Abstract:
A downhole tractor is provided that includes a motor, at least one rotatable screw powered by the motor, and a chain engageable with the at least one rotatable screw, whereby rotation of the at least one rotatable screw moves the chain.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally pertains to power transmissions systems, and more particularly to a system in which power is transferred from a power source via a rotatable screw member to a chain or other flexible drive member, such as a continuous drive belt. 
   2. Description of the Related Art 
   It is known within a variety of fields to use a drive sprocket to transfer rotational power from a power source, such as a motor, to a chain or other belt-type drive component. One example of this type of system is the drive system on a military tank in which power is transferred through a gear box to a drive sprocket which in turns drives the tracks of the tank. A similar example, but on a much more miniaturized scale, can be found in the oil and gas industry, in which downhole tractors include tracks that are driven by a drive sprocket. An example of a downhole tractor is shown in U.S. Pat. No. 4,670,862. Downhole tractors of this type are typically used to convey logging devices and other devices in horizontal and highly-deviated wells. As will become apparent from the following description and discussion, however, the present invention, which is also directed to systems for transferring rotary motion to an endless flexible device, overcomes the deficiencies of the previous devices and constitutes an improved and more efficient drive system. 
   SUMMARY OF THE INVENTION 
   In a broad aspect, the present invention is directed to systems in which rotary motion is used to drive a chain or other endless flexible device through the interaction of a rotatable screw with the chain or flexible device, as opposed to transferring power through a drive sprocket to the chain. In another aspect, the present invention may be a chain drive assembly comprising: a rotatable screw and a chain, the screw being engageable with the chain such that rotation of the screw causes linear movement of the chain. Another feature of this aspect of the invention may be that the assembly of claim  1 , wherein the screw is a power screw, and further including a plurality of threaded components connected to the chain and threadably engageable with the power screw. Another feature of this aspect of the invention may be that the threaded components are split nuts. Another feature of this aspect of the invention may be that the threaded components are quarter nuts. Another feature of this aspect of the invention may be that the screw is a lead screw and the chain includes a plurality of rollers adapted for interacting engagement with the lead screw. 
   In another aspect, the present invention may be a chain drive assembly comprising a first rotatable screw, a second rotatable screw, and a chain, the first and second rotatable screws positioned in generally parallel relationship on opposite sides of the chain and engageable with the chain such that rotation of the first and second screws in opposite directions causes linear movement of the chain. Another feature of this aspect of the invention may be that each of the first and second screws is a power screw, and may further include a plurality of threaded components connected to first and second edges of the chain, the plurality of threaded components on the first edge of the chain being threadably engageable with the first power screw, and the plurality of threaded components on the second edge of the chain being threadably engageable with the second power screw. Another feature of this aspect of the invention may be that the threaded components are split nuts. Another feature of this aspect of the invention may be that the threaded components are quarter nuts. Another feature of this aspect of the invention may be that each of the first and second screws is a lead screw and the chain includes a plurality of rollers rotatably mounted along first and second edges of the chain, the plurality of rollers along the first edge of the chain being engageable with the first lead screw, and the plurality of rollers along the second edge of the chain being engageable with the second lead screw. Another feature of this aspect of the invention may be that the rollers are mounted to link pins that form part of the chain. Another feature of this aspect of the invention may be that the rollers are mounted to axles that are generally perpendicular to an upper surface of the chain. Another feature of this aspect of the invention may be that the rollers are adapted for rolling engagement with thread flanges on the screws. 
   In yet another aspect, the present invention may be a chain drive assembly comprising: a first and a second chain, and a first, a second, a third and a fourth rotatable screw, the first and second rotatable screws positioned in generally parallel relationship on opposite sides of the first chain and engageable with the first chain such that rotation of the first and second screws in opposite directions causes linear movement of the first chain, and the third and fourth rotatable screws positioned in generally parallel relationship on opposite sides of the second chain and engageable with the second chain such that rotation of the third and fourth screws in opposite directions causes linear movement of the second chain. 
