Patent Publication Number: US-5890403-A

Title: Braking mechanism for power tongs

Description:
FIELD OF THE INVENTION 
     The present invention relates to power tongs typically used in the oil and gas industry to make up and break apart threaded joints on pipe, casing and similar tubular members. In particular, the present invention deals with an improvement to the braking mechanism found on most power tongs. The present invention also deals with a mechanism allowing a power tong operator to reverse, from a remote location, the direction in which the power tong is rotating the tubular members. 
     BACKGROUND OF THE INVENTION 
     Power tongs have been in existence for many years and are generally employed in the oil and gas industry to grip and rotate tubular members, such as drill pipe. It is necessary to grip drill pipe with high compressive forces while applying a high degree of torque in order to break apart or tighten threaded pipe connections. An example of a conventional power tong can be seen in U.S. Pat. No. 4,084,453 to Eckel. Most current power tong designs such as Eckel include an open slot or throat, through which the drill pipe is passed in order to place the power tong in position around the pipe. Typically power tong designs employ a cam mechanism for converting a portion of the torque into a gripping (compressive) force normal to the pipe. This conversion is often accomplished utilizing a power-driven ring gear having an interior cam surface. The ring gear will also have an opening corresponding in size to the throat of the power tong. The ring gear is rotatively positioned between an upper and a lower cage plate which form a jaw carrier. The cage plates are also rotatively positioned in the power tongs and also have openings corresponding with the throat of the power tong. Multiple jaw members are typically secured between the upper and lower cage plates in such a manner that relative movement between the ring gear and cage plates causes the jaws to ride upon the cam surfaces and to close on the drill pipe. After the jaws have closed on the drill pipe, the cage plates and the ring gear will then rotate as a unit to apply torque to the drill pipe. 
     In order to initially hold the cages plates stationary while the ring gear rotates sufficiently to close the jaws, a brake band typically encircles the upper cage plate. The brake band applies an initial frictional force to the cage plate, holding the cage plates stationary and allowing the ring gear to move relative to the cage plates. Generally, the brake band is adjustable such that it may be tightened or loosened in order to vary the amount of frictional force applied to the cage plate. After relative rotation begins and the jaws mount the cam surfaces and close on the drill pipe, the jaws will begin to transfer torque to the cage plates which will eventually overcome the resisting frictional force of the brake band. Because the cam surfaces translate torque into radial force, a higher torque needed to overcome the resistance of the brake band will result in a higher radial force being placed on the drill pipe. Therefore the frictional resistance of the brake band may be adjusted to regulate the radial load placed on the drill pipe. 
     While the prior art brake band has accomplished its intended function, it is very inefficient. After the cage plates and ring gear begin moving as a unit to rotate the drill pipe, the friction caused by the brake band now provides undesirable resistance to the torque being applied to the drill pipe and the energy expended by the power tongs in overcoming this resistance is wasted. Also, since it is primarily the resistance of the brake band which determines the radial force with which the jaws grip the drill pipe, this force cannot be varied without stopping work to adjust the brake band. Furthermore, it is difficult to insure the brake band will provide a constant resisting force to the cage plate. For example, grease or a similar substance may be deposited between the cage plate and brake band thereby lowering the frictional resistance of the brake band. Additionally, brake bands wear over time and do not grip as tightly as originally designed. This wearing tendency is often caused by the fact that the cage plate does not present a continuous surface but has an opening corresponding to the throat of the power tong. The contact of this opening with the brake band during rotation of the cage plate causes excessive wear and damage to the brake band. 
