Patent Publication Number: US-9885349-B2

Title: Pumping unit and counterbalance system for pumping units

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 13/097,754 filed Apr. 29, 2011, and entitled “Cross-Jack Counterbalance System,” which claimed the benefit of U.S. Provisional Patent Application 61/332,766 filed May 8, 2010 of the same title. The Applicant claims the benefit of U.S. patent application Ser. No. 13/097,754 under 35 U.S.C. § 120, and claims the benefit of U.S. Provisional Patent Application No. 61/332,766 under 35 U.S.C. § 119(e). The entire content of each of these prior patent applications is incorporated herein by this reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates pumping units such as those used for artificial lift in oil wells. More particularly, the invention relates to counterbalance systems for use with pumping units as well as to pumping units which incorporate the counterbalance systems. 
     BACKGROUND OF THE INVENTION 
     In the field of oil and gas production, a reciprocating beam pumping unit or “pumpjack” (referred to herein simply as a “pumping unit”) is a form of counterbalanced reciprocating device used to drive a downhole pump to lift reservoir fluids in wells with insufficient bottom-hole pressure to lift reservoir fluids to the surface at a desired production rate. The pumping unit is an above ground drive unit that includes a rotating motor to drive a crankshaft (typically through a reducing gear arrangement) and a suitable linkage arrangement between the crankshaft and a walking beam. In operation, the walking beam is driven to pivot back and forth about a suitable pivot structure to provide an effective mechanical reciprocating motion to drive the downhole reciprocating pump. The downhole pump can be within the well hundreds or even thousands of feet below the surface. The connection to the above ground pumping unit is through a series of elongated interconnected rods known as sucker rods which extend typically through a string of production tubing from the surface to the location of the downhole pump. The reciprocating action of the above ground pumping unit raises and lowers the entire length of the sucker rods to drive the downhole pump to lift reservoir fluids which have entered the well bore. 
     On each upstroke of the pumping unit, the pumping unit must lift not only the entire length of sucker rods and the reciprocating portion of the downhole pump, but also the entire column of reservoir fluids in the production tubing. The weight lifted with each stroke can sometimes exceed twenty thousand pounds. As a practical matter, lifting such weight requires a counterweight arrangement on the pumping unit. One type of counterweight arrangement includes counterweights located on the walking beam, opposite the pivot point from the downhole weight. These “beam-balanced” pumping units suffer from reduced stroke length and reduced pump capacity. Also, they become relatively unbalanced at larger angles of walking beam tilt. Therefore, they have been limited to use in relatively shallow wells. Another type of counterweight arrangement, which can be used with or without counterweights located on the walking beam, includes counterweights located on the rotating crankshaft of the driving reduction gear. In these “crank-balanced” pumping units the rotation of the counterweights mounted on the crankshaft has a horizontal force vector in all but the twelve and six o&#39;clock positions. Reducing this to horizontal and vertical vectors only, fifty percent (50%) of the work used to move the counterweights is horizontal, and thus ineffective. Because the drive unit directly effects counterweight movement and indirectly moves the rod-pump complex through the walking beam, only the vertical forces on the counterweights produce vertical movement on the downhole pump components. Thus fifty percent (50%) of the power drive unit work expended in a crank-balanced pumping unit can be ineffective in pump output. Frequency driving the drive motor for the pumping unit differentially in different portions of the rotational cycle can reduce power consumption during less vertically efficient portions of the rotational stroke cycle. However, because the vertical and horizontal components cannot be completely isolated, the reduction of ineffective power consumption is limited. 
     Even with prior art counterweight arrangements, driving the pumping unit requires considerable energy input from the motor, which is commonly an electric motor. Because the upward stroke of the pumping unit and downhole reciprocating pump produce a relatively low volume of pumped fluids, between 5-40 liters per stroke, long run-times for these pumps can consume relatively large amounts of energy. This energy consumption is part of the calculated “lift cost,” which reflects the relative efficiency and profitability of such production wells. 
     Attempts to improve artificial lift systems which utilize a pumping unit have included improvements in materials of construction, design improvements in critical components such as bearing surfaces, reduction gearing, variations in counter-weight balancing, stroke mechanics and overall harmonics of rotary and reciprocal actions, and the use of frequency drive systems to more efficiently match mechanical harmonics to motor drive output throughout the pump cycle. Also, reducing downhole weights which must be lifted with each pumping cycle by use of lighter weight components can reduce pumping work directly. The benefits of lighter weight downhole components sometimes are off-set by increased failure rates and reduced capacity. 