   In still another aspect, the present invention may be a vehicle comprising: a motor; a rotatable screw powered by the motor; a flexible track engaged with the screw, whereby rotation of the screw causes movement of the track. Another feature of this aspect of the invention may be that the flexible track is a chain. Another feature of this aspect of the invention may be that the screw is a power screw and the chain further includes a plurality of threaded members that are engageable with the screw. Another feature of this aspect of the invention may be that the threaded members are portions of threaded nuts. Another feature of this aspect of the invention may be that the threaded members are split nuts. Another feature of this aspect of the invention may be that the threaded members are quarter nuts. Another feature of this aspect of the invention may be that the screw is a lead screw and the chain further includes a plurality of rollers that are adapted for rolling engagement with a thread flange of the lead screw. Another feature of this aspect of the invention may be that the vehicle is a downhole tractor. 
   This section to be supplemented upon completion of new claims. 
   In another aspect, the present invention may be a method of converting rotary motion into linear motion comprising: providing a rotatable screw, providing a flexible track, engaging the screw with the flexible track, and rotating the screw so as to move the flexible track. 
   Other features, aspects and advantages of the present invention will become apparent from the following discussion 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a specific embodiment of a downhole tractor that includes a specific embodiment of a chain drive system in accordance with the present invention, and shown in a deployed position. 
       FIG. 2  is a perspective view of a section of chain that may be used in the system shown in  FIG. 1 , and illustrates the attachment of a plurality of split nuts to the chain. 
       FIG. 3  is a cross-sectional view taken along line  3 — 3  of  FIG. 1 . 
       FIG. 4  is a side view, partially in cross-section, of another specific embodiment of a chain drive system in accordance with the present invention, which includes quarter nuts attached to opposite sides of a chain, which nuts engage threads of generally parallel power screws that are disposed on opposite sides of the chain. 
       FIG. 5  is a cross-sectional view taken along line  5 — 5  of  FIG. 4 . 
       FIG. 6  is a perspective view of a section of the chain shown in  FIG. 4 , and illustrates the attachment of a plurality of quarter nuts to the chain. 
       FIG. 7  is a perspective view of another specific embodiment of a chain drive system in accordance with the present invention, which includes rollers mounted to the chain pins, which rollers engage helices of generally parallel lead screws that are disposed on opposite sides of the chain. 
       FIG. 8  is a perspective view showing a portion of the chain shown in  FIG. 7 , from which the rollers are more easily seen. 
       FIG. 9  is a perspective view showing an alternative design for connecting rollers to a chain. 
       FIG. 10  is an end view of the design shown in  FIG. 9 , and further illustrating the rollers engaged with two lead screws that are disposed in generally parallel relationship on opposite side of the chain to which the rollers are secured. 
       FIG. 11  is a perspective view showing an alternative embodiment of a chain drive system constructed in accordance with the present invention. 
       FIG. 12  is a top view of the system shown in  FIG. 11 . 
       FIG. 13  is a cross-sectional view taken along line  13 — 13  of  FIG. 12 . 
   

   While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings in detail, wherein like numerals denote identical elements throughout the several views, there is shown in  FIG. 1  a specific embodiment of a downhole tractor  10  that includes a specific embodiment of a chain drive system constructed in accordance with the present invention, as will be more fully explained below. In  FIG. 1 , the tractor  10  is shown in a deployed position and engaged with an open, or non-cased, well bore  12 . The tractor  10  may include a main motor  14  disposed within a housing  16 . The main motor  14  is adapted to rotate a screw  18 , such as a power screw or a lead screw, for example. The screw  18  includes a central longitudinal axis that may be generally co-axial with a central longitudinal axis of a tool string (not shown) to which the tractor  10  is connected. The screw  18  may be directly or indirectly mounted or coupled to the motor shaft. The tractor  10  also includes, in this embodiment, a first and a second track assembly  20  and  22 , each of which includes a flexible track  24  or endless belt rotatably disposed about four wheels  26  (e.g., idler sprockets) mounted to a collapsible parallelogram frame  28 . In a specific embodiment, the flexible track  24  may be a chain, as further discussed below. Each frame  28  may be connected by a link  30  to a telescoping arm  32  that is connected to and powered by an auxiliary motor  34  disposed within the housing  16 . The arm  32  is shown in a fully-extended position in  FIG. 1 , which position coincides with each track  24  being engaged with the open well bore  12 . When the arm  32  is retracted, each track assembly  20 / 22  is pulled into a retracted position (not shown). The links  30  are shown in dashed lines in their retracted positions, which correspond to the retracted position of the track assemblies  20 / 22 . 
