Abstract:
A jack for leveling a pool table fits under at least one leg of the table. The jack has a circular base disk and a circular upper disk, with a gear wheel disposed between the disks. Opposing surfaces on the disks and the gear wheel have pairs of cooperating circular ramped grooves therein, with each pair of grooves receiving a ball. A worm gear has teeth in geared connection with the teeth of the gear wheel. Turning the worm gear causes the gear wheel to rotate, which causes the balls to roll in their respective pair of grooves, thus creating an axial motion of the gear wheel and upper disk. By selectively turning the worm gear in a clockwise or counter-clockwise direction, the axial motion may be used to selectively raise or lower the pool table, in order to level the playing surface.

Description:
TECHNICAL FIELD 
     This invention relates to a device for leveling large objects. More particularly, the invention is a jack which is retrofittable to existing pool tables, and utilizes three disks having ramps of varying depth in which two sets of balls travel, thereby selectively creating an axial lifting or lowering action to level the pool table. 
     BACKGROUND ART 
     It is quite common for the playing surface of a pool table to require leveling. This need can arise because the floor on which the table sits is not level, or because the table itself is not level, or both. Numerous attempts have been made to provide solutions to this leveling problem. 
     Perhaps the most common and well-known solution is leveling the table by placing shims under the feet of one or more of the table&#39;s support legs. The shims may often simply be folded pieces of paper, match book covers, or even slivers of wood. Some shims are even sold at retail, with hard rubber disks being popular, for example. As leveling solutions, shims are simple and somewhat effective, but are generally lacking in durability and ease of use. 
     Another simple solution is through the use of a leveling foot, a device which is often used on chairs, tables, and other furniture to extend their legs. The leveling foot includes a threaded shaft having a larger diameter circular foot on its lower end, and with its upper end extending into a threaded sleeve in the leg of a table, for example. By turning the foot in the desired direction, the shaft may be caused to extend from or retreat into the leg. In this way, the leg may be made effectively longer or shorter, thereby raising or lowering the table as desired. While this is an effective approach for some furniture, it is less effective for pool tables. For one thing, it requires that the leveling foot be un-weighted while the foot of the device is rotated. This works best with lighter furniture such as chairs or light tables. However, pool tables often weigh several hundred to a thousand pounds or more, which would make un-weighting the leveling foot awkward at best. In addition, a leveling foot would be quite difficult for the average user to retrofit to a pool table, further limiting its usefulness. 
     A basic leveling foot arrangement is depicted in U.S. Pat. No. 7,654,911 B2 to Cartwright. Cartwright uses an internally-threaded insert  24 , which includes a threaded metal sleeve with a flange  25  at one end. A hole must be drilled in the bottom of the table leg to allow the insert to be placed in the leg. This allows the rod  22  of leveling foot  26  to be inserted into sleeve  30 , with the entire combination being fitted into the furniture leg  14 . Knob  28  is provided to allow the leveling foot to be operated while the device is still weighted. However, the knob of Cartwright is better suited for use with lighter furniture, as turning the knob while supporting the large weight of a pool table would be difficult. 
     A variation of the leveling foot approach is shown in U.S. Pat. No. 6,729,590 B2 to Gabriel. The device of Gabriel is designed such that it could be operated while still supporting the weight of a pool table. This capability is accomplished by providing a worm gear for driving a driven gear, which in turn drives an elevation shaft to raise or lower an object. However, operating the Gabriel device while still supporting the weight of a heavy pool table would cause large amounts of friction between the threads of the worm gear and those of the driven gear. This degree of friction would require a relatively large amount of force to overcome, and would also necessitate that the device be constructed of steel or other hard metal. 
     Other variations of the leveling foot approach are disclosed in U.S. Pat. No. 1,417,639 to Sterner; and U.S. Pat. No. 3,653,341 to Nielsen. Each of those devices provides a way to drive the leveling foot while still supporting the weight of the table. However, all of the leveling foot devices suffer from friction problems similar to those found with Gabriel. In addition, none of the devices, including Gabriel, are suitable for retrofitting to an existing pool table having no leveling capabilities. Therefore, the devices must be included in the legs of a table when it is sold, which may be seen as an unnecessary added expense by potential buyers of the table. In addition, home pool table legs are often thin at the bottom, making them further unsuited for enclosing a leveling foot device. 
