Abstract:
A portable rowing machine is disclosed that includes a tubular frame consisting of two parallel tubes positioned and supported by floor brackets, a seat that is removably situated on top of the tubular frame and slidable along the tubular frame on rollers mounted to the underside of the seat, swivel arms attached to brackets and extending perpendicularly outward from the tubular frame, and oar arms attached to the ends of the swivel arms. The tubular frame includes a telescoping portion enabling adjustment of the length of the frame from a compact storage size to a size desired for a users leg length. The swivel arm brackets are pivotally positionable in a storage position and a working position. The oar arms are adjustable in length to suit the users body size. A clutch bearing and band brake assembly provides oar arm motion resistance comparable to rowing a boat.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to exercise machines and more particularly to a portable rowing machine. 
     BACKGROUND OF THE INVENTION 
     Many variations on exercise machines are well known in the prior art. One particular type of exercise machine that remains very popular is the rowing machine. Generally, rowing machines include a frame upon which a seat is slidably engaged, foot rests at one end of the frame and rowing arms attached to the frame extending out to the side and providing resistance to movement in one direction to simulate the rowing motion developed when one rows a small boat. 
     Rowing machines provide a rigorous workout since the arm, back and leg muscles must be used to overcome the resistance presented by a rowing machine. In addition, abdominal muscles receive a good workout when rowing takes place. 
     Some of the drawbacks of rowing machines known in the prior art include excessive weight, bulkiness and designs that are not readily folded up or collapsed to reduce the size of the machine (height, width and length) for easy storage thereof. 
     Thus, what is needed is a rowing machine that provides all of the exercise features of rowing a boat that is a rugged and sturdy design yet readily collapsible and folded up for convenient storing of the rowing machine when not in use. 
     SUMMARY OF THE INVENTION 
     A portable rowing machine, according to one aspect of the present invention, comprises a frame including a first tube having a first end and a second end, a second tube having a first end and a second end, a first support bracket attached to said first tube and said second tube near said first end of said first and second tubes, and a second support bracket attached to said first tube and said second tube at a location between said first end and said second end of said first tube and said second tube and spaced apart from said first support bracket, said first support bracket and said second support bracket maintaining said first tube and said second tube in a horizontal parallel relationship at a first predetermined distance, an arm bracket attached to said first tube and said second tube at a location between said first and said second support brackets, a first arm having a proximal end and a distal end and attached to said arm bracket at the proximal end of said first arm and extending substantially perpendicularly outward from said first tube, said first arm including a first hinge attached to the distal end of said first arm, a second arm having a proximal end and a distal end and attached to said arm bracket at the proximal end of said second swivel arm and extending substantially perpendicularly outward from said second tube, said second arm including a second hinge attached to the distal end of said second arm, a first clutch bearing having an inner race and an outer race, and wherein the inner race of said first clutch bearing is attached to said first hinge, a second clutch bearing having an inner race and an outer race, and wherein said inner race of said second clutch bearing is attached to said second hinge, first brake means positioned in contact with said outer race of said first clutch bearing, said first brake means creating a frictional resistive force with said outer race of said first clutch bearing, second brake means positioned in contact with said outer race of said second clutch bearing, said second brake means creating a frictional resistive force with said outer race of said second clutch bearing, a first oar arm attached to said first brake means, a second oar arm attached to said second brake means, a seat bracket including a horizontal planar member, a first pair of rollers and a second pair of rollers rotatably mounted to the underside of said planar member, wherein said first pair of rollers are spaced apart said first predetermined distance from said second pair of rollers, and said first and said second pair of rollers having a surface profile adapted to engage said first and said second tubes, and wherein said seat bracket is situated so that said first pair of rollers engages and rests upon said first tube and said second pair of rollers engages and rests upon said second tube, a seat cushion attached to said horizontal planar member, and a horizontal foot rest attached to said second support bracket. 
     One object of the present invention is to provide an improved portable rowing machine. 
     Another object of the present invention is to provide a portable rowing machine that is configurable in an operating configuration and configurable into a storage configuration having a small footprint. 
     Yet another object of the present invention is to provide a portable rowing machine that includes an oar arm mechanism that operates to accurately recreate actual rowing resistance. 
     These and other objects of the present invention will become more apparent from the following figures and description of the preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top perspective view of a portable rowing machine according to the present invention shown in a fully extended operational configuration. 
         FIG. 2  is a side elevational view of the portable rowing machine of  FIG. 1 . 
