Patent Publication Number: US-6663081-B1

Title: Safety mechanism for use with a power unit and a hydraulic jack

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Applications have also been filed directed to a power unit convertible into a jack, a lift bridge, a lift arm assembly, and leveling pads, as described in the present specification. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for lifting and supporting an object i.e. a corner of an automobile; particularly to a two part jacking system including a power unit that can be used to place and elevate a jack stand. The inventor of the present invention is a pioneer of the two part jacking system holding numerous issued patents for a two part jacking system and related products and processes as described below. 
     Briefly, the two part jacking system consists of a power unit and a set of separate mechanical jack stands. Examples of the two part jacking system and power unit are described in detail in U.S. Pat. No. Re. 32,715 and U.S. Pat. No. 4,589,630. Some examples of the jack stands are described in detail in U.S. Pat. Nos. 4,553,772; 4,490,264; 5,110,089; 5,183,235 and 5,379,974. The stands are capable of being vertically extended and retracted from the garage floor or road surface and, when extended, can be locked in place at any desired position by a ratchet and pawl assembly. The power unit has a mobile chassis adapted to carry a plurality of the jack stands, and has a pair of lift arms adapted to mate with the outermost jack stand for placement and removal. 
     In use, the mobile power unit is operated entirely from its handle. It is maneuvered under a vehicle to place a jack stand in a desired location for lift and supporting the vehicle. The power unit is activated from the handle, and this jack stand is then vertically extended to the desired height, thus lift the vehicle on the stand. By operating the controls at the end of the handle, the operator can cause the power unit to disengage from the stand, and the stand will remain locked in its extended supporting position under the vehicle. In this manner, the vehicle is raised and supported on a separate mechanical stand without transferring the vehicle from a jack to a stand and without the operator placing any part of his body under the vehicle or having to touch the stand itself. After the stand is raised and locked in place to support the vehicle or other load in a lifted position, the power unit lift arms are lowered and the power unit is disengaged from the stand and pulled away, leaving the stand in position supporting the load. Another jack stand, carried within the chassis, is automatically transferred to the forward end the chassis for placement at another desired location of the vehicle or for use to lift and support another vehicle. 
     To lower the vehicle and remove the stand, the power unit is maneuvered to reengage with the stand. The engagement causes any existing jack stands carried within the chassis to be automatically transferred rearward within the chassis. By manually operating a control at the end of the handle, the user can cause the power unit to reengage with the stand, and to disengage the ratchet locking mechanism of the stand and lower the stand to its original position. The power unit remains engaged with the stand and can be pulled away from the vehicle with the stand carried within the chassis. 
     The original power units were adapted to carry one or two jack stands within the chassis for consumer use; and up to four jack stands within the chassis for commercial use. Additional jack stands could be purchased and arranged at various stations on the garage floor to reload the power unit, so that a single power unit could be utilized to efficiently place and actuate numerous jack stands. However, it was found that many commercial users would utilize all of their available jack stands, and the power unit was thereafter useless until another jack stand was available to be extracted and reused. It was determined that it would be highly desirable if the power unit could also function as a lift device when no jack stands were available, and when the power unit was separated from the source of jack stands. 
     It was also discovered that consumer projects usually required only two supports, and it was determined that it would be highly desirable to have a consumer system consisting of one jack stand and a power unit that could also be utilized as a lift and supporting device. 
     It was also discovered that commercial users routinely provide emergency field service, and the weight and extended chassis length (for carrying four jack stands) was somewhat unwieldy and difficult to be loaded, transported and maneuvered for such emergency field services. It was determined that it would be highly desirable to have a compact commercial power unit for shop use; and that could also efficiently be used as a stand alone lift and supporting device, as well as a power unit for use with a jack stand, for such field service. 
     The two part system efficiently and effectively eliminates the need to use a cumbersome hydraulic floor jack as a jack stand. However, some consumer and even professional users carelessly and hastily use the hydraulic jack as a supporting device until the project is completed. Hydraulic jacks have suffered from the disadvantage of hydraulic bleed, making performance in supporting a load over an extended period of time, unpredictable and hazardous for the user. It was determined that it would be highly desirable to provide a hydraulic lift device with a reliable mechanical mechanism for securing the lift arm so that it would support the load even upon hydraulic bleed or other failure of the hydraulic lift system. 
