Abstract:
An elastomeric gripping element is supported and mounted by a housing and is configured to have an interference fit under normal conditions with the surface of the object to be gripped. The housing and the surface to be gripped are separated by a gap. For gripping mating with the surface, the elastomeric gripping element is initially stretched by operative means in the housing element in an axial direction parallel to the generating element of the comating surface to be gripped, with an attendant reduction in its cross-sectional thickness perpendicular to the direction of stretch. In this manner, the gripping element interference with the comating surface of the object to be gripped is minimized or eliminated for installation. When positioning for the installation mating is accomplished, the gripping element tension is selectively released, permitting the gripping element to attempt to return to an unstretched position and thereby to assume a preloaded position against the comating surface. Release of the gripped object is accomplished by retensioning the elastomeric gripping element. The gripping element may be repeatably disengaged and reengaged by selectably restretching the gripping element and then selectably releasing it upon completion of repositioning.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application, pursuant to 35 U.S.C. 111(b), claims the benefit of the earlier filing date of provisional application Ser. No. 60/184,171 filed Feb. 21, 2000, and entitled “Method and Apparatus for Gripping Objects”. The present application is related to provisional application Ser. No. 60/184,169 also filed Feb. 21, 2000 and entitled “Novel Method and Apparatus for Sealing.” 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates to a method and apparatus for selectively gripping and releasing objects. More particularly, the invention relates to an elastomeric gripping element mounted in a housing and configured to have an interference fit under normal conditions with the surface of the object to be gripped. 
     BACKGROUND OF THE INVENTION 
     There are a number of devices used to grip shafts, pipes, and other objects, some of which have been in use for a number of years. Almost all of the gripping devices currently being used operate in an active manner. An “active” operating device is one that is normally not in a gripping configuration, but must be selectively and actively forced into gripping an object. In contrast, “passive” devices normally exist “at rest” in a gripping mode. Such passive devices must be selectively operated to cause them to not grip an object. 
     Tubular collets or split rings which obtain their flexibility by provision of one or more slots in a metallic tube wall parallel to the tube axis and which change the gripping surface diameter by wedging on conical surfaces due to application of axial loads constitute a large, general class of gripping devices. Examples of this class of device are illustrated in several patents such as Knox U.S. Pat. No. 2,962,096; Richey U.S. Pat. No. 4,105,262; Russell U.S. Pat. No. 4,438,822; Reneau U.S. Pat. No. 4,728,125; and Nagano et al. West Germany Patent 24 39 100. 
     These collet or split ring devices are active devices, requiring the application of force to distort a normally nongripping element into a gripping configuration. Such devices normally have a very limited range of diameters which they can grip. When such devices are forced to distort too much they undergo permanent deformation. For example, collets can normally provide only limited gripping without being permanently distorted. 
     A similar class of active device uses a solid metallic ring or tube extension which fits very closely to the surface to be gripped and wedges conically tapered surfaces under the action of axial loads to effect gripping. The solid metallic ring is forced against the gripped surface by the wedging action. Such devices require a careful control of diameters of the gripping and gripped surfaces in order to avoid permanent distortions to the gripping ring. Examples of such devices are the Amlok devices, obtainable from Advanced Machine and Engineering, Rockford, Ill. and devices obtainable from Hänchen Hydraulic Gmbh, Ostfildem, Germany. 
     The Mapeco shaft coupling (Mapeco Products, Locust Valley, N.Y.) operates with the same type of solid ring gripping mechanism as the Amlok and Hänchen devices. However, the Mapeco device must be actively actuated by hydraulic pressure to grip. 
     Another class of gripping devices produces metal-to-metal gripping engagement for shafts by means of active hydraulically induced bulging of a gripping sleeve to cause it to distort into engagement with the gripped object. The Amlok hydraulic squeeze bushing (Advanced Machine and Engineering, Rockford, Ill.) requires active maintenance of hydraulic pressure in order to maintain its grip. The ETP bushing (Zero-Max/Helland Motion Control Products, Minneapolis, Minn.) uses a permanently entrapped somewhat compressible fluid to induce clamping. Yet the fluid must be constantly pressurized by a piston actuated by screws. Both types of bulging sleeve can operate only over very small gripping diameter ranges. Similarly, Amlok clamp disks and rings operate by selectably applied active direct compression of the gripped object, thereby permitting development of friction on the contact interface. 