     SUMMARY OF THE INVENTION 
     The current invention includes a counterbalance system to counterbalance the downhole weight connected to a reciprocating pumping unit to more efficiently offset the vertical translations of the downhole weights with each stroke of the reciprocating pump unit. This substantially eliminates a significant portion of ineffective work associated with previous pumping units, thus reducing the overall work of pumping, energy consumption, and, consequently, lift costs. 
     A counterbalance system within the scope of the present invention may be used with a pre-existing pumping unit, or may be integrated with a newly manufactured pumping unit. Thus the present invention encompasses counterbalance systems for use with pumping units and to new or retrofitted pumping units including such a counterbalance system. The present invention also encompasses methods for counterbalancing a pumping unit. 
     In one embodiment of the present invention, a counterbalance system includes an outrigger support structure, a first elongated outrigger member, and a second elongated outrigger member. The outrigger support structure is adapted to be mounted in an operating position on a walking beam of a pumping unit. In this operating position the outrigger support structure extends transverse to the walking beam from a first lateral side of the counterbalance system at a first side of the walking beam to a second lateral side of the counterbalance system at a second side of the walking beam opposite the first side of the walking beam. The outrigger support structure has a mounting axis which aligns in a vertical plane with the longitudinal axis of the walking beam when the outrigger support structure is mounted in the operating position on the walking beam. The first elongated outrigger member is connected to the outrigger support structure at the first lateral side of the counterbalance system with a suspension end of the first outrigger member positioned on the first lateral side of the counterbalance system. The second outrigger member is also connected to the outrigger support structure, but is connected at the second lateral side of the counterbalance system with a suspension end of the second outrigger member positioned on the second lateral side of the counterbalance system. Both the first outrigger member and the second outrigger member are connected to the outrigger support structure so that the respective longitudinal axis of each outrigger member extends along the mounting axis. A first suspension element may be connected to the first outrigger member so as to depend from the suspension end of the first outrigger member when the outrigger support structure is in the operating position. A second suspension element may be connected to the second outrigger member so as to depend from the suspension end of the second outrigger member when the outrigger support structure is in the operating position. Suitable counterbalance weights may be connected to the first suspension element while additional counterbalance weights may be connected to the second suspension element. These counterbalance weights connected to the first and second suspension elements may be used to counterbalance the downhole weight on the walking beam. 
     As will be discussed further below in connection with the drawings, a counterbalance system according to this embodiment allows the downhole weight on the walking beam to be counterbalanced without requiring horizontal movement of the counterweights. Reducing or eliminating horizontal movement of the counterweights reduces the load on the pumping unit motor and thus reduces the power consumed by the pumping unit motor to reduce lift cost. 
     In another aspect of the invention a first outrigger complex which includes the first outrigger member and a second outrigger complex which includes the second outrigger member may be mounted together with the outrigger support structure to improve the efficiency of the pumping unit. In particular, the first outrigger complex, the second outrigger complex, and the outrigger support structure may be mounted on the walking beam so that the first and second outrigger beams are positioned below the level of the walking beam and axially along the walking beam so as to place the center of gravity of a combined walking beam complex, outrigger support structure, first outrigger complex, and second outrigger complex at a pivot axis for the walking beam of the pumping unit. 
     As used in this disclosure and the accompanying claims, the term “walking beam complex” is used to refer to the walking beam and elements which may be connected to the walking beam other than the present counterbalance system and the downhole weight. In particular, the walking beam complex includes a structure known as a “horse head” connected to the walking beam and from which the downhole weight is suspended in operation of the pumping unit. Also, the terms “first outrigger complex” and “second outrigger complex” will be used to refer to the respective outrigger member and additional elements supported by the respective outrigger member other than the counterweights. In particular the first outrigger complex may include a first counterbalance head from which counterweights are suspended, and the second outrigger complex may include a second counterbalance head from which counterweights are suspended. 
