Abstract:
A bell crank and a mechanical link are included in an aerial lift system to mechanically constrain motion of a two-boom aerial lift mechanism to prevent the aerial lift system from accessing unstable positions. Typically an aerial lift is coupled to a vehicle via a turntable. The turntable includes a counterweight on a rearward side of the turntable to balance the booms and associated hardware as they are extended in a forward direction. If the booms are in certain rearward, or even vertical positions, the counterweight can cause the vehicle to tip and such unstable positions are to be avoided. By providing the bell crank proximate the pivot joint between upper and lower booms, with the bell crank coupling at pivot joints with: the lower boom, the mechanical link, and an upper hydraulic cylinder, the booms of the aerial lift system are mechanically constrained to avoid vehicle tipping.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to aerial lift systems and vehicles provided with such lift systems. 
         [0003]    2. Background Art 
         [0004]    One type of aerial lift includes a vehicle onto which a pivotable turntable is mounted, with the turntable having a series of adjustable, and possibly, extendable booms onto which a platform or basket is mounted. Typically, a lower and an upper boom are used to position a platform or basket away from the vehicle to a desired location so that a person within the basket or platform may perform work or maintenance on an object that is otherwise difficult to access. 
         [0005]    It is desirable for the boom to allow the basket to extend beyond the footprint of the vehicle. However, as the basket moves with respect to the vehicle, the center of mass of the vehicle system, i.e., including the boom, basket, the person or persons in the basket and whatever other cargo is in the basket, migrates. Several measures can be taken to overcome a tendency of the vehicle to tip when the boom is extended away from the vehicle. 
         [0006]    For example, the weight and/or footprint of the vehicle are increased so that the vehicle system is more stable. However, it is undesirable to increase the weight of the vehicle because it makes transporting the vehicle to a work site that much more difficult. Also, if the surface near the work site is unstable, such as may result from a presence of sand or mud, the more that the vehicle system weighs, the more likely the vehicle will become stuck during maneuvering. Another measure used to stabilize the vehicle in the elevated position calls for increasing the track width and/or the wheel base, whereby the foot print of the vehicle is increased, which improves stability. However, the larger the footprint, the less maneuverable the vehicle becomes. Also, a wider vehicle is prevented from accessing certain locations that it may have otherwise been able to access. 
         [0007]    Another commonly employed measure involves providing a counterweight on the turntable so that the counterweight rotates with the turntable. The counterweight balances the boom extending in a direction away from the counterweight, which is called a “forward” direction regardless of the angle of the rotation of the turntable. Thus, the forward and rearward directions are defined with respect to the turntable, not with respect to the vehicle or any object that is being accessed from the basket. However, the counterweight provides an undesirable imbalance force when the upper boom is rotated in a rearward direction. Thus, this measure commonly includes taking additional measures to prevent the boom from moving too far rearward. 
         [0008]    In some other designs, the positions of the booms are controlled by hydraulic cylinders. By knowing the extent that the hydraulic cylinders are extended, the combinations of boom positions leading to tipping can be avoided. However, such a system relies on having sensors to measure positions of the hydraulic cylinders, a controller, and frequent calibration of the sensors to ensure sufficient measurement accuracy. 
         [0009]    In yet another prior design, an upright member is provided between the lower and upper booms. The upright is actively controlled via hydraulic feedback to maintain it in a vertical position. A disadvantage of such a system is that it requires additional hydraulic cylinders managed by complex valving and additional sensors. Furthermore, the system may require periodic calibration. Also, by introducing an intermediate link, i.e., the upright, additional play is introduced. The amount of play is exacerbated at the operator&#39;s station in the ‘basket. Such play undermines the operator&#39;s sense of security. 
