Patent Publication Number: US-6341665-B1

Title: Retractable counterweight for straight-boom aerial work platform

Description:
FIELD OF THE INVENTION 
     The present invention relates to load lifting devices, especially such devices wherein a boom pivots in order to lift or reposition a load. An example of such a device is an aerial work platform. 
     BACKGROUND OF THE INVENTION 
     A machine which lifts a load typically experiences forces which tend to tip the machine in the direction of the load as the load is lifted. This occurs, for example, in cranes and aerial work platforms. Conventionally, a counterweight may be provided for the machine in a position which tends to tip the machine in an opposite direction. This provides for better balance during operation, and less stress on certain portions of the apparatus. 
     In cranes, it is conventional to lift a load by means of a cable, taking up the cable in order to lift a load. It is known to provide a counterweight which shifts in a direction away from the load as the load is lifted in order to balance the crane during the lifting operation. Typically, a crane serves the purpose of lifting and moving a load from one place to another. 
     A aerial work platform is an example of a machine which lifts a load by pivoting a boom which supports the load. Also, it is typical that the load is supported in varying positions for extended periods of time, rather than merely being lifted from one place and set down in another. The boom may be pivoted in a generally upward direction to lift the load, an in an opposite direction to lower the load or position the load at a lesser height. 
     In such an apparatus, wherein the load supported by the boom may be considered to be positioned to what will be considered the “front” of the apparatus, the load tends to tip the machine forwardly. A counterweight may typically be positioned at an opposite side of the apparatus at a position which would tend to tip the machine rearwardly. This tends to roughly balance the machine. 
     However, as the load is lifted by the pivoting boom, the mass of the load and the boom moves in a rearward direction, closer to the balance point of the machine. As a result, the force which tends to tip the machine in a forward direction is reduced progressively as the load is lifted. If the counterweight remains stationary, the net force which may tend to tip the machine rearwardly will increase. If the change in balance is of substantial magnitude, an imbalance situation may occur. 
     It is known to be desirable to shift the counterweight as the load is lifted in order to better balance the apparatus. It is also desirable to lower the center the gravity of the counterweight as the load is lifted in order to maintain a lower overall center gravity of the apparatus, thus further enhancing stability. 
     OBJECTS OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a mounting arrangement for a counterweight in a load lifting apparatus which permits the counterweight to shift in a manner to optimally balance the apparatus as a load is lifted or lowered. A particular object is to achieve this result in a highly efficient manner. 
     A further object of the invention is to provide such an apparatus wherein the counterweight is movable over a range of motion sufficient to optimally balance the apparatus. 
     A specific object of the invention is to provide an aerial work platform which remains optimally balanced regardless of the position of the platform, and regardless of the direction to which the platform has been rotated with respect to the supporting chassis of the aerial work platform apparatus. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved, in accordance with the present invention, by providing a load lifting apparatus comprising a boom which is pivotable about a horizontal axis for lifting a load, a first end of the boom extending to one side of the horizontal axis being adapted to lift a load, the boom comprising a second end extending from the horizontal axis, a linearly shiftable counterweight, and a link connecting the second end of the boom to the counterweight which shifts the counterweight in a first direction when the boom is pivoted to lift a load and in a second direction when the boom is pivoted to lower the load. The counterweight may also be shifted upwardly and downwardly in response to movement of the boom and load. In preferred embodiments, the boom and counterweight are mounted on a portion of the apparatus which is rotatable about a vertical axis. 
     An apparatus in accordance with the invention may alternatively comprise a hydraulically driven device, a pneumatically driven device, or an electrically driven device for shifting the counterweight in response to raising and lowering of the boom. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention, as well as the particular advantages of the invention, will be described with reference to the accompanying drawings in which: 
     FIG. 1 is a side view of a typical aerial work platform of the straight boom type; 
     FIG. 2A is a rear perspective view of an aerial work platform in accordance with the invention, illustrating a shiftable counterweight in accordance with the invention; 
     FIG. 2B is a rear perspective view, similar to FIG. 2A, wherein one portion of the shiftable counterweight of FIG. 2A is removed for clarity of illustration; 
     FIG. 3 is a rear view of a counterweight according to a preferred embodiment of the invention; 
     FIG. 4 is a lower rear perspective view of an apparatus in accordance with the invention illustrating the counterweight and link of one preferred embodiment; 
     FIGS. 5A-5C are views, partly in section, of the embodiment of FIGS. 2A-4 with the boom in a lowered position, intermediate position and fully raised position, respectively; 
     FIG. 6A is side view, partly in section, of a second embodiment of the present invention comprising a hydraulic device for shifting the counterweight, illustrating the boom in a substantially horizontal position; 
     FIG. 6B is another view of the embodiment of FIG. 6A, showing the boom in a raised position; 
     FIG. 7 is a schematic illustration of a master-slave hydraulic piston and cylinder arrangement suitable for the embodiment of FIGS. 6A-6B; 
     FIG. 8 illustrates another alternative drive arrangement for shifting the counterweight in an apparatus in accordance with the invention; and 
     FIG. 9 illustrates a further alternative drive arrangement for shifting a counterweight in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a side view of a typical aerial work platform of the straight boom type, designated generally by reference numeral  10 . This apparatus includes a chassis  12  supported on wheels  14 . A rotatable turret  16  supports a boom  18 . Turret  16  rotates about a vertical axis. Boom  18  is pivotable about a horizontal axis  20  whereby it may be raised and lowered. At one end, boom  18  supports a platform  22  for supporting and lifting one or more persons. 