     After the desired torque has been applied to a drill pipe joint, the direction of the ring gear&#39;s rotation is reversed to allow the jaws to back off of the cam surfaces and release the pipe. Again the brake band holds the cage plate stationary during the ring gear&#39;s initial reverse rotation. Once the jaws have backed off the cam surfaces, it is necessary for the ring gear and cage plates to move in unison to align the openings of the cage plates and the ring gear with the throat of the power tongs. This is accomplished by the interaction of a backing lug on the ring gear and a backing pin inserted into the upper cage plate. The upper cage plate will have two apertures for receiving the backing pin. The backing pin is inserted into one of the two apertures depending on which direction the ring gear will be applying torque to the drill pipe. The ring gear will rotate relative to the cage plates until the backing pin contacts the backing lug. At this point, the openings of the ring gear and cage plates will be aligned and the interaction of the backing pin and backing lug will cause the ring gear and cage plates to rotate together. The operator of the power tong continues to rotate the ring gear in order to observe when the openings of the ring gear and cage plates are aligned with the throat of the power tongs. This alignment is necessary to back the power tongs off of the drill pipe. 
     This leads to another disadvantage encountered in prior art power tongs when the tong operator desires to reverse the rotational direction of the power tong. To do so, the operator must manually switch the backing pin to the alternate aperture. This may be very difficult in certain drilling operation, such as when operations are being carried out by an automated pipe handling system and the operator controlling the power tong is working from a control station some distance from the tong. The remote controlling of power tongs presents another problem in that the operator may not be able to view the throat of the power tong from his location. Thus the operator is not able to visually align the openings of the ring gear and cage plates with the power tong&#39;s throat in order to back the power tong off of a drill pipe. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of this invention to provide a selectively releasable braking apparatus for use in conjunction with a power tong. 
     It is another object of this invention to provide a braking apparatus allowing a selective radial load to be placed on the drill pipe or other tubular member. 
     It is still another object of this invention to provide an apparatus which allows the direction of rotation of the power tong to be reversed from a remote location without the manual operation of a backing pin. 
     It is a further object of this invention to provide an apparatus which will automatically align the throat of the power tong and the openings in the ring gear and the cage plates without the operator needing to view the power tong throat. 
     Accordingly, a selectively releasable braking apparatus for a power tong is provided. The selectively releasable braking apparatus comprises a braking ring operatively connected to a cage plate of the power tong and a braking surface mechanically connected to the cage plate. The apparatus further has a gripping device capable of selectively gripping and holding the braking surface and has a gear train transferring torque from said brake ring to said braking surface. 
     Furthermore, in a power tong having a throat, a ring gear with an opening, and a cage plate with an opening, the present invention provides an apparatus for aligning the openings in the ring gear and the cage plate with the throat of the power tong and remotely reversing the direction of rotation of the ring gear and cage plate. The apparatus comprises a stop device preventing relative rotation of the ring gear and the cage plate until a preselected torque load is placed on said stop device. The apparatus also has a selectively releasable cage plate brake and a cage plate latch selectively preventing rotation of the cage plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a prior art power tong with the upper cage plate removed. 
     FIG. 2 is a top view of a prior art power tong showing the brake band and reversing pin. 
     FIG. 3 is a cross-sectional view of a prior art power tong illustrating the cage plates&#39; positioning with respect to the ring gear. 
     FIG. 4 is a cross-sectional view of the present invention attached to a conventional power tong body. 
     FIG. 5 is a plane view of the present invention. 
     FIG. 6 is a cross-sectional view of the present invention with the cover plates removed. 
     FIG. 7 is a plane view of the present invention with the cover plates removed illustrating the secondary braking gears. 
     FIG. 8 is a cross-sectional view of an alternate embodiment of the present invention. 
     FIG. 9 is a plane view of the alternate embodiment illustrating the device for selectively operating the brake band. 
     FIG. 10 is a detailed view of a preferred braking surface gripping device of the present invention. 
     FIG. 11 is a schematic view of a fluid circuit used in conjunction with the present invention. 
     FIG. 12 is an alternate embodiment of the present invention illustrating the braking ring integrally formed on the cage plate. 