       SUMMARY 
       [0010]    To solve at least one problem in the prior art, an aerial lift system is disclosed which includes a vehicle, a turntable coupled to the vehicle, a lower boom coupled to the turntable at a first turntable pivot, a lower linear actuator coupled between the turntable and the lower boom, and a mechanical link coupled to the turntable at a second turntable pivot. The turntable is coupled to the vehicle with an axis of rotation of the turntable being substantially vertical. The upper side of the turntable includes pivot joints for at least a lower boom and a mechanical link. The axis of rotation about these first turntable and second turntable pivots, in some embodiments, is substantially perpendicular to the axis of rotation of the turntable with respect to the vehicle. A bell crank is coupled to: the lower boom at a first bell-crank pivot and to the mechanical link at a second bell-crank pivot. The system includes an upper boom coupled to the lower boom at a boom-to-boom pivot. The system also includes an upper linear actuator coupled to the bell crank at a third bell crank pivot and coupled to the upper boom. The linear actuators are any suitable linear actuator such a hydraulic cylinder. The mechanical link, in some embodiments, includes two members. The lower linear actuator passes between the two members of the mechanical link as it couples between the lower boom and the turntable. 
         [0011]    By providing the mechanical link and the bell crank, the upper and lower booms are constrained in such a manner that they are prevented from accessing positions in which vehicle tipping may occur. This presents advantages over prior designs. For example, in one embodiment of the invention in which the lower boom is coupled to the upper boom, there are fewer pivot points than some prior art systems. Thus, there is less play in the system at the operator basket, which may extend 50 feet or more from the vehicle, during positioning of the basket or due to the wind acting upon the basket. The operator feels more secure and comfortable when the basket bounces to a lesser degree. 
         [0012]    At least one embodiment of the invention obviates the need for complicated electronic/hydraulic control and hydraulic actuators used to limit rear instability. Furthermore, the mechanical system does not need calibration, as needed with a system using control valves and actuators. Furthermore, service intervals for some embodiments of the invention described by the present disclosure, which are mechanically based, are likely longer than service intervals for prior systems which may include sensors, controllers, and complex valving. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows an aerial lift system in which the booms are in a stowed position for transportation and in which the upper and lower hydraulic cylinders are fully retracted; 
           [0014]      FIG. 2  shows an aerial lift in which the upper hydraulic cylinder is fully extended and the lower hydraulic cylinder is fully retracted; 
           [0015]      FIG. 3  shows an aerial lift in which the upper and lower hydraulic cylinders are fully extended; and 
           [0016]      FIG. 4  shows an aerial lift in which the upper hydraulic cylinder is fully retracted and the lower hydraulic cylinder is fully extended. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    As shown in  FIGS. 1 through 4 , an aerial lift system  10  includes a vehicle  12  with an aerial lift  13 . According to the embodiment depicted in  FIG. 1 , vehicle  12  is a wheeled vehicle. Alternatively, vehicle  12  may be a tracked vehicle. In some embodiments, vehicle  12  is not self-propelled. A turntable  14  is rotatably mounted on vehicle  12 . Alternatively, element  14  is a support member which is fixed to vehicle  12 . A lower boom  16  is coupled to turntable  14  via a pivot joint  18 , at or near to a proximal end of the lower boom  16 . A lower hydraulic cylinder  20  is pivotally coupled between turntable  14  and lower boom  16 . Generally, a distal end of the lower hydraulic cylinder  20  is coupled to the lower boom  16  at some distance from the proximal end of the boom  16 . 
         [0018]    In one embodiment, a proximal end of the lower hydraulic cylinder  20  extends through a slot in turntable  14  to provide stowage space for lower hydraulic cylinder  20 . Lower hydraulic cylinder  20  is trunnion that is in turn mounted to the turntable in one embodiment, with the trunnion joint being located along the body of lower hydraulic cylinder  20 , i.e., not connected at the proximal end of lower hydraulic cylinder  20 . A mechanical link  22  is also coupled to turntable  14  via a pivot joint  23 . In one embodiment, mechanical link  22  comprises two members  24  and  26 . In the embodiment shown in  FIG. 1 , lower hydraulic cylinder  20  passes between members  24  and  26  of mechanical link  22 . A second end of lower boom  16  pivotally couples with an upper boom  28  at a pivot  29   
         [0019]    According to one embodiment, aerial lift  13  includes a bell crank  30  which is pivotally coupled to mechanical link  22  and lower boom  16 . Bell crank  30  is also pivotally coupled to an upper hydraulic cylinder  32  at one end of the upper hydraulic cylinder with the other end of upper hydraulic cylinder  32  coupled to upper boom  28 . Upper hydraulic cylinder  32  couples with upper boom  28  away from either end of upper boom  28 . Upper boom  28  is coupled to lower boom  16  at a close end of upper boom  28 . At a far end of upper boom  28 , a basket assembly  34  is coupled. Basket assembly  34  includes a platform and/or cage in which an operator, and possibly tools and/or cargo, may be lifted. Basket assembly  34  includes pivotal links and hydraulic cylinders so that it may be raised, lowered, extended, etc. with respect to the far end of the upper boom  28 . Furthermore, basket assembly  34  may include components to provide users with automated, or semi-automated, leveling to ensure that basket assembly  34  remains substantially level. In the embodiment shown in  FIG. 1 , an extendible link  36  is provided between the basket assembly  34  and the upper boom  28 . The extendible link  36  provides users with apparatus for extending a length of the upper boom  28 , and thus extending a reach of the basket assembly  34 . 
