Patent Publication Number: US-2023141730-A1

Title: Cut-off saw

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 16/018,219, filed Jun. 26, 2018, which claims priority to U.S. Provisional Patent Application No. 62/524,770, filed Jun. 26, 2017, the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present disclosure relates to cut-off saws, and more particularly to battery powered cut-off saws. 
     SUMMARY OF THE INVENTION 
     In one aspect, a power tool includes a housing and a support arm extending from the housing and defining a longitudinal axis. The support arm includes a first arm portion coupled to the housing and a second arm portion coupled to the first arm portion. The power tool further includes a drive pulley coupled to the first arm portion. The drive pulley defines a first axis. A driven pulley is coupled to the second arm portion and defines a second axis spaced from the first axis. The power tool also includes a synchronous belt connecting the drive pulley and the driven pulley. The second arm portion is movable relative to the first arm portion between a first configuration and a second configuration offset 180 degrees from the first configuration about the longitudinal axis. A distance between the first axis and the second axis is substantially the same in both the first configuration and the second configuration. 
     In another aspect, a power tool includes a housing, a motor supported within the housing, the motor having an output shaft operable at maximum speed greater than 10,000 revolutions per minute, and a battery configured to power the motor, the battery having an output voltage greater than 40 volts. A support arm extends from the housing and defines a longitudinal axis. The support arm includes a first arm portion coupled to the housing and a second arm portion coupled to the first arm portion. A drive pulley is coupled to the output shaft, which is rotatably coupled to the first arm portion. The drive pulley defines a first axis. A driven pulley is coupled to the second arm portion and defines a second axis spaced from the first axis. The power tool also includes a synchronous belt connecting the drive pulley and the driven pulley, and a cutting wheel coupled to the driven pulley, the cutting wheel having a diameter greater than 9 inches. The second arm portion is movable relative to the first arm portion between a first configuration and a second configuration offset 180 degrees from the first configuration about the longitudinal axis, thereby relocating the cutting wheel to another side of the support arm. A distance between the first axis and the second axis is substantially the same in both the first configuration and the second configuration. 
     In another aspect, a power tool includes a housing having an upper portion and a lower portion, a motor supported within the lower portion, a battery configured to provide power to the motor, and a battery receptacle disposed on the upper portion of the housing, the battery being removably coupled to the receptacle. The battery receptacle includes a guide rail defining an insertion and removal axis of the battery, a recessed portion adjacent the guide rail, the recessed portion having a drainage surface forming an acute included angle with the insertion and removal axis, and a drainage hole located proximate an end of the drainage surface. The drainage surface is configured to direct fluid that infiltrates an interface between the battery and the battery receptacle toward the drainage hole. The power tool further includes a support arm extending from the housing and defining a longitudinal axis, the support arm including a first arm portion coupled to the housing and a second arm portion coupled to the first arm portion. A drive pulley is coupled to the first arm portion and defines first axis. A driven pulley is coupled to the second arm portion and defines a second axis spaced from the first axis. The power tool also includes a synchronous belt connecting the drive pulley and the driven pulley. The second arm portion is movable relative to the first arm portion between a first configuration and a second configuration offset 180 degrees from the first configuration about the longitudinal axis, and a distance between the first axis and the second axis is substantially the same in both the first configuration and the second configuration. 
     In another aspect, a power tool includes a housing having an upper portion and a lower portion, a motor supported within the lower portion, a battery configured to provide power to the motor, a battery receptacle disposed on the upper portion of the housing. The battery is removably coupled to the receptacle. The battery receptacle includes a guide rail defining an insertion and removal axis of the battery and a recessed portion adjacent the guide rail. The recessed portion has a drainage surface forming an acute included angle with the insertion and removal axis. The battery receptacle also includes a drainage hole located proximate an end of the drainage surface. The drainage surface is configured to direct fluid that infiltrates an interface between the battery and the battery receptacle toward the drainage hole. 
     In another aspect, a cut-off saw includes a housing, a motor supported within the housing, the motor having an output shaft operable at maximum speed greater than 10,000 revolutions per minute, and a battery configured to power the motor. The battery has an output voltage greater than 40 volts. The cut-off saw also includes a drive pulley coupled to the output shaft, a driven pulley connected to the drive pulley by a synchronous belt, and a cutting wheel coupled to the driven pulley. The cutting wheel has a diameter greater than 9 inches. 
     Other aspects of the disclosure will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a cut-off saw according to an embodiment of the disclosure. 
         FIG.  2    is a cross-sectional view of the saw of  FIG.  1   , taken along line  2 - 2  in  FIG.  1   . 
         FIG.  3    is a perspective view of the saw of  FIG.  1   , illustrating a drive train of the saw. 
         FIG.  4    is a perspective view illustrating a battery receptacle of the saw of  FIG.  1   . 
         FIG.  5    is a cross-sectional view taken along line  5 - 5  in  FIG.  4   . 
         FIG.  6 A  is a perspective view illustrating a drainage surface of the battery receptacle of  FIG.  4   . 
         FIG.  6 B  is a cross-sectional view illustrating a water drainage path. 
         FIG.  7    is a cross-sectional view taken along line  7 - 7  in  FIG.  1   . 
         FIG.  8    is a cross-sectional view taken along line  8 - 8  in  FIG.  1   . 
         FIG.  9    is an exploded view of a support arm of the saw of  FIG.  1   . 
         FIG.  10    is a perspective view of the support arm of  FIG.  9    in a first configuration. 
         FIG.  11    is a perspective view of the support arm of  FIG.  9    in a second configuration. 
         FIG.  12    is a perspective view of a portion of the saw of  FIG.  1    with the support arm in the first configuration. 
         FIG.  13    is a perspective view of a portion of the saw of  FIG.  1    with the support arm in the second configuration. 
         FIG.  14    is another perspective view of the portion of the saw of  FIG.  13   . 
         FIG.  15    is an exploded view of a portion of the saw of  FIG.  1   . 
     