     In view of the foregoing problems and desirable features of a two part lift and supporting system, it is an object of the present invention to provide a power unit for use with a jack stand, that is readily convertible for use directly as a load-lifting jack. 
     It is another object to provide a power unit that is automatically adapted for use with a jack stand when engaged with a jack stand, and automatically adapted for use as a load-lifting jack when not engaged with a jack stand. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are accomplished by a safety mechanism and a hydraulic floor jack including a generally rectangular chassis having a forward end and a rearward end and a left flange and tight flange extending upwardly from the chassis. Each flange has a longitudinal retaining slot extending horizontally from the rearward end to about midway along the flange. The chassis has a lift arm assembly including a left lift arm and a right left arm acting in parallel having forward ends and rearward ends and interconnected near the midpoints thereof by a lateral pivotal push bar. The rearward ends of the lift arms are connected by an axel slidably retained within the longitudinal slots in the flanges. 
     The lifting device includes a left connecting arm and right connecting arm acting in parallel having forward ends and rearward ends. The forward ends of the connecting arms are pivotally connected near the forward end of the respective flange. The rearward end of the connecting arms are pivotally connected near the midpoint of the respective lift arm. 
     A hydraulic actuator provides the lift and has a rearward end pivotally mounted at the rearward end of the chassis, and has a forward end attached to the lateral pivotal push bar. The hydraulic actuator is adapted to be extendable whereby the forward ends of the lift arms are raised and the rearward ends of the lifting arms are translated forward along the longitudinal slots of the flanges. 
     The safety mechanism comprises a toothed rack bar and a mechanical dog. The toothed rack bar extends horizontally within the chassis and is adjacent to the longitudinal slots in the flanges. The mechanical dog is pivotally mounted on the axel interconnecting the rearward end of the lift arms, and is adapted to be engagable with a corresponding tooth of the rack bar. The engagement of the dog in the rack bar mechanically secures the rearward end of the lift arms to the chassis, independent of any force of the hydraulic actuator. The dog further is adapted to be releasable from the rack bar so that the rearward end of the lift arms can be translated rearward, to facilitate lowering the jack. 
     A tension spring is advantageously utilized in the release linkage of the safety mechanism so that the dog is not released until the lift arm is pushed slightly forward, for an additional safety feature. The safety mechanism can be adapted to various configurations of floor jacks and can have a variety of suitable lever and cable release mechanisms. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the novel features of the invention are set forth in the appended claims, the invention will be better understood along with other features thereof from the following detailed description taken in conjunction with the drawings, in which: 
     FIG. 1 is perspective view of the present invention, showing a power unit for use with a jack stand positioned within the chassis, and a bridge displaced rearward and not in use; 
     FIG. 2 is a perspective view of the bridge of the present invention; 
     FIG. 3 is a perspective view, similar to FIG. 1, showing the power unit utilizing the bridge positioned forwardly for use as a jack; 
     FIG. 4 is a perspective view of a typical jack stand (in an elevated position) that can be positioned, elevated and lowered by the power unit of FIG. 1; 
     FIG. 5 is a perspective view, similar to FIG. 1, showing the bridge displaced toward the rearward ends of the lift arms to show the forward ends of the lift arm assembly; 
     FIG. 6 is a top plan view of a portion of a lift arm for use with an automatically forward positioned bridge; 
     FIG. 7 is a sectional view taken along  7 — 7  of FIG. 6, and showing a first embodiment of the automatically forward positioned bridge on the forward end of the lift arm; 
     FIG. 7A is a sectional view taken along  7 A— 7 A of FIG. 6, and showing a second embodiment of the automatically forward positioned bridge on the forward end of the lift arm; 