         [0020]    A counterweight  38  (shown in  FIGS. 2-4 ), may be included. In some embodiments, counterweight  38  has a counterweight portion located on both sides of the lower boom  16 . For illustration purposes, only the counterweight portion behind lower boom  16  is shown in  FIGS. 2-4 . 
         [0021]    In the preceding paragraphs, embodiments showing some of the elements of the present invention have been described. In the succeeding discussion, the system characteristics leading to improved operation are described in regards to  FIGS. 2-4 , in which the aerial lift is shown with the hydraulic cylinders in extreme positions. 
         [0022]    In  FIG. 2 , upper hydraulic cylinder  32  is shown in a fully extended position while lower hydraulic cylinder  20  is shown in a fully retracted position. In practice, upper boom  28  and lower boom  16  can be adjusted to position basket assembly  34  close to the desired location, i.e., rough location. To more finely adjust the basket to placement into the desired location, basket assembly  34  can be raised, lowered, and rotated independently of upper boom  28  and lower boom  16 . As shown in  FIG. 2 , basket assembly  34  is in a lower position, but can be raised as needed to obtain further height above vehicle  12 . 
         [0023]    Generally, bell crank  30  has three pivots: a lower boom pivot  30   a , a mechanical link pivot  30   b , and an upper hydraulic cylinder pivot  30   c . As shown in  FIG. 2 , upper hydraulic cylinder pivot  30   c  is below mechanical link  22 . This limits the extent to which upper hydraulic cylinder  32  can extend and thus limits the rotation of upper boom  28  in the rearward direction. 
         [0024]    In  FIG. 3 , both upper hydraulic cylinder  32  and lower hydraulic cylinder  20  are shown in their fully extended positions, i.e., a mostly vertical position. As discussed above, basket assembly  34  can be raised from the position shown in  FIG. 3  to attain a slightly higher end position. Furthermore, extendible link  36  can be extended to its furthest position. When lower hydraulic cylinder  20  is extended, lower boom  16  is caused to rotate about pivot joint  18 . Lower boom  16  acts on bell crank  30 , the motion of which is constrained by being coupled to mechanical link  22 . The pivot joint of bell crank  30 , which is coupled to upper hydraulic cylinder  32 , is rotated so that it is in the interior of the obtuse angle formed between upper boom  28  and lower boom  16 . 
         [0025]    In  FIG. 2 , the angle between lower boom  16  and upper boom  28  is about 45 degrees; whereas, in  FIG. 3 , the angle between lower boom  16  and upper boom  28  is about 135 degrees. In both  FIGS. 2 and 3 , upper hydraulic cylinder  32  is fully extended. It is the position of bell crank  30 , as influenced by the position of lower boom  16  and mechanical link  22 , which causes the different relative positions of the two booms. The relative angle between the two booms, described in regards to  FIGS. 2 and 3 , is not intended to be limiting, but is provided instead for illustrative purposes only. The relative lengths of the booms, the mounting positions of the hydraulic cylinders on the booms, the size and location of the pivot joints on the bell crank, the position of the mechanical link in relation to the lower boom, the extensions of the hydraulic cylinders, and other parameters are parameters which influence the angle in between the booms. Many alternative combinations of such parameters are within the scope of the present disclosure and many would provide for other angles between the booms. 