     Accordingly, platform  22  carries a load which may be supported at a broad range of positions in order to enable the persons on the platform to perform tasks at locations which might otherwise be inaccessible. In order to stabilize the apparatus and prevent the apparatus from tipping as a result of the load on the platform  22 , the chassis and turret are typically designed to have a relatively large mass. Particularly, a rearward portion  24  of turret  16  will often include a counterweight of substantial mass. The presence of such mass, particularly at rearward portion  24 , generally prevents tipping of the apparatus with platform  22  situated throughout a significant range of positions. 
     However, it is desirable to provide more precise balance for the aerial work platform apparatus by providing a movable or shiftable counterweight. This enhances the balance of the apparatus during use, and may expand the range of utility of the apparatus. 
     FIGS. 2A,  2 B,  3  and  4  are illustrative of a first embodiment of an apparatus according to the invention. FIGS. 2A and 2B are partial illustrations of the apparatus, showing only those elements which are important to the present invention. FIG. 3 illustrates the counterweight of the embodiment of FIGS. 2A-2B. FIG. 4 shows the counterweight mounted on the apparatus and connected to a link for shifting the counterweight in response to movement of the boom, as will be described in greater detail hereinafter. 
     This first embodiment of the invention comprises a turret  30  which is rotatably mounted on a chassis (not shown). As will be described, turret  30  supports counterweights in accordance with the invention. Turret  30  may also support other devices which are not illustrated. The apparatus further comprises a boom  32  which is pivotally mounted at a boom pivot axis  34  (FIG.  2 B). 
     The counterweight  36  in the embodiment illustrated includes symmetrical portions  36 A and  36 B positioned on opposite sides of turret  30 . The illustrated shape of the counterweight portions is exemplary, and should not be considered as limiting. Parts  36 A and  36 B of the counterweight are joined, in this preferred embodiment, by a table or plate  38 . Consequently, all portions of the counterweight will move in unison, as will be described in greater detail hereinafter. 
     The counterweight further comprises wheels  40 . In the illustrated embodiment, wheels  40  comprise sets of wheels attached to table  38 . Again, this arrangement is not limiting, but is only exemplary. Wheels  40  are positioned on tracks  42  located on each side of turret  30 . As illustrated, tracks  42  are formed by the edges of a plate  44  (FIG. 4) attached to the underside of turret  30 . Such a structure for tracks  42  is convenient, but not limiting. Tracks  42  may comprise any form of flanges, rails, etc. associated with turret  30 . 
     In this first embodiment, boom  32  comprises a portion  32 ′ which extends generally rearwardly from pivot axis  34 . Boom portion  32 ′ is pivotally connected to a link  46  at a joint  48 . Link  46  comprises two parts arranged symmetrically in the illustrated embodiment. These parts act in unison, and the specific form of link  46  is not limited to such an arrangement or configuration. 
     Link  46  is also joined at a second pivotable joint  50  to counterweight table  38 . In the illustrated embodiment, link  46  is connected to table  38  via a flange  52  which is attached to the lower side of table  38 . The connection between boom  32  and counterweight  36  via link  46  and the above-described related elements causes the counterweight to shift upon movement of the boom, as will be described with reference to FIGS. 5A-5C. 
     FIG. 5A illustrates boom  32  in a fully lowered position, wherein the platform of the aerial work platform (not shown) is similarly fully lowered. A piston and cylinder device  54  is connected between turret  30  and boom  32  for raising a lowering the boom in a well known manner. Counterweight  36  is at its rear-most position on turret  30 , supported by wheels  40  on tracks  42 . The platform portion of the aerial work platform (not shown) at the end of boom  32  is considered to be at a “forward” or “front” position. 