     FIG. 8 is a sectional view taken along  8 — 8  of FIG. 6, showing the first embodiment of the bridge supported on leveling pads at the forward end of the lift arms; 
     FIG. 8A is a sectional view taken along  8 A— 8 A of FIG. 6, showing the second embodiment of the bridge supported on leveling pads at the forward end of the lift arms; 
     FIG. 9 is a sectional view, similar to FIG. 7A, and showing the bridge pushed toward the rearward end of the lift arms (automatically) by the jack stand; 
     FIG. 10 is a perspective view of selected components of the power unit of FIG. 1, showing the lift arm assembly, a leveling pad and a connecting arm; 
     FIG. 11 is a sectional view taken along  11 — 11  of FIG. 10, showing a recess and a compression spring within a guide track of the lift arm; 
     FIG. 12 is a sectional view taken along  12 — 12  of FIG. 10, showing the leveling pad; 
     FIG. 13 is a side elevational view of the rearward end of a lift arm showing a means for securing the rearward end of the lift arm, engaged so that a hydraulic jack can be safely used as a jack stand; 
     FIG. 14 is a side elevational view, similar to FIG. 12, showing the means for securing the rearward end of the lift arm in the disengaged position; 
     FIG. 15 is a top plan view of a portion of the power unit, showing a means for securing the rearward ends of the lift arms of a power unit, that is automatically engaged when the power unit is used without a jack stand; 
     FIG. 16 is a sectional view taken along  16 — 16  of FIG. 15 showing the securing means automatically engaged; and 
     FIG. 17 is a sectional view, similar to FIG. 15, showing the securing means automatically disengaged by a jack stand (shown in phantom lines) actuating a control bar. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Figures and the following specification may describe and define several distinctive inventions that are interrelated within a lifting and supporting system, and may be included in divisional patents (or pending applications) having distinctive sets of claims directed to the respective invention. Also, the power unit and jack stands are discussed and described in terms of an automotive jack system, but it should be understood that the system is not limited to automotive uses and can be utilized for lifting and supporting any type of load. 
     Power Unit Convertible Into A Load-Lifting Jack 
     Referring first to FIGS. 1-5, there is illustrated a mobile power unit  20  of the present invention for conventional use with a jack stand  22  (see FIG.  4 ), that is readily convertible for use as a load-lifting jack by a lift bridge  24 . The bridge is shown in FIG. 1 on the power unit in a displaced inoperative position; and is shown in FIG. 3 in the forward position to convert the power unit for use as a load-lifting jack. The lift bridge is a key element and feature of the present invention, and is shown separately from the power unit in FIG.  2 . 
     The power unit includes a generally rectangular mobile chassis  26  having a forward end  28  primarily for receiving, placing and retrieving the jack stand  22 , and a rearward end  30  including a pair of wheels  32  for providing mobility and maneuverability for the chassis by a handle assembly  34 . The jack stand  22  includes a base having a pair of engagement lugs  21  and enclosing means for releasing the telescopic ratcheting ram of the jack and a lift collar  23  for lifting the ratcheting ram by the power unit. The forward end  28  of the chassis has a “U” shaped opening  29  adapted to slide over the base of the jack stand  22  and under the engagement lugs  21  to carry and control the jack stand. The chassis includes a jack actuation mechanism  37  for engagement with the lugs  21  of the jack stand, to control the lifting and lowering of the jack stand. The handle assembly is also used to control and pump a hydraulic actuator  36  at the rearward end of the chassis and to actuate the mechanisms  37 , at the forward end of the chassis, to control the engagement, release and elevation of the jack stand, as fully described in the prior art patents enumerated in the BACKGROUND OF THE INVENTION. The handle assembly is also used to actuate a release of a locking safety device to be described later in detail. The chassis includes a left flange  38  and a right flange  40  extending generally vertically upward from the sides of the chassis for supporting a lift arm assembly  42 . 