     The friction problems associated with the Gabriel, Sterner, Nielsen, and Cartwright devices could be overcome by utilizing an arrangement of balls or rollers traveling in lifting ramps or grooves between two surfaces as may be seen in U.S. Pat. No. 7,878,543 B2 to Bodtker et al.; U.S. Pat. No. 7,252,017 to Kramer; and U.S. Pat. No. 5,106,349 to Botterill et al. In each of the foregoing patents, two opposing plates or disks have lifting ramps in which balls travel when one of the plates is rotated. As the balls rise or sink on the lifting ramps, an axial motion is created, which may be used to raise or lower a supported object such as a pool table. 
     However, in order to raise or lower a pool table, at least three plates would be required. This is due to the fact that the upper and lower plates would necessarily be non-rotatable while the device was bearing the weight of the table. Yet the aforementioned patents disclose devices with only two plates having opposing grooves in which balls would travel. Therefore, it is necessary to provide some means for handling the friction between the surfaces of the two plates having no opposing grooves between them. One way to handle this friction is disclosed in U.S. Pat. No. 8,662,260 B2 to Baldeosingh et al., which uses a thrust bearing to reduce friction. U.S. Pat. No. 4,016,957 to Osujo et al. uses Teflon for a wear surface, while U.S. Pat. No. 7,735,612 Pozivilko et al. uses a Boss washer and a retaining washer as a bearing surface. 
     While all of these solutions are effective in handling friction between plates, they each require additional parts or material which does not contribute directly to the lifting function of the device. In addition, having two plate surfaces with no grooves therein makes the device unnecessarily thicker, for a given amount of lift. This is counter-productive, since it is of critical importance that the height of the device be kept as low as possible, while still producing sufficient lift. A low height is necessary for any device which is to be retrofitted to the foot of an existing pool table leg, as the overall height of the pool table cannot be excessively increased without changing the look and feel of the game to the players, which would make the device unacceptable. It is thus critical for a retrofittable leveling device to seek the most lift with the least height possible. This is especially true since the weight of a pool table may exceed a thousand pounds. 
     Some of the foregoing problems are alleviated by U.S. Pat. No. 5,713,446 to Organek et al; and U.S. Pat. No. 5,078,249 to Botterill. The devices of Organek and Botterill provide three plates, with  2  sets of balls traveling in two sets of opposing grooves. In this configuration, the friction between the second set of opposing surfaces is handled by the second set of balls themselves, without the need for additional parts merely to handle the friction. The second set of balls also provides a lifting action, thereby making more effective use of the height of the device to produce lift. 
     However, the configuration of Organek and Botterill results in essentially maximizing the required thickness of the control plate, and thereby unnecessarily increases the overall height of the device for any given lift provided. This unwanted result occurs because both devices “stack” the grooves on the upper surface of the control plate directly over the grooves on the lower surface of the control plate, bunk bed style. In particular, the deep end of each lower groove is directly underneath the deep end of a respective upper groove. This means that the thickness of the control plate must be equal to twice the deepest depth of a groove, plus a minimum material thickness between two grooves. This would not work well for a retrofittable leveling device for a pool table, to be placed under one or more legs of the table. Such a retrofittable device would necessarily be capable of generating the required lift, without being so thick as to disturb the look and feel of the game by adding excessive height to the playing surface of the table. 
     There is thus a need for a leveling device which is capable of producing sufficient force to raise and lower a pool table of substantial weight. The device would be retrofittable to an existing pool table, without a need for the owner to perform complex tasks, such as drilling a hole in a pool table leg in order to insert components of the device. The use of the leveling device should also not disturb the aesthetics of the table itself. When installed under a leg of the table, the thickness of the device would ideally add as little to the height of the table as possible, while still maintaining the capability to raise and lower the table easily. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a jack for leveling a pool table is provided. The jack includes a circular base disk having an upper surface, a circular upper disk having a lower surface, and a circular gear wheel disposed between the disks and having a center hole preferably a circular center hole. The gear wheel has a lower surface opposing the base disk upper surface to form a first pair of opposing surfaces, and an upper surface opposing the upper disk lower surface to form a second pair of opposing surfaces. The gear wheel further includes gear teeth on its circumference, in geared connection with a worm gear. A hub projects from a first disk of the disks and extends through the center hole of the gear wheel. The gear wheel is rotatably mounted to the hub, and is capable of axial motion thereon. A hub engagement member is mounted to the second disk of the disks for engaging the hub to lock the disks rotationally in relation to each other. The upper surface of the base disk has a plurality of grooves, with the grooves following a circular arc along the longitudinal centerline of the base disk grooves, and having a radius extending from the center of the base disk upper surface to the centerline. 