     As shown in FIG. 5A, counterweight  36  is also at the highest position along tracks  42 . Tracks  42  are inclined downwardly and forwardly, as is apparent from FIG.  5 A. 
     FIG. 5B illustrates the apparatus of FIG. 5A after the boom  32  has been partially raised. Hydraulic fluid has been supplied under pressure to device  54  in order to raise the boom and platform. As a result, boom  32  pivots in a counter clockwise direction about pivot axis  34 , raising the boom and platform. Boom portion  32 ′ also pivots in a counter clockwise direction. 
     As the boom and platform are raised by pivoting about axis  34 , the center of mass of the combined boom and platform moves inwardly toward the vertical axis of rotation  56  of turret  30 . Simultaneously, as boom portion  32 ′ rotates in a counter clockwise direction, boom portion  32 ′ and link  46  move counterweight  36  along track  42 . 
     As a result, as boom  32  pivots to raise the platform, which moves the platform inwardly toward axis  56 , counterweight  36  is also moved inwardly toward axis  56  but from an opposite direction. Consequently, as the platform and its load moves inwardly toward axis  56 , reducing the forces which tend to tip the aerial work platform apparatus forwardly, counterweight  36  moves inwardly from the rear of the apparatus, simultaneously reducing the balancing force which tends to tip the apparatus rearwardly. As a result, overall balance of the apparatus is maintained and enhanced throughout movement of the platform. 
     Additionally, because tracks  42  are inclined, as the platform and its load are raised, the counterweight is lowered. Raising the platform tends to de-stabilize the apparatus by raising its overall center of gravity. This is efficiently and continuously counteracted and balanced by the lowering of the counterweight  36  along inclined tracks  42 , which tends to lower the overall center of gravity of the apparatus. In this additional way, an apparatus according to the invention maintains optimal balance throughout the range of motion of the boom and platform. 
     FIG. 5C is an additional view, similar to FIGS. 5A and 5B, showing the apparatus with boom  32  in the fully raised position. When the boom is fully raised in this manner, the platform is positioned substantially above, or relatively near the center of balance of the apparatus which, for the sake of the present description, will be assumed to be at or near vertical axis  56 . As a result, the weight of the boom and platform exerts relatively little force which would tend to tip the apparatus forward. In accordance with the invention, counterweight  36  is also moved to a position relatively close to vertical axis  56  whereat is imposes relatively little force which would tend to tip the apparatus rearwardly. Thus, optimum balance is maintained. Additionally, as illustrated in FIG. 5C, counterweight  36  is at its lowest point when the boom  32  is fully raised and the platform is at its highest point. This further enhances stability and balance of the apparatus. 
     As can be seen in FIGS. 2A-2B,  4  and  5 A- 5 C, boom portion  32 ′ extends from boom pivot axis  34  in a direction which is not parallel to the direction of boom portion  32 . Stated somewhat differently, boom portion  32 ′ extends at an angle from a plane which contains boom portion  32 . In operation, this orientation of boom portion  32 ′ enables the combination of boom portion  32 ′ and link  46  to shift counterweight  36  over a longer range along tracks  42 . This enhances the operation of the apparatus by facilitating optimum balance over a broader range of movement for the boom and platform. 
     FIGS. 6A-6B illustrate an alternate embodiment of the invention. Like the first-described embodiment, this embodiment includes a turret  30  mounted on a chassis  12 . Boom  32  is pivotally mounted on turret  30  at pivot axis  34 . This embodiment similarly includes movable counterweight  36  including portions secured to counterweight table  38 , all of which is supported by wheels  40  on tracks  42 . This embodiment also includes a hydraulic cylinder and piston lifting device  54  for raising and lowering the boom. This embodiment does not require, however, boom portion  32 , link  46  and associated elements. 
     The embodiment of FIGS. 6A-6B comprises a second piston and cylinder device  58  for moving the counterweight  36  as the boom is pivoted. Device  58  is connected at point  60  to turret  30 , and at point  62  to counterweight  36  by suitable coupling means. 
     Boom  32  is illustrated in FIG. 6A in approximately a horizontal position, raised slightly from its lowest position. In this horizontal position, the piston of device  54  is partially extended, and the piston of device  58  is partially retracted. Accordingly, with boom  32  slightly elevated in this manner, counterweight  36  has been moved a short distance downwardly and forwardly along tracks  42 . 
