You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    The field of the invention is a lockable swivel that can be used in a drill string for rotation thereof when the swivel is locked and in wireline applications to let the drill string rotate freely when the wireline is operated. 
       BACKGROUND OF THE INVENTION 
       [0002]    Previously, if the operator desired to rotate the drill string during wireline operations, the wireline was pulled from the well bore and the entry devices were disengaged from the drill string. The removal of the wireline could be avoided if an inline swivel was placed in the drill string between the wireline device and the rotary table. This arrangement would permit rotation to be accomplished with a wireline in place, but effectively disengaged the top-drive unit from its preferred role of providing both lifting power and rotation to the drill string. 
         [0003]    Using prior conventional technology, the drill pipe was separated and raised high above the rig floor on each run in order to change out tools. Although the pipe can be rotated, the operator could not circulate or reciprocate the pipe during these periods. Circulation was achieved by adding a pump-in sub and another T.I.W. safety valve immediately above the existing T.I.W. valve; which, however, put the disconnect or break point between the upper T.I.W. valve and the swivel several feet above the rig floor creating a safety hazard while operating the rig tongs. 
         [0004]    Further, since the tool strings must be stripped in and out beneath the upper assembly, a lubricator or tool protection device could not be used and all tools and explosives were brought onto the rig floor unshielded and unconfined. In the event of an inadvertent detonation of the explosive string shot or perforators, all personnel on the rig floor were totally exposed to this unnecessary life-threatening hazard. 
         [0005]    Once rigged-up and going in the hole using conventional technology such as the Boyd side-entry sub, the wireline passed through the acute angle in the side entry sub. This caused excessive wearing of the wireline and creates sever grooving in the sub. The single rubber pack-off, which is commonly used with this system, is very susceptible to leaking and/or line gripping and stoppage during pump-down operations. The system cannot be used when working under surface pressure and with the need to utilize a grease injector and wireline blow out preventers (BOPs). 
         [0006]    During pipe recovery operations, both right and left-hand torque must be worked down-hole using the rig tongs. This is a procedure has long been recognized to be one of the greatest safety hazards to be encountered during pipe recovery operations. When using this prior technology, pipe tongs were attached to the drill string and secured to the rig to hold torque that had been put into the drill string from the rotary table or top drive unit. 
         [0007]    With the present invention, this torque can be maintained while continuing circulation and wireline operation. 
         [0008]    USP RE41,759  FIG. 4  provided an apparatus which would allow the connection of various wireline devices  106  to be placed in the drill string  100  between the top drive unit  102  and the rotary table  114  of a conventional drilling rig throughout wireline operations. Such devices  106  as the Boyd Borehole Drill Pipe Continuous Side Entry or Exit Apparatus (such as described in U.S. Reissue Pat. No. 33,150) or the Top Entry Sub Arrangement (as described in U.S. Pat. No. 5,284,210) may both be utilized for various wireline operations. 
         [0009]    Referring to  FIG. 4 , what this reference disclosed is a lockable in-line swivel device  110  which is selectively engaged by the operator to permit or inhibit rotational movement provided by a top drive unit  102  to be transmitted through the swivel  110  to the pipe string  112  and to allow disengagement of the locked swivel  110  so that rotation may be accomplished by the rotary table  114  simultaneously with the wireline operations. 
         [0010]    This design permitted the wireline entry devices  106  to be left in the drill string  100  during all operations involving the wireline operation avoiding the time consuming makeup and disengagement of the entry tools  106  required to safely permit entry of the wireline into the well bore. If rotation and longitudinal movement is desired, the wireline alone was removed from the wellbore, but the entry tool  106  remained in place and the swivel  110  is locked to provide transmission of all rotation through the swivel  110  into the pipe string  112 . 
         [0011]    At other times, the operator using a top-drive unit  102  could pick up the drill string  100  and yet maintain torque which has been put into the pipe string  112  in pipe recovery operations. This was done by engaging the swivel  110  in locked position and picking up with the top drive unit  102 . As the torque is worked through the drill string  100 , additional wireline operations and the operator would set the drill string  100  down, disengage the swivel  110 , continue to rotate with the rotary table  114  and continue the wireline operations. 