     The lower surface of the gear wheel has a plurality of grooves, including one groove for each of the base disk grooves, with the lower gear wheel surface grooves following a circular arc along the longitudinal centerline of the gear wheel lower surface grooves, and having a radius extending from the center of the gear wheel lower surface to the centerline, with the radius being equal to the radius of the base disk upper surface grooves. A plurality of grooves are provided in the upper surface of the gear wheel, with the grooves following a circular arc along the longitudinal centerline of the gear wheel upper surface grooves, and having a radius extending from the center of the gear wheel upper surface to the centerline. 
     The lower surface of the upper disk has a plurality of grooves, including one groove for each of the upper gear wheel surface grooves, with the upper disk grooves following a circular arc along the longitudinal centerline of the upper disk grooves, and having a radius extending from the center of the lower surface of the upper disk to the centerline, and with the radius being equal to the radius of the gear wheel upper surface grooves. 
     Each of the grooves has a deep end and a shallow end, and a ramp extending between those ends, and the radius of the gear wheel lower surface grooves and the radius of the gear wheel upper surface grooves are unequal. Each of the grooves cooperates with a groove from its opposing surface to form an opposing pair of grooves. 
     There is a ball disposed in each pair of opposing grooves for rolling movement therein. A worm gear is provided and has threads in geared connection with the teeth of the gear wheel, so that rotating the worm gear in a selected clockwise or counterclockwise direction causes the gear wheel to rotate in a corresponding selected direction. This in turn causes each ball to roll in its groove and axially move the first and second opposing surfaces either towards each other or away from each other, thereby enabling an axial lowering or lifting movement of the jack. 
     Optionally, a central retaining bolt may be mounted at a proximal end to the base disk, with the retaining bolt having a head at a distal end. A central retaining bolt sleeve extends through the gear wheel and the upper disk. The retaining bolt sleeve has flanges which abut with the head to prevent further axial lifting movement when the lifting movement reaches a preselected maximum. 
     In view of the foregoing, several advantages of the present invention are readily apparent. A jack is provided which is capable of producing sufficient force to raise a heavy pool table. The device is retrofittable to an existing pool table, without a need for the owner to perform complex tasks, such as drilling a hole in a pool table leg in order to insert components of the device. The use of the jack also does not disturb the aesthetics of the table itself. When installed under a leg of the table, the thickness of the device adds as little to the height of the table as possible, while still maintaining the capability to raise and lower the table easily. 