     Non-split mechanical ring gripping devices may be actively forced under application of axial loads into gripping by flexurally deforming into contact with the gripped surface. Speith hydraulic actuated clamping sleeves (Advanced Machine and Engineering, Rockford, Ill.) uses a circumferentially convoluted sleeve for a flexural gripping device, whereas Russell (U.S. Pat. No. 4,438,822) uses an array of Belleville springs for gripping. Both types of device have only a very limited range of gripping diameters without undergoing permanent deformation. 
     A very common type of gripping device termed a ‘slip’ is based upon wedging of one or more discrete wedges of either planar or arcuate construction. Examples of such gripping devices can be obtained from Stewart &amp; Stevenson, Houston, Tex. and Morgrip Products, Walsall, England. The wedges of these devices are normally actively biased into engagement with the gripped object by gravity or springs. Such slips are unidirectional gripping devices which will resist motion in the direction which tightens the wedge, but will release for motions which will loosen the wedge. Most slips have relatively steep wedge angles so that they are self-releasing when subjected to reversed axial loads. In addition, some slips come with separately operable release mechanisms which pull the wedges out of engagement. The Stewart &amp; Stevenson slips for their conductor pipe connector are of a conventional construction, but are not readily releasable. Oilfield drill pipe slips are a more typical construction. The Morgrip Pipe Clamp uses wedged rolling balls as slips in a manner similar to a common type of one-way clutch. Slips are used to grip objects which have a relatively large size variation capability. One major disadvantage with many slips is induced damage to the gripped surface from teeth on the face of the slips or, for the Morgrip Pipe Clamp, from the balls. 
     Knox U.S. Pat. No. 2,962,096 and Russell U.S. Pat. No. 4,438,822 disclose rubber rings which are actively axially compressed to grip. The Knox rubber ring is intended to seal against a pipe, but in the process provides some level of gripping. Both devices function similarly to the expandable rubber bottle stoppers which are actively caused to expand to seal and grip by axial squeezing applied by a camming lever. 
     Nixon U.S. Pat. No. 4,121,675 works similarly to the Russell rubber gripper, but utilizes knitted metal instead of rubber. Rubber collets are commonly used in machine shops to grip drills or tool shanks. These devices use active axial compression of the rubber element against a cylindrical case with a self-releasing conically tapered back wall to cause the rubber to distort to induce gripping. Normally, radial steel inserts embedded in the rubber are used to grip the object, rather than using the rubber directly. Rubber collets accurately and effectively grip over a large diameter range. 
     Richey U.S. Pat. No. 4,131,167 discloses an active helical spring gripping mechanism which uses twisting of the spring to cause it to grip a cylinder. The gripping is through friction developed in a manner somewhat comparable to a wrap spring one-way clutch, but the spring ends must be actively held in a tightly wound condition to grip. 
     Russell U.S. Pat. No. 4,438,822 discloses the only known passive device. This Russell device has a passive torsional spring gripper which normally has an interference fit with the surface to be gripped. The spring is twisted to get it to release. Both this device and that of Richey can experience difficulty with the initial establishment of gripping due to buildup of friction not permitting full engagement with the gripped object over the full length of the helix. Additionally, both devices are sensitive to vibrations and are not well suited for axial load resistance. 
     Thus, a need exists for a passive preloading gripping device that does not rely on applying external mechanical force to efficiently initiate or maintain the gripping action on an object. 
     SUMMARY OF THE INVENTION 
     The invention contemplates a simple, inexpensive device for solving the problems and disadvantages of the prior approaches discussed above. The present invention provides a rapid, reliable and accurate gripping of objects without the application of an external mechanical force to efficiently initiate or maintain the gripping action on an object. 
