Abstract:
A screw coupling between two bodies, one having an external screw thread and another having an internal screw thread complementary thereto, which screw threads are superimposed onto two base surfaces monotonously narrowing and widening respectively in the direction of a central axis, the longitudinal section of each of these screw threads relative to the central axis being a periodic univalent function which is continuous at least at the inflection points. The screw coupling has the following features: the first and second derivatives of the longitudinal section of each screw thread along the base surface is a continuous function; the second derivative of the longitudinal section of each screw thread relative to the central axis is a continuous function at least in the region of the inflection points; and the screw threads are in mutual contact over a substantial surface area differing from line contact.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a screw coupling between two bodies, the one of which has an external screw thread and another has an internal screw thread complementary thereto, which screw threads are superimposed onto two for instance truncated conical base surfaces monotonously narrowing and widening respectively in the direction of a central axis, the longitudinal section of each of these screw threads relative to the central axis being a periodic univalent function which is continuous at least at the inflection points. 
     2. Description of Related Art 
     Such a screw coupling has been known and commercially available for many years. 
     This known coupling comprises screw threads with a longitudinal section relative to the central axis which consists of mutually alternating circular arcs which are in most cases identical. 
     The drawback of such a known screw coupling is that the leads of the screw threads are in line contact with each other, whereby the contact pressure is great in the active mode of the coupling and may result in plastic deformation of the contact surfaces. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to embody a screw coupling of the described type such that the contact zones do not make line contact but surface contact with each other, whereby the contact pressure is substantially reduced and plastic deformation thereof must be considered almost precluded. 
     It is a further object of the invention to embody a screw coupling of the described type such that there is no danger of leakage in the case of a sealing function for medium under pressure. 
     According to the invention the above stated objects are primarily realized with the screw coupling of the stated type, which has the feature that the first derivative of the longitudinal section of each screw thread along the base surface is a continuous function; 
     the second derivative of the longitudinal section of each screw thread along the base surface is a continuous function; 
     the second derivative of the longitudinal section of each screw thread relative to the central axis is a continuous function at least in the region of the inflection points; and 
     the screw threads are in mutual contact over a substantial surface area differing from line contact. 
     In a determined embodiment the screw connection can have the special feature that both screw threads are substantially identical. 
     According to the invention the screw connection can further have the special feature that the function is symmetrical along the base surface. 
     An embodiment which satisfies said stated options simultaneously has the special feature that the function is a sine function. 
     Another embodiment has the special feature that the function is calculated on the basis of a Fourier series. 
     For practical reasons and in order to prevent sharp edges and transitions which could result in excessive material stresses and breakage, the screw coupling according to the invention can have the special feature that the harmonics above a chosen maximum ranking number, for instance 5, are ignored. The omission of the harmonics takes place during the design process in the direction from the higher to the lower-order terms, until the technically achievable accuracy is achieved. 
     A practical embodiment has the feature that the screw coupling is dimensioned such that, after insertion of the external screw thread into the internal screw thread, the bodies must be rotated through an angle of rotation of less than 360° relative to each other in order to block a further rotation. 
     According to another aspect of the invention, the screw connection has the feature that the angle of rotation lies in the range of about 70°-240°. 
     This latter embodiment can further have the feature that the angle of rotation lies in the range of about 120°-180°. 
     On the basis of materials science considerations the screw connection can have the feature that the half-apex angle of the cone tangent of the base surfaces is considerably smaller at each axial position than the natural shear angle of the applied materials under torsional stress. 
     This latter embodiment can have the particular feature that said shear angle amounts to 45° and the half-apex angle has a value of a maximum of about 30°. 
     This latter variant preferably has the special feature that the value of the apex angle amounts to about 15°±50%. 
     As stated above, it is one of the objects of the invention to design a screw coupling such that it naturally forms an excellent seal for medium under pressure. It may nevertheless be advantageous to improve the medium sealing still further. For this purpose the screw coupling according to the invention can have the special feature that sealing means are added to the screw coupling, for instance a cover layer of flexible material such as a rubber or a PTFE, or a viscous or pasty material, for instance Molykote® 
     For instance in the case where the bodies are rods which are mutually coupled by means of a screw coupling according to the invention, it can be important, at least at the position of the screw coupling, to increase the bending stiffness of the screw coupling. In this respect the screw coupling according to the invention can have the feature that each of the two bodies has a contact surface, which two contact surfaces press against each other in the active mode of the screw coupling and thus increase the bending stiffness of the screw coupling. 
