Self-centering clamp for down-hole tubulars
BACKGROUND OF THE INVENTION 
     The present invention relates to a device for clamping oil well and water well tubulars and rods, including but not limited to drill pipe, drill collars, well casing, production tubing, sucker rods, pump column pipe, and the like, all of which tubulars, pipes and rods are referred to herein simply as &#34;down-hole tubulars&#34;. More particularly, this invention relates to such a clamp which precisely centers down-hole tubulars of varying diameters. 
     In well drilling and well completion operations it is often necessary to lift and to align lengths of down-hole tubulars precisely. For example, in oil or water well drilling, multiple lengths of drill pipe must often be raised from a horizontal position at or near ground level to a vertical position aligned with the center line of the well. Such lifting and aligning operations require some type of clamp for securely holding the tubular in place as it is lifted. When a pivotably mounted transfer arm is used, this clamp must support large loads in several different orientations. 
     Compounding the problem is the fact that each joint or length of down-hole tubular must be closely aligned with a string of such tubulars after it has been lifted to the vertical position, as when a string of drill pipe or casing is being made up, for example. A clamp for such purposes should preferably provide, without any adjustment, the necessary alignment for down-hole tubulars having various diameters. Proper alignment has been a problem for many such clamps of the prior art, particularly those employing pivoted clamping jaws. 
     When pivoted clamping jaws are used, there is a tendency for the center of the clamped down-hole tubular to vary as a function of the diameter of the tubular being clamped. This problem may be alleviated somewhat by using guided jaws in conjunction with symmetrically moving pivoted rocker arms. Such an arrangement is shown in a machine tool clamp described by Lorenz in U.S. Pat. No. 3,386,726. In the Lorenz clamp the guided jaws are free to translate with respect to the pivotably mounted rocker arms as the clamp closes. 
     This approach, however, suffers from the disadvantage that loads are not symmetrically distributed in the clamp for the full range of clamp positions. As the guided jaw translates with respect to the rocker arm, the center of clamping force on the jaw moves. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a self-centering clamp for down-hole tubulars which avoids these and other disadvantages of the prior art. 
     The general object of this invention is to provide a clamp for down-hole tubulars which precisely clamps and centers such tubulars in such a manner that in each case the tubular is clamped with its central axis at a substantially constant position with respect to the clamp, in spite of variations in the diameter of the clamped tubular. 
     Another object of this invention is to provide a sturdy clamp which symmetrically bears the clamping forces associated with clamping and holding down-hole tubulars having a range of diameters. 
     Yet another object of this invention is to provide a clamp having the aforementioned self-centering and symmetrical load bearing features which can clamp down-hole tubulars having a predetermined range of diameters without requiring manual adjustment or replacement of component parts, thereby speeding and facilitating both drilling and well service operations. 
     Yet another object of this invention is to provide a clamp having the aforementioned self-centering and symmetrical load bearing features which is compact and avoids complex positioning linkages such that the clamp can be rotatably mounted to a transfer arm, thereby facilitating gravity loading and unloading of the clamp, as well as the use of automated or semi-automated loading and unloading systems. 
     According to this invention, these and other objects are achieved by providing a clamp having two opposed clamping members which are guided along a first line which passes through the clamping axis. Means are provided for positioning the opposed clamping members along the first line such that the clamping members are maintained substantially equidistant from the clamping axis. Preferably this interconnecting means comprises a pivot member having first and second spaced attachment points, means for pivotably mounting the pivot member to the clamp, means for connecting the first clamping member to the first attachment point such that movement of the first clamping member along the first line causes the pivot member to rotate, and means for connecting the second clamping member to the second attachment point such that rotation of the pivot member causes the second clamping member to move along the first line to maintain the first and second clamping members substantially equidistant from the clamping axis. 
     In that both clamping members are guided along a line which passes through the clamping axis, the clamp of this invention provides substantially symmetrical load bearing capabilities for down-hole tubulars having a wide range of diameters. This facilitates the design of a clamp which is sturdy yet not unduly heavy due to the need to withstand assymetrical clamping loads. 
     Another advantage of this invention is that down-hole tubulars of varying diameters can be accurately clamped and centered about the same clamping axis. This facilitates precise alignment of the clamped length of down-hole tubular with other lengths, such as in a drill string or a production string, for example. 
     The clamp of this invention provides the further advantage that the clamp itself can be embodied in a sturdy, compact structure which avoids the need for gear mechanisms, which may be subject to failure under adverse conditions of field use. In addition, the clamp of this invention provides the further advantage that no manual adjustment or replacement of parts is required to obtain the precise centering and symmetrical clamping features described above, even when down-hole tubulars of varying diameters are clamped. 