     One aspect of the present invention is a gripping apparatus having a housing, an elastomeric gripping element anchored to one end of the housing, a reciprocable movable end attached to the gripping element, and a means for reciprocably moving the movable end between a first position and a second position, such that when the movable end is in the first position the gripping element is stretched and when the movable end is in the second position the gripping element is relaxed. 
     Another aspect of the present invention is a gripping device having an elastomeric gripping element securely attached to a static anchor end and a moveable end, where the movable end is moved in either direction with a hydraulically actuated piston. 
     Yet another aspect of the invention is a gripping apparatus having an elastomeric gripping assembly that has an elastomeric gripping element containing an embedded antiextrusion device, a static anchor end and a movable end. The movable end can be moved from its original position to stretch the elastomeric gripping element and can be returned to its original position to relax the gripping element and release the object being gripped. 
     Still yet another aspect of the invention is a process for gripping an object comprising the steps of: (a) selecting the object to be gripped by a gripping apparatus having a housing, an elastomeric gripping element anchored to one end of the housing, a reciprocable movable end attached to the gripping element, and a means for reciprocably moving the movable end between a first position and a second position, such that when the movable end is in the first position the gripping element is stretched and when the movable end is in the second position the gripping element is relaxed; (b) moving the movable end of the gripping apparatus to the first position to stretch the gripping element; (c) inserting the object to be gripped into the interior of the gripping apparatus; and (d) moving the movable end toward the second position to bias the gripping element against the object to be gripped. 
     The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or redesigning the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are believed to be characteristic of the invention, both as to its structure and methods of operation, together with the objects and advantages thereof, will be better understood from the following description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a side view of one embodiment of a gripping device; 
     FIG. 2 shows an axial, frontal view of the gripping device of FIG. 1; 
     FIG. 3 is a longitudinal sectional view of the gripping device of FIG. 1 with a relaxed gripping element; 
     FIG. 4 is a quarter-sectional view of a molded gripping assembly having an at-rest stretchable gripping element; 
     FIG. 5 is a longitudinal sectional view of the gripping device of FIG. 1 with a stretched gripping element ready to engage an object; and 
     FIG. 6 is a longitudinal sectional view of the gripping device of FIG. 1 with the gripping element biased against and gripping a pipe. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a passive preloading gripping device that does not rely on applying or maintaining external mechanical force to efficiently initiate or maintain its gripping action on an object. 
     Referring now to the drawings, and initially to FIGS. 1 and 2, it is pointed out that like reference characters designate like or similar parts throughout the drawings. The Figures, or drawings, are not intended to be to scale. For example, purely for the sake of greater clarity in the drawings, wall thickness and spacing are not dimensioned as they actually exist in the assembled embodiment. 
     FIGS. 1 and 2 illustrate a gripping device  10  mounted in a tubular housing  12 . The exterior of tubular housing  12  has an increased diameter at its upper end (the right side of the drawing) to accommodate the actuating means for the device and to structurally support mirror image integral ears  13  with through hole lifting eyes  14  provided for handling purposes. First hydraulic actuation port  15  and second hydraulic actuation port  16  are connected to sealed chambers inside the body for selective operation of the mechanism of this device. 
     The description of the internals of gripping device  10  is common to its operating states shown in FIGS. 4,  5 , and  6 . Referring now to FIG. 3 (with the bottom of the device  10  on the left), female thread  18  with a large thread relief extends to downwardly facing lower transverse shoulder  19 . Main through bore  20  extends approximately halfway through housing  12  from lower transverse shoulder  19  to upwardly facing small shoulder  21 . 
     First polished counterbore  24  is located above small transverse shoulder  21 . Second polished counterbore  25  is located above first counterbore  24  and connected by upwardly looking large transverse shoulder  27 . First hydraulic actuation port  15  intersects second counterbore  25  adjacent large transverse shoulder  27 . Second hydraulic actuation port  16  also intersects second counterbore  25  near its upper end. Female thread  30  is located at the upper end of housing  12 , where its thread relief is joined to second counterbore  25  by conically tapered transition  31 . 