     The placement of the center of gravity of the combined walking beam complex, outrigger support structure, first outrigger complex, and second outrigger complex at or near the pivot axis for the walking beam reduces induced torques in the pumping unit arising from the misalignment of the center of gravity of the walking beam and the pivot axis of the walking beam which is present in many pumping units. This reduction in induced torques reduces the power requirements of the pumping unit and thus reduces lift costs. 
     Embodiments of a counterbalance system in accordance with the present invention do not preclude the use of frequency driving to summarily improve efficiency. Furthermore, embodiments of a counterbalance system within the scope of the invention do not require significant changes in a pre-existing pumping unit. Removal of the crankshaft counterweights (which can be used on the present counterbalance system counterweights) with rebalancing of the pumping unit with the present counterbalance system in place are all that is required to effect the gain in efficiency. In scaled prototype studies the gain in efficiency was well over fifty percent (50%). In addition to lift cost savings through reduced energy consumption, unloading the crankshaft loads can reduce maintenance costs and increase component longevity and reduce component failure. 
     In another aspect of the present invention, adjustable heads may be used as the counterbalance heads for the first and second outrigger members. In particular, counterbalance heads may include an arrangement for adjusting the curvature of the face of the head to minimize or eliminate horizontal movement of the counterweights suspended from the first and second outrigger members. Also, the curved adjustable head may be mounted so that it may be tilted with respect to the end of the respective outrigger beam and can be raised or lowered to adjust for vertical height with respect to the center of pivot. An adjustable head as disclosed herein may be used not only as a counterbalance head but also as the head connected at the front end of the walking beam, independent of a counterbalance system as disclosed herein. 
     These and other advantages and features of the invention will be apparent from the following description of illustrative embodiments, considered along with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a prior art pumping unit showing the translational vectors of counterweight movement with each pump stroke. 
         FIG. 2  is a schematic side view of the pumping unit of  FIG. 1  with a counterbalance system according to one embodiment of the present invention mounted in an operating position. 
         FIG. 3  is a schematic rear view of the pumping unit and counterbalance system shown in  FIG. 2 . 
         FIG. 4  is a schematic side view of the counterbalance system shown in  FIGS. 2 and 3 . 
         FIG. 5  is a schematic plan view of the counterbalance system shown in  FIGS. 2-4 . 
         FIG. 6  is a schematic of the bottom view of the counterbalance system shown in  FIGS. 2-5 . 
         FIG. 7  is a schematic side view showing an embodiment of an adjustable suspension bolt of an embodiment of the adjustable head shown in  FIGS. 2-6 . 
         FIG. 8  is a schematic side view of the adjustable head shown in  FIGS. 2-6 . 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In the following description  FIG. 1  will be used to describe a prior art pumping unit and to illustrate aspects of the prior art pumping unit which lead to certain operational inefficiencies.  FIGS. 2 and 3  will then be used to describe a counterbalance system according to certain embodiments of the present invention as mounted in an operating position on the prior art pumping unit of  FIG. 1 .  FIGS. 4-8  show the example counterbalance system separate from a pumping unit. 
     Referring to  FIG. 1 , a pumping unit  9  includes a walking beam  10  pivotally mounted on a walking beam support  12  (also referred to as a “Samson post”). In this arrangement, walking beam  10  may pivot about a walking beam axis WA extending perpendicular to the plane of the drawing. A horse head  11  is connected at a front end of walking beam  10 . Pumping unit  9  also includes a motor  13  (also may also be referred to as a “prime mover”) which is operably connected via a drive belt to a reducing gear arrangement  14 . In operation, motor  13  drives a crank  15  about a crankshaft axis CA. The lower end of a Pitman arm  16  is connected to the crank  15  via a suitable bearing arrangement and the upper end of the Pitman arm is connected with a pivoting arrangement to walking beam  10 . In this prior art crank-balanced pumping unit  9  a counterbalance weight  17  is connected to the crank arm  15  to provide a counterbalance for the downhole weight which is suspended from horse head  11  and illustrated diagrammatically as weight  18 . 
     Although not apparent from the side view of  FIG. 1 , it should be appreciated that a separate crank arm is connected to the reducing gear crankshaft at the opposite side of the reducing gear  14  to which crank arm  15  is connected. This opposite side crank arm carries its own counterweight similar to counterweight  17  and is connected for reciprocating the walking beam  10  by an additional Pitman arm connected to the opposite side crank arm. The two spaced apart Pitman arms are connected to a transverse member commonly known as an “equalizer bar” which provides the desired linkage between the Pitman arms and walking beam  10  (the equalizer bar  19  is shown in the view of  FIG. 3 ). 