     FIG. 13 is a plane view of the alternate embodiment showing the secondary braking gears. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     FIGS. 1-3 illustrate a prior art power tong 101. FIG. 1 illustrates power tong 101 is of the type having an open throat 112. FIG. 1 shows power tong 101 with the cover plate and top cage plate removed in order to show the main internal components positioned within frame 102 of power tong 101. Frame 102 contains a series of rollers 103 running along the inner periphery of the front end of frame 102. Ring gear 104 is positioned between and supported by rollers 103 such that ring gear 104 may rotate within frame 102. Both the cage plates 106 and 105 (seen in FIG. 3) and ring gear 104 have openings which correspond in size to throat 112. The outer periphery of ring gear 104 will have a series of gear teeth 108 (shown schematically) positioned thereon. Gear teeth 108 will engage the cogs of drive train 110 in order to impart torque to ring gear 104. The inner periphery of ring gear 104 will also have a plurality of cam surfaces 114 formed thereon which will operate to open and close jaws 116 (seen in FIG. 2). Still viewing FIG. 1, ring gear 104 will further have channel 109 formed on its upper and lower surfaces. Channel 109 is sized to engage roller bearings 115 (seen in FIG. 3) which are connected to the cages plates 106 and 105. It will be understood that when ring gear 104 is assembled in power tong 101 between upper and lower cage plates 106 and 105, ring gear 104 is able to rotate relative to cage plates 106 and 105 on roller bearings 115. In order to hold the cage plates stationary while ring gear 104 rotates, a brake band 117 (seen in FIG. 2) will surround a portion of the periphery of upper cage plate 106 and impart a frictional resistance to upper cage plate 106. The amount of frictional resistance imparted may be adjusted by tightening screws 123 on brake band 117. However, the degree to which ring gear 104 is able to rotate relative to cage plates 106 and 105 is limited. After jaws 116 have mounted cam surfaces 114, further rotation of ring gear 104 will impart sufficient torque to cages plates 106 and 105 to overcome the frictional resistance of brake band 117. Thereafter, ring gear 104 and cage plates 106 and 105 rotate together to apply torque to the tubular member. 
     In order to align the openings in cage plates 106 and 105 and ring gear 104 with the power tong throat 112, the power tong employs the backing lug 125 seen in FIG. 1. Backing lug 125 is positioned on ring gear 104 and will engage backing pin 120 as seen in FIG. 3. As best seen in FIG. 2, backing pin 120 will be positionable in either one of backing pin apertures 121 or 122. Backing pin apertures 121 and 122 are positioned on either side of backing lug 125 when the openings of ring gear 104 and cages plates 106 and 105 are aligned. Retaining bolt 119 will allow backing pin 120 enough movement to be switched from pin aperture 121 to 122 while insuring backing pin 120 cannot be completely separated from cage plate 106. As discussed above, while generally carrying out its intended function, this prior art power tong has numerous disadvantages. 
     It is an object of the present invention to provide a selectively releasable braking apparatus which eliminates many of the disadvantages of the prior art brake band. Turning to FIG. 4, the braking apparatus 1 is illustrated as mounted on the lower cage plate 30 of a conventional power tong. For simplicity, the only parts of the conventional power tong shown are upper cage plate 29, lower cage plate 30, ring gear 28 and the rear portion of power tong body plates 35. The braking apparatus 1 will have a brake ring 25 which is positioned between an upper brake ring plate 14 and a lower brake ring plate 15. A spacer or adapter 33 will be positioned between cage plate 30 and upper bake ring plate 14. Roller bearings 20 will be attached to brake ring plates 14 and 15 by bolts 21. Roller bearings 20 will ride in groove 26 such that brake ring 25 is rotatively mounted between brake ring plates 14 and 15. A series of bolts 24 will connect brake ring 25 to bottom cage plate 30 such that these components must rotate together. Brake ring 25 will have gear teeth 27 which will engage secondary braking gears 23 as best seen in FIG. 7. The secondary braking gears 23 will in turn engage primary braking gear 12. Returning to FIG. 4, primary braking gear 12 will engage teeth 13 of brake disc stem 6 such that torque may be transferred from braking gear 12 to a braking surface 4, which in the embodiment shown is brake disc 5. It will be understood that this gear train arrangement allows torque imparted to cage plate 30 to be transferred to brake disc 5. It will also be understood that the term &#34;gear train&#34; is not limited to a particular number or configuration of gears. Further, any number of alternative gear trains could be used as long as these gear trains were capable of transferring torque from cage plate 30 to a braking surface 4 such as brake disc 5. As best seen in FIG. 4, primary braking gear 12 and brake disc stem 6 will be contained between plates 14 and 10. 