     The lift arm assembly (see also FIG. 10) includes a pair of left and right lift arms  44  acting in parallel having forward ends  46  and rearward ends  48  and interconnected near the midpoints thereof by a lateral push bar  50 , and is adapted to be nested on the chassis  26 . A pair of connecting arms  52  acting in parallel and having forward ends  54  is pivotally connected to the respective flange  38 , 40  with a suitable shoulder rivet fastener  56 , and having rearward ends  58  connected to the respective lift arm (near the midpoint thereof) and sharing a common axis  60  with the push bar. The chassis further includes a longitudinal retaining means, illustrated as a pair of generally horizontal slots  62  extending from near the rearward end of the respective flange to about the midpoint thereof (a distance otherwise corresponding to the range of travel of the rearward end  48  of the lift arm). The rearward ends of the lift arms are interconnected by an axel  63  that is slidably retained within the respective slot of the respective flange; with the forward ends of the lift arms  46  extending freely toward the forward end of the chassis. The longitudinal retaining means can alternatively be provided in the chassis by a suitable horizontally extended upper flange, a channel, or upper rack and pinion mechanism, etc. in alternative embodiments. 
     The hydraulic actuator  36  has a rearward end  64  pivotally mounted to the rearward end of the chassis (see FIG. 1) and has a forward ram end  66  attached to the lateral push bar  50 . When the actuator is extended, the forward ends  46  of the lift arms  44  are raised (by upward rotation of the connecting arm  52 ) and the rearward ends  48  of the lift arms with the axel  63  are translated forward along the slots  62  in the respective flange. In a preferred embodiment, the length of the connecting arms  52  are about half the length of the lift arms; and they are pivotally connected near the center of the lift arm, whereby the forward ends of the lift arms are elevated substantially vertically by the rotation of the connecting arm during the entire range of the lifting operations. 
     The Lift Bridge 
     The lift bridge  24  is illustrated as a separate component in FIG. 2, and shown on the power unit in FIGS. 1,  3  and  5 , as a generally rectangular load bearing plate  68  oriented horizontally and having a forward end  70 , a rearward end  72 , left and right sides  74 , a top surface  76  and a bottom surface  78 . The bridge is adapted to be positioned on the forward ends  46  of the lift arms  44  “to Bridge the open span between the lift arms”, so that the power unit  20  can then be convertible for use as a load-lifting jack. As discussed in the BACKGROUND OF THE INVENTION, The power unit has heretofore been utilized only for use to engage the lift collar  23  of a jack stand  22  and for placing, lifting and retrieving the jack stand. It is readily understood from FIG. 3 that actuation of the hydraulic actuator  36  would raise and lower the load bearing lift bridge  24 , along with the lift arms, for use of the power unit as a hydraulic lifting jack, to greatly expand and enhance the utility of the power unit. The bridge typically, but not essentially, includes a screw-out load bearing saddle plate  82 . The saddle plate including a large threaded shaft  83  extending vertically downward therefrom, and adapted to engage a corresponding threaded aperture in the center of plate  68  of the bridge. Such screw-out saddle plates have been utilized in conventional jacks and are adjustable (up to about six inches) to compensate for the variable clearance heights (i.e. the clearance at the lift point for a low sports car, and the clearance at the lift point of a pickup truck) to further increase the utility of the load-lifting jack of the present invention. 
     A basic embodiment of the lift bridge  24  is a simple plate that is adapted to be positioned by hand by the user into the desired position on the forward ends of the lift arms  44 . The forward ends  46  of the lift arms include members adapted to engage the bottom surface or sides of the bridge. A suitable member is shown (see FIGS. 5 and 10) as leveling pad  84  having a horizontal guide flange  86 . The bridge further includes suitable channels  88  formed into the inner sides thereof and adapted to engage the guide flanges  86  when the bridge is properly positioned on the forward ends of the lift arms. When it is desirable to convert the power unit back for use with a jack stand, the bridge is readily displaced from the forward ends of the lift arms and stored conveniently at a rearward position on the lift arm (as shown in FIGS.  1  and  5 ), or alternatively stored at the handle assembly  34 , or removed entirely from the power unit. 
     The lift bridge  24  is efficiently produced by a metallic casting incorporating the desired recesses and flanges, as well as any other desired features, i.e. a central aperture therein with suitable reinforcing boss, or strengthening ribs or gussets for added strength or for other specific applications. The basic bridge, positioned by the operator, is incorporated into efficient new products; however, it is particularly useful as an accessory for retrofitting the numerous power units currently in use with jack stands, to be convertible for use as load-lifting jacks. 