         FIG. 3  is an end view of the portable rowing machine of  FIG. 1 . 
         FIG. 4  is a top perspective view of the portable rowing machine of  FIG. 1  in a storage configuration and having some parts not shown. 
         FIG. 5  is a plan view of the rowing machine shown in  FIG. 4 . 
         FIG. 6  is a side elevational view of the rowing machine shown in  FIG. 4 . 
         FIG. 7  is an end view of the rowing machine shown in  FIG. 4 . 
         FIG. 8  is a top perspective view of a swivel arm-oar assembly shown in  FIG. 4 . 
         FIG. 9  is a plan view of a swivel arm-oar assembly. 
         FIG. 10  is a partial cutaway side elevational view of a swivel arm-oar assembly. 
         FIG. 11  is a plan view of the brake-clutch bearing assembly  50 . 
         FIG. 12  is a cross-sectional view of the brake-clutch bearing assembly  50  looking in the direction of the arrows labeled A-A in  FIG. 9 . 
         FIG. 13  is a top perspective view of the slide rail assembly with some items removed. 
         FIG. 14  is a bottom perspective view of the seat bracket depicting the rollers mounted to the underside thereof. 
         FIG. 15  is a top perspective view of rail plate bracket  17 . 
         FIG. 16  is a front elevational view of release block  42 . 
         FIG. 17  is a front perspective view of release block  42 . 
         FIG. 18  is a front perspective view of band brake holder  76 . 
         FIG. 19  is a front perspective view of cylindrical hinge portion  46   b.    
         FIG. 20  is a close up elevational view of an oar  54  and oar extension  56  of  FIG. 1  including a cross-section of compression nut  75 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring now to  FIGS. 1 ,  2  and  3 , a portable rowing machine  10  according to the present invention is shown.  FIG. 1  is a perspective view of rowing machine  10 .  FIG. 2  is a side view of rowing machine  10 .  FIG. 3  is an end view looking from the foot end of rowing machine  10 .  FIGS. 1-3  depict rowing machine  10  in operational configuration. Rowing machine  10  includes a frame consisting of tubes  12  and  14  which are situated in a parallel arrangement as shown with foot plate brackets  16  and  18  providing vertical support from below to tubes  12  and  14  as well as establishing parallel spacing of tubes  12  and  14 . Foot plate brackets  16  and  18  are attached (preferably welded) to tubes  12  and  14  and provide vertical support from below as well as establishing fixed parallel spacing between tubes  12  and  14 . In addition, brackets  16  and  18  include apertures for mounting rubber feet  20  to the underside of brackets  16  and  18 . Rail plate bracket  17  is also attached to the underside of tubes  12  and  14  at the location shown. Tubes  22  and  24  are radially sized for insertion into tubes  12  and  14  and are slidably received therein to create a telescoping assembly. Slide rail bracket  26  is attached at the distal ends of tubes  22  and  24  as shown to maintain tubes  22  and  24  in parallel spaced apart alignment. Bracket  26  includes apertures therein for mounting rubber feet  20  to the underside thereof. Bracket  26  also includes ears  30  projecting above tubes  22  and  24  wherein foot rod  32  is attached. Foot rod spacers  34  are hollow tubes positioned over rod  32  and include foot strap brackets  36  situated at both ends of foot rod spacers  34 . Two flexible foot straps  38  are attached as shown to the four foot strap brackets  36  and are adjustable in length to receive a variety of foot sizes between strap  38  and foot rod spacer  34 . Brackets  36  and spacers  34  are retained on tube  32  by way of c-clips  40  engaging slots at the distal ends of tube  32 . 
     Release block  42  is vertically movable between bracket  17  and tubes  22  and  24 . Release block  42  is situated on vertically oriented pins attached to bracket  17  and a spring (shown in  FIG. 16 ) urges block  42  upwards in contact with tubes  22  and  24 . Release block  42  also includes locating pins (shown in  FIG. 17 ) extending upward therefrom that engage a series of holes or apertures (shown in  FIG. 13 ) machined in the underside of tubes  22  and  24  to provide a plurality of locking positions for tubes  22  and  24  as they are extended or telescoped outward from within tubes  12  and  14 . The flat surfaces on the uppermost surfaces of tubes  22  and  24  provide an area upon which numerals are placed (either using permanent markings, adhesive labels or by way of machining) that enable the user of machine  10  to quickly extend tubes  22  and  24  outward from a storage position where bracket  24  is adjacent bracket  17  to a desired operating position based on the height and leg length of the user of machine  10 . More detail regarding release block  42  is shown in  FIGS. 16 and 17 . 