     As motor  13  drivers reducing gear  14  to rotate crank  15  and the opposite side crank about axis CA, the connection between Pitman arm  16  (and the opposite side Pitman arm) and walking beam  10  causes a reciprocating pivoting motion in the walking beam  10  about walking beam axis WA. The vertical motion about the curved face of horse head  11  imparts a vertical reciprocating motion on the downhole weight, which, in an actual implementation of a pumping unit, comprises a string of sucker rods, reciprocating downhole pump components, and a column of reservoir fluids in the well above the level of the downhole pump. 
       FIG. 1  also shows a circle centered on crankshaft axis CA which indicates the arc of movement of counterweight  17  as crank  15  pivots about crankshaft axis CA in the course of operation of pumping unit  9 . A horizontal line H in  FIG. 1  indicates the horizontal deviation of counterweight  17  as it rotates about crankshaft axis CA, while a vertical line V shows the vertical deviation of the counterweight as it rotates about axis CA. 
       FIG. 1  further shows how walking beam  10  is mounted above walking beam pivot axis WA in the prior art pumping unit  9 . This mounting of walking beam  10  above walking beam pivot axis WA necessarily places the center of gravity of the walking beam complex at a point which is offset from walking beam pivot axis WA (the walking beam complex being made up of walking beam  10  and horse head  11  along with the equalizer bar (not shown in  FIG. 1 ) at the rear of the walking beam to which the Pitman arms connect). This placement of the walking beam complex center of gravity at a point offset from the walking beam pivot axis WA causes induced torques as the walking beam pivots in its pumping cycle about walking beam axis WA. As noted in the summary section and as will be described further below, a counterbalance system according to some embodiments of the present invention may be mounted on a walking beam such as walking beam  10  so as to effectively shift the center of gravity of the walking beam complex and mounted elements of the counterbalance so that it aligns with the walking beam pivot axis WA. 
     Referring to  FIGS. 2-6 , a counterbalance system according to one embodiment of the present invention includes first and second outrigger members  21  supported parallel to each other on either side of walking beam  10  by outrigger mounts  22 . In the illustrated embodiment the outrigger members  21  are longitudinally slideable through mounts  22  so that the outrigger members can be positioned and adjusted longitudinally to effect the balance desired between the counterweight system and downhole weight across pivot axis WA. Because the outrigger members  21  are connected in this embodiment so as to straddle pivot axis WA fore and aft, the outrigger members may be structurally substantial to support the counterweights described further below without unduly complicating the balance of the composite weights across pivot axis WA. 
     Each outrigger member  21  terminates on the end opposite horse head  11  of walking beam  10  in mounting components which attach a respective counterbalance head  40  to that end of the respective outrigger member. The end of the respective outrigger member  21  to which the respective counterbalance head is attached may be referred to as the suspension end of the respective outrigger member. In the illustrated example embodiment of the counterbalance system, each counterbalance head  40  comprises an adjustable head structure and the mounting components for the respective counterbalance head allows further adjustments as will be described further below. A respective counterbalance weight, in this illustrated case made up of counterweight components  34 ,  35 , and  36 , is suspended from the respective counterbalance head  40  by suspensor cables  33  which comprise the counterbalance system suspension elements in this form of the invention. 
     As shown particularly in  FIG. 2 , outrigger members  21  (one on either side of walking beam  10 ) are positioned by an outrigger support structure made up of transverse beams  23  and  24  below the vertical level of walking beam pivot axis WA. The composite weights of the outrigger members  21  and other components making up each outrigger complex below the vertical level of pivot axis WA combined with the weights associated with the walking beam complex may be used to produce a center of gravity for the combined structure that corresponds to walking beam pivot axis WA. This reduces unwanted, induced torque forces as the walking beam tilts without mounting the walking beam  10  itself differently on walking beam support  12 . This adjustment of the center of gravity of the combined structure of the walking beam complex, support structure (beams  23 ,  24 , and mounts  22 ), and each outrigger member complex (each made up of a respective outrigger member  21  and counterbalance head  40 ) can dramatically reduce the work associated with articulating the loaded walking beam  10 . 