     During initial development and experimental use of the lift bridge, it was discovered that an unattached bridge was sometimes misplaced, and sometimes required extra time and effort to properly position and remove the bridge during alternate jack stand/lifting jack operations. Continued development resulted in a preferred embodiments in which the bridge is slidably retained within guide tracks extending along the upper surface of the lift arms, so that the lift bridge is integral with the power unit and can be readily positioned by hand into the forward position and alternatively into the displaced position while retained within the tracks of the lift arms. A more preferred embodiment further includes a forward biasing means (i.e. a suitable elastomeric member or a suitable compression spring) so that the lift bridge is automatically urged into a slide forward position on the lift arms whenever a jack stand is not engaged within the chassis; and further, so that the presence of a jack stand within the chassis will automatically push the lift bridge rearward (overcoming the forward bias of the spring) along the lift arm to a displaced position. 
     The Lift Arm Assembly-Automatic Slide Forward Bridge 
     Referring also to FIGS. 6-8, a first embodiment of the lift bridge  24  is described in which the lift arms  44  include upper guide tracks, shown as recessed channels  90  extending from the forward ends thereof to a rearward location past the midpoint thereof (past the attachment of the lateral push bar  50 ). The bridge includes a pair of guide members  92  extending downward from the rearward bottom surface thereof and adapted to fit within the recessed channels of the tracks. The guide members  92  and recessed channels  90  each have suitable shapes to retain the rearward end of the bridge loosely within the guide tracks, and are adapted to facilitate the traverse of the bridge along the upper surface of the lift arm. As shown in FIG. 8, a suitable shape of the guide member  92  of the bridge is an inverted “T” that is retained within a “C” shaped recessed channel  90  of the lift arm. The recessed channel typically extends completely through the forward end of the lift arm for the initial insertion of the guide members into the channel, and is thereafter confined within the track by a fastener that secures the lift pad (and closes the forward end of the channel). This retention means can operate smoothly with very loose tolerances and is quite rugged and reliable within the garage environment. A variety of complementary engaging shapes can be suitable adapted for this type of retention means. The bridge is retained by the engagement of the guide members  92  within the recessed channels  90  whenever the bridge is displaced rearward along the lift arm. The bridge remains integral with the lift arms and functions quite smoothly by hand manipulation from the forward position to the displaced position. 
     Referring now to FIGS. 6,  7 A,  8 A and  9 , another embodiment of the bridge  24  is described in which the lift arms  44  include upper guide tracks, again shown as recessed channels  90  extending from the forward ends thereof to a rearward location past the midpoint thereof The guide tracks each have a suitable shape and internal surface to retain a follower member  94 , with the follower member having a guide pin  96  extending upward from the opening of the channel and adapted to traverse along the upper surface of the lift arm. As shown in FIG. 8A, the follower member  94  has a suitable inverted “T” cross section retained within a “C” shaped recessed channel  90 . The bridge further includes a pair of recessed slots  98  in the rearward bottom thereof adapted to engage the guide pins  96  to facilitate movement of the lift bridge along the track from the forward position to the displaced position. As also shown in FIG. 8A, the guide pins  96  have a suitable “T” shape adapted to fit within a suitable “C” shape of the recessed slots  98  in the bottom of the bridge. The inverted “T” shape of the follower member and the “T” shape of the guide pin form a generally “T” shaped cross section of the follower member to suitable interconnect the bridge within the recessed channel. The bridge is retained by the engagement of the guide pins  96  of the follower member and the recessed slots  98  whenever the bridge is displaced rearward along the lift arm. When the bridge is transferred to the forward end of the lift arms, the side channels  88  of the bridge slide over the guide flanges  86  of the leveling pads  84  until the bridge is fully positioned thereon. This follower member, guide pin, recessed slot, retention means is similarly operable with loose tolerances, and is rugged and reliable in the work environment. With the foregoing components, the lift bridge remains integral with the lift arms and functions quite smoothly by the hand manipulation from the forward position to the rearward displaced position on the power unit. 