     Swivel arms  44  are pivotally attached to rail plate bracket  17  and extend substantially perpendicularly outward from and slightly above tubes  12  and  14  in the operational position shown in  FIGS. 1-3 . Washers or bushing spacers  45  are positioned on both sides of swivel arm  44  adjacent the attachment point to bracket  17  to reduce friction between swivel arms  44  and bracket  17  and provide support for swivel arms  44 . Hinges  46  are mounted to the distal end of arms  44 . Band brakes  48  and clutch bearing assemblies  50  are mounted on hinges  46  and are pivotal about pivot pin  46   a . Clutch bearing assemblies  50  include band brakes  48  which make frictional contact with the outer bearing race of clutch bearings situated within assemblies  50 . Brake adjustment knobs  52  provide a mechanism for mechanical adjustment of the frictional or resistive force created between clutch bearing assemblies  50  and band brakes  48 . Band brake covers  53  are mounted over clutch bearing assemblies  50  to enclose band brake  48  and clutch bearing assembly  50  components. Oars  54  are attached to band brakes  48  via band brake holders  76 . The free ends of oars  54  extend inwards and over toward tubes  22  and  24 . Oars  54  include oar extensions  56  that are telescoping tubes situated within oars  54  and axially movable and lockable within oars  54 . Compression nuts  75  and split rings (shown in  FIG. 20 ) within nuts  75  provide a locking mechanism to secure oar extensions  56  in any axial telescoping position with respect to oars  54 . Compression nuts  75  and corresponding split rings are products manufactured by Valco/Valley Tool and Die, Inc. of North Royalton, Ohio (www.valcocleve.com), part numbers NC-100 and NC-1087, that function to lock the axial position of oar extension  56  with respect to oar tube  54 . Handle grips  58  are attached over the distal ends of oar extensions  56  and provide a convenient gripping surface for the user of machine  10 . 
     Roller bracket  60  includes four rollers  62  having concave surfaces that are rotatably attached as shown and which are situated to engage and rest upon the upper surfaces of tubes  12  and  14 . Bracket  60  also includes a horizontal surface upon which seat cushion  64  is mounted. Rollers  62  enable bracket  60  and cushion  64  to move freely along the upper surface of tubes  12  and  14  when an operator is seated thereon and operating rowing machine  10 . 
     Locking pins  66  are linearly movable in slots  68  formed in swivel arms  44 . Locking pin retainers  70  are attached to the underside of arms  44  and include springs (not shown) that urge locking pins  66  toward rail plate bracket  17 . Rail plate bracket  17  includes slots  17   a  and  17   b  (see  FIG. 15 ) for receiving locking pins  66 . Swivel arms  44  are positioned in one of two positions, a storage position (shown in  FIGS. 4-6 ) and an extended or operating position (shown in  FIGS. 1-3 ). Moving locking pins  66  toward corresponding hinges  46  releases swivel arms  44  for rotational movement into either the operating position or the storage position. Upon moving swivel arms  44  into one of the two available rotational positions, locking pins  66  reengage slots  17   a  or  17   b  (see  FIGS. 4 and 15 ) in bracket  17  to lock swivel arms  44  in position. 
     Operationally speaking, a user depresses release block  42 , slides tubes  22  and  24  in a telescoping fashion outward from within tubes  12  and  14  to a desired leg length position, sits upon cushion  64 , places his feet inside straps  38 , grips handles  58  and slides seat bracket  60  along tubes  12  and  14  while pulling the handles  58 . Clutch bearing assemblies  50  enable a free movement of the oars  54  toward bracket  26  and present a resistance to movement in the opposite direction of movement. The resistance to movement of the oars when moved away from bracket  26  toward bracket  16  is created by the band brake and clutch bearing interaction which will be further discussed in detail below. When the user is finished with exercising on rowing machine  10 , locking pins  66  are moved outward to enable swivel arms  44  to pivotally rotate into a position adjacent tubes  12  and  14  and release block  42  is depressed to enable tubes  22  and  24  to slide within tubes  12  and  14  thereby reducing the width, height and length of rowing machine  10  for storage purposes. 