    
    
     Before any embodiments of the disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The disclosure is capable of supporting other embodiments and of being practiced or of being carried out in various ways. 
     DETAILED DESCRIPTION 
       FIG.  1    illustrates a handheld power tool  10 , which is a cut-off saw in the illustrated embodiment. The saw  10  includes a housing  15 , a support arm  20  coupled to and extending from the housing  15 , a cutting wheel  25  carried by the support arm  20 , and a guard  30  covering a portion of the circumference of the cutting wheel  25 . The cutting wheel  25  can be a blade, an abrasive disk, or any other rotatable element capable of removing material from a workpiece. In the illustrated embodiment, the cutting wheel  25  has a diameter greater than 9 inches and is preferably 14 inches in diameter. In other embodiments, the cutting wheel  25  can be between about 10 inches and about 16 inches in diameter. 
     With reference to  FIG.  15   , the guard  30  is rotatably coupled to the support arm  20  to provide a variety of operating positions that expose different circumferential portions of the cutting wheel  25 . This advantageously allows the saw  10  to be used in a variety of cutting positions. In the illustrated embodiment, a stop member  31  is coupled for co-rotation with the guard  30  and includes first, second, and third projections  32   a ,  32   b ,  32   c  that extend in a radially outward direction. A first travel region  33   a  is defined between the first and second projections  32   a ,  32   b , and a second travel region  33   b  is defined between the first and third projections  32   a ,  32   c . In the illustrated embodiment, a post  34  (which may be, for example, the head of a cap screw) projects from the support arm  20  into one of the travel regions  33   a ,  33   b , depending on an installation position of the guard  30 . 
     With continued reference to  FIG.  15   , the post  34  is engageable with the projections  32   a ,  32   b  to limit movement of the guard  30  to first and second rotational positions that define the ends of the first travel region  33   a . Alternatively, the post  34  is engageable with the projections  32   a ,  32   c  to limit movement of the guard  30  to third and fourth rotational positions that define the ends of the second travel region  33   b . In some embodiments, a user may remove and reposition the guard  30  to align the post  34  with either the first travel region  33   a  or the second travel region  33   b , to permit movement of the guard  30  between the first and second rotational positions or the third and fourth rotational positions, respectively. In the illustrated embodiment, an elastomeric gasket  36  is disposed between the guard  30  and the support arm  20 . The gasket  36  includes axially protruding portions  37  that are received in corresponding recesses  38  in the guard  30  such that the gasket  36  is coupled for co-rotation with the guard  30 . The gasket provides a frictional engagement between the guard  30  and the support arm  20  to inhibit inadvertent rotation of the guard  30 . 
     Referring again to  FIG.  1   , the illustrated housing  15  is a clamshell housing having left and right cooperating halves  35 ,  40 . A first or rear handle  45  extends from a rear portion of the housing  15  in a direction generally opposite the support arm  20 . A trigger  50  for operating the saw  10  is located on the rear handle  45 . In the illustrated embodiment, the saw  10  also includes a second or forward handle  55  that wraps around an upper portion of the housing  15 . The forward handle  55  and the rear handle  45  provide grip areas to facilitate two-handed operation of the saw  10 . 
     Referring to  FIG.  2   , the saw  10  further includes a motor housing  60  formed within the housing  15  at a front, lower portion of the housing  15 . An electric motor (not shown) is mounted in the motor housing  60 . The motor is preferably a brushless direct-current (“BLDC”) motor. Operation of the motor is governed by a motor control system  65  including a printed circuit board (“PCB”)  70 . 
     With reference to  FIGS.  1  and  2   , the illustrated saw  10  is a cordless electric saw and includes a battery  75  that provides power to the motor. The battery  75  is removably coupled to a battery receptacle  80 , which is located on the upper portion of the housing  15  in the illustrated embodiment ( FIG.  1   ). As such, the forward handle  55  at least partially surrounds the battery receptacle  80  and the battery  75 , when the battery  75  is coupled to the receptacle  80 . In other embodiments, the saw  10  may be a corded electric saw configured to receive power from a wall outlet or other remote power source. The illustrated battery  75  is a power tool battery pack and includes a battery housing  85  and a plurality of rechargeable battery cells  90  ( FIG.  2   ) disposed within the housing  85 . The battery cells  90  are lithium-based battery cells but can alternatively have any other suitable chemistry. In the illustrated embodiment, the battery  75  has a nominal output voltage of about 80V. In other embodiments, the battery  75  can have a different nominal voltage, such as, for example, 36V, 40V, 72V, between 36V and about 80V, or greater than 40V. 