   The manner in which the screw  18  causes the tracks  24  to move will now be explained. In a specific embodiment, in which each flexible track  24  is a chain, as shown in  FIG. 2 , a series of split nuts  36  may be secured to a chain  38 , such as by mounting to link plates  40  on top of the chain  38 . The split nuts  36  are preferably of appropriate size and thread dimensions so as to be engageable with threads on the screw  18 , which in this embodiment is a power screw. With reference to  FIG. 1 , the tracks  24  are mounted in relation to the screw  18  such that the split nuts  36  will engage the screw  18 . As such, when the screw  18  is rotated, the splits nuts  36 , by virtue of being threadably engaged with the screw  18 , will travel downwardly along the screw  18 . This will result in movement of the chain  38  in a direction generally along the central axis of the screw  18 , and will thus also result in rotation of the tracks  24 /chain  38  in a first direction as indicated by the arrows in  FIG. 1 . This will cause the tractor  10  to move downwardly within the well bore  12 . The direction of rotation of the tracks  24 /chain  38  may be reversed by reversing the rotation of the screw  18 , which will cause the tractor  10  to reverse its direction of movement and move upwardly within the well bore  12 . A gearbox (not shown) may also be provided between the motor  14  and the screw  18 , depending on the lead of the screw  18  and design specifications. 
   Referring to  FIG. 3 , which is a cross-sectional view taken along line  3 — 3  of  FIG. 1 , it can be seen that, in another specific embodiment, the tractor  10  may be provided with three track assemblies  20 ,  22 , and  23 , all driven by the single screw  18 . Alternatively, the tractor  10  may be provided with a single track assembly. While the chain path shown in  FIG. 1  is in the shape of a parallelogram, it may also be in other shapes, such as a triangle or trapezoid. The thread length of the screw  18  may be equal to the length of a chain link, or equal to the length of the interacting chain, or longer if desired. In a specific embodiment, the thread of the screw  18  may be trapezoidal, square, triangular, round or rectangular. The screw  18  may also be provided with multiple thread starts.  FIG. 3  also illustrates that the housing  16  may be provided with one or more channels  42  for passage of any variety of signals, such as through electric or fiber optic cables, and/or hydraulic fluid. 
   Another specific embodiment of the present invention is shown in  FIG. 4 , wherein a motor  44  is adapted to transfer rotary power through a gear box  46  to a first gear  48  that is meshed with a second gear  50 . The second gear  50  is mounted to a first shaft  52 . A third gear  56  and a first screw  60  are also mounted to the first shaft  52 . The third gear  56  is meshed with a fourth gear  58  that is mounted to a second shaft  54 . A second screw  62  is also mounted to the second shaft  54 . The first and second shafts  52  and  54 , and the first and second screws  60  and  62 , are disposed in generally parallel relationship. A flexible track, such as a chain  64 , is disposed between the first and second shafts  52  and  54 , and engaged with the first and second screws  60  and  62 . More specifically, as best shown in  FIGS. 5 and 6 , a plurality of quarter nuts  66  are secured to opposed edges  68  and  70  of the chain  64 . The chain  64  is disposed between the first and second screws  60  and  62  such that the quarter nuts  66  will engage threads on the corresponding screws  60  and  62 . When the motor  44  is energized, rotary power will be transferred through the gear box  46  to rotate the first gear  48 , which will rotate the second gear  50 , which will rotate the third gear  56  and the first screw  60  in a first direction, as indicated by arrow  72 . Movement of the third gear  56  will simultaneously cause the fourth gear  58  and the second screw  62  to rotate in a second direction, as indicated by arrow  74 , which is opposite of the direction in which first screw  60  rotates. As the screws  60  and  62  rotate in opposite directions, the quarter nuts  66  are threadably moved along the rotating screws  60  and  62 , thereby resulting in movement of the chain  64  in the direction of arrow  76 , and generally along central axes of the screws  60  and  62 . 