     Referring now to  FIGS. 4-6 , portable rowing machine  10  is shown configured into the storage position.  FIG. 4  is a perspective view,  FIG. 5  is a plan view, and  FIG. 6  is a side elevational view of rowing machine  10  with several items such as seat cushion  64 , rubber feet  20 , foot straps  38 , band brakes  48  and handle grips  58  removed from each figure. Tubes  22  and  24 , attached to slide rail bracket  26 , are shown in the retracted or storage telescoping position. The storage position shown for the foot support area of machine  10  is realized (from the extended position) by depressing release block  42  downwards and applying horizontal force to bracket  26  in the direction of bracket  17  thereby sliding tubes  22  and  24  into tubes  12  and  14 , respectively. Swivel arms  44  are shown in the storage position with oars  54  positioned adjacent arms  44  to achieve the most compact height footprint for rowing machine  10 . Locking pins  66 , movable in slots  68 , are shown positioned in slot  17   b  of bracket  17  (see  FIG. 15 ) so that swivel arms  44  are secured in the storage position shown. Slots  17   a  are engaged by pins  66  when arms  44  are extended outward. Band brake covers  53  are attached to band brake holders  76  (shown in more detail in  FIG. 18 ). 
     Also shown in  FIGS. 4-6  are roller bracket  60 , rollers  62 , brackets  16  and  18 , oar extensions  56 , hinges  46 , clutch bearing assemblies  50 , foot rod  32 , foot rod spacers  34 , and foot strap brackets  36 , compression nuts  75 , washers or bushings  45  and locking pin retainers  70 . 
     Referring now to  FIG. 7 , an enlarged end view of rowing machine  10  is shown from the slide rail bracket  26  end thereof. Some items such as handle grips  58 , band brakes  48 , clutch bearings  51 , foot straps  38  and rubber feet  20  are not shown in this figure. Tubes  22  and  24  are shown axially positioned within tubes  12  and  14 , respectively. Tubes  22  and  24  are attached to slide rail bracket  26  as shown. Bushing washers  45  are positioned on upper and lower surfaces of swivel arms  44  where arms  44  are pivotally attached to bracket  17 . Nut and bolt fasteners  71  pivotally secure swivel arms  44  and washers  45  in place on bracket  17 . Swivel arms  44  are shown positioned in a storage position. Hinges  46  are attached to swivel arms  44 . Rollers  62  are shown attached to roller bracket  60  and positioned to rotatably engage the upper surfaces of tubes  12  and  14 . Oars  54  and oar extensions  56  are shown positioned adjacent swivel arms  44  in a storage position depicting the typical minimized height and minimized width required for storage of rowing machine  10 . Lock nuts  79  are shown situated on the end of rods  74  (both shown in  FIG. 10 ). Locking pins  66 , brake covers  53 , release block  42 , foot rod  32 , foot rod spacers  34 , compression nuts  75 , locking pin retainers  70  and foot strap brackets  36  are also shown. 
     Referring now to  FIGS. 8-10 , additional details for the swivel arm-oar assemblies of  FIG. 1-6  are shown.  FIG. 8  is a perspective view,  FIG. 9  is a plan view, and  FIG. 10  is a partial cutaway side view of a swivel arm-oar assembly. Some items are not shown in  FIGS. 8 and 10 , in particular, band brake  48 , clutch bearing  51 , compression nut  75  and handle grips  58 . The components of both swivel arm-oar assemblies of rowing machine  10  are identical in components and construction with the only difference being the operating direction (clockwise versus counter-clockwise) of the clutch bearing installed in each assembly  50 . Inverted installation of the clutch-bearing  51  ( FIG. 12 ) provides this rotational difference in the assembly of the swivel arm-oar assemblies. Locking pin retainer  70  is attached to the underside of swivel arm  44  to retain locking pins  66  in slot  68  of swivel arm  44 . A spring (not shown) situated within locking pin retainer  70  urges locking pin  66  into the position shown. Hinge  46  is attached to swivel arm  44  to enable pivotal movement of arm  44  with respect to oar  54  about hinge pin  46   a . Oar extension  56  is situated within oar  54  and is movable axially within oar  54  to provide an adjustable length rowing oar. Oar extension  56  receives oar stop  72  ( FIG. 10 ) axially within oar extension  56 . Oar extension  56  is inwardly radially crimped onto grooves  73  in oar stop  72  to secure oar stop  72  within oar extension  56 . Oar stop  72  is constructed of nylon, Delrin or other similar material. Adjustment of overall oar length is achieved by rotating compression nut  75  (see  FIGS. 1-3 ) and adjusting the axial position of oar extension  56  with respect to oar  54 , then rotating compression nut  75  in a clockwise direction to lock the compression nut and split ring ( FIG. 20 ) between oar  54  and oar extension  56 . Rod  74  is threaded at both ends and mounted in a threaded aperture in band brake holder bracket  76  at one end. Lock nut  79  is attached to the opposing threaded end of rod  74 . Oar extension  56  is retained on rod  74  and within oar  54  by oar stop  72  which moves freely on rod  74  and contacts lock nut  79  when oar extension  56  is moved axially away from hinge  46 . Brake covers  53  are attached using screws  77  to band brake holder  76 . Band brake adjustment knob  52  is also shown in  FIG. 9 . Compression nuts  75  (shown in  FIGS. 1-3 ) are threaded onto threaded portions  54   a  (shown in  FIGS. 8-10 ) of oar  54 . 