     Referring now to FIGS. 6,  7 ,  7 A and  9 , the more preferred embodiments are described wherein the upper recessed channels  90  further include biasing means, shown as compression springs  100  to urge the respective guide member  92  or follower members  94  to the forward ends of the channels of the lift arms. Thus, the lift bridge  24  is automatically urged to the forward ends of the lift arm; and conversely, a force on the forward end  70  of the lift bridge pushes the respective guide member or follower member rearward within the channel to compress the spring. The compression springs are adapted to provide sufficient force to position the bridge at the forward ends of the lift arms (when the lift arms are in a generally horizontal orientation), and further adapted to be readily compressible by the routine engagement of the forward end of the bridge with a base or lifting pad of a jack stand  22  within the forward end of the chassis. The springs  100  are suitably inserted into the channels  90  at the forwards end thereof, prior to insertion of the respective guide member  92  or follower member  94 . 
     The springs  100  are retained within the channels  90  (see also FIG. 11) and automatically expanded to the full span of the channel (see FIGS. 6,  7  and  7 A) along with the respective guide members  92  or follower members  94 , whenever there is no jack stand  22  positioned within the chassis of the power unit, as in FIGS. 3 and 7. The power unit with the automatic slide forward bridge positioned at the ends of the lift arms is thus automatically converted for use as a lifting jack. 
     The springs  100  are shown fully compressed within the channels in FIG. 9, by the engagement of the jack stand  22  and correspond to the power unit shown in FIG.  1 . The power unit with the automatic slide forward bridge forced rearward by the engagement of a jack stand, is thus automatically converted for use with the jack stand. 
     The foregoing left lift arm, right lift arm and lateral push bar are efficiently manufactured as a single integral casting to form the lift arm assembly  42 . The guide track recessed channels  90  are also efficiently and reliably directly formed into the casting. This casting assures reliable precise alignment of the lift arms and push bar, and further assures high strength and durability of the assembly. The assembly can be completed with minimum machining of the apertures and interactive surfaces, for final assembly with the mating components. 
     Leveling Pads 
     It is highly desirable for the forward ends of the lift arms to maintain a uniform generally level platform (parallel with the chassis) throughout the lifting operation. This is important for proper engagement of the forward ends of the lift arms  44 with the lift collar  23  of the jack stand  22  for use as a power unit; and also for engagement of the lift arms with the bridge  24  for level contact with the object to be lifted, for use as a jack. Basic leveling means can be incorporated, i.e. a spherical (or circular) ball and socket engagement between the pairs of components that permits relative rotation with changes in the angle of the lift arms. Another example of a basic leveling means can include a pivotal member having a heavy weight extended downward that tends to remain plumb during changes in the angle of the lift arms. 
     Referring particularly to FIGS. 3,  5 , and  10 , a preferred embodiment of a leveling means is shown that includes the pair of leveling pads  84  attached to the outer side of the forward ends  46  of the lift arms  44 . The leveling pad comprises a vertically oriented rectangular plate  102  having an upper surface  104  aligned with the upper surface of the lift arm, a forward end  106  and a central horizontal aperture therein pivotally attached coaxial at  108  with an aperture of the respective lift arm. As previously discussed, the leveling pad further includes a guide flange  86  extending horizontally from the upper surface thereof and adapted to engage the respective channel  88  in the inner side of the bridge. A lever arm  110  extends forward and downward from the plate, and has a length corresponding to the length of the pivotal connecting arm  52 . A pair of connecting rods  112  interconnects the end of the lever arms to a point  114  on the pivotal connecting arm  52 , so that as the angle between the forward portion of the lift arm and the connecting arm increases, the angle between the lift arm and the lever arm  110  decreases so that the upper surface of the leveling pads remains horizontal during the entire ranged of elevation of the lift arm. The connecting rod  112  does not carry any significant load and is used only to rotate the leveling pads and is thus a relatively small member that is retained by suitable washers and cotter pins  116 . 