     The saw  10  includes a drive assembly  100  for transmitting torque from the motor to the cutting wheel  25  ( FIG.  3   ). The drive assembly  100  includes a drive pulley  105  fixed to an output shaft (not shown) of the motor, a driven pulley  110  connected to the drive pulley  105  by a belt  115 , a spindle  120  fixed to the driven pulley  110  ( FIG.  7   ), and a clamp assembly  125  coupled to the spindle  120 . In some embodiments, a clutch mechanism may be provided between the output shaft and the drive pulley  105  to selectively interrupt torque transfer between the output shaft and the drive pulley  105 . The clamp assembly  125  includes clamping disks  130   a ,  130   b  that hold the cutting wheel  25 . 
     With reference to  FIGS.  1 - 3   , the drive pulley  105  defines a first rotational axis  135 , and the driven pulley  110  defines a second rotational axis  140  spaced from the first rotational axis ( FIG.  3   ). The support arm  20  includes a first arm portion  145  coupled to the housing  15  and a second arm portion  150  coupled to the first arm portion  145 . In the illustrated embodiment, the first arm portion  145  includes a mount  155  to which the motor is directly fastened ( FIG.  2   ). The output shaft of the motor extends through the first arm portion  145  to the drive pulley  105  ( FIG.  3   ). The spindle  120  extends through the second arm portion  150  and is supported by two bearings  160 . The driven pulley  110  and the clamp assembly  125  are located on opposite sides of the second arm portion  150 . In the illustrated embodiment, first and second covers  165 ,  170  ( FIG.  1   ) are secured to the first and second arm portions  145 ,  150  to enclose the drive assembly  100  during ordinary operation. The covers  165 ,  170  are coupled to the respective arm portions  145 ,  150  by screws, but can be attached via a snap fit or any other suitable manner in other embodiments. 
     With reference to  FIG.  12   , the illustrated belt  115  is a synchronous belt having a plurality of teeth  173  extending laterally across a width of the belt  115 . The teeth  173  are engageable with corresponding teeth  175  on the driven pulley  110  and the drive pulley  105 . The toothed engagement between the synchronous belt  115  and the pulleys  105 ,  110  prevents the belt  115  from slipping under high loads as may occur with a v-belt. In addition, the relatively flat profile of the synchronous belt  115  allows the drive pulley  105  to be smaller in diameter when compared with a v-belt configuration. As such, a higher reduction can be achieved between the drive pulley  105  and the driven pulley  110 . For example, in some embodiments, the drive pulley  105  and the driven pulley  110  may be sized to provide a  4 : 1  reduction from the motor output shaft to the spindle  120 . In other embodiments, the drive pulley  105  and the driven pulley  110  may be sized to provide between a  3 : 1  and a  5 : 1  reduction from the motor output shaft to the spindle  120 . 
     This relatively high reduction ratio advantageously eliminates the need for a separate gearbox or gear reduction stage between the motor output shaft and the drive pulley  105 , thereby improving mechanical efficiency and reducing the size, cost, and weight of the drive assembly  100 . In the illustrated embodiment, the drive assembly  100  has a mechanical efficiency (i.e. a ratio of power at the spindle  120  to power at the output shaft of the motor) between about 95 percent and about 98 percent. In contrast, a drive assembly requiring a gearbox may have a mechanical efficiency of only about 92 percent or less. The relatively high reduction ratio also can allow the motor to spin at a higher rate compared to v-belt and direct drive configurations, which can improve cooling and performance. In some embodiments, the motor has a maximum output speed greater than 10,000 RPM. In other embodiments, the motor has a maximum output speed between about 10,000 RPM and about 30,000 RPM. In the illustrated embodiment, the motor has a maximum output speed of about 20,000 RPM. Finally, the synchronous belt  115  advantageously does not require tensioning. Accordingly, the saw  10  need not include means for adjusting the tension of the belt  115 , which reduces the weight, complexity, and cost of the drive assembly  100 . In addition, the saw&#39;s performance will stay relatively consistent over the lifetime of the belt  115 . In contrast, v-belts typically stretch after a period of ordinary operation and must be manually or automatically tensioned from time to time to prevent slippage. 
     The drive assembly  100  of the saw  10  advantageously provides for quieter operation than typical cut-off saws. Table 1 lists sound pressure levels in decibels (dBa) measured during operation of the saw  10 . The sound pressure levels were measured when operating the saw  10  with a diamond cutting wheel  25 , a composite cutting wheel  25 , and with no cutting wheel  25  attached. The sound pressure levels were measured in two locations: at the front of the saw  10 , and at a typical operator position (i.e. above and behind the rear handle  45 ). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 SOUND PRESSURE LEVEL TESTS 
               
