary arm essentially anywhere within an arcuate area about lift pedestal  101  or a similar base. The area is generally defined by a radius equal to the combined length of the articulated arms and further dependant on any travel-limits such as bumpers or stops at the pivot points. Cable  110  is secured to or within the take-up mechanism of actuator  118  and passes through pulley assembly  108  (e.g., one or two pulleys) and then over a pulley  112  at the distal end of the secondary arm and align with end effector  114  and accordingly load  116 . Cable  110 , as used herein, may include stranded or solid cable, rope, line or wire, as well as chain, strap, hose or other member for transmitting a tensile lifting force between an actuator and a load. 
     Pulley assembly  108 , as shown in  FIG. 6 , is strategically oriented such that the center of the assembly freely pivots about clevis pin  106 , and the assembly is permitted to pivot in response to cable  110 , as the cable wraps about the pulley in cooperation with the translational movement of secondary arm  104 . Generally, the angular orientation of assembly  108  is a function of the position of the secondary arm relative to the primary arm, and may be characterized as the centerline of the pulleys (line  105 ,  FIG. 2 ) in assembly  108  generally bisecting the included angle formed between the primary and secondary arms,  102  and  104  respectively. 
     Having described the basic operation of the lift assembly  103  and the associated mechanical elements, attention is now turned to various aspects of the embodiments designed to compensate for the various forces discussed above. Counterproductive forces are experienced when cable  110  moves off of the centerline of secondary arm  104 —when the secondary arm is pivoted relative to the primary arm. As previously noted in  FIGS. 2-6 , cable  110  has an intrinsic propensity to deviate from the centerline as a function of the wrap about either one of the pulleys  108 . However, by altering the droop angle of arm  104 , it is possible to raise and lower load  116  to create a counterbalancing force and thereby control both the radial moment and translation forces to a point where they counteract and substantially cancel the extraneous forces. As seen in  FIGS. 7 and 8  one implementation relies predominantly on a cam  120  interposed between the articulated arms and is securely attached to primary arm  102  about clevis pin  106 . Cam follower  122  is rotationally attached to secondary arm  104  via a lower clevis member  107   a  and is responsible for causing the angular displacement of arm  104 , in a vertical direction, as it reacts to the arcuate profile of cam  120 . Accordingly, as arm  104  pivots about pin  106  it now has two degrees of freedom, horizontally to move the load within the operating region, and vertically as a force modifier caused by the cam profile. 
     The effectiveness of the cam/cam follower arrangement depicted in  FIGS. 7-13  in counteracting or correcting for the adverse forces is directly reliant on the cam profile, which may be modeled based upon various load ranges and which may also be empirically derived based on, or characterized by, observation and experimentation with a specific embodiment. It should be understood that the cam and cam follower may also be associated with the top clevis member and/or with the primary arm, albeit possibly with greater risk of failure of components in the arm/pivot, and that alternative mechanical means may be similarly employed such as an inclined plane, elliptical pivot pin, and the like, to achieve the desired angular vertical adjustment of the secondary arm relative to the primary arm. Also to be noted is a desirable degree of swivel or play, as the clevis pin is tilting in and out, between the inner and outer race of an upper ball type bearing that is present between upper clevis  107   b  and the pin  106 . Such a bearing is consequently identified as self-aligning in order to accommodate angular misalignments of clevis pin  106  and is particularly suitable for the present application where deflections or misalignment are intentionally developed. 
     As will be further appreciated, the cam profile is necessarily a function of the length and geometry of the articulated arms  104  and  104 , the pulley assembly  108 , as well as the weight of the load suspended from the free end of the secondary arm. One embodiment of a cam profile is depicted in  FIG. 9 . The example cam radius graph of  FIG. 10  (shown for a single-direction pivot only) is a generalization showing the cam radius or slope of the lobe versus the angular position of the arms relative to one another. 
     Turning to  FIGS. 11-13 , clevis pin  106  and bearing  124  are shown as being moveable along the centerline of secondary arm  104  (dashed line in  FIGS. 12 and 13 ) within bearing slot  126 . Bearing slot  126  is aligned with its long axis also along the same longitudinal axis as cam follower  122  and allows lower clevis member  107   a  to translate, in response to motion developed by cam  120 , only in the direction of the center axis of secondary arm  104 . Accordingly, the upper clevis pin bearing (not shown) is constrained within arm  102 , and affords a vertical movement pivot point for the upper clevis member ( 107   b ) as the lower clevis member  107   a  “skews” as a result of the cam induced vertical motion of arm  104 —hence the preference for a self-aligning upper bearing that is compliant with off axis tilt of clevis members  107   a,b.    
     As depicted in  FIGS. 12 and 13 , as arm  104  rotates, cam follower  122  remains engaged with cam  120  and causes the lower clevis member  107   a  to move in and out along the axis depicted, thereby resulting in a raising-lowering of the distal end of the secondary arm. In one embodiment, cam  120  is oriented such that the cam radius at a first rate of change increases to cause secondary arm  104  to pivot upwardly from being in-line with the primary arm and through a 90-degree position and then generally decreases at a greater rate of change in radius until the arm  104  becomes almost completely folded back.  FIG. 9  graphically characterizes one possible example of the cam radius (translating to resultant vertical motion of the distal end of lift arm assembly  100 ) as a function of the cam induced angle between the primary and secondary arms. It will be appreciated that empirical information may result in a cam profile that is similar or entirely different to that depicted. 
     Furthermore, as suggested previously, a cam profile may be applicable for a particular lift configuration (e.g., size/construction) and a different profile may be applicable for alternative configurations. It is also believed that the cam profile is likely to be applicable only to a range of loads, such that alternative profiles may need to be used when larger or smaller loads are suspended from the end of the secondary arm via the cable. And, customized cam profiles may also be used so that the operation or performance of the articulated arm lift may be adjusted or tuned to a particular application (e.g., having one or more detents or similar structures on the cam to prevent or encourage movement to certain angles between the primary and secondary arms. 
     In recapitulation, disclosed is a method and apparatus for compensating for inherent forces developed within an articulated arm lifting mechanism. The disclosed technique employs a cam or equivalent mechanism to provide a counteracting force by adjusting the elevation of the secondary arm and load as the load is traversed. 
     It is, therefore, apparent that there has been provided, in accordance with the present invention, a method and apparatus for adjusting the angular position of one member of a jib crane with articulating arms to counteract forces tending to resist movement of the arms relative to one another or through certain positions and thereby improve the lift system. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.