     Annular bottom retainer ring  36  has transverse upper and lower faces and a smooth through bore to clear the outer diameter of any cylindrical objects which will be gripped by this device  10 . Male thread  37  on the exterior of bottom retainer ring  36  engages female thread  18  of housing  12  to retain the internals of gripping device  10  on the lower end. Spanner holes  38  are provided on the lower face of bottom retainer ring  36  to facilitate assembly. 
     Gripper assembly  40 , as seen in FIG. 4, consists of gripper anchor  42 , elastomeric gripper element  44 , and movable gripper end  46 . Gripper assembly  40  is positioned within main through bore  20  of device  10 . Annular gripper anchor  42  has a smooth through bore the same diameter as that of bottom retainer ring  36  and a transverse lower face  32  provided with multiple spanner holes  39 . Gripper anchor  42  has a stepped outer profile with a larger cylindrical surface  33  located below a smaller cylindrical face  35 , said faces being separated by an upwardly facing transverse shoulder  29 . As shown in FIG. 3, the upwardly facing transverse shoulder  29  of gripper anchor  42  abuts lower transverse shoulder  19  of the housing  12 , and the smaller outer cylindrical face  35  of gripper anchor  42  closely fits within main through bore  20  of housing  12 . 
     The upper transverse face  28  of gripper anchor  42  has an optional radiused undercut face groove  57  to provide enhanced bonding for attachment of the elastomer of elastomeric gripper element  44 . The elastomeric gripper element  44  may also have a substance having a high frictional coefficient, such as silica flour, embedded in its inner surface  66  that will comate with the object being gripped. In addition, the elastomeric gripping element may have one or more antiextrusion devices  63  embedded in and bonded to the elastomeric matrix of gripping element  44  to provide the gripping element  44  with increased strength and stability. A preferred embodiment will have more than one antiextrusion devices  63  embedded in the gripping element  44 , with at least one at each end. If only one antiextrusion device is embedded in the gripping element  44 , it would preferably be located at the lower end close to the gripper anchor  42 . The antiextrusion device is further described in copending application entitled Antiextrusion Device filed Feb. 21, 2001 which is incorporated herein by reference. The inner diameter of such antiextrusion devices would be only slightly more than that of the gripping surface  66  of the gripping element. 
     Annular movable gripper end  46  has a groove  22  on its lower transverse face  26  similar to that on the upper transverse face  28  of gripper anchor  42 . The outer cylindrical face of movable gripper end  46  is stepped, with the lower cylindrical face  43  closely fitting to the main through bore  20  of housing  12  and the upper cylindrical face having a reduced diameter male thread  47  with a thread relief A transverse face  23  of movable gripper end  46  connects the lower cylindrical face  43  with the thread relief adjacent male thread  47 , while an upper transverse face  17  connects male thread  47  with the through bore of the part. Movable gripper end  46  has the same smooth through bore as that of bottom retainer ring  36 . 
     Elastomeric gripper element  44  is molded onto gripper anchor  42  and movable gripper end  46 . The outer diameter of gripper element  44  closely fits to main through bore  20  of housing  12 , while the inner cylindrical face of gripper element  44  is smaller than the minimum size cylinder which will be gripped by this device. Transitional tapered lead-ins will connect the inner cylindrical face  66  of gripper element  44  with the bores of the gripper anchor  42  and movable gripper end  46 . At least a tapered lead-in from the gripper anchor  42  to the gripping element  44  should be used to effect a progressive interference fit against a gripped object by the elastomeric gripping element. 
     Turning now to FIG. 3, piston  52  has an annular construction and is positioned upwardly from and connected to movable gripper end  46 . Piston  52  has a head section  92  which has the largest outer diameter. Head section  92  is positioned between first reduced outer diameter section  94  on the lower side of piston  52  and second reduced outer diameter section  96  on the upper end of piston  52 . 
     The head section  92  of piston  52  is a cylindrical surface carrying a male O-ring groove  54  in which O-ring  55  is positioned so that it can seal between piston  52  and second polished counterbore  25  of housing  12 . First reduced outer diameter cylindrical section  94  is sized to slide freely within the first polished counterbore  24  of housing  12 ; male O-ring groove  56  carrying O-ring  57  is positioned intermediately in first reduced outer diameter cylindrical section  94 . Second reduced outer diameter cylindrical section  96  of piston  52  has the same diameter as the first section  94  and has a first hydraulic chamber  72  between it and second reduced outer diameter cylindrical section  96 . Spanner holes  59  in upper end transverse face  95  permit application of torque to the piston  52  for assembly of the piston to the gripping element  44 . 
