Abstract:
In a pivoting-jaw power tong having a rotary gear with substantially linear camming surfaces and pivoting jaw members with cam followers for engagement with the camming surfaces upon rotation of the rotary gear relative to the jaw members, each cam follower has at least one contact surface configured for sliding engagement with a primary camming surface. The provision of sliding contact surfaces reduces contact pressures between the cam followers and the camming surfaces, thus reducing wear. The cam followers may be formed integrally with the jaw members, but preferably will be rollers which can swivel relative to their respective jaw members to facilitate substantially uniform sliding contract between the cam follower contact surfaces and the camming surfaces of the rotary gear. In alternative embodiments, the camming surfaces may have a slight concave curvature, with the cam followers&#39; contact surfaces being correspondingly convexly curved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit, pursuant to 35 U.S.C. 119(e), of U.S. Provisional Application No. 60/806,409, filed on Jun. 30, 2006, and said provisional application is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates in general to power tongs for gripping oilfield tubulars to facilitate make-up or break-out of threaded connections between tubulars. In particular, the invention relates to pivoting-jaw power tongs having rotary gears with improved camming surface geometry.  
       BACKGROUND OF THE INVENTION  
       [0003]     Power tongs are well known in the field of drilling and servicing oil and gas wells. Drill pipe and production tubing are typically provided in the form of round steel pipe (commonly referred to as tubulars) with threaded ends for connecting tubulars into a drill string or a production string, depending on the operation being conducted. The term “make-up” is commonly used to refer to the process of connecting tubulars to each other (i.e., “making up” a threaded connection), and the term “break-out” refers to the process of disconnecting tubulars (i.e., “breaking out” a threaded connection). Well drilling and well servicing involve both make-up and break-out functions, for a variety of purposes well known in the field.  
         [0004]     Make-up or break-out of a threaded joint requires that the tubular on each side of the joint be firmly gripped so that the tubulars can be contra-rotated relative to each other, either clockwise or counterclockwise, to make up or break out the joint as desired. This gripping function is commonly carried out using a power tong in each of the tubulars. Power tongs typically have either sliding jaw assemblies or pivoting jaw assemblies, and the present invention is particularly referable to the pivoting-jaw type. There are numerous known types or models of pivoting-jaw power tongs, but they generally have the common features of a partial-circle (or “C-shaped”) rotary gear, the internal surface of which defines a camming surface, and a jaw assembly disposed inside the rotary gear and having two or more cam followers that ride against the camming surface of the rotary gear. The cam followers may be in the form of rollers which turn around suitable axles or pivot pins, or they may be formed integrally with the jaws. The jaw assembly has several (typically four) dies—i.e., elements which are toothed or otherwise adapted for grippingly engaging the circumferential outer surface of a tubular member by effectively biting into the steel surface of the tubular when forced against the tubular. The geometry of the camming surface is adapted such that when the rotary gear is rotated around the jaw assembly, either clockwise or counterclockwise away from a neutral position, the dies are used into gripping contact with the outer surface of the tubular. When the tubulars on each side of the joint have been thus engaged by respective power tongs, the tongs may be rotated relative to each other in the desired mode, thus making up or breaking out the joint as desired.  
         [0005]     One example of a prior art pivoting-jaw power tong is disclosed in Canadian Patent No. 1,125,737 issued on Jun. 15, 1982 to Farr et al. (and corresponding to U.S. Pat. No. 4,350,062). As with typical pivoting-jaw power tongs, the camming surface of the Farr device includes a pair of opposed recesses (or neutral zones) such that when the rotary gear is rotated to a neutral position wherein when each cam follower has moved into one of the recesses, the jaws spread apart so as to allow the jaws to receive a tubular. On either side of each recess, the camming surface has a circularly-curved primary camming surface. These primary camming surfaces are configured such that when the rotary gear is rotated in either direction away from the neutral position, each jaw is rotated inward. As rotation of the rotary gear increases, the jaws close on the tubular, causing the dies to bite into the tubular. When the tubular is firmly gripped by the dies, the tubular can be rotated so as to be connected to or disconnected from (as the case may be) another tubular.  