     According to yet another aspect of the invention, the screw coupling comprises locking means for locking the screw coupling against undesired relative return rotation of the bodies. 
     This screw coupling can have the particular feature that the locking means comprise a ring with non-round inner surface which is slidable and securable over the screw coupling, which inner surface co-acts with corresponding, complementarily formed outer surfaces around the respective screw threads, which outer surfaces lie at least substantially in one imaginary plane in the active mode of the screw coupling. 
     Practical and easy, to produce is an embodiment in which said inner surface and said outer surfaces are flat. 
     The above stated two embodiments relate to a “hard” locking, which offers no rotational freedom but which, from a certain angle of tolerance, totally blocks a return displacement of the screw connection in a manner comparable to a hard stop. 
     The rotation locking can also take place in “soft” manner. In such an embodiment the screw coupling according to the invention can for instance have the special feature that the locking means comprise friction means. 
     According to this latter aspect of the invention, the screw coupling can have the special feature that the friction means comprise a friction element present on the one body, for instance a ring of friction material, for instance a rubber-like material, in the region of the screw coupling, which friction element is added to one of the two bodies; which friction element exerts some pressure on an opposite contact surface of the other body in the active mode of the screw coupling. 
     The screw connection according to the invention can be successfully applied for instance for use in surgical procedures on the bone structure. A screw connection intended for this purpose can thus have the special feature that the one body is a screw comprising a counter-sinkable head of at least more or less truncated conical main shape which corresponds to the base surface, and a threaded end with a screw thread which is superimposed onto a cylindrical base surface, wherein said main plane has the same central axis as said base surface and the pitch of the one screw thread is the same as that of the other screw thread. 
     This screw connection can particularly have the special feature that the screw thread superimposed onto the cylindrical base surface is of discrete type. “Discrete type” is understood to mean a screw thread which does not have roughly the same longitudinal sectional form at every position but wherein the screw thread is of discontinuous type and consists of a helical series of rounded protrusions similar to small segments of a sphere. Such a structure is advantageous in bone surgery because such a discrete screw thread loads the bone, which can be provided beforehand with a borehole, locally with a relatively high pressure. This enhances the bone growth and thus aids recovery of the patient. 
     The geometry of the protrusions, i.e. their form and dimensions, is preferably chosen such that, after insertion of the screw into a pre-drilled hole, the elastic limit of the bone is not exceeded, or at least not substantially. 
     The protrusions can be manufactured with any suitable method. A new, possibly suitable method consist of the so-called laser cusing. 
     A screw coupling according to the invention can in practice be effected with little difficulty due to the co-action between the for instance more or less truncated conical recess and protrusion. It can nevertheless be advantageous in some circumstances if a central locating pin is added to the external screw thread and a central cavity is added to the internal screw thread, into which the locating pin fits and into which the locating pin can already be inserted before the screw threads come into mutual contact. 
     According to yet another aspect of the invention, the screw coupling according to the invention has the feature that at least one of the screw threads has at least one zone extending along the base surface which is free of at least the most protruding part of the screw thread and thus forms a passage for a medium, for instance cooling medium. 
     A truncated conical base surface with substantially any desired apex angle can be provided with a screw thread by a suitable machining process, for instance by milling, grinding, spark machining, electrochemical machining or other suitable processes. The screw connection can particularly have the special feature that a screw thread is formed by making use of a rotatingly driven bit, a spherical milling head, spark machining tool or the like, modelled in accordance with the desired shape of the screw thread, which is driven in both axial direction and radial direction and runs successively through a rotating path relative to the relevant body. 
     The invention finds application in various technical fields. Two shafts can for instance be coupled to each other collinearly and coaxially such that they are connected to each other completely free of play. The one shaft can for instance be a motor shaft which is coupled via a coupling according to the invention to the shaft of a rotor with a chosen function, for instance the rotor of a centrifugal pump. According to the invention such a screw connection between two shafts can be effected exceptionally easily, for instance by bringing the two shafts into mutual contact at the position of the screw threads and performing a relative rotation through for instance 180°, wherein the relative direction of rotation of the motor shaft corresponds to the direction in which the motor drives this motor shaft during use. Without further provisions it is necessary to prevent the motor rotating in the opposite direction, since the screw connection would otherwise come loose. Use can be made of a hard rotation locking of the above stated type. In this case the driving can take place in both directions. 