     Referring now to  FIG. 11 , a plan view of bearing assembly  50 , with the upper brake cover  53  removed, is shown. Bearing assembly  50  includes band brake  48 , hinge portion  46   b , brake covers  53  (shown in  FIG. 9 ) and clutch bearing  51 . Band brake  48  includes a braking material  48   a  attached about the inner periphery thereof. Band brake  48  is attached to band brake holder  76  via threaded rod  86  and nut  88 . Brake adjustment knob  52  is screwed onto threaded rod  86  to enable adjustment of the braking force applied by band brake  48  to the outer periphery or outer race of clutch bearing  51 . Clutch bearing  51  is positioned over a cylindrical part of hinge portion  46   b  and shaft key  90  is positioned in a keyway in the inner race surface of bearing  51  that is aligned with a keyway  92  in cylindrical portion of hinge portion  46   b  at location  92 . 
     Referring now to  FIG. 12 , a cross-sectional view of clutch bearing assembly  50  looking in the direction of the arrows labeled A-A of  FIG. 9  is shown. Clutch bearing  51  is situated on cylindrical portion of hinge  46   b  (see  FIG. 19 ). Band brake  48  and braking material  48   a  are situated about the external cylindrical surface or outer race of clutch bearing  51 . Band brake covers  53  are situated above and below brake  48  and positioned by bushings or spacers  78  as shown. Spacer  82  is situated between upper band brake cover  53  and band brake cap  80  to prevent vertical movement of bushings  78  and bearing  51 . Screw  84  secures spacer  82  and bushings  78  in position without applying any force to the inner race of bearing  51 . Shaft key  90  is positioned in a keyway in the inner race of bearing  51  and a keyway in cylindrical portion  46   b  at location  92  to prevent rotational movement of the inner race of clutch bearing  51 . Brake covers  53  move freely about cylindrical portion  46   b  yet are retained in position by bushings or spacers  78 . Clutch bearing  51  is a model number CSK30-P manufactured by Xinchang Peak Bearings Co., LTD of Qingshan Industrial Park, Xinchang, Zhejiang 312500, China. Clutch bearings that permit rotation in one direction and lock in the opposing rotational direction are common and other sources for such products are well known in the art. Operationally speaking, when an oar arm is rotated by the user, brake covers  53  and band brake  48  are also rotated about the central axis of the cylindrical part of hinge portion  46   b . Clutch bearing  51  allows free movement in one direction, yet locks up in the opposite direction, thus non-frictional motion is enabled for the brake  48  and clutch bearing  51  rotation in a first rotational direction, and the opposing direction rotation causes clutch bearing  51  to lock the inner race to the outer race and the user must overcome the friction between band brake  48  and the outer race of bearing  51  to move the oar arm in that direction thereby creating a rowing comparable motion resistance. 
     Referring now to  FIG. 13 , a perspective view of the assembly of slide rail bracket  26  and tubes  22  and  24  is shown. Slide rail plugs  27  are secured within the ends of tubes  22  and  24  to provide a friction reducing and mechanical centering function in relation to tubes  12  and  14  of  FIGS. 1-3 . Slide rail plugs  27  also serve to prevent inadvertent disassembly of tubes  22  and  24  from within tubes  12  and  14 . An array of apertures  22   a  and  24   a  are machined in the lower surfaces of tubes  22  and  24 , respectively. Apertures  22   a  and  24   a  are engaged by locating pins situated in release block  42  (see  FIG. 17 ). Foot rod  32 , foot rod spacers  34 , and foot strap brackets  36  are also shown. 