     In an initial concept, the leveling mechanism was designed with the forward portion of the lift arm in a level orientation, with the lever arm  110  extending downward at an angle corresponding to the angle of the connecting arms  52  (about 20 degrees). The mechanism was conceived to operate as a parallelogram having the length of the connecting rods  112  equal to the distance from the aperture  108  to the common axis  60  of the lift arm; and the distance from the common axis  60  to the connection point  114  on the connecting arms  52  to be equal to the length of the lever arm  110 . However, in an initial embodiment, it was necessary to slightly modify the component relationships to empirically perfect the desired level platform. In the example of the embodiment, the length of the lift arm  44  is about 12 inches, the distance from the aperture  108  to the common axis  60  is about 5.50 inches, the length of the lever arm  110  is about 2 inches, the connecting point  114  is about 1.05 inches from the common axis  60 , and the connecting rods are about 6.25 inches in length. Although the concept of the parallelogram is believed to have merit, the above relative lengths and connection points are suitable to produce the desired level platform on the lift arm and can be proportioned for other embodiments having lift arms with a different length. 
     This leveling mechanism was developed in conjunction with the power unit and bridge for use as a jack. However, it should be understood that this feature is adaptable to all conventional floor jacks and pivotal linkages. 
     Safe Jack Securing Mechanism 
     As discussed in the BACKGROUND OF THE INVENTION, a hydraulic jack is dependent upon a series of fluid valves and seals and carefully aligned piston and ram components. With time or excessive use, these seals tend to leak fluid and loose hydraulic pressure known as “hydraulic bleed” and the jack becomes unreliable and unsafe. In spite of excessive use and wear, some hydraulic jacks can function in the dynamic mode with repeated pumping to extend the ram, and can lift a load; however, without the repeated pumping of the actuator, the ram can not sustain a static load for an extended period of time, without a block or mechanical stand, and the jack fails to support the load. 
     Another feature of the present invention is a hydraulic floor jack that includes a mechanical securing mechanism that converts the jack into a safe mechanical jack stand. This mechanism was developed in conjunction with the power unit and bridge for use as a safe jack, and for a safe jack stand. However, it should be understood that this feature is adaptable to all conventional hydraulic floor jacks. 
     Referring now to FIGS. 1,  13  and  14 , a basic embodiment of a safe securing mechanism is shown in which the lift arm assembly is extended by the hydraulic actuator  36  and the rearward ends of the lift arms  44  and axle  63  are translated along the chassis within the retaining slots  62  in the flanges of the chassis, as previously discussed. The securing mechanism comprises a toothed rack bar  118  extending horizontally along the chassis and adjacent to the retaining slot; and a mechanical dog  120  pivotally mounted on the rearward end  48  of the lift arm and adapted to be engageable with a corresponding tooth  122  of the rack bar. The securing mechanism can function by securing only one of the rearward ends of the lift arms, but preferably is adapted to secure both of the rearward ends of the lift arms for balance of the loads and as an added safety factor for the lifting and supporting device. 
     The rack bar is typically formed of steel plate or casting about 0.5 inches wide and extends the length of the slot. The bar does not require welding and is efficiently attached to the base of the chassis with conventional rivet fasteners  126 . The teeth  122  are typically evenly spaced and can have a variety of suitable shapes, however the teeth are preferably inclined toward the forward end of the chassis. The dog is suitably formed from a steel plate or casting and the weight of the extended dog is naturally forced by gravity onto the toothed bar. The engagement of the dog with the toothed bar can be further ensured by a coaxial torsion spring (not shown) exerting downward pressure on the dog. The dog is thus adapted to be engaged with a corresponding tooth of the rack bar at all times (unless intentionally disengaged), to mechanically secure the rearward ends of the lift arms to the chassis independent of any force of the hydraulic cylinder on the lift arm. 