   Another specific embodiment of the present invention is shown in  FIG. 7 . The embodiment in  FIG. 7  is similar to the embodiment shown in  FIG. 4  insofar as both embodiments include generally parallel screws that rotate in opposite directions and engage a chain that is disposed between the screws. But the embodiment in  FIG. 7  employs first and second lead screws  78  and  80 , whereas the screws  60  and  62  disclosed in  FIG. 4  are power screws. Each lead screw  78  and  80  includes a corresponding helix, or thread flange,  79  and  81 . Another difference between the embodiments in  FIGS. 4 and 7  pertains to the manner in which the screws  78  and  80  engage with the chain  83 . In  FIG. 7 , the power screws  78  and  80  engage rollers  82  that are rotatably mounted to pins  84  that hold links of the chain  83  together, which pins  84  may be extended from their normal lengths to accommodate the rollers  82 . The rollers  82  and pins  84  are more easily seen in  FIG. 8 . As shown in  FIG. 7 , in a specific embodiment, the chain  83  is positioned in relation to the first and second lead screws  78  and  80  such that a roller  82  is positioned between consecutive turns of each thread flange  79  and  80 . In operation, when the lead screws  78  and  80  are simultaneously rotated in opposite directions, the rollers  82  will roll along the corresponding thread flange  79 / 81  in the direction of arrow  86 , which will result in movement of the chain  83  in the direction of arrow  86 , in a general direction along the central longitudinal axes of the screws  78  and  80 . Although not shown in  FIG. 7 , rotary movement may be imparted to the screws  78  and  80  in any known manner, including, for example, by use of the motor  44  and gearbox  46  configuration shown in  FIG. 4 . 
   An alternative design for mounting the rollers  82  to the chain  83  is shown in  FIGS. 9 and 10 . Instead of mounting the rollers  82  to the pins  84  that form part of the chain  83 , as shown in  FIG. 8 , in the embodiment shown in  FIGS. 9 and 10  the rollers  82  are mounted to axles  88  that are mounted to a link plate  90 . In a specific embodiment, the axles  88  may be mounted such that they are generally perpendicular to an upper surface of the chain  83 . The link plate  90  is secured to the chain  83  in any known manner. As best shown in  FIG. 10 , the rollers  82  are mounted so as to engage and roll along the thread flanges  79 / 81  of the screws  78 / 80 , thereby resulting in movement of the chain  83  in the same manner as discussed above in connection with  FIG. 7 . 
   Another specific embodiment of the present invention in which rollers and lead screws are used is illustrated in  FIGS. 11-13 . As best shown in  FIG. 11 , in this embodiment, a motor  92  powers a central drive gear  94  which is meshed with a first gear  96  and a second gear  98 . The first gear  96  is mounted to a first shaft  100  on one side of the motor  92 , and the second gear  98  is mounted to a second shaft  102  on an opposite side of the motor  92 . A third gear  104  is also mounted to the first shaft  100 , and is meshed with a fourth gear  106 , which is mounted on a third shaft  108 . The third shaft  108  is disposed below and generally parallel to the first shaft  100 . A fifth gear  110  is mounted to the second shaft  102 , and is meshed with a sixth gear  112 , which is mounted on a fourth shaft  114 . The fourth shaft  114  is disposed above and generally parallel to the second shaft  102 . All of the shafts are preferably rotatably mounted within bearings in a manner known in the art. It can be seen that rotation of the central drive gear  94  will result in rotation of each of the shafts  100 ,  102 ,  108  and  114  in various directions of rotation, as will be better understood from the further description below. A lead screw is directly or indirectly coupled to each shaft, to include the screw and corresponding shaft being formed as a single unitary structure. More specifically, a first lead screw  116  is coupled to the first shaft  100 ; a second lead screw  118  is coupled to the second shaft  102 ; a third lead screw  120  is coupled to the third shaft  108 ; and a fourth lead screw  122  is coupled to the fourth shaft  114 . 
   As best shown in  FIG. 13 , this specific embodiment of the present invention is designed to drive two flexible tracks, or chains, namely a first chain  124  and a second chain  126 . While this embodiment is shown with two chains, that number should not be taken as a limitation, as this embodiment could easily be adapted for use with other numbers of chains. In this specific embodiment, as shown in  FIGS. 11 and 12 , the chains  124  and  126  are equipped with rollers  128  mounted in the same manner discussed above and depicted in  FIG. 8 . The rollers  128  are positioned relative to the lead screws  116 ,  118 ,  120  and  122  so as to move the chains  124  and  126  in the same manner as described above and shown in  FIG. 7 . In more particular, the first chain  124  is disposed between and powered by the first lead screw  116  and the fourth lead screw  122 ; and the second chain  126  is disposed between and powered by the second lead screw  118  and the third lead screw  120 . It is further noted that the helices, or threaded flanges, on the respective lead screws  116 ,  118 ,  120  and  122  are appropriately disposed in counter- and/or like rotating directional relationships to one another so as to work in unison to move the respective rollers  128  and chains  124 / 126  in the desired direction, as will be readily understood by those of ordinary skill in the art. In a specific embodiment, the first and fourth screws  116  and  122  rotate in opposite directions, the second and third screws  118  and  120  rotate in opposite directions, the second and fourth screws  118  and  122  rotate in opposite directions, and the first and third screws  116  and  120  rotate in opposite directions. 