            
           
           
               
               
               
            
               
                   
                 Measurement Location 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Cutting Wheel Material 
                 Front of Tool 
                 Operator Position 
               
               
                   
                   
               
               
                   
                 Diamond 
                 88.2 dBa 
                 86.3 dBa 
               
               
                   
                 Composite 
                 75.9 dBa 
                 79.3 dBa 
               
               
                   
                 No Cutting Wheel 
                 75.3 dBa 
                 77.9 dBa 
               
               
                   
                   
               
            
           
         
       
     
     As evident from the data in Table 1, the saw  10  produces less than 90 dBa during operation. In some embodiments, the saw  10  produces less than 85 dBa during operation. In some embodiments, the saw  10  produces less than 80 dBa during operation. In contrast, it has been found that other cut-off saws on the market produce more than 95 dBa during operation. Human perception of sound pressure is such that an increase of 10 dBa sounds approximately twice as loud. Accordingly, it is evident that the saw  10  would be perceived by an operator as significantly quieter than other cut-off saws. 
     The saw  10  also advantageously produces less vibration than typical cut-off saws. Table 2 lists hand-arm vibration (HAV) values for the saw  10 . Accelerometers were positioned on the rear handle  45  (Location #1) and on the forward handle  55  (Location #2). The HAV values were determined during a wet plunge cutting operation and during no-load operation using an HVM100 Human Vibration Meter produced by LARSON DAVIS. The accelerometers measured acceleration along all three axes, and the HVM100 calculated the HAV values based on vector sums of the measured accelerations. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 VIBRATION TESTS 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Plunge 
                 Plunge 
                   