     It is readily seen in FIG. 13 that any downward force on the lift arm (by a failed hydraulic actuator) which would tend to push the rearward end of the lift arm (to the right in FIG. 13) would be securely locked by the engagement of the dog within a corresponding tooth of the bar, whereby the hydraulic jack can be operable for use as a safe jack stand that can not fail. The forwardly inclined teeth of the bar and the weighted (or torsion sprung ) engagement of the dog, allows the lift arm to ratchet forward within the slot as the lift arm is raised upward (to the left in FIG.  13 ), but securely lock against any downward movement (to the right in FIG.  13 ), unless the dog is actuated into the disengaged position as shown in FIG.  14 . 
     The dog of the securing mechanism can be actuated to be engaged or disengaged by suitable latches, detent means, cables and linkage (not shown), directly by an operator. Preferably, the dog further includes a spring release linkage  124 , shown in FIGS. 13 and 14, at the upper periphery of the dog that can be operated from the handle assembly  34  to release the securing means from the lift arm. The shape of the dog and teeth are preferably designed so that any rearward force on the lift arm forces the dog to be securely locked into the tooth of the bar. To release the mechanism, the linkage is actuated and the tension spring exert a rotational torque on the dog; however, the dog can not be actually disengaged until the force of the load is relieved from the dog, by slightly extending the actuator and raising the lift arm. The securing mechanism is released when not needed to secure the jack and is naturally released when the job is completed and the jack is intentionally lowered and removed. 
     Securing Mechanism Adapted to the Power Unit with Bridge 
     Referring to FIGS. 15-17, the securing mechanism is shown adapted to the power unit of the present invention to automatically lock the lift arm  44  against rearward translation within the slots  62  when the power unit is used as a jack and jack stand. As previously discussed, the securing mechanism can be adapted to only one lift arm, but is preferably adapted to both of the lift arms of the power unit. The toothed rack bars  118  are shown positioned adjacent to the retaining slots  62  in the flanges of the chassis, inboard of the lift arms  44  and are secured to the chassis  26  by suitable rivet fasteners  126 . The mechanical dogs  120  are mounted coaxial with the rearward ends and axel  63  of the lift arms and are normally in contact with the teeth  122  of the bar (see FIG. 16) as previously discussed. 
     The securing mechanism is automated by a pair of slip rings  128  that are coaxially mounded around the lateral push bar  50  having a pair of tension springs  130  attached to the upper portion  132  thereof, and interconnected to an upper peripheral portion  134  of the dogs. The normal position of the slip rings is such that the tension springs are slack and fully contracted with no tension thereon, and the dogs are engaged with the toothed rack bar as shown in FIG.  16 . The foregoing normal position corresponds to the power unit not engaged with a jack stand  22 , and the bridge  24  is in the automatic slide forward position on the forward ends of the lift arms (see FIG.  3 ), and the power unit is adapted for use as a jack and jack stand. The power unit can thus be operated as a jack and raised to a desired level and positioned as a jack stand, as previously discussed, with the dogs automatically ratcheted forward within the teeth of the bar, and securely locked against any rearward movement of the lift arms. 
     The slip rings  128  include a linkage mechanism comprising a control lever  135  extending from the lower periphery of the slip rings, and interconnected by a control bar  136 . When a jack stand  22  (see FIG. 4) is engaged into the chassis of the power unit, a portion of the base thereof (shown in phantom lines,  22 ) contacts the control bar  136  and rotates the slip rings about 45 degrees counter-clockwise (about one inch at the periphery thereof) and places the spring  130  in tension, and applies a corresponding rotational force of the periphery of the dog  120 . However, if there is any rearward force on the dog, the dog remains forced into engagement in the tooth of the bar until the actuator is extended to slightly lift the load; then the tension of the spring is sufficient to rotate the dog, and thus the safety mechanism can not accidentally release the load. The slip rings can further include suitable stops (i.e. on the lift arm or chassis) to confine the range of rotation thereof to prevent any over extension of the spring  130 . 
     It may also be convenient to manually release the safety mechanism when the power unit is being lowered or repositioned without a load. A suitable manual release is provided with a linkage  138  attached to the control bar  136 , and extending to a suitable lever at the top of the handle assembly  34  (not shown). 
     While specific embodiments and examples of the present invention have been illustrated and described herein, it is realized that modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the spirit and scope of the invention.