     Still referring to FIG. 3, piston  52  through bore has the same diameter as bottom retainer ring  36 . Lower end transverse face of piston  52  is counterbored and provided with female thread  60 , a thread relief, and a transverse shoulder  91  between the thread relief and the through bore. Piston  52  is threadedly connected to movable gripper end  46  by male thread  47  and female thread  60 . 
     Annular top retainer ring  62  has a through bore closely mating to the second reduced outer diameter cylindrical section  96  of piston  52 . Female O-ring groove  64  carrying O-ring  65  is positioned near the lower end of the through bore of top retainer ring  62 . The upper transverse face  97  of top retainer ring  62  is provided with spanner holes  98  for assembly purposes. Male thread  67  is located on the largest outer diameter cylindrical section of top retainer ring  62  adjacent the upper transverse face  97 . Male thread  67  is engaged with female thread  30  of housing  12  to retain the internals of the gripper device  10 . 
     Below male thread  67  is located second reduced outer diameter cylindrical segment  93  of top retainer ring  62 , with male O-ring groove  68  near its lower end and O-ring  69  positioned therein. The second reduced outer diameter cylindrical segment  93  of top retainer ring  62  closely fits to the second polished bore  25  of housing  12 , so O-ring  69  seals against the second polished bore  25  of housing  12 . The second reduced outer diameter cylindrical segment  93  of top retainer ring  62  is joined to the through bore by a transverse lower face  99 . 
     An annular first hydraulic chamber  72 , accessible through first hydraulic port  15 , is defined between O-rings  55  and  57 , with piston  52  as the chamber inner wall and housing  12  as its outer wall. Second hydraulic port  16  intersects second polished counterbore  25  of housing  12  below O-ring  69  on the lower end of top retainer ring  62 . An annular second hydraulic chamber  74 , accessible through second hydraulic port  16 , is defined between O-rings  55 ,  65 , and  69  with piston  52  as the chamber inner wall and housing  12  as its outer wall. 
     Referring to FIG. 6, a length of pipe  75  is shown inserted within the bore of the gripping device  10  and engaged by gripping element  44 . 
     Operation of the Embodiments of the Invention 
     The gripping device  10  is shown in FIG. 3 in a relaxed, inactivated state. The gripping device in FIG. 3 has no hydraulic pressure applied to piston  52  through either hydraulic port  15  or  16 , so the elastomeric gripping element  44  is untensioned and free to assume its as-molded shape. The as-molded shape of elastomeric gripping element  44  is such that it will, when relaxed, have a substantial interference fit with the smallest cylindrical object which it is designed to grip. 
     The same gripping device  10  is shown in FIG. 5 in its stretched configuration for receiving installation of a cylindrical object such as a pipe into its bore preparatory for gripping said object. Normally, the object to be gripped is inserted through the lower end of the gripping device, and during lifting the object will be supported in a manner such that the axis of the gripping device is vertical. The gripping device is typically supported by suitable means, such as lifting cables attached to the lifting eyes  14  when it is being used for gripping. However, it should be noted that the preceding conditions are not requirements. 
     The configuration of FIG. 5 is attained by applying and maintaining hydraulic pressure to hydraulic port  15  and, hence, to chamber  72  in order to cause piston  52  to be forced upwardly (to the right of the drawings). When piston  52  is forced upwardly, the elastomer of elastomeric gripping element  44  is stretched, since the gripper anchor  42  is restrained by lower transverse shoulder  19  of housing  12 . The pressure applied to first hydraulic port  15  must be sufficient to cause sufficient stretch in elastomeric gripping element  44  so that its cross-sectional radial thickness will be sufficiently reduced (i.e., its inner diameter increased) at its inner diameter section to eliminate the fit interference of its unstretched state shown in FIG. 3 with the object, such as pipe  75 , to be gripped. 