   One notable difference between the designs of  FIGS. 7 and 11  versus the designs of  FIGS. 1 and 4  relates to the relative mechanical efficiency of the designs. When the designs are compared in this light, it is noted that the “rolling friction” associated with the use of the rollers  82 / 128  to engage the lead screws in  FIGS. 7 and 11  is less than the “sliding friction” associated with the use of the split and quarter nuts  36 / 66  to engage the power screws  18 / 60 / 62  in  FIGS. 1 and 4 . For that reason, the “rolling” design of the type shown in  FIGS. 7 and 11  is preferred over the “sliding” designs of the type shown in  FIGS. 1 and 4 . 
   The drive systems of the present invention as described hereinabove may be employed in a wide variety of applications, including for example in downhole tractors for use in oil and gas exploration. As space and mechanical packaging area are at a premium when operating in a downhole well environment, the present invention is advantageous in comparison to previous downhole tractors. A conventional sprocket-drive system in this environment would be limited under abrasive downhole oil and gas conditions in efficiency, reliability and strength. Another advantage of the present invention is that turning rotary motion into linear motion in a plane perpendicular to the rotary motion is avoided. Such a constraint occurs due to the position of the gearbox and drive motor for downhole tractors, in which the gearbox and motor are co-axial with the tool string axis whereas the axis of the drive sprocket is perpendicular to the axis of rotary motion. But through use of the present invention, power is transferred to the chain through a screw, which is on axis with the tool string axis, resulting in linear movement of the chain, without converting rotary motion into an axis perpendicular to the tool string axis. This approach inherently lessens the gear reduction required from the motor to the screw since the screw pitch can be adjusted to change the final drive ratio of the chain, and, to the extent a gearbox is used or desired, will result in a more simplified and reliable gearbox. 
   The present invention may be equally applicable in other contexts. For example, a chain driven by a drive screw could replace the mechanism in a conventional chain-type garage door opening system or in a similar but more heavy-duty mechanical system where space is an issue or in which sprockets may not be used or where the forces applied to the chain links must be distributed differently to avoid sprocket failure. This invention may also have an application in any chain-drive system where limitations in space and power require the drive motor to be aligned in the axis of the chain direction and/or where a 90-degree change of torque direction with respect to the motor is not possible. By use of the present invention, the track element of a device such as a snowmobile, chain-type digging machine or conveying machine may be reduced in size by replacing a sprocket-drive system with a screw-driven chain system. A tractor-mounted trenching device is an example of a design where the torque supplied by the tractor&#39;s power take off is at a 90-degree angle in relation to the torque direction required by a sprocket-driven trenching chain. Use of the present invention in this situation would simplify the transmission of such a device, thereby making it more economical to manufacture. 
   Another advantage of the present invention over sprocket-driven tracks is that a constant speed of the track or chain is achieved with the present invention whereas the speed of a sprocket-driven track will oscillate or surge. As will be understood by those of skill in the art, sprockets include driving teeth with valleys or troughs between each tooth. The speed of the sprocket-driven chain will be at its maximum when the driven portion of the chain is in contact with the top of the teeth, and at its minimum when in contact with the valleys. With the present invention, however, since the screw thread or helix flange is in constant and continuous contact with the nuts or rollers, there is no oscillation or surging and thus the speed of the driven chain or track remains constant. 
   It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials or embodiments shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. For example, while the invention has been described and illustrated through use of a multiplex roller chain, the invention is not so limited, but also encompasses other types of chains, such as simplex roller chains, silent chains, and track chains like on a military tank, and may further include other types of flexible devices, such as a belt or rope. As another example, while the embodiments of  FIGS. 1 and 4  use split nuts  36  and quarter nuts  66 , respectively, to engage a screw in a sliding friction type manner, other mechanical protrusions, such as monolithic rubbing blocks made of a low friction metal or plastic material, may be used to engage a screw. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.