     Additional advantages of this invention will become apparent from the description which follows, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the leveling device in the closed position; 
         FIG. 2  is a perspective view of the device in the open position; 
         FIG. 3  is a perspective view of the hand tool for turning the worm gear; 
         FIG. 4  is a top view showing the hand tool of  FIG. 4 , and the insertion of the tool into the device; 
         FIG. 5  is a perspective view of the device in place under the foot of a pool table leg; 
         FIG. 6  is an exploded view of the device, showing the upper surfaces of the gear wheel and the disks, and associated parts of the device; 
         FIG. 7  is an exploded view of the device, showing the lower surfaces of the gear wheel and the disks, and associated parts of the device; 
         FIG. 8  is a perspective view of the upper surface of the gear wheel, with the balls in the deep end of their respective grooves; 
         FIG. 9  is a perspective view of the upper surface of the gear wheel, with the balls in the shallow end of their respective grooves; 
         FIG. 10  is a cross-sectional view of the device in the open position; 
         FIG. 11  is a cross-sectional view of the device in the closed position; 
         FIG. 12  is a top transparent view of the gear wheel, showing the upper and lower grooves directly adjacent to one another; 
         FIG. 13  is a top transparent view of the gear wheel, showing the upper and lower grooves offset from one another; 
         FIG. 14  is a top transparent view of the gear wheel, showing the upper and lower grooves offset from one another, and with the upper grooves overlapping the lower grooves; 
         FIG. 15  is a cross-sectional view of the gear wheel shown in  FIG. 12 ; 
         FIG. 16  is a cross-sectional view of the gear wheel shown in  FIG. 13 ; and 
         FIG. 17  is a cross-sectional view of the gear wheel shown in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIGS. 1 and 2 , a leveling jack  1  according to the present invention is depicted.  FIG. 1  shows the device  1  in the closed position  2 , while  FIG. 2  shows the device  1  in the fully open position  4 . In the fully open position  4 , the upper portion  6  of the device is separated from the base portion  8  by a gap  10 . The gap  10  is equal to the lift provided by the device  1  in its open position  4 . As shown in  FIGS. 3 and 4 , the device  1  is activated by a hand tool  12 , by inserting the tip  13  of the tool into the tip receptacle  14  and turning the handle  15  of the tool. As shown, the tip  13  is in the form of a hex ball driver, rather than a more traditional Allen wrench, for example. This hex ball driver shape allows the tool  12  to be inserted at an oblique angle to the floor, thereby providing greater clearance for the operator&#39;s hand in operating the tool. 
     Turning now to  FIG. 5 , the device  1  is shown in place under the foot  16  of a pool table leg  18 . It will be noted that the device  1  has approximately the same diameter  20  as the diameter  22  of the pool table foot  16 . This is an important consideration for the aesthetics of using the device  1  under the foot of a pool table leg. Pool tables which merit the use and expense of a precise leveling device such as the present invention, typically with one device under each of the table&#39;s four corner legs, are often themselves expensive and visually striking pieces of furniture. Therefore, it is desirable that the device  1  be as unobtrusive as possible, so as not to disturb the aesthetics of the pool table. 
     Experience and aesthetic sense dictate that the most visually pleasing and unobtrusive diameter  20  for the device  1  is a diameter which matches the diameter  22  of the foot  16 . Achieving this aesthetic match between the respective diameters of the foot  16  and the device  1  is made considerably easier by the fact that a de facto industry standard diameter of three inches has been adopted for the feet of most home pool table legs. Therefore, the device  1  is typically manufactured with a diameter  20  of three inches, or very close to that diameter. As will be readily appreciated, such a size constraint complicates the design of any device to be used to precisely lift pool tables, which can sometimes weigh well over a thousand pounds. Of course, a larger device could much more easily be utilized to lift such a heavy object, but the constraints on the maximum diameter of the device do not always allow that option. 
     In addition to the foregoing aesthetic constraints on the diameter  20  of the device  1 , there are also practical and physical limits placed on the height  24  of the device. Of course, the overall height  24  of the device  1  will vary at any given time, depending upon how much lift is being provided by the device at the moment. Physically, the maximum height  24  must not be so great as to cause the pool table to wobble. On a more practical level, in operation the height  24  of the device  1  must not cause such an increase in the overall height of the pool table so as to be unacceptable to the players using the table. 
     One reason for the practical limits on the height of the device is that it may reasonably be anticipated that many users of the device will be “serious” players, since less serious players would most likely not invest in a set (typically four to a set) of devices designed to level their table to a precise degree. Such serious players would not want to have the feel of their game disrupted by an excessive change in the height of the table&#39;s playing surface. As an example, the height  24  of the preferred embodiment of the device  1  is one-half inch in the closed position  2 , and seven-eighths of an inch in the fully open position  4 . As was the case with the small diameter  20 , the critical limitations on the maximum height  24  of the device  1  thus greatly complicate the problem of achieving large amounts of lift from a device with such size limitations. 