     FIG. 6 shows gripping device  10  holding a pipe  75 . The pressure in first hydraulic chamber  72  has been released through first hydraulic port  15 , permitting the elastomer of elastomeric gripping element  44  to rebound inwardly and downwardly (towards the left of the Figure) in an attempt to resume its unstressed as-molded state. Because the diameter and ovality of the pipe  75  are controlled by factory tolerances to lie within a known range, the inner diameter of the elastomeric gripping element  44  is deliberately molded sufficiently smaller than the minimum pipe size to ensure an interference fit with the pipe  75 . Thus, in the process of attempting to return to its molded shape from its stretched position, the elastomeric gripping element  44  will assume a position such that it conforms to the local contours of the pipe  75  and presses strongly against it in a radial direction to effect a highly preloaded interfacial contact. The elastomer cannot fully rebound with the pipe present, due to the essentially incompressible nature of the elastomer. Thus, piston  52  does not fully return to its unloaded position shown in FIG. 3 when the pipe  75  is present. 
     The high interfacial contact stresses of the gripper element  44  on pipe  75  permit the development of proportionately high frictional forces on the same interface, particularly since the elastomer will be selected on the basis of having a high coefficient of friction. The high interfacial frictional forces permit using the clamp as a reliable device to grip the object. If pipe  75  moves downward following initial gripping, the attendant frictional drag on the elastomer forces the elastomer into even more intimate contact with higher interfacial stresses and, hence, better gripping. Release of the gripped object is simply accomplished by reapplying pressure to the first hydraulic chamber  72  in order to restretch gripper element  44  and thereby eliminate its interference fit. 
     For long gripper elements, it is possible that excessive friction drag may occur between gripper element  44  and either pipe  75  or the main through bore  20  of housing  12 , thereby interfering with developing adequate compressive forces on the elastomer/pipe interface. It is very simple in such a case to remedy the problem by applying temporary hydraulic pressure to second hydraulic chamber  74  through second hydraulic port  16  and thereby overcome the frictional drag which would otherwise prevent the elastomeric element from fully seating against the pipe. It is not necessary to maintain the hydraulic pressure on second hydraulic chamber  74  to ensure adequate gripping. An alternative or supplemental method of avoiding the frictional drag problem is to slightly taper the inner diameter of the contact surface of gripping element  44 , between the tapered lead-ins, so that the inner diameter of the gripping surface is slightly larger on its upper end. With this modification, the seating of elastomeric gripper element  44  against pipe  75  will proceed progressively upwardly from the bottom to the top, thereby aiding in obtaining proper seating. Additionally, downward axial loads from pipe  75  on gripper element  44  during lifting aid in seating the elastomer against the pipe. 
     Longer gripper elements may also demonstrate a tendency to ‘neck down’ in the middle when subjected to high tensions. This tendency is easily controlled by using integrally bonded rigid intermediate rings  112  which either partially or fully segment the elastomer to radially stiffen the gripping element  44 . Such rigid intermediate rings  112 , shown for clarity only in FIG. 5, cannot have outer diameters larger than that of the main through bore  20  of housing  12  nor can their inner diameters be less than the clearance diameter for the gripped cylindrical object. Gripping may be enhanced by integrally bonding high friction elements into the elastomeric matrix of the gripping element or laminating a high friction surface material to the internal surface of the elastomeric gripping element  44 . 
     Advantages of This Invention 
     The advantages of this invention accrue primarily from: a) the molded shape of the elastomeric gripping element allowing the gripping element to be designed to have a non-marring interference fit with a wide range of object sizes and shapes, b) stretching of the elastomeric gripping element to avoid significant fit interference when the object to be gripped is being inserted into the device  10 , and c) the ability of the gripping element to attempt to return to its as-molded shape and thereby passively assume a presqueezed condition against its comating object surface simply by releasing the installation tension on the elastomeric gripping element. 
     Conventional gripping devices rely upon active elements which are not molded or formed to have an interference fit and are installed with no interference fit but then must be actively compressed to cause interference with their comating surface. 