     To grip the braking surface 4, apparatus 1 has a gripping device 11 which is capable of gripping and holding the braking surface 4. In the embodiment shown in FIG. 4, this gripping device 11 is a selective gripping device comprising brake caliper 7 having an internal caliper finger to grip brake disc 5. FIG. 10 illustrates how caliper finger 8 of brake caliper 7 will press against brake disc 5 in order to exert a gripping force by seizing brake disc 5 between caliper finger 8 and brake pad 17. Caliper finger 8 and brake pad 17 will be constructed of any conventional material which is wear resistant but also has a high coefficient of friction. Brake caliper 8 will be biased in the closed position such that finger 8 and pad 17 will grip brake disc 5 until a counter-acting force causes caliper finger 8 to release brake disc 5. In the embodiment of FIG. 10, the biasing force will be provided by spring 18 and the counter-acting force will be provided by the piston of dual piston cylinder 9 positioned on brake caliper 7. It can be seen that spring 18 biases the piston of dual piston cylinder 9 in the closed position (i.e. gripping the baking surface 4). Dual piston cylinder 9 will also include a smaller piston head 9b and a larger piston head 9a. Larger piston head 9a allows spring 18 to be overcome and braking surface 4 to be released by a lower pressure source than required by smaller piston head 9b. While not shown, hydraulic fluid lines or other conventional means will provide the activating force to dual piston cylinders 9. The function of this low pressure/high pressure release mechanism will be explained in greater detail below. However, the present invention is intended to include within its scope any alternate release mechanism. The release mechanisms could include single piston devices activated at one pressure level or non-hydraulic devices that release the gripping device 11&#39;s hold on braking surface 4. 
     An alternate embodiment of the braking apparatus 1 is seen in FIGS. 8 and 9. FIG. 8 illustrates a braking surface 4 comprising a brake drum 60 operatively connected to the gear train of the braking apparatus 1. The gripping device 11 engaging brake drum 60 will be formed by a selective brake band 65 positioned about the perimeter of brake drum 60. Brake band 65 will not entirely encompass the circumference of brake drum 60, but will leave a slight gap between its terminal ends as best seen in FIG. 9. Each end will have end projection 67 or 68 extending therefrom. Similar to the caliper fingers 8 described above, end projections 67 and 68 will be biased together by a spring or similar device in a brake band releasing piston assembly 66. While not shown, releasing piston assembly 66 will be constructed similarly to gripping device 11 seen in FIG. 10. A spring in releasing piston assembly 66 will bias end projections 67 and 68 together, constricting brake band 65 which then grasps brake drum 60 and prevent its rotation. In order to expand selective brake band 65, hydraulic fluid will be supplied to releasing piston assembly 66 and internal pistons will then overcome the spring biasing end projections 67 and 68 together. In the embodiment shown, constricting piston assembly 66 also has a dual piston cylinder similar to that explained in reference to brake caliper 7. Releasing piston assembly 66 may receive hydraulic fluid from any conventional hydraulic circuit or the novel hydraulic circuit described below. 
     The scope of the present invention is intended to include any mechanism for selectively constricting brake band 65 or causing gripping device 11 to grip brake disc 5. Such a mechanism could include a threaded or screw device which may be electrically or hydraulically operated in order to allow control from a remote location. Alternatively, a conventional threaded or screw device that is manually tightened or loosened could be employed. While a braking apparatus 1 using a manually operated tightening device on brake band 65 or gripping device 11 could not be remotely controlled, such a braking apparatus 1 would still have significant advantages over the prior art. The brake band would not wear and loosen as in the prior art and grease or other substances on cage plate 30 would not cause slippage of braking apparatus 1. 