     Referring now to  FIGS. 6 through 9 , the main internal working parts of the device  1  are shown. A base disk  30  and an upper disk  31  have a gear wheel  32  between them. Optional non-slip covers  34 ,  35  are preferably provided to aid in preventing slipping of the device  1  on the floor, and also to prevent slipping of the pool table foot  16  on the device  1 . The covers  34 ,  35  are preferably made of rubber or other resilient non-slip material, and are mounted to the base disk  30  and the upper disk  31  respectively. Gear wheel  32  has a circular center hole  37  which receives hub  36 , thus allowing the gear wheel  32  to rotate freely around the hub  36 , as well as to move axially up and down thereon. Preferably, the hub  36  is mounted to the base disk  30 , although it could optionally be mounted to the upper disk  31 . Advantageously, a tight tolerance is maintained between the hub  36  and the center hole  37 , so as to eliminate wobbling in the device  1 , and to facilitate the alignment of the worm gear threads  72  with the gear wheel teeth  74 . Spline sleeve  38  is mounted opposite the hub  36 , either to upper disk  31  when the hub is mounted to the base disk  30 , or to base disk  30  if the hub were to be optionally mounted to the upper disk  31 . Spline sleeve  38  has male splines  39  which fit into hub slots  40 , to rotationally lock the base disk  30  and the upper disk  31  in place relative to one another. In this way, the gear wheel  32  may rotate freely, while the base disk  30  and upper disk  31  remain rotationally fixed. 
     As best seen in  FIGS. 6 and 7 , in the preferred embodiment the upper surface  41  of base disk  30  includes a series of three grooves  42 , each of which is paired with an associated groove  43  on the lower surface  44  of gear wheel  32 . The centerline  46  of each of the grooves  42 ,  43  is a circular arc which is concentric with the circular shape of the respective disk or gear wheel on which the groove  42 ,  43  resides. Each base groove  42  has a radius  47  equal to the radius  48  of its respective opposing groove  43 . A set of three balls  49  is provided, with one ball  49  rolling within each pair of opposing grooves  42 ,  43  when the device is activated, as will be described. 
     In like fashion, opposing grooves  51 ,  52  are also provided on the upper surface  54  of gear wheel  32 , and on lower surface  50  of upper disk  31 . These grooves  51 ,  52  also each follow a circular arc  56 , and have respective radii  57 ,  58  which are equal to each other. A set of three balls  59  is provided, with one ball  59  rolling within each pair of opposing grooves  51 ,  52  when the device is activated. It should be noted that while a set of three grooves  42 ,  43 ,  51 ,  52  have been provided on each level, sets of two, four, or any number of grooves might be utilized. The use of three grooves has been found to provide an optimal combination of leverage and stability, and is thus the preferred number of grooves for the device  1 . 
     Each groove  42 ,  43 ,  51 ,  52  has a deep end  60  and a shallow end  62 , with a continuous inclined ramp  64  extending between those ends. As illustrated in  FIGS. 8 and 9 , each of the balls  59  begin in their respective deep end  60  when the device  1  is in the closed position  2 , and travel toward the shallow end  62  as the lifting action of the device  1  is activated, reaching the shallow end  62  when the device  1  is in the fully opened position  4 . This travel from deep end  60  to shallow end  62  also occurs in the same way with respect to balls  49 . For stability purposes, the shallow end  62  maintains a minimum depth, so that the balls  49 ,  59  are at all times restrained within their respective groove. For example, for balls of one-quarter inch diameter, a shallow end depth of one thirty-second inch has been found to be effective in optimizing lift, while at the same time providing sufficient depth so that the balls do not slide outside of their respective grooves. 
     Worm gear  70  is provided, and includes threads  72  for interacting with the teeth  74  of the gear wheel  32 . As may also be seen in  FIGS. 10 and 11 , the worm gear  70  fits into cylindrical casing  76 , in close proximity to the gear wheel  32 . The worm gear  70  is restrained from moving axially in the casing  76  by stop pin  78 , which slides into groove  80  on the side of worm gear  70 . The stop pin  78  in turn fits snugly into stop pin hole  82  in the casing  76 , and is thereby held in place. This arrangement between the stop pin  78  and the worm gear  70  allows the worm gear to rotate freely in the casing  76 , while the stop pin provides a low-friction bearing surface to prevent the worm gear from moving axially in of the casing  76  as the worm gear rotates. In addition, the casing  76  prevents lateral movement of the worm gear  70  as it rotates. By thus preventing both linear and lateral motion of the worm gear  70 , this configuration acts to maintain the synchronous relationship between the worm gear threads  72  and the gear wheel teeth  74 . 