                   
               
               
                   
                 Wet Cut, 
                 Wet Cut, 
                 No Load, 
                 No Load, 
               
               
                   
                 Location #1 
                 Location #2 
                 Location #1 
                 Location #2 
               
               
                   
                 (m/s 2 ) 
                 (m/s 2 ) 
                 (m/s 2 ) 
                 (m/s 2 ) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Trial 1 
                 6.18 
                 6.30 
                 2.83 
                 3.25 
               
               
                 Trial 2 
                 6.00 
                 6.06 
                 2.76 
                 2.65 
               
               
                 Trial 3 
                 5.69 
                 6.06 
                 2.85 
                 2.73 
               
               
                 Trial 4 
                 5.80 
                 5.41 
                 2.84 
                 2.74 
               
               
                 Trial 5 
                 4.99 
                 7.01 
                 2.85 
                 2.79 
               
               
                 Average 
                 5.73 
                 6.17 
                 2.83 
                 2.83 
               
               
                   
               
            
           
         
       
     
     As evident from the data in Table 2, the saw  10  may produce an average no-load HAV between about 2.7 m/s 2  and about 3.0 m/s 2  at either or both the front handle  55  and the rear handle  45 . For example, the illustrated saw  10  produces an average no-load HAV of 2.83 m/s 2  at both the front handle  55  and the rear handle  45 . In other embodiments, the average no-load HAV at the front handle  55  or the rear handle  45  may differ. In some embodiments, the saw  10  may produce an average plunge cut HAV between about 5 m/s 2  and about 7 m/s 2 , or between about 5 m/s 2  and about 6.2 m/s 2  in other embodiments, at either or both the front handle  55  and the rear handle  45 . For example, the illustrated saw  10  produces an average plunge cut HAV less than about 6.2 m/s 2  at the front handle  55  and less than about 5.8 m/s 2  at the rear handle  45 . 
     With reference to  FIG.  1   , the illustrated saw  10  further includes a fluid distribution system  200 . The fluid distribution system  200  includes a connector  205  coupled to the lower portion of the housing  15 , a control valve  210  coupled to the forward handle  55 , and a distributor  215  coupled to the guard  30 . A supply line (not shown) can be attached to the connector  205  to provide fluid such as water to the fluid distribution system  200  from an external source (not shown). A first line (not shown) extends from the connector  205  to the control valve  210 , and a second line (not shown) extends from the control valve  210  to the distributor  215 . In the illustrated embodiment, the distributor  215  includes a pair of spray nozzles  220  disposed on opposite sides of the guard  30  connected by a supply line  222 . The spray nozzles  220  are operable to discharge fluid onto each side of the cutting wheel  25  for cooling, lubrication, and dust abatement. In the illustrated embodiment, an auxiliary handle  225  is attached to the guard  30  through which a portion of the supply line  222  extends that can be grasped by a user to facilitate adjusting an angular position of the guard  30 . However, the handle  225  may alternatively be located elsewhere on the guard  30  remote from the supply line  222 . 
     Referring to  FIGS.  4 - 6 B , the battery receptacle  80  includes a drainage path  230  ( FIGS.  6 A and  6 B ) to direct fluid (e.g., from the fluid distribution system  200 ) from the interface between the battery housing  85  and the battery receptacle  80 . The illustrated battery receptacle  80  includes a pair of guide rails  235  that define an insertion and removal axis  240  of the battery  75  ( FIG.  5   ). The battery receptacle  80  further includes a recessed portion  245  between the guide rails  235 . When the battery  75  is positioned in the battery receptacle  80 , a lower-most surface of the battery  75  is positioned adjacent the recessed portion  245  of the receptacle  80 . The recessed portion  245  of the receptacle  80  has an angled drainage surface  250  that defines an axis  255 , which forms an acute included angle A 1  with the insertion and removal axis  240 . In some embodiments, the angle A 1  is between about 0.5 degrees and about 5 degrees. The drainage surface  250  leads to a drainage hole  265  located at an intersection between the left and right housing halves  35 ,  40  ( FIGS.  4  and  6 A ). The drainage hole  265  communicates with a closed passage  270  that extends laterally through the housing  15 , to the exterior of the housing  15 . 