         [0026]    In  FIG. 3 , hydraulic lines  40  and  42  are shown coupled to bosses  41  and  43  on upper hydraulic cylinder  32 . Bosses  41  and  43  and hydraulic lines  40  and  42  are not shown in  FIGS. 1 ,  2 , and  4  for illustration simplicity. When supplying fluid via hydraulic line  40 , the upper hydraulic cylinder  32  extends. When supplying fluid via hydraulic line  42 , the upper hydraulic cylinder  32  contracts. Lower hydraulic cylinder  20  has analogous hydraulic lines  44  and  46 . A lower operator-controlled actuator  48  and an upper operator-controlled actuator  50  may be provided for operation from within the basket assembly  34 . In one embodiment, actuators  48  and  50  are push-pull levers. Actuators  48  and  50  control hydraulic pressure supplied in lines  40 ,  42 ,  44 , and  46 . Other embodiments, including electronically actuated actuators, may be used. 
         [0027]    In  FIG. 4 , the combination of lower hydraulic cylinder  20  fully extended and upper hydraulic cylinder  32  fully retracted is shown. Bell crank  30  is in the same position in  FIG. 4  as in  FIG. 3 . Both  FIGS. 3 and 4  show lower boom  16  in the furthest forward position accessible (which is approximately 25 degrees rotated toward the rearward direction from a vertical axis in the embodiments shown in  FIGS. 3 and 4 ). With upper hydraulic cylinder  32  fully retracted, this shows a position in which basket assembly  34  is far forward. In this example, counterweight  38  counterbalances the weight of basket assembly  34 , upper boom  28 , and whatever cargo is in basket assembly  34 . 
         [0028]    Combinations of the extreme positions of hydraulic cylinders  20  and  32  are shown in  FIGS. 1-4 . Bell crank  30  and mechanical link  22  influence the range of motion of the lower boom  16  and upper boom  28  such that a significant range of motion is provided in such positions illustrated in  FIGS. 1-4  as well as intermediate positions of cylinders  20  and  32 . However, bell crank  30  and mechanical link  22  inhibit accessing unstable positions of the aerial lift system, i.e., positions potentially leading to vehicle tipping. 
         [0029]    By adding bell crank  30  and mechanical link  22 , as shown in  FIGS. 1-4 , aerial lift  13  is prevented from accessing rearward positions that might result in the vehicle tipping. The disclosed system provides an advantage over prior art systems in that vehicle tipping is prevented by a purely mechanical system . In prior art systems, at least position sensors and a control system are used to ensure that tipping positions are not accessed. Such systems depend on the sensors providing an accurate measurement to the control system, and the control system maintaining control over the hydraulic cylinders. According to embodiments of the present invention, tipping is prevented without relying on sensors and actuators. 
         [0030]    According to embodiments of the present disclosure, upper boom  28  and lower boom  16  are directly linked with their range of motion controlled via hydraulic cylinders,  20  and  32 , mechanical link  22 , and bell crank  30 . In some prior art systems, an additional link is included between upper and lower booms. As each link between the turntable and the basket adds to the amount of play experienced at the basket, it is an advantage, according to some embodiments of the present disclosure, that no such additional link is employed. 
         [0031]    Another advantage, according to the present disclosure, is that the operator can directly control hydraulic cylinders  20  and  32  without relying on an electronic controller. In some prior art systems, a controller is interposed between operator controls and linear actuators to ensure that undesirable positions of associated booms are not accessed. In other prior systems, an additional link is provided between upper and lower booms and a complex control scheme is employed to ensure that undesirable positions are not accessed. 
         [0032]    While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. For example, hydraulic cylinders  20  and  32  are shown in  FIGS. 1-4 . However, any type of linear actuator, such as electro-mechanical motors (such as a stepper motor), a linear motor, etc. can be used in place of hydraulic cylinders. Where one or more embodiments have been described as providing advantages or being preferred over other embodiments and/or over prior art in regard to one or more desired characteristics, one of ordinary skill in the art will recognize that compromises, additions, subtractions or other modifications may be made among various features to achieve desired system attributes, which may depend on the specific application or implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described as being less desirable relative to other embodiments with respect to one or more characteristics are not outside the scope of the disclosure as claimed.