         [0012]    The swivel provided to accomplish the above operations is shown in  FIGS. 1-3  of this patent. The design features a multi-component body with spaced splines with the lower spline juxtaposed in an overlapping relation to a retainer that holds the upper housing assembly of body parts to the lower housing with fairly primitive rotary bearing design and no thrust bearing capability. The present invention improves on the shortcomings of this design providing several unique features that reduce the number of housing parts to allow the use of a unitary upper and lower body with an internal retaining ring to secure the actuating piston to the upper housing. The design adds thrust bearing capability nested between pairs of tapered surfaces on one side between the housings and on the other between the lower housing and the end cap. Spherical bearings are used to facilitate relative rotation. These and other features of the present invention will be more readily appreciated by a review of the description of the preferred embodiment and the associated drawing while recognizing that the full scope of the invention is to be found in the appended claims. 
         [0013]    Also relevant in the lockable swivel field is U.S. Pat. No. 6,378,630  FIG. 6  where flow through the swivel compresses a stack of Belleville washers to engage splines and lock the swivel as long as flow displaces a piston to overcome the resisting spring force. Other references show swivels in general and swivels that combine a wireline entry fitting into a common housing or swivels that lock in one rotational direction and allow relative rotation in an opposite direction. Some examples of these references are U.S. Pat. Nos. 6,553,825; 5,996,712; 6,796,191; 8,171,991; 6,915,865; 6,994,628; 7,793,731; 7,316,276; 7,168,498; 7,377,316; 7,392,850; 8,210,268; 4,074,775 FIG. 2B; 7,011,162 FIGS. 2 a  and 2B; 4,781,359; 4,715,454 and 7,857,058. 
       SUMMARY OF THE INVENTION 
       [0014]    A lockable swivel features a unitary upper housing connected to a unitary lower housing with a connector to hold the housing components together for relative or tandem rotation. A hydraulically actuated piston shifts spaced apart splines axially to selectively engage splines on both housings for the locked position and to release the splines in both housing for the unlocked position. A thrust bearing in the lower housing is disposed between two pairs of tapered surfaces with the upper pair disposed on the housings and the lower pair between the connector and the lower housing. The thrust bearing is disposed between a pair of spherical bearings with the upper radial bearing retained by the upper housing and the lower radial bearing retained by the connector. An internal ring retains the piston to the upper housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a half section half exterior view of the locking swivel in the locked piston down position; and 
           [0016]      FIG. 2  is the view of  FIG. 1  in the piston up or unlocked position. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Referring to  FIG. 1  upper body  12  is secured to lower body  13  by end cap or connector  15 . A screw or other fastener  4  retains a threaded connection  30  between the upper body  12  and the connector  15 . A retainer  14  is secured at thread  32  to the upper body  12  and that threaded connection is maintained by a circlip  11 . A swivel piston  19  can move down until bottoming on the retainer  14  as shown in  FIG. 1  and can move axially in the opposite direction until bottoming out by essentially eliminating the volume of chamber  42  when contacting an end surface that defines that chamber. Near the upper end  36  are o-ring  22  and backup ring  25  to retain pressure in the central passage  38 . A cavity  40  that varies in volume is defined between seals  22  and  23 . Seal  23  and upper seal  24  define a variable volume cavity  42  into which a fitting  1 ,  3  is connected for communicating hydraulic pressure that enlarges cavity  42  while pushing down on piston area  44  to move the piston  19  to the  FIG. 1  position where it is bottomed on the retainer ring  14 . Another connection  1 ,  2  extends into cavity  46  which is shown in  FIG. 1  in its smallest volume position. Lower seal  24  and seal  23  mounted to the retainer  14  seal this variable volume space. Adding hydraulic pressure into fittings  1 ,  2  and removing hydraulic pressure from fitting  1 ,  3  advances the piston  19  up toward and into contact with a top surface that defines chamber  42 . 