     The embodiments described above employ a separate braking ring which is bolted or otherwise attached to the cage plate of a conventional power tong. Another embodiment of the present invention is shown in FIGS. 12 and 13 and illustrates another manner in which the braking ring may be operatively connected to the cage plate of a power tong. This embodiment comprises a novel cage plate 31 with the braking ring 25 formed directly thereon. The gear teeth 32 of cage plate 31 will engage the secondary gears 23 of the gear train and thereby transfer torque to the braking surface 11 as described in the above embodiments. Cage plate 31 eliminates the need for a separate braking ring, adapter, and braking ring plates. The remaining components of braking apparatus 1 are the same for this embodiment as for those described above. 
     In operation, the power tong will be positioned such that the tubular member is in the throat of the power tong. To allow the jaws to grip the tubular member, it is necessary to rotate the ring gear 28 while holding the cage plates 29 and 30 stationary. The braking apparatus accomplishes this function by braking disc 5 preventing rotation of cage plates 29 and 30 while the drive train of the power tong rotates ring gear 28. After ring gear 28 has rotated sufficiently relative to cage plates 29 and 30 to close the tong jaws, further increases in torque will begin to increase the radial force the jaws are exerting on the tubular member. By controlling how much torque is applied to the ring gear 28 before the braking apparatus 1 releases cage plates 29 and 30, the present invention may control the amount of radial force placed on the tubular member by the jaws. This ability to control the radial force on the tubular member is of particular importance when the power tong is working stainless steel or Corrosion Resistant Alloy (CRA) tubular members. Contrary to the conventional toothed jaw members used to grip regular carbon steel tubular members, the face of jaw members gripping CRA tubular members must be comparatively smooth in order not to damage the costly CRA tubular member. Therefore the power tong must apply higher radial loads to the CRA tubular members to prevent slippage between the smooth jaw surfaces and the tubular member. The selective braking apparatus of the present invention allows the cage plate to be held as long as needed to apply the appropriate radial force to the tubular member. As a further advantage, the release of brake disc 5 after the appropriate radial force has been obtained eliminates practically all frictional resistance caused by a braking mechanism and all power may be used to apply torque to the tubular member. 
     As discussed above, it is another object of this invention to provide an apparatus which will automatically align the throat of the power tong and the openings in the ring gear and the cage plates without the operator needing to view the power tong throat. As seen in FIG. 4, this apparatus will include a stop device which in FIG. 4 is detent means 36 for selectively locking the cage plates 29 and 30 and ring gear 28 together; and the apparatus will also include a selectively engagable cage plate latch 40 which selectively prevents the cage plates 29 and 30 from rotating relative to the tong body. The apparatus will also incorporate the braking apparatus 1 described above. 
     Detent means 36 includes a ball 37 mounted in a bore 34 formed in ring gear 28. Ball 37 is biased downward by a spring 39 such that ball 37 will engage seat 38 when bore 34 and seat 38 are aligned. The embodiment shown will have a single detent means 36 which when engaged, will align the openings in cage plates 29 and 30 and ring gear 28. While only one detent means 36 is shown in the Figures, it will be understood that multiple detent means 36 could also be employed in alternate embodiments. The detent means 36 resists relative torque between cage plates 29 and 30 and ring gear 28 and causes these members to rotate together until a threshold torque overcomes spring 39 of detent means 36 and forces ball 37 into bore 34. Thereafter, cage plates 29 and 30 and ring gear 28 will be free to rotate relative to one another until detent means 36 is again engaged on the realignment of the openings of cage plates 29 and 30 and ring gear 28. 
     A cage plate latch 40 is also fixedly positioned on the power tong body and will selectively engage cage plate 29. Cage plate latch 40 will comprise a cylinder 79 having a piston 81 and a spring 80. Attached to piston 81 is a latch pin 82 which is biased toward a latch slot 83 formed on cage plate 29. When latch pin 82 engages latch slot 83, cage plate 29 will be prevented from rotating. To release cage plate latch 40, hydraulic fluid will be pumped into cylinder 79, forcing piston 81 rearward and disengaging latch pin 82 from latch slot 83. As long as fluid pressure is being supplied to cylinder 79, spring 80 will not return latch pin 82 to latch slot 83 and cage plate latch 40 does not inhibit rotation of cage plates 29 and 30. 