     The outrigger member mounts  22  are part of the outrigger support structure which connect the outrigger members  21  to the walking beam  10  and are located on either side of walking beam fore and aft of pivot axis WA in the illustrated embodiment. These structurally substantial, channel-like mounts  22  on a respective side of walking beam  10  focally encase and support the respective outrigger member  21  while allowing the outrigger member to slide within the channels to adjust the longitudinal position of the outrigger member relative to the mounts. The each mount  22  is attached on a respective lateral side of the counterbalance system to the outrigger support structure, and particularly to a respective end of transverse beam  24  of the outrigger support structure. In total, the illustrated embodiment includes four mounts  22 ; two on opposite sides of pivot axis WA (and two on either side of the walking beam  10 ). To fix the respective outrigger member  21  at the desired longitudinal position on mounts  22 , the mounts may incorporate fixation screws (not shown) which engage the respective outrigger beam in a suitable fashion. 
     Transverse beams  23  and  24  extend transverse to walking beam  10  and together provide a structure from which outrigger members  21  may be suspended from the walking beam. A separate set of transverse beams  23  and  24  is provided fore and aft of walking beam pivot axis WA in the illustrated embodiment. In each set, beam  23  extends over the top of walking beam  10  with beam segments  24  on either side of the walking beam. In the illustrated example, and as shown in the views of  FIGS. 5 and 6 , each beam segment  23  is associated with a flange  38  which represents an area of increased contact surface with the walking beam to facilitate bolting or clamping of the transverse beam arrangement (made up of beams  23  and  24 ) to the walking beam in a stable fashion to prevent the counterbalance arrangement from rotating about the longitudinal axis of the walking beam. Each set of transverse beam segments  24 , one on each side of walking beam  10 , and the bridging beam segment  23  forms a saddle around the adjacent walking beam section. A connecting strap (not shown) may pass under the said section of walking beam to affix to each beam section  24 , in effect enclosing around the walking beam section. It will be appreciated that the height of the transverse beam section  24  in the illustrated counterbalance system determines the relative vertical position of the outrigger members  21  and functions to shift the center of gravity as described above. It will also be noted by comparing  FIGS. 5 and 3  that the outrigger support structure defines a mounting axis MA, and that this mounting axis aligns in a vertical plane with the longitudinal axis of the walking beam  10  when the counterbalance system is mounted in the operating position on the walking beam. 
     In the illustrated example shown particularly in  FIGS. 2 and 3 , the counterbalance weight is provided by a respective set of components  34 ,  35 , and  36  suspended from the suspension end of a respective one of the outrigger members  21 . In particular, each set of counterweight components includes an equalizer bar  36 , a mounting bar  34 , and a counterbalance weight  35 . The transverse equalizer bar  36  is suspended from a respective outrigger member  21  on suspension cables  33  and provides a stable connection point for connecting mounting bar  34 . The mounting bar in turns provide connection points for connecting the counterbalance weight  35 . It will be noted that in the installation shown in  FIGS. 2 and 3 , the counterweight components  34 ,  35 , and  36  overhang the reducer gear  14 ; however, the width between the two sets of counterweight components prevents interference with the reducer gear  14  while providing adequate stroke movement and ground clearance. 
     Counterweights  35  can be recycled from their previous crankshaft placement on the pre-existing pumping unit or can be OEM weights provided with the counterbalance system. If the counterweights  35  are recycled from the pre-existing pumping unit, the mounting points on the transverse equalizer bar  36  may be customized to match those of the pumping unit crank  15 . Using dedicated OEM counterweights can standardize the mounting points on the transverse equalizer bar  36 . The amount of counterweight used can be equal to that required for typical crankshaft balancing. Completion of balance with the counterbalance system in place can be accomplished by sliding the outrigger members  21  forward or aft relative to the mounts  22  and walking beam  10 , thereby moving these counterweights to reach the desired balance end point. 
     Each illustrated counterbalance head  40  is adjustably connected to its respective outrigger member  21 . Tilt adjustment is facilitated by hinge pin  26  and tilt adjustment screws  25  above and below the hinge pin. In particular curved head complex tilt adjustment screws  25  pass through threaded holes in a terminal rigid mount  25   a  on the top and bottom of the respective outrigger member  21  to press against the mounting plate  27  which tilts by means of the hinge pin  26 . Screw adjustment allows proper positioning of the counterbalance head to facilitate vertical translation accuracy of the suspension cables  33  and the counterbalance components  34 ,  35 , and  36 . 