         [0006]     The primary camming surfaces of the Farr device have different curvature radii on either side of each recess, and this is considered to have certain advantages over typical prior art pivoting-jaw power tongs in which the same curvature radius is used for all of the primary camming surfaces. However, it has been observed that power tongs with circular primary camming surfaces are prone to reduced effectiveness as the dies, pins, rollers, and/or camming surfaces become worn. In such circumstances, the force with which the dies are urged against the surface of the tubular is not uniform, so the dies grip the tubular with variable effectiveness, and in the worst case a die may grip the tubular with little or no effectiveness at all. In such cases only three of the dies will be effectively gripping the tubular instead of four, and this condition tends to causing warping and/or marking of the tubular. This tends to be particular problem for the dies located near the pivot points of the jaws.  
         [0007]     U.S. Pat. No. 6,988,428 (Kathan) discloses an improved pivoting-jaw power tong with enhanced capability for reliable and effective gripping engagement of tubular members in cases where dies, pins, rollers, and/or camming surfaces are worn. In the pivoting-jaw power tong of U.S. Pat. No. 6,988,428, the primary camming surfaces of the rotary gear have a novel geometry. Each primary camming surface has a substantially linear configuration, rather than a circularly-curved configuration. More specifically, each primary camming surface is oriented so as to form an acute angle with a radial line extending from the rotary gear to the point where the primary camming surface meets its corresponding neutral recess. By virtue of this geometric configuration, the radial distance to a point on any of the primary camming surfaces reduces, in substantially linear fashion, with increased distance away from the neutral recess. Therefore, increased rotation of the ring gear in either direction away from the neutral position will cause both jaws to rotate further inward, thus increasing the force that the dies on the free ends of the jaws will exert on a tubular being engaged by the apparatus.  
         [0008]     When using a power tong having such linearly-configured primary camming surfaces, the dies will engage the tubular with considerably increased uniformity and effectiveness, even when the dies, pins, cam followers (e.g., rollers), and/or camming surfaces are worn. In prior art power tongs with curved camming surfaces (such as in the Farr reference), there is a relatively small “sweet spot” or optimal contact zone on the camming surface corresponding to each cam follower such that each die will exert maximum gripping force on the tubular when the cam followers are at their corresponding sweet spots. This works well when the apparatus is new, without any wear to the various components. All of the cam followers will hit their sweet spots at the same time (i.e., when the rotary gear is in a specific optimal position), because the distance from the rotary gear&#39;s center of rotation to the sweet spot is the same for all of the cam followers, the distance from the center of rotation to the face of the cam followers is constant, and the distance from the center of rotation to the die contact surfaces is constant.  
         [0009]     Continued use of the power tong inevitably results in wear to the components, however, and a certain amount of “play” develops in the mechanism. This causes changes in the geometrical relationship between the com followers, dies, and/or camming surfaces, such that the cam follower can no longer hit their sweet spots at the came time. In effect, one or more of the sweet spots become shifted to a different position on the camming surface because of the wear, and the geometric relationship between the sweet spots no longer coincides precisely with the geometric relationship between the cam followers. Even small amounts of play can thus result in reduced gripping force being applied to the tubular at one or more of the dies, such that the tubular is not gripped uniformly.  
         [0010]     This undesirable condition cannot be effectively remedied by further rotation of the rotary gear, because the tangential angle between the cam followers and the camming surfaces (which may be referred to as the “tangential contact angle”) changes as the cam followers move away from their sweet spots or optimal contact zones on the camming surfaces, due to the fact that the camming surfaces are curved. However, when substantially linear camming surfaces are used, the tangential contact angle will be substantially the same for all cam followers regardless of the position of the rotary gear, and will not be materially altered by normal operational wear to the various components of the power tong.  
         [0011]     In the power tong of U.S. Pat. No. 6,988,428, the linear camming surfaces are oriented such that the radial distance from the center of rotation to the camming surface decreases in substantially linear fashion as the camming surfaces propagate away from the neutral recesses in the camming surfaces. By virtue of this camming surface geometry, rotation of the rotary gear in either direction away from the neutral position results in a linearly progressive reduction of the distance from the center of rotation of the points where the cam followers contact the camming surfaces, with the rate of reduction varying in proportion to the angular displacement of the rotary gear, regardless of how far the cam followers may be displaced away from the neutral recesses.  