         [0018]    Piston  19  has an upper spline  48  engaged with a spline  50  on the upper body  12  in the  FIG. 1  locked position. At the same time the piston  19  has a lower spline  54  that engages a mating spline  52  in the  FIG. 1  position. Thus the piston  19  locks the upper housing  12  to the lower housing  13  for tandem rotation when the two spline pairs are engaged. As stated before with hydraulic pressure applied to chamber  46  and removed from chamber  42  the piston  19  rises and the spline pairs  52 ,  54  disengage so that the housing components  12  and  13  can relatively rotate. Spline pairs  48  and  50  remain engaged. Such relative rotation is facilitated by the upper and lower preferably spherical bearings  9  which can also include any type of rolling element bearing such as ball, roller or needle among others, that are disposed on opposite sides of the thrust bearing  8 . The upper bearing  9  is externally retained by the lower end  56  of the upper body  12  on two adjacent sides that are perpendicular to each other and surfaces  60  and  62  on the lower body  13 . The lower bearing is retained externally by adjacent surfaces  64  and  66  on the connector  15  and surface  68  on the lower body as well as an adjacent surface on the end cap  16  that is retained with screws  6  to the connector  15 . A grease fitting  7  allows adding grease to the lower spherical bearing  9 . 
         [0019]    Thrust bearing  8  is externally retained by adjacent perpendicular surfaces  74  and  76  on the connector  15  and  70  and  72  on the lower body  13 . Thrust bearing  8  is straddled by mating sloping surfaces on opposed sides. Above the bearing  8  surface  80  on the lower end  56  of the upper body  12  sits in opposition to surface  82  on the lower body. Below the bearing  8  surface  84  on the lower body  13  is opposed to mating surface  86  on the connector  15 . Surfaces  84  and  86  are not intended to contact. In one option the pair of mating sloping surfaces  80  and  82  above bearing  8  can contact and in that variation the upper radial bearing  9  can be replaced with a floating roller bearing to transmit the axial component of a thrust load while the radial component is absorbed by the upper body  12 . 
         [0020]    In preferred alternative to dealing with tensile thrust, as it is very unusual to have compressive thrust loads in such devices, the surfaces  84  and  86  or  80  and  82  do not contact and the thrust load is taken by the upper spherical bearing  9 . The mating pairs of sloping surfaces allow the use of a larger thrust bearing  8  than the radial bearings  9  that straddle it above and below. 
         [0021]    The piston  19  can be pinned at  5  to allow external indication of the position of the piston  19 . Piston  19  can also be shear pinned to the upper body  12  for run in to prevent accidental movement of the piston  19  until a predetermined force is applied in chamber  42 . 
         [0022]    Those skilled in the art can now appreciate several features and variations thereof as depicted in  FIG. 1 .  FIG. 2  is identical to  FIG. 1  with the piston  19  in the raised position toward a travel stop that is the top radial surface that defines chamber  42 , to permit relative rotation between the bodies  12  and  13 . The bodies  12  and  13  are each made of a single component. The piston  19  is retained by an internal ring  14  located near a thick portion of the upper body  12 . Spaced apart spherical bearings  9  straddle an even larger thrust bearing  8 . The upper splines mate adjacent a thick portion of the upper housing  12  where there are no weak points such as threaded body connections. The lower splines mate within the retainer ring  14  to lend support to the lower end of the piston  19 . The bearing assembly  8 ,  9  is axially spaced from the meshing splines. The upper body takes a radial component from thrust loading and transfers the axial component to the thrust bearing  9 . 
         [0023]    While the piston is illustrated as hydraulically externally driven in opposed directions those skilled in the art will appreciate that the piston can be alternatively actuated with flow cycles therethrough that in combination with a j-slot mechanism can put the piston  19  in the splines locked and unlocked positions in situations where hydraulic power systems are not available. In this case the piston is acted on by a spring return to work against the force generated with fluid flow. On the other hand the piston can have unequal piston areas and can be moved against a spring bias with simply applied pressure and removal of the applied pressure. 
         [0024]    The driving force can also be locally available rig air. The spacing of the bearing assembly that comprises bearings  8  and  9  axially spaced from the retainer  14  allow the use of larger bearings without adding unduly to the diameter of the housings while at the same time providing additional supporting wall for the bearings. 
         [0025]    The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims appended hereto.

Summary:
A swivel apparatus includes upper and lower housings having a passage therethrough operably connected with a coupling for selective relative rotation; at least one movable piston having a first position where the housings are rotationally locked to each other and a second position where the housings are free for relative rotation; the piston is retained to the upper housing with a retainer secured within and discrete from the upper housing, and the retainer is axially spaced from a bearing assembly.