     Also important is the possibility of optionally incorporating a controlled passage, for instance for a cooling medium, which can discharge heat from the motor or the rotor. A row of wave crests lying in a line can for instance thus be flattened to some extent, or a channel can extend more or less along the cone surface. 
     The applications of the screw connection according to the invention are numerous. The screw connection according to the invention can for instance find application in the context of hose couplings and pipe couplings. 
     The screw coupling can thus have the special feature that the screw coupling is a coupling between two pipes through which medium can flow, the mutually connecting inner surfaces of which pipes connect to each other without any appreciable narrowing, obstruction or interruption, such that medium flowing through the pipes can pass over the screw coupling with negligible disruption. 
     In a surprising and highly effective embodiment the screw coupling according to the invention has the special feature that the base surfaces are flat, therefore take the form of truncated cones with apex angles of 2×90°. 
     This latter embodiment can have the special feature that the screw coupling is dimensioned such that, after insertion of the one screw thread into the other screw thread, the bodies must be rotated relative to each other through an angle of rotation of less than 90°, preferably less than 45°, still more preferably less than 22°, in order to block a further rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side view of a prior art conical screw; 
         FIG. 2  shows detail II in which a complementary screw thread is also shown for the purpose of elucidating the above stated drawbacks of the prior art; 
         FIG. 3  shows a view corresponding to  FIG. 1  of a conical screw according to the invention; 
         FIG. 4  shows a view corresponding to  FIG. 2  of two co-acting screw threads according to the invention, which shows that the mutually facing surfaces lie against each other over their whole surface area; 
         FIG. 5  shows a cross-section through a coupling between two shaft with a screw connection according to the invention; 
         FIG. 6  shows the coupling of  FIG. 5  with a friction ring serving as rotation locking; 
         FIG. 7  shows a cross-section through an alternative shaft coupling with internal mechanical locking, in non-coupled situation; 
         FIG. 8A  shows a longitudinal section through the screw connection in  FIG. 7  in active mode; 
         FIG. 8B  shows a longitudinal section corresponding to  FIG. 8A  of a variant in which the rotation locking takes place from the other side; 
         FIG. 8C  shows a longitudinal section corresponding to  FIGS. 8A and 8B  through a variant in which rotation locking takes place from the underside using a round rod; 
         FIG. 8D  shows the cross-section VIII-VIII of  FIG. 8C ; 
         FIG. 9  shows a side view of a conical screw with grooves serving as medium passages; 
         FIG. 10  shows an end view of the screw according to  FIG. 9 ; 
         FIG. 11  shows a longitudinal section through a pipe coupling for two thick-walled pipes in uncoupled situation; 
         FIG. 12  is a top view of the pipe coupling according to  FIG. 11  in coupled, active mode and after locking with a non-round locking ring; 
         FIG. 13  shows the cross section III-III according to  FIG. 12 ; 
         FIG. 14  shows a sleeve which, together with two pipes for coupling, forms two screw couplings according to the invention with rotation locking by means of socket screws; 
         FIG. 15  shows a cross-section through a coupling sleeve according to the invention for coupling of two garden hoses; 
         FIG. 16  shows a cross-section of similar type as drawn in  FIG. 15 , but now dimensioned for thick-walled rubber hoses; 
         FIG. 17  shows a longitudinal section through a broken bone with a fixing plate with screws according to the invention for mutually fixing the adjacent parts of a broken bone; 
         FIG. 18  shows a detail on larger scale of a self-drilling and self-tapping screw in an insert at 90′; 
         FIG. 19  shows an insert for drilling respectively screwing at an angle of about 90°−15°=75°; 
         FIG. 20  shows a cross-section corresponding to  FIG. 19  through a variant with an angle of about 90°−30°=60°; 
         FIG. 21  shows a cross-section through a pin as used in a broken leg and a broken ankle, with various types of screw; 