     In a lifting operation, beginning in the closed position  4 , each ball  49 ,  59  is in the deep end  60  of its respective groove  42 ,  43 ,  51 ,  52 . This may best be seen in  FIG. 8 , where each spherical ball  59  has its lower hemispherical half  90  entirely contained in its respective groove  51 , while the hemispherical upper half  92  extends out of the groove  51 . In this position, the upper half  92  of each ball  59  will be entirely contained in the deep end  60  of groove  52  in the upper disk  31 . As may be seen in  FIG. 11 , showing the device in the closed position  2 , each of the deep ends  60  of the upper grooves  51 ,  52  has a semi-circular cross-section  94 ,  96 , which is exactly adapted to contain its respective hemispherical half  90 ,  92  of ball  59 . Thus, when the device  1  is closed as in  FIG. 11 , the two semi-circular cross sections  94 ,  96  combine to produce one perfectly circular cross-section  98 , which is exactly sized and shaped for containing an entire spherical ball  59 . 
     To activate the lifting operation of the device  1 , the worm gear  70  is turned in a clockwise direction, using the hand tool  12 . A single clockwise turn of the worm gear  70  moves the gear wheel  32  one tooth  74  in a clockwise direction. In the preferred embodiment as shown in  FIGS. 8 and 9 , the gear wheel  32  has 172 teeth, resulting in a great deal of leverage. Referring to the upper set of balls  59  in  FIGS. 8 and 9 , as the gear wheel  32  turns, the upper set of balls  59  move from their deep end  60  to shallow end  62  in the grooves  51 ,  52 . As the balls move from the deep end  60  to shallow end  62 , upper disk  31  is forced to move axially away from the gear wheel  32 . At the same time, in response to the same gear wheel rotation the lower set of balls  49  move in like fashion from deep to shallow in their opposing grooves  42 ,  43 , additionally forcing the base disk  30  and the gear wheel  32  axially apart. This separation of the opposing surfaces  41 ,  44 ,  50 ,  54  provides an axial lifting movement of the opposing surfaces  41 ,  44 ,  50 ,  54 , thereby moving the pool table leg upward to aid in leveling the table. A lowering motion of the opposing surfaces  41 ,  44 ,  50 ,  54  may be produced by selectively rotating the worm gear  70  in the opposite direction, thereby moving opposing surfaces axially closer to one another and lowering the height of the pool table. 
     An examination of some actual dimensions will be instructive in gaining perspective on the above-described operation of the device  1 . In one instance of the preferred embodiment, the balls  49 ,  59  are one-quarter inch in diameter. Therefore, in order to accept exactly one-half of the ball in the closed position  2 , the deep end  60  of each groove  42 ,  43 ,  51 ,  52  must be one-eighth inch deep. As discussed earlier, with balls  49 ,  59  of one-quarter inch diameter, a shallow end  62  having a depth of one thirty-second of an inch may optionally be utilized. 
     Beginning in the closed position  2 , the balls  49 ,  59  move from the deep end  60  toward the shallow end  62 , in response to the turning of the gear wheel  32 . As previously noted, each clockwise turn of the worm gear  70  turns the gear wheel  32  one tooth in the same direction. Thus, with the foregoing configuration and with a gear wheel  32  having 172 teeth, for each turn of the worm gear  70  the device  1  will provide approximately 0.004 inches of lift, which is approximately the thickness of an ordinary piece of printer paper. This amount of lift per turn of the worm gear is of interest for comparison purposes, as it is common for pool table owners to use pieces of paper as shims to provide a makeshift way of leveling their pool table surfaces. 
     For the same configuration as just discussed, the device  1  will provide a maximum lift of three eighths of an inch in moving from a closed position  2  to a completely open position  4 . This maximum lift may be calculated, starting from the fact that the balls  49 ,  59  move from the deep end  60  to the shallow end  62  in each of four sets of grooves  42 ,  43 ,  51 ,  52 . Thus, in each groove  42 ,  43 ,  51 ,  52 , the ball  49 ,  59  moves from a depth of one eighth inch in the deep end  60 , to a depth of one thirty-second of an inch in the shallow end  62 , an axial movement of three thirty-seconds of an inch. Thus, each set of grooves  42 ,  43 ,  51 ,  52  provides three thirty-seconds of an inch of lift as the device  1  moves from closed to fully open. Since there are four sets of grooves  42 ,  43 ,  51 ,  52  in the device  1 , the total lift provided is four times as great as three thirty-seconds of an inch, or three eighths of an inch. 