         [0012]     The use of such linear camming surfaces results in a much larger sweet spot corresponding to each cam follower. This camming surface geometry allows the power tong to automatically adjust for wear in the mechanism such that the contact force between the cam followers and the camming surfaces—and, therefore, the gripping force applied by the dies to the tubular—will be substantially constant at all locations.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The invention has found that the effectiveness of the power tong of U.S. Pat. No. 6,988,428 can be enhanced by using beveled cam followers, so as to provide a substantially flat contact area when a cam follower is brought into contact with one of the linear camming surfaces. This geometry has the effect of significantly reducing the contact pressure between the cam follower and the camming surface, as compared to the typical case in known power tongs where the cam follower is a circular roller or has a curved profile. In such cases, the contact area between the cam follower and the camming surface will be comparatively small, in the nature of a concentrated point load or line load. In the present invention, however, both contact surfaces are substantially flat or planar, and substantially parallel to each other. Accordingly, for a given force being transferred perpendicularly across the contact area, the contract pressure will be considerably less than it would be in prior art tongs where the same force is applied much like a concentrated point load or linear load.  
         [0014]     The use of substantially flat cam follower surfaces can reduce or even eliminate localized deformations in the camming surfaces and the cam followers. The high contact pressures resulting from the use of round or roller-type cam followers cam cause localized plastic deformation of the surficial layers of metal in the contact zones of each of these elements, possibly causing the formation of grooves or ruts in the camming surfaces and/or wear in the cam followers. These undesirable effects can over time result in the need for increased rotation of the rotary gear in order to develop satisfactory gripping forces between the dies and a pipe joint being made up or broken out with the power tong.  
         [0015]     Accordingly, the present invention may be described in general terms as an improvement for a power tong with linear or substantially linear camming surfaces, with the improvement comprising a cam follower having one or more substantially flat contact surfaces configured and oriented to coma into sliding and substantially uniform contact with one of the linear camming surfaces. Preferably the cam follower will be developed so as to have two such substantially flat contact surfaces, oriented such that the cam follower will come into sliding and substantially uniform contact with one of the linear camming surfaces regardless of which direction the rotary gear is being turned (i.e., whether for purposes of make-up or break-out).  
         [0016]     The one or more substantially flat contact surfaces can be provided in a cam follower that is formed integrally with a power tong jaw. In such cases, the angular orientation of the flat contact surfaces will typically be selected so as to optimize the uniformity of contact with their corresponding linear camming surfaces under maximum load conditions.  
         [0017]     Alternatively, the one or more substantially flat contact surfaces can be provided in the roller of a roller-type cam follower that is rotatably mounted to a power tong jaw. In this embodiment, the roller can rotate to accommodate the slight changes in the angular relationship between the jaws and their corresponding linear camming surfaces as the rotary gear turns. This results in automatic adjustment of the substantially flat contact surfaces of the cam followers such that they will be in optimally uniform contact with the corresponding linear camming surfaces, despite angular movements of the jaws.  
         [0018]     The substantially flat contact surfaces of the cam followers may be coated or laminated with a material that facilitates sliding, while also being capable of withstanding, without excessive deformation, the compressive stress acting over the contact area with the linear camming surfaces.  
         [0019]     As will be appreciated from the preceding discussion, the term “linear camming surface”, as used in this patent specification, is not to be constructed as requiring perfectly planar camming surfaces, but are to be interpreted as covering substantially linear camming surfaces as well. Similarly, references to “flat cam follower surfaces” are to be interpreted as covering substantially flat cam follower surfaces, rather than being limited to perfectly planar cam follower surfaces.  
         [0020]     In alternative embodiments of the invention, the camming surfaces may have a slight concave curvature, with the cam follower surfaces having a corresponding convex curvature to facilitate sliding and substantially uniform contact with the curved camming surfaces.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Embodiments of the present invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which:  
         [0022]      FIG. 1  is an isometric view of a prior art power tong in accordance with U.S. Pat. No. 6,988,428, shown with the rotary gear in the neutral position such that a tubular can pass through the throat opening when the hinged doors have been opened.  
         [0023]      FIG. 2  is a perspective view of the prior art rotary gear of the power tong of  FIG. 1 .  
         [0024]      FIG. 3  is a plan view of the prior art rotary gear pf  FIG. 2 .  
         [0025]      FIG. 4  is an enlarged plan view of the prior art rotary gear of  FIG. 2 , illustrating the geometric configuration of the camming surfaces.  
         [0026]      FIG. 5  is a plan view of the rotary gear and pivoting jaws of the prior art power tong of  FIG. 1 , shown with the rotary gear in the open position in which a tubular can pass through the throat of the rotary gear and into position between the pivoting jaws.  
         [0027]      FIG. 6  is a plan view of the prior art rotary gear and pivoting jaws as in  FIG. 5 , shown with the rotary gear rotated counterclockwise from the neutral position with a tubular positioned inside the pivoting jaws, and with the jaws&#39; dies beginning to engage and the tubular.  