     Prestretching an elastomeric gripper element for its installation adjacent a comating object surface so that its cross-section thickness is reduced permits very high but controllable presqueezes for ensuring reliable gripping. Having an elastomeric gripper element which is sized to always assume an interference fit against its comating object surface in attempting to return to its molded shape following stretching ensures that the gripper element will always be sufficiently biased against its comating surface due to the locked-in stresses in the elastomer. This interfacial bias against the gripped object is maintained passively by the tendency of the elastomer to return to its molded, minimal energy shape. Thus, the interfacial biasing force of this invention is obtained by a passive means rather than the active means or gravity relied on by current devices. The gripper device described herein is always passively urged (i.e., without outside intervention) to have adequate presqueeze on the object interface in spite of elastomer shrinkage or creep. This maintenance of proper presqueeze with shrinkage or creep is not feasible with conventional elastomeric grippers without actively recompressing the gripping element. Recompression of the gripping element is often impractical and the need for recompression is typically unrecognized until it is too late and the gripper has failed during use. 
     Furthermore, in contrast to the conventional active elastomeric grippers, the level of presqueeze for the grippers of this invention is controllable by selection of the gripping element&#39;s general geometry, the elastomeric compound from which it is constructed, and the minimum amount of interference fit designed into the gripping device  10 . In contrast, conventional active grippers frequently are overcompressed by installation personnel when presqueeze is applied, with the result that the pipe or other gripped object may be locally necked down in an excessive manner. This situation is particularly difficult to avoid with screws applying the active loading on conventional grippers, even when jack screw torsions are carefully controlled, since screw and elastomer friction are highly variable and unknown. 
     The ability to overcome friction, which resists the gripping element assuming its as-molded condition after release of the installation tension, by means of temporarily hydraulically biasing the actuating piston downwardly to overcome the friction, is another strong advantage of this apparatus. This approach to gripping is applicable to both male and female gripping devices and is applicable to a variety of cross-sectional shapes of the gripped member. For picking up gravity loads, it is preferable for the tensioning of the elastomer to be applied upwardly so that the gravity load will contribute to grip performance. 
     By using a smooth elastomer without aggressively abrasive additives, this type of gripping device will not mar sensitive surfaces. The active surface of the elastomeric gripper element may be mildly ridged or waffled so that water or other problematic materials can be excluded from the heavily preloaded elastomer/comating surface interface and good frictional properties thereby maintained. 
     A particular advantage of this type of gripping device is that it can be designed to grip a wider range of object sizes than a collet type of gripper. Additionally, this design is compact, robust, does not require intricate or precision machining, and is inexpensive. The passive gripping device is a much safer approach to handling dangerous objects than the traditional active gripping devices. Release of the gripped object is also particularly simple compared to slip-type grippers, which are prone to jamming. 
     It readily may be understood that the gripping device of this invention may be somewhat changed from what is shown for this embodiment without departing from this invention. For instance, the bonding surfaces of the gripper anchor upper end and the movable gripper end can varied from the types shown in the drawings for this invention without exceeding the limits of this invention. Similarly, the gripper embodiment can be adapted readily to both planar or near planar or irregularly shaped objects. For instance, a one-sided planar gripper having the basic design characteristics of this device and operated by a conventional hydraulic piston and which entraps a planar object against a static planar surface on its obverse side is consistent with this invention. The gripping device of this invention is not limited to only tubular objects. 
     The gripping device shown in the drawings of this patent can be everted so that the operative features are mounted on a mandrel, rather than in an outer housing, so that a male gripping device is also consistent with the principles of this invention. The stretching of the gripping element also can be performed by wedging, camming, or other suitable means without departing from this invention. Multiple hydraulic cylinders or cylinders with arcuate or lunate or unusually shaped pistons can also be used for hydraulic tensioning of the gripping element for installation. These tensioning variations are desirable for semicircular or other irregularly shaped objects. 
     Thus, having described several embodiments of the gripping device and its use, it is believed that other modifications, variations, and changes will be suggested to those skilled in the art in view of the description set forth above. It is therefore to be understood that all such variations, modifications, and changes are believed to fall within the scope of the invention as defined in the appended claims.