     As previously discussed, one of the main problems confronting the device  1  of the present invention is creating large amounts of leverage for very precise lifting, while still adhering to severe constraints on the device&#39;s diameter  20  and height  24 . In that context, positioning of the grooves  43 ,  51  on the gear wheel  32  can have a significant impact. For instance, the grooves  43 ,  51  of the gear wheel  32  could have equal radii  48 ,  58 , and be “stacked,” with one directly on top of the other in the gear wheel, bunk-bed style. That configuration would allow the largest possible radii  48 ,  58 , for a device  1  of a particular diameter  20 . With grooves  43 ,  51  thus having maximized and equal radii  48 ,  58 , the grooves could be made as long as possible for that particular diameter  20 , thus maximizing their leverage. However, maximizing the available leverage of the grooves in this way comes with one significant disadvantage. Having the deep ends  60  of the grooves  43 ,  51  stacked one on top of the other in that fashion would require a gear wheel  32  which was nearly twice as thick as a gear wheel having only one set of grooves. This would significantly add to the overall height  24  of the device  1  which can be undesirable. 
     Referring now to  FIGS. 12-17 , three alternative solutions are presented to the foregoing problem of configuring the gear wheel grooves  43 ,  51  so as to optimize the radii  48 ,  58  and the height  24 , while remaining within the size constraints placed on the device  1 . In one solution, as seen in  FIGS. 12 and 15 , the grooves  43 ,  51  may be placed side-by-side in the gear wheel  32 . This configuration has the advantage of minimizing the required thickness  100  of the gear wheel  32 . In this configuration, the gear wheel  32  must only be thick enough to accommodate the depth  102  of the deep end  60  of either groove  43 ,  51 , plus a minimum wall thickness  104  required for structural integrity of the device  1 . This represents the absolute minimum thickness  100  for the gear wheel  32 , since neither the deep end  60  nor the selected minimum wall thickness  104  can be further reduced. In practice, when the device is constructed of aluminum, this minimum wall thickness  104  has been determined to be approximately 0.030 inches. The drawback of this configuration, however, is that the maximum possible radius  48  of the lower grooves  43  is significantly reduced by having the deep ends  60  of the grooves  43 ,  51  side-by-side. Reducing the maximum radius  48  in this way inherently reduces the maximum length of the groove  43 , which in turn reduces the maximum leverage possible with that groove. 
     Referring now to  FIGS. 13 and 16 , a second configuration of the gear wheel grooves  43 ,  51  is depicted. The grooves  43 ,  51  have radial mid-points  106 ,  108 , which are points on the centerline  110 ,  112  of the respective groove, half-way between the deep end  60  and the shallow end  62  of that groove. In the configuration as shown, the mid-points  106 ,  108  are radially offset from each other, with the beneficial result that the deep ends  60  of the grooves  43 ,  51  are no longer positioned side-by-side. Instead, for example, where the grooves  43 ,  51  are offset by a central offset angle  114  of sixty degrees, the deep end  60  of groove  43  is positioned adjacent to the mid-point  108  of groove  51 ; and likewise, the deep end  60  of groove  51  is positioned next to the mid-point  106  of groove  43 . While sets of three grooves on each surface have been found to be preferred, other numbers of grooves are feasible on each surface. Most generally, the number of grooves in a set may be designated generally as “n,” in which case the preferred offset angle would then be equal to 360/2n. 
     The offset configuration of  FIGS. 13 and 16  allows the maximum potential radius  48  of groove  43  to be significantly increased, as compared to the maximum radius allowed by the configuration of  FIGS. 12 and 15 . Increasing the radius  48  in this way allows groove  43  to be made longer, thereby increasing its leverage without increasing the thickness  100  of the gear wheel  32 . While a radial offset angle  114  of sixty degrees provides optimal results, any offset at all will allow an increase in the radius  48 . This is due to the fact that the deep ends  60  of grooves  43 ,  51  would no longer be side-by-side, thereby removing a major limitation on the length of the radius  48 . 