         [0028]      FIG. 7  is a plan view of the prior art rotary gear and pivoting jaws as in  FIG. 5 , shown with the rotary gear rotated further counterclockwise, with the front pair of dies securely engaging and gripping the tubular.  
         [0029]      FIG. 8  is a plan view of the rotary gear and pivoting jaws generally similar to those shown in  FIGS. 5, 6 , and  7 , but with each cam follower having flat-surfaced sliding contact surfaces in accordance with one embodiment of the present invention.  
         [0030]      FIG. 9  is a plan view of a rotary gear with pivoting jaws having roller-type cam followers with sliding camming surfaces in accordance with an alternative embodiment of the invention, shown with the rear pair of dies beginning to engage a tubular.  
         [0031]      FIG. 10  is a plan view of the rotary gear and pivoting jaws as in  FIG. 9 , shown with the rotary gear rotated clockwise relative to  FIG. 9 , with all dies securely engaging and gripping the tubular. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     1. Prior Art Power Tong  
       [0032]      FIG. 1  generally illustrates an assembled prior art power tong  10  in accordance with U.S. Pat. No. 6,988,428. With the exception of the configuration of the camming surfaces of the rotary gear (which is not visible in  FIG. 1 ), the construction of power tong  10  is largely similar to known power tongs. A generally-C-shaped gear housing  12  has doors  14  which can be swung open about hinge points  18  using handles  16  (as indicated by the broken arrows in  FIG. 1 ) so as to provide an opening into a central space  19  within gear housing  12 . A pair of jaw members  20  (typically of generally arcuate shape) are pivotably mounted within gear housing  12 . As shown in  FIG. 5 , each jaw member  20  has a pivot end  20 A, a free end  20 B, an inner side  20 C disposed toward central space  19 , and an outer side  20 D. The pivot end  20 A of each jaw member  20  is pivotably mounted to gear housing  12  by means of a pivot pin  22 , at a point opposite the opening into central space  19 . Dies  30 , for grippingly engaging a tubular, are mounted on the inner sides  20 C of each member  20  near each end thereof. Additional features of jaw members  20  are shown in  FIGS. 5, 6 , and  7 , and described in greater detail further on in this specification.  
         [0033]     Disposed within gear housing  12  (but not shown in  FIG. 1 ) is a generally C-shaped rotary gear  40 , exemplary embodiments of which are is illustrated in  FIGS. 2-7 . Rotary gear  40  has a circular perimeter with a plurality of gear teeth  41 . Rotary gear  40  also has an inner surface  42  which encloses a central space  48  of sufficient size to enclose jaw members  20  without interference. The perimeter of rotary gear  40  is interrupted by a throat opening  46  which provides access to central space  48 . Rotary gear  40  is mounted within gear housing  12  so as to surround jaw members  20  (as best seen in  FIGS. 5-7 ), and so as to be rotated within gear housing  12  about center axis C of rotary gear  40 . The power tong  10  includes means for rotating rotary gear  40 , and such means may be of any suitable type well known in the field of the invention. For exemplary purposes,  FIG. 1  illustrated power tong  10  with hydraulic actuation means  100 , comprising a hydraulic motor  110  which rotates a pinion gear (not shown) that engages gear teeth  41  so as to rotate rotary gear  40  clockwise or counterclockwise as desired, by means of hydraulic valve control levers  112 . Hydraulic lines  114  lead from hydraulic actuation means  100  to a hydraulic pump (not shown) associated with the drilling rig or service rig on which power tong  10  is being used.  FIG. 1  also illustrates a lifting ring  120  of a type that may be used for suspending power tong  10  from the rig&#39;s hoist.  
         [0034]     The three-dimensional configuration of rotary gear  40  is illustrated in  FIG. 2 . As shown in plan view in  FIG. 3 , inner surface  42  is of a generally circular configuration, but includes several geometrically distinct portions. If rotary gear  40  as illustrated in  FIG. 3  is analogized to a clock face, with throat opening  46  at approximately six o&#39;clock, a first neutral recess  43 A is formed in inner surface  42  at approximately nine o&#39;clock, and a second neutral recess  43 B is formed in inner surface  42  at approximately three o&#39;clock. The purpose of these neutral recesses will be explained later in this document.  