     By employing braking apparatus 1, cage plate latch 40, and detent means 36 in a specific sequence of operation, the throat of a power tong can be automatically aligned with the openings in the ring gear 28 and the cage plates 29 and 30 without the operator needing to view the power tong throat. FIG. 11 represents schematically one embodiment of a fluid circuit which would control the interaction of cage plate latch 40, braking apparatus 1, and detent means 36. Control circuit 3 will have a high pressure source 41 and a low pressure source 42 of hydraulic fluid. High pressure source 41 will power hydraulic motor 45 when the power tongs are closing on and applying the high torque needed to make-up or break apart a tubular member joint. Low pressure source 42 will also supply fluid to motor 45 but for the purpose of releasing the tubular member and realigning the power tong throat with the openings on the ring gear 28 and cage plates 29 and 30. Motor 45 will have two lines 48 and 49 which will cause the motor 45 to supply torque in the clockwise or counter-clockwise direction, depending on through which line fluid is supplied. When fluid is being supplied by one line, for example line 48, then the other line (49) will act as a return line allowing the fluid to complete a circuit back to source 41. Fluid pressure source 41 will have lines 41a and 41b for supplying fluid to lines 48 and 49 respectively. Fluid pressure source 42 will have lines 42a and 42b for supplying fluid to lines 49 and 48 respectively. 
     In operation, the power tong will be positioned such that the tubular member is in the throat of the power tong. To allow the jaws to grip the tubular member, it is necessary to rotate the ring gear 28 while holding the cage plates 29 and 30 stationary. To rotate the tubular member in a given direction (arbitrarily designated clock-wise), a valve will be opened at supply source 41 allowing high pressure fluid to flow through line 41a. Fluid will flow through pilot operated check valve 43b and line 48 to supply power to motor 45. Fluid will return by way of line 49 and 41b. Fluid is able to pass through pilot operated check valve 43a because a cross-over line (shown by dashed lines) will displace the ball of pilot operated check valve 43a and open valve 43a to returning fluid. As fluid is being supplied to motor 45, fluid will simultaneously flow through shuttle valve 44 towards cage plate latch 40 and small piston head 9b of dual piston cylinder 9 in gripping device 11. The latch spring 80 on cage plate latch 40 is sized such that it will be overcome by the fluid pressure and release cage plates 29 and 30 prior to small piston head 9b releasing braking surface 4 which is braking disc 5 in the embodiment shown in FIG. 4. As pressure increases, the torque applied by motor 45 to ring gear 28 increases. Because braking disc 5 is preventing rotation of cage plates 29 and 30, the torque applied to ring gear 28 is tending to cause relative rotation between cage plates 29 and 30 and ring gear 28 and tending to overcome detent means 36. As the torque threshold for overcoming detent means 36 is reached, ball 37 will be forced into bore 34 and ring gear 28 will begin to rotate relative to cage plates 29 and 30. However, cage plates 29 and 30 are still held stationary since small piston head 9a is sized to insure that the fluid pressure needed to produce the torque overcoming the release threshold of detent means 36 is less than the fluid pressure necessary to overcome spring 18 and release braking disc 5. After ring gear 28 has rotated sufficiently relative to cage plates 29 and 30 to close the tong jaws, further increases in torque will begin to increase the radial force the jaws are exerting on the tubular member. The rise in torque corresponds to an increase in fluid pressure which will at a predetermined point be sufficient to overcome spring 18 and cause brake caliper 7 to release brake disc 5. With the release of brake disc 5, there is no longer a frictional resistance caused by a braking apparatus 1 and all power may be applied as torque on the tubular member. 
     Various valves will prevent the flow of fluid through incorrect lines. When fluid is traveling through line 41a, shuttle valve 44 will prevent fluid from entering line 41b and disrupting the return flow from motor 45. Similarly, pilot operated check valves 47a and 47b will prevent high pressure fluid from entering the low pressure side of fluid circuit 3. Pilot operated check valves 47a and 47b will also have cross-over lines (shown as dashed lines) allowing low pressure fluid to circulate through motor 45. 