     These and other objects and attendant advantages of the present invention will be better understood by reference to the following detailed description taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view in partial cutaway of a preferred embodiment of the self-centering clamp of this invention . 
     FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG. 1. 
     FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1. 
     FIG. 4a is a cross-sectional view taken along line 4--4 of FIG. 3 showing the clamp in the open position with the jaws extended to the largest extent possible. 
     FIG. 4b is a cross-sectional view corresponding to that of FIG. 4a showing the clamp in a closed position. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to the drawings, FIG. 1 shows a side view of a preferred embodiment of the self-centering clamp of this invention. This clamp 10 is provided with a frame 40 which forms the basic structure of the clamp 10. This frame 40 is a weldment made up of a number of component plates, including opposed parallel side plates 42,44 which are separated by opposed parallel edge plates 46,48. In addition, brace plates 49 are provided to further strengthen the frame 40 and to maintain the side plates 42,44 in a parallel relationship. 
     As best shown in FIG. 3, the side plates 42,44 and the edge plates 46,48 cooperate to form a rigid boxlike structure at one end of the clamp 10. This boxlike structure guides the movement of the opposed jaws 20,22, as will be explained in detail below. The frame 40 forms the main structure of the clamp 10 and in this preferred embodiment is welded up from component plates of steel which are for the most part one-half inch in thickness. The edge plates 46,48 are preferably 3/4 of an inch in thickness. 
     In this preferred embodiment, elongated slots 50,52 are formed in the edge plates 48, as best shown in FIG. 4a. In addition, a rounded oblong slot 92,94 is formed in each side plate 44,42, respectively. In this preferred embodiment, each of these oblong slots 92,94 measures 2.50 inches in width and 2.875 inches in length. 
     Mounted within the square guide channels formed by the frame 40 are two opposed clamping members or jaws 20,22. Each jaw 20,22 is substantially prevented from twisting by the respective portions of the frame 40, and is constrained to movement along a straight line such that each jaw 20,22 remains centered about a clamping axis which passes through the central, longitudinal axis of the down-hole tubular 12 being clamped. As used herein, the term &#34;clamping axis&#34; denotes the central longitudinal axis of the clamped down-hole tubular. Each jaw 20,22 defines a clamping surface 28,30 adjacent the front end of the jaw, and includes a pin 24,26 at the rear end of the jaw. Preferably, each clamping surface 28,30 is provided with four elongated hardened inserts which serve to securely grip down-hole tubulars clamped in the clamp 10. 
     As best shown in FIG. 4a, the clamp 10 also includes a pair of opposed rocker arms 60,62. Each rocker arm defines four separate holes spaced along the length of the rocker arm 60,62. In FIG. 4a, these four holes are designated by reference numerals 64, 66, 68 and 70. Each rocker arm 60,62 is pivotably connected to a respective one of the jaws 20,22 by means of the pin 24,26 which passes through the hole 64 in the respective rocker arm 60,62. Each rocker arm 60,62 is in turn pivotably mounted between a pair of spaced, parallel tension members 80,82 by means of pins 84,86. As best shown in FIGS. 2, 3 and 4a, these tension members 80,82 are in turn mounted to a pin 90 which is held in place in the oblong slots 92,94 formed in the frame 40. As will be explained below, the slots 92,94 allow a limited amount of longitudinal movement of the pin 90. 
     In this preferred embodiment the pin 90 is a solid steel pin 2.500 inches in diameter. Because of the close fit between the diameter of the pin 90 and the width of the oblong slots 92,94 along the length of the tension members 80,82 the pin 90 is substantially restrained from movement along the length of the tension members 80,82, yet it is permitted an excursion of approximately 0.375 inches along a line which passes through the center of the pin 90 and the clamped tubular member 12. 
     A sleeve 100 is pivotably mounted on the pin 90 between the tension members 80,82. In this preferred embodiment, a pivot plate 102 is secured to the sleeve 100 such that the two are free to pivot about the pin 90 as a unit. This pivot plate 102 defines two spaced attachment point bores 104,106. The pivot plate 102 is interconnected to each of the rocker arms 60,62 by means of links 110,112. As best shown in FIGS. 2 and 4a, a pair of parallel links 110 are pivotably mounted to the rocker arm 60 by means of a pin 114 which extends through the hole 66 in the rocker arm 60. In addition, these links 110 are pivotably secured to the pivot plate 102 via a pin 118 which passes through the bore 104 in the pivot plate 102. Similarly, a pair of parallel links 112 are mounted pivotably to the pivot plate 102 by means of a pin 120 which passes through the bore 106, and at the other end to the rocker arm 62 by means of a pin 116 which passes through the hole 66 in the rocker arm 62. 