         [0035]     First neutral recess  43 A is contiguous with inner surface  42  at points W and X, and second neutral recess  43 B is contiguous with inner surface  42  at points Y and Z. Inner surface  42  includes substantially linear (i.e., substantially planar) primary camming surfaces  44 W,  44 X,  44 Y, and  44 Z which propagate away from transition points W, X, Y, and Z respectively, as shown in  FIG. 3 . Primary camming surfaces  44 W,  44 X,  44 Y, and  44 Z are indicated in  FIG. 3  as having lengths L W , L X , L Y , and L Z  respectively. The magnitude of these lengths will be dependent on the particular requirements of a given power tong  10 .  
         [0036]     The geometric characteristics of primary camming surfaces  44 W,  44 X,  44 Y, and  44 Z are illustrated in  FIG. 4 . A radial line R extending from center axis C to transition point W, X, Y, or Z will form an acute angle (A W , A X , A Y , or A Z ) with the corresponding primary camming surface  44 W,  44 X,  44 Y, or  44 Z. By virtue of this geometric characteristic, the radial distance from center axis C to a point on a given primary camming surface decreases in substantially linear fashion according to the distance away from the corresponding transition point. Acute angles A W , A X , A Y , or A Z  will typically be between eighty and ninety degrees, but the precise magnitude of these angles will be determined to suit the particular requirements of a given application (including, for example, the size of tubular T on which the power tong  10  is to be used).  
         [0037]      FIGS. 5, 6 , and  7  illustrate rotary gear  40  with jaw members  20  disposed within central space  48 , and within gear housing  12  (the components of which are omitted from  FIGS. 5, 6 , and  7  for clarity). As previously mentioned, the pivot end  20 A of each jaw member  20  pivots about a pivot pin  22  mounted to gear housing  12  at a point generally opposite throat opening  46 . Each jaw member  20  has a cam-following element (or cam follower)  24  associated with outer side  20 D. In the embodiments shown in  FIGS. 5, 6 , and  7 , cam followers  24  are provided in the form of protuberances formed integrally with jaw members  20 . Alternatively, cam followers  24  may be in the form of rollers rotatably mounted to their corresponding jaw members  20  using suitable axles or pivot pins. In any event, jaw members  20 , cam followers  24 , inner surface  42 , and neutral recesses  43 A and  43 B are configured and arranged such that jaw members  20  can pivot outward into an open position (as illustrated in  FIG. 5 ) in which each cam follower  24  is disposed within a corresponding neutral recess ( 43 A or  43 B), and in which a tubular T can pass through throat opening  46  of rotary gear  40  into central space  48 , and so as to be substantially concentric with center axis C of rotary gear  40 .  
         [0038]     The basic operation of the prior art power tong  10  of U.S. Pat. No. 6,988,428 may be understood with reference to  FIGS. 6 and 7 . In  FIG. 6 , with a tubular T positioned between jaw members  20 , rotary gear  40  has been rotated counterclockwise (as indicated by the curved arrows) relative to jaw members  20  (and relative to gear housing  12 ). The rotation of rotary gear  40  has forced cam followers  24  out if their corresponding neutral recesses  43 A and  43 B, such that they engage opposing primary camming surfaces  44   W  and  44   Z . As a result, the free ends  20 B of jaw members  20  have rotated inward to the point that dies  30 A and  30 B have begun to engage the cylindrical outer surface of tubular T. Because primary camming surfaces  44   W  and  44   Z  are of linear (or planar) configuration as previously described, further counterclockwise rotation of rotary gear  40  causes further inward rotation of jaw members  20  such that dies  30 A and  30 B bite into tubular T as shown in  FIG. 7 . Tubular T is thus securely gripped by jaw members  20 , thereby facilitating rotation of tubular T relative to an adjoining tubular.  
         [0039]     In similar fashion, clockwise rotation of rotary gear  40  (from the open position) would cause cam followers  24  to engage opposing linear primary camming surfaces  44   X  and  44   Y .  
         [0040]     In the embodiment described above, inner surface  42  of rotary gear  40  defines linear primary camming surfaces adjacent to each neutral recess. Alternative embodiments may have only one opposing pair of linear primary camming surfaces (i.e.,  44   W  and  44   Z , or  44   X  and  44   Y ), with the other opposing pair of camming surfaces being of a different configuration (e.g., curved).  