     A third configuration of the grooves  43 ,  51  may be seen by reference to  FIGS. 14 and 17 . In this configuration, the grooves  43 ,  51  are radially offset as previously discussed, preferably with an offset angle  114  of sixty degrees. In addition, the lower grooves  43  have been moved outward on gear wheel  32  from their position in  FIGS. 13 and 16 . In this position, the upper grooves  51  overlap the lower grooves  43 , as best seen in  FIG. 17 . Moving the grooves  43  further outward in this way increases the radius  48  of the grooves  43 , which allows the grooves  43  to be made longer, thereby increasing their leverage without increasing the thickness  100  of the gear wheel  32 . 
     The overlap of the upper grooves  51  over the lower grooves  43  is made possible by the offsetting of the grooves  43 ,  51  from one another. As best seen in  FIG. 15 , when the grooves are not offset, they can be placed no closer to one another than shown in  FIG. 15 , due to the requirement of maintaining a minimum wall thickness  104  between the grooves. As may be seen from the configuration of  FIG. 15 , when the deep ends  60  of the grooves  43 ,  51  are side-by-side, there is no room remaining to move groove  43  underneath groove  51  without increasing the thickness  100  of the gear wheel  32 . However, as may be seen in  FIGS. 13 and 16 , offsetting the grooves  43 ,  51  makes it possible to move the lower grooves  43  under the upper grooves  51 , so that the upper grooves  51  overlap the lower grooves  43  as shown in  FIG. 17 . In this way, offsetting the grooves  43 ,  51  from one another allows the radius  48  to be increased without increasing the thickness  100  of the gear wheel  32 . It is noted that it is a matter of choice as to which of the grooves  43 ,  51  have a smaller radius  48 ,  58  in this offset and overlapping configuration. Either selection will produce the desired result, and the upper grooves  51  will overlap the lower grooves  43  in either case. 
     Referring now to  FIGS. 10 and 11 , the operation of the optional central retaining bolt  116  is illustrated. When the device  1  is in use, the bolt  116  acts to selectively limit the maximum separation of the disks  30 ,  31  from the gear wheel  32 . This is an important function, because the lifting movement represented by this separation should not be permitted to become so large that the balls  49 ,  59  travel so far that they are no longer contained within the shallow ends  62  of their respective grooves  42 ,  43 ,  51 ,  52 . This limitation on the lifting movement could be accomplished by the user not exceeding the lifting limits of the device, but mechanical limits are more reliable. Thus the lifting movement ideally would be mechanically limited to a preselected maximum, in order to maintain the balls  49 ,  59  in their respective grooves. This limiting action may be accomplished by the head  120  on the bolt  116 , which abuts with flanges  122  on the central retaining bolt sleeve  124  to stop any further separation when the device  1  has reached the fully open position  4 . Optional return spring  126  is mounted within the bolt sleeve  124 , and is compressed between the head  120  and the flanges  122  as the device  1  is opened, thus adding a return force to aid in closing the device  1 . This action is useful when the device  1  is opened, but is not bearing weight, since it is necessary to keep pressure on the balls at all times so that they will roll properly when the device is activated. As will be readily appreciated, no such additional return force is needed when a pool table is already in place on the device. 
     In practice, a single device under one corner leg of a typical four or six-legged pool table may be sufficient to level the playing surface of the table. This can occur, for example, when just one quadrant of the playing surface is in need of raising in order to satisfactorily level the table. Ideally, however, a system of four jacks is deployed, with one jack under each corner leg of the table. Use of a system of four jacks in this manner ensures that the playing surface may readily be leveled at any time, regardless of the location of any needed lifting or lowering. 
     One variation of the foregoing system of jacks occurs when one corner of the table is higher than the other corners. Such a situation is typically due to fluctuations in the level of the floor upon which the table rests. When this occurs, a shim or spacer may be placed under the leg at the highest corner, with jacks under the remaining three corner legs. 
     This invention has been described in detail with reference to particular embodiments thereof, but it will be understood that various other modifications can be effected within the spirit and scope of the invention.