     A hydraulic cylinder 130 is mounted between the rocker arms 60,62 by means of pins 132,134 which pass through the holes 70 in the end portions of the rocker arms 60,62. In this preferred embodiment, the links 110,112 and the tension members 80,82 are formed of 1/2-inch plate steel, and the pivot plate 102 and the rocker arms 60,62 are formed of 11/2-inch plate steel. 
     Having described the structure of this preferred embodiment of the clamp of this invention, its operation can now be discussed in connection with FIGS. 4a and 4b. FIG. 4a shows the clamp 10 with the jaws 20,22 retracted to permit large diameter down-hole tubulars to be loaded into the clamp 10. FIG. 4b shows the same clamp with the jaws 20,22 moved towards one another to clamp a small diameter down-hole tubular. As can be seen by comparing FIGS. 4a and 4b, the hydraulic cylinder 130 acts to pivot the rocker arms 60,62 about the pins 86,84, and thereby to position the jaws 20,22. As explained above, the frame 40 acts as a guide to restrict movement of the jaws 20,22 to linear movement such that the center line of the jaws remains centered on the desired clamping axis for the tubular 12. 
     The elongation of the slots 92,94 allows the tension members 80,82 to approach the jaws 20,22 when the jaws are positioned close to one another, while also allowing the tension members 80,82 to move away from the jaws 20,22 when the jaws are moved away from each other. This movement of the pin 90 and the tension members 80,82 is important, for it allows the clamping loads to be transferred symmetrically from the jaws 20,22 to the rocker arms 60,62 while allowing the jaws 20,22 to move in precisely a straight line. In effect, the elongation of the slots 92,94 accomodates the varying separation between the tension members 80,82 and the jaws 20,22 caused by the varying angle of the rocker arms 60,62. 
     The links 110,112 and the pivot plate 102 serve to center the jaws 20,22 simply and reliably on the desired clamping axis. When the jaws 20,22 move from the open position shown in FIG. 4a to the closed position shown in FIG. 4b, the links 110,112 serve to rotate the pivot plate 102 from the position shown in FIG. 4a to the position shown in FIG. 4b. As the pivot plate 102 rotates about the pin 90, it insures that the rocker arms 60,62 maintain substantially symmetrical positions with respect to the desired clamping axis. This insures that the jaws 20,22 are symmetrically placed about the desired clamping axis throughout the range of travel of the jaws 20,22. Thus, the clamp 10 centers the jaws 20,22 with the required precision about the desired clamping axis for a wide range of travel of the jaws 20,22. This means that both large diameter down-hole tubulars and small diameter down-hole tubulars can be reliably clamped about the same clamping axis, automatically and without the need for manual adjustment or replacement of component parts of the clamp 10. The preferred embodiment shown in the drawings reliably and automatically centers clamped tubulars having diameters as large as 91/2 inches and as small as 31/2 inches substantially about the same clamping axis. 
     One important advantage of the self-centering clamp of this invention is that it can be embodied in a compact, rigid clamp which is relatively low in cost and easy to manufacture. The clamp of this invention can be used in many applications related to the handling of down-hole tubulars. For example, a pair of spaced clamps 10 can be mounted to a drill rig transfer arm to clamp down-hole tubulars as they are moved between a horizontal and a vertical position with the arm. These clamps can be used to releasably clamp down-hole tubulars having wide range diameters to the transfer arm. Because each clamped tubular is centered about substantially the same clamping axis, the clamp of this invention allows tubulars to be precisely centered with other tubulars, as when necessary to make up a drill string, for example. 
     A second important application for the self-centering clamp of this invention is in make-up or break-out devices used to make and break threaded joints between adjacent lengths of down-hole tubulars. Because the clamp of this invention is self-centering for a wide range of clamped tubulars, make-up or break-out devices utilizing this clamp can be used with a wide range of tubulars without need for any adjustment to provide for proper centering of the clamp. 
     An additional advantage of this preferred embodiment is that it is simple and rugged, and well suited to bear the heavy loads to which such clamps are routinely subjected. The novel linkage of this embodiment is compact in that none of the linkage extends beyond the hydraulic cylinder. Furthermore, this embodiment is easily manufactured from readily worked materials such as plate steel. The link and pivot plate arrangement is reliable, and it entirely avoids the need for a gear linkage. 
     Of course, it should be understood that various changes and modifications to the preferred embodiment described above will be apparent to those skilled in the art. For example, the clamp of this invention can be scaled as appropriate to accomodate tubulars of the desired sizes and to bear associated stresses and strains reliably. Such changes and modifications can be made without departing from the true spirit and scope of the present invention, and it is therefore intended that the following claims be interpreted to cover all such changes and modifications.