     With reference to  FIG.  8   , the saw  10  further includes a closed cooling path  300  that extends through the housing  15 . The illustrated housing  15  includes an aperture  305  that extends laterally through the housing  15 , at a position between the rear handle  45  and the motor housing  60 . Slotted air intake openings  310  line the aperture  305  and communicate with the interior of the housing  15 . In the illustrated embodiment, the aperture  305  has a generally pentagonal or five-sided cross-section, and the air intake openings  310  are positioned on three of the five sides of the aperture  305 . The position of the air intake openings  310  in the aperture  305  helps to shield the openings  310  from fluid, dust, and debris present during operation of the saw  10 . In other embodiments, the air intake openings  310  may be arranged and positioned differently. Additionally, the saw  10  may not include the aperture  305 , and may draw intake air from other locations, such as from proximate the rear handle  45 . 
     With continued reference to  FIG.  8   , the air intake openings  310  communicate with an air space  315  that is separated from the interior of the motor housing  60  by a wall  320 . Air drawn through the air intake openings  310  is routed along the cooling path  300  by the wall  320  and various other walls and baffles, which direct the air past a finned heat sink  325  to cool the PCB  70 . After passing over the heat sink  325 , the air can enter the motor housing  60 , cooling the motor before being discharged through slotted exhaust openings  330  located on the bottom portion of the housing  15 . A fan (not shown) is provided with the motor to induce the airflow along the cooling path  300  during operation of the saw  10 . 
     Referring to  FIGS.  3  and  9 - 14   , the support arm  20  of the saw  10  is adjustable between a first or inboard configuration ( FIGS.  3 ,  10 , and  12   ) in which the cutting wheel  25  is generally aligned with a longitudinal mid-plane of the saw  10  and a second or outboard configuration ( FIGS.  11 ,  13 , and  14   ) in which the cutting wheel  25  is offset from the longitudinal mid-plane of the saw  10 . In the illustrated embodiment, the support arm  20  is adjustable between the first and second configurations by rotating the second arm portion  150  by 180 degrees about a longitudinal axis  400  of the support arm  20  ( FIGS.  10  and  11   ). The first arm portion  145  includes first and second opposite, lateral sides  405 ,  410 . The second arm portion includes a flange  415  having an inner side  420  ( FIG.  9   ). In the first configuration, the inner side  420  of the flange abuts the first lateral side  405  of the first arm portion  145  ( FIG.  10   ), and in the second configuration, the inner side  420  of the flange  415  abuts the second lateral side  410  of the first arm portion  145  ( FIG.  11   ). In the illustrated embodiment, the second arm portion  150  is removably secured to the first arm portion  145  by three fastener assemblies  430  (i.e. nut and bolt assemblies) that extend through corresponding bores  435   a ,  435   b  in the first and second arm portions  145 ,  150  ( FIG.  9   ). To facilitate proper alignment of the second arm portion  150  relative to the first arm portion  145 , the second arm portion  150  includes first and second locator pins  440   a ,  440   b  that are received within corresponding first and second locator openings  445   a ,  445   b  in the first arm portion  145 . In the illustrated embodiment, the first locator opening  445   a  is oval shaped and elongated along the longitudinal axis  400  to allow for tolerance variations. 
     The first arm portion  145  includes a drive opening  450  that defines a first axis  455  coaxial with the rotational axis  135  of the drive pulley  105 , and the second arm portion  150  includes a spindle opening  460  that defines a second axis  465  coaxial with the rotational axis  140  of the driven pulley  110  ( FIG.  9   ). A distance D between the first and second axes  455 ,  465  is substantially the same, regardless of whether the support arm  20  is in the first configuration ( FIG.  12   ) or the second configuration ( FIG.  13   ). Thus, the belt  115  extends the same span and a user need not adjust the tension of the belt  115  when changing the support arm  20  between the first and second configurations. 
     Various features of the invention are set forth in the following claims.