       2. Power Tong Having Cam Followers with Sliding Contact Surfaces  
       [0041]     The present invention may be understood with reference to  FIG. 8 , which illustrates a rotary gear  40  and pivoting jaw members  20  largely similar to those of the prior art power tong shown in  FIGS. 5, 6 , and  7 . In accordance with the present invention, however, each jaw member  20  incorporates a cam follower  240  with a first substantially flat sliding contact surface  250  and a second substantially flat sliding contact surface  260 . The first sliding contact surface  250  are oriented to come into sliding and substantially uniform contact with linear primary camming surfaces  44   W  and  44   Z  when rotary gear  40  is being turned counterclockwise relative to jaw members  20  (as in  FIG. 8 ), and second sliding contact surfaces  260  are oriented to come into sliding and substantially uniform contact with linear primary camming surfaces  44   X  and  44   Y  when rotary gear  40  is being turned clockwise relative to jaw members  20 .  
         [0042]      FIGS. 9 and 10  illustrate an alternative embodiment of the present invention (for simplicity, gear teeth  41  of rotary gear  40  are not shown in  FIGS. 9 and 10 ). In the embodiment of  FIGS. 9 and 10 , each jaw member  120  has a roller-type cam follower  130 , rotatably or swivellably mounted to the jaw member  120  by means of a swivel pin  132 . Each roller-type cam follower  130  has a first substantially flat contact surface  150  and a second substantially flat sliding contact surface  160 . First siding contact surfaces  150  are oriented to come into sliding and substantially uniform contact with linear primary camming surfaces  44   W  and  44   Z  when rotary gear  40  is being turned counterclockwise relative to jaw members  120 , and second contact surfaces  160  are oriented to come into sliding and substantially uniform contact with linear primary camming surfaces  44   X  and  44   Y  when rotary gear  40  is being turned clockwise relative to jaw members  120  (as in  FIGS. 9 and 10 ).  
         [0043]     In  FIG. 9 , a tubular T has been positioned between jaw members  120 , and rotary gear  40  has been rotated clockwise such that first contact surfaces  150  of roller-type cam followers  130  have begun to slide counterclockwise along corresponding primary camming surfaces  44   X  and  44   Y , and such that rear dies  30 A of jaw members  120  have begun to grippingly engage tubular T. In the embodiment shown in  FIG. 9 , each jaw member  120  has an additional die  30 C disposed between front and rear dies  30 A and  30 B, to enhance overall gripping effectiveness of the power tong. However, the inclusion of central dies  30 C is not essential to the present invention, and in fact the number and configuration of the dies are entirely incidental to the invention.  
         [0044]     In  FIG. 10 , rotary gear  40  has been rotated further clockwise such that the first substantially flat sliding contact surface  150  of such roller-type cam followers  130  now fully and slidingly engages either of primary camming surfaces  44   X  and  44   Y . As shown, cam followers  130  have moved further counterclockwise along primary camming surfaces  44   X  and  44   Y  such that, due to the geometric characteristics of primary camming surfaces  44   X  and  44   Y  as previously described, all of the dies  30 A,  30 B, and  30 C have come into secure gripping engagement with tubular T.  
         [0045]     Roller-type cam followers  130  may optionally be mounted to jaw members  120  in association with biasing means such as a spring (not shown), to orient first and second sliding contact surfaces  150  and  160  for optimal engagement with primary camming surfaces  44   X ,  44   Y ,  44   W , and  44   Z  as the case may be, depending on the direction of rotation of rotary gear  40 . In alternative embodiments, each roller-type cam follower  130  could have only a single sliding contact surface, with biasing means provided to orient the single sliding contact surface for engagement with primary camming surface  44   X ,  44   Y ,  44   W , or  44   Z  as the case may be.  
         [0046]     As previously discussed, primary camming surfaces  44   X ,  44   Y ,  44   W , and  44   Z  may have a slight concave curvature, with first and second contact surfaces  150  and  160  being correspondingly curved for sliding and substantially uniform contact with curved primary camming surfaces  44   X ,  44   Y ,  44   W , and  44   Z . In such alternative embodiments, primary camming surfaces  44   X ,  44   Y ,  44   W , and  44   Z  will retain essentially the same geometric characteristics as described in the context of substantially linear primary camming surfaces with reference to  FIG. 4 , but with acute angles A W , A X , A Y , and A Z  being formed with reference to chord lines between the beginning and end points of the corresponding curved primary camming surface  44 W,  44 X,  44 Y, or  44 Z. The radial distance from center axis C to a point on a given primary camming surface will still decrease in substantially linear fashion according to the distance away from the corresponding transition point, in spite of the slight curvature of the primary camming surfaces.  
         [0047]     It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the essential concept of the invention, and all such modifications are intended to come within the scope of the present invention.  
         [0048]     In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following that word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element.