     FIG. 6B illustrates the apparatus of FIG. 6A with boom  32  in the fully raised position. As illustrated, with the boom in this position, piston-cylinder device  54  is fully extended, while piston-cylinder device  58  is fully retracted, moving the counterweight  36  fully forward and downward along tracks  42 . 
     Piston and cylinder device  54  is in a master-slave relationship with piston and cylinder device  58 . As fluid is pumped into piston-cylinder device  54  in order to extend the piston and raise the boom, fluid flows from the piston side of that device through a conduit  64  into the piston side of device  58 , causing device  58  to retract its piston. This moves counterweight  36  downwardly and forwardly along tracks  42 . As this occurs, fluid leaves the opposite side of device  58  through a second conduit  66 , flowing into the opposite side of device  54 . When the boom is lowered, flow is in the opposite direction. The piston of device  58  is extended and the counterweight  36  is moved upwardly and rearwardly along tracks  42 . 
     FIG. 7 further illustrates a master-slave arrangement suitable for hydraulic devices  54  and  58  of the embodiment of FIGS. 6A-6B. FIG. 7 illustrates the fact that hydraulic cylinder  58  for shifting the counterweight may actually comprise two parts,  58 A and  58 B, operating in parallel. Such an arrangement may be desirable with a construction as described above, comprising counterweight portions symmetrically arranged on opposite sides of the turret. In such a construction, it may be desirable to include symmetrically-operating piston and cylinder devices  58 A and  58 B connected to counterweight portions  36 A and  36 B, respectively. Such an arrangement makes it relatively easy to maintain the counterweight in alignment as it is moved along tracks  42 . However, the invention is not limited to a device comprising any specific number of devices for shifting the counterweight. 
     FIG. 8 illustrates an alternate embodiment of means for shifting the counterweight in accordance with the invention. This embodiment comprises a pair of reversible motors  68 A and  68 B suitably mounted on turret  30 . The motors reversibly drive a pair of screws  70 A and  70 B. The screws cooperate with nuts  72 A and  72 B, respectively, attached to counterweights  36 A and  36 B. As motors  68 A and  68 B are driven in one direction or the other, the cooperation of screws  70 A and  70 B with nuts  72 A and  72 B will cause the counterweight portions to shift linearly along tracks  42 , as previously described. 
     The arrangement of FIG. 8 comprises pairs of motors, drive screws, etc., corresponding to counterweight portions  36 A and  36 B of the preferred embodiments described above. Again, however, the invention is not limited to such a symmetrical arrangement, but contemplates structures having a single drive device or more than two drive devices. 
     Motors  68 A and  68 B may be powered electrically, hydraulically, or pneumatically. The position and direction of drive of the motors may be controlled by sensors which provide signals representing the position (elevation) of boom  32 . Such sensors might determine, for example, the angle of rotation of the boom about pivot axis  34  from its lowermost position, the actual inclination of the boom, the actual height of the platform from ground level, the actual radial distance of the platform from a selected point on the apparatus, or any other parameter which may be utilized for control of the motors. Such control can be achieves by, for example, a microprocessor-controlled circuit for driving the motors and, thus, positioning the counterweight. 
     FIG. 9 illustrates yet another embodiment of means for shifting the counterweight. This embodiment comprises a rack and pinion drive arrangement which includes a pair of reversible motors  74 A and  74 B associated with turret  30  and connected to a corresponding pair of pinions  76 A and  76 B. The pinions engage racks  78 A and  78 B, respectively, associated with counterweight portions  36 A and  36 B. 
     As illustrated, the motors are hydraulic or pneumatic reversible motors, driven from a supply  82  of fluid under pressure. The direction of fluid flow is controlled by a pair of three-way valves  80 A and  80 B. In the position illustrated, valves  80 A and  80 B are closed, and no fluid is flowing to the motors. These valves may be shifted to cause the motors to rotate either clockwise or counterclockwise in a well know manner. Fluid flowing through the motors returns to the supply via a reservoir  84 . 
     As with the embodiment of FIG. 8, valves  80 A and  80 B may be controlled by devices which sense the angle or position of the boom and/or the platform of the aerial work platform apparatus. Reversible motors  74 A and  74 B need not be hydraulic or pneumatic, but may be electrically driven. As with the previously-described embodiments, the embodiment of FIG. 9 is also not limited to an arrangement comprising two symmetrically-disposed drives, but also contemplates a single drive for a counterweight, or more than two drives. 
     The invention has, thus, been described with reference to several embodiments. This description should not be considered as limiting, however, inasmuch as the invention contemplates variations of structure and proportion of elements consistent with the objectives heretofore described, the invention being defined solely by the appended claims.