     After the desired torque load has been applied to the tubular member, the valve supplying fluid from high pressure source 41 will be closed and the pressure in the system will be allowed to dissipate. The spring 80 in cage plate latch 40 will again move latch pin 82 into engagement with latch slot 83 and spring 18 will urge braking caliper 7 to grip braking disc 5. Since the openings of the cage plate and ring gear will be in whatever arbitrary position they were in when the tongs stopped rotating the tubular member, these members will not be aligned with the throat of the power tong. To effect this alignment so that the power tong may be backed off the tubular member, low pressure hydraulic fluid will be introduced into the hydraulic circuit from low pressure source 42. As the above example supplied high pressure fluid to line 48 in order to apply torque to the tubular member, fluid must be supplied to line 49 in order to reverse the direction of the power tong&#39;s rotation. Therefore fluid flows from line 42a through pilot operated check valve 47b to line 49. Shuttle valve 46 allows fluid to enter line 51 but prevents fluid from interfering with the return flow from line 48. Pilot operated check valve 43a will prevent the fluid from entering the high pressure side of the circuit. Pilot operated check valve 47a will allow the return flow of fluid from motor 45 since cross-over lines have displaced the ball in pilot operated check valve 47a. It can be seen that fluid from low pressure supply 42 does not flow into line 50 to release cage plate latch 40, but fluid does flow to line 51 and large piston head 9a. The fluid supplied to motor 45 will cause motor 45 to begin rotating ring gear 28. However, the pressure does not produce sufficient force on piston head 9b to overcome spring 18 and release brake disc 5. Therefore cage plates 29 and 30 will be held in place and ring gear 28 continues to rotate until seat 38 of detent means 36 is aligned with bore 34 containing ball 37. Ball 37 will then be forced into seat 38 by spring 39 and ring gear 28 will be coupled with cage plates 29 and 30. At this point, the openings of cage plates 29 and 30 and ring gear 28 will be aligned. Because of the resistance imparted by the braking apparatus 1 to ring gear 28 through cage plates 29 and 30, torque resisting motor 45 will increase in conjunction with an increase in pressure in the hydraulic line. If braking apparatus 1 did not release cage plates 29 and 30, the increasing torque would eventually overcome detent means 36. However, piston head 9a is sized such that the pressure will overcome spring 18 prior to motor 45 generating sufficient torque to overcome detent means 36. Once piston head 9b overcomes spring 18 and braking apparatus 1 releases cage plates 29 and 30, the cage plates 29 and 30 and ring gear 28 will begin to rotate together. As discussed, no pressure has been supplied to cage plate latch 40 and therefore latch pin 82 (seen in FIG. 4) is biased against cage plate 29 by latch spring 80. As cage plate 29 continues to rotate, cage plate latch pin 82 will come into alignment with latch slot 83 and engage latch slot 83, thereby aligning the openings of cage plates 29 and 30 and ring gear 28 with the throat of the power tongs. Once again, the resistance to rotation will cause motor 45 to place higher torque on ring gear 28 which would eventually overcome detent means 36. However, cross-over relief valves 55 of fluid circuit 3 will divert fluid to the opposing line of motor 45 before the pressure is sufficient to overcome detent means 36. Since the openings of cage plates 29 and 30 and ring gear 28 are aligned with the throat of the power tong, the power tong may be backed off the tubular member and engage the next tubular member in the making up or breaking apart process. 
     Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. For example, it is envisioned that the function of the hydraulic circuit described above could be accomplished by other means. One such means could be an electo-mechanical means supplying a high source of power and a low source of power to the tong motor. Therefore the description of a high and low power source includes both electrical sources and fluid pressure sources. Similarly, the timed releasing of the cage plate latch and the braking apparatus could be carried out by solenoids or similar electo-mechanical devices. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications which fall within the true spirit and scope of the invention.