     The illustrated counterbalance head mounting arrangement also facilitates adjustment to the vertical height of the respective counterbalance head  40  relative to the walking beam pivot axis. This vertical adjustment arrangement includes a vertical height adjustment screw  28  which is threaded through a hole in a structurally dense plate that comprises the top portion of the respective counterbalance head  40 . The vertical height adjustment screw  28  may be threaded downwardly to push down on the terminal end mounting flange  27 , thus raising the counterbalance head through an articulating slot on the counterbalance head mounting plate  27   a  into which mounting flange  27  slides. The top plate through which adjustment screw  28  passes has two grooved channels in the top surface that receives and transmits the suspending cables  33  which originate at the terminal ends of the outrigger members  21 . The top plate also affixes the chain conduit  29  that directs and supports the suspending cables against the curved front face of the counterbalance head  40 . The top structural plate is rigidly affixed to the counterbalance head mounting plate  27   a.    
     The illustrated counterbalance heads  40  have a front face which is adjustable to approximate different curvatures to match the curvature of the face of the horse head  11 . The adjustability is facilitated by the separate front face plates  31  which make up the front face of the respective counterbalance head  40 . The position of each plate  31  is adjustable via bolts  30  which are provided to bridge between the plate  27   a  and the respective face plate  31 . At the back plate  27   a , a respective bolt  30  is threaded through a bushing nut which can be rotated to lengthen and shorten that respective bolt. The threaded bolts  30  terminate at the other end into threaded holes in articulating pins that pass through fixed bushings affixed to the back of the front face plates  31 . Bolts  30  act as structural members for the respective face plate  31 . By lengthening or shortening these bolts in concert over the span of the curved head complex, the curvature of the front face of the curved head complex can be changed. These bolts form structural rows on either side of the given counterbalance head  40 . 
     The front face plates  31  of the respective counterbalance head  40  are curved, channeled structural plates which in unison create the front face of the counterbalance head and collectively define collectively the radius of curvature of the front face of the counterbalance head. The air bag structural members  32  affix or abut against the rear of the front face plates  31  to reinforce the rigidity of the plate section. To improve the continuity of the non-contiguous collective curvatures of the front face plates, a specialized chain conduit  29  travels along grooved channels on each side of the front face plates  31 . The chain conduit  29  affixes to the top plate of the head complex and follows channel-like grooves within the individual front face plates and are held in position by pressure from the overlying suspension cable within a spine formed channel of the chain. An excess of chain extends beyond the lower edge of the counterbalance head and redundantly extends and affixes to the bottom edge of the curved head mounting back plate. As the head expands with steeping of the radius of curvature, the excess chain moves onto the front face, which keeps the cable groove continuous. Each curved counterbalance head has two grooved chain conduits for two cables per head. 
     Air bag structural members  32  form a central line within the counterbalance head to create an adjustable reinforcement of the adjustable counterbalance head to resist compressive collapse from forces of the loaded suspension cables and the outrigger beams as the downward force of the counterweights resists deflection by the curved front surface of the curved face of the counterbalance head. These air bag structural members  32  can be inflated or deflated through needle valves to maintain the desired radius of curvature as adjusted by the suspension bolts  30  while imparting adequate structural rigidity. 
     The suspension cables  33  originate as a loop through an eyelet in the terminal end of the respective outrigger member  21 , follow ring eyelets at the top of outrigger member where the loops close atop the beams with cable clamping devices, and extend over the top of the counterbalance head to follow the grooves and chain conduits as described above. As the walking beam  10  tilts, the radius of curvature of the counterbalance head  40  keeps the suspension cable within the prescribed vertical path. The suspension cables  33  terminate in eyelets on the transverse equalizer bar  36  of the respective counterbalance arrangement ( 34 ,  35 , and  36 . 
     As used herein, whether in the above description or the following claims, the terms “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, that is, to mean including but not limited to. Any use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another, or the temporal order in which acts of a method are performed. Rather, unless specifically stated otherwise, such ordinal terms are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term). 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit the scope of the invention. Various other embodiments and modifications to these preferred embodiments may be made by those skilled in the art without departing from the scope of the present invention.