     Referring now to  FIG. 14 , a perspective view of the underside of roller bracket  60  is shown. Rollers  62  are rotatably attached via fasteners  61  to the underside of bracket  60  as shown and spaced apart to correspond with the separation distance between tubes  12  and  14  (see  FIG. 1 ). Apertures  60   a  provide a series of through holes for attaching seat cushion  64  ( FIG. 1 ) to bracket  60 . 
     Referring now to  FIG. 15 , a perspective view of rail plate bracket  17  with riser blocks removed is shown. Rail plate bracket  17  includes slots  17   a  and  17   b  that receive locking pins  66  ( FIGS. 1-3 ) for positioning of swivel arms  44  into one of two positions, namely a storage position and an open or operational position for rowing machine  10 . Apertures  17   c  provide a mounting location for the attachment of riser blocks (shown in  FIG. 1 ) that form a part of rail plate bracket  17  and are disposed between tubes  12  and  14  and rail plate bracket  17 . Apertures  17   d  receive pins  47  (see  FIG. 17 ) mounted in release block  42  to retain release block  42  in position over bracket  17  and adjacent tubes  12  and  14 . Apertures  17   e  receive rubber feet  20 . Recesses  17   f  receive return springs  43  (shown in  FIG. 16 ) that engage apertures  42   a  (see  FIG. 16 ) in release block  42  urging release block  42  upward. Swivel arms  44  ( FIG. 1 ) are attached in apertures  17   g.    
     Referring now to  FIGS. 16 and 17 , a front view and a top perspective view of release block  42  of  FIGS. 1-6  are shown, respectively. Apertures  42   a  receive springs  43  that engage apertures  17   f  in rail plate bracket  17  ( FIG. 15 ). Apertures  42   b  in block  42  receive pins  47  which extend upward through holes (not shown) in tubes  12  and  14  to engage adjacent pairs of positioning holes  22   a  and  24   a  situated along the underside of tubes  22  and  24  (see  FIG. 13 ) thereby locking the extension position of tubes  22  and  24  with respect to tubes  12  and  14  of  FIG. 1 . Pins  47  are situated in apertures  17   d  of bracket  17 . 
     Referring now to  FIG. 18 , a perspective view of band brake holder  76  is shown. Holder  76  provides a mechanical attachment mechanism for affixing oar arms  54  to the band brakes  48  and band brake covers  53 . Oars  54  are attached over cylindrical portion  94  and secured via a locking pin inserted into aperture  96 . Apertures  98  receive bolts or screws that secure band brake covers  53  to band brake holder  76 . Threaded aperture  100  receives threaded rod  86  which forms a portion of the band brake adjustment mechanism. Threaded aperture  99  receives rod  74  shown in  FIG. 10 . 
     Referring now to  FIG. 19 , a perspective view of hinge portion  46   b  that forms a part of hinge  46  is shown. Clutch bearing  51  is installed over cylindrical portion  102 . Keyway  92  receives shaft key  90  ( FIG. 12 ) to prevent rotation of the inner race of clutch bearing  51 . A hinge pin is installed at location  46   a  about which the two parts of hinge  46  are pivotally assembled. Threaded aperture  104  receives screw  84  ( FIG. 12 ). 
     Referring now to  FIG. 20 , a partial front elevational view of oar  54  and oar extension  56  is shown. In this figure, particular detail is provided for compression nut  75  and split ring  106  shown in cross-section. Oar extension  56  is disposed within oar  54  and axially movable therein when compression nut  75  is loosened. The interior of compression nut  75  includes an axial taper in the area adjacent split ring  106 . Nut  75  applies inward axial pressure to split ring  106  when nut  75  is tightened onto threads at  54   a  in oar  54 . The force applied to split ring  106  serves to lock the position of oar extension  56  with respect to oar  54  thereby enabling locking telescoping adjustment of the overall oar length for rowing machine  10 . Such telescopic tubular extension locking mechanisms are known in painting and cleaning extension handle products of the prior art. 
     Most of the components of rowing machine  10  that are structural in nature are fabricated from high strength metals such as aluminum or steel. For the unlimited budget customer, exotic metals such as titanium may be used. The use of aluminum for brackets and tubes serves to reduce the overall weight of machine  10 . Oar arms  54  and oar arm extensions  56  tubing is preferably made from steel tubing as these components are subjected to significant stresses when the user operates machine  10 . Bushings and friction reducing washers are fabricated from delrin, nylon or other similar materials having low friction contact properties and light weight yet also having high strength characteristics. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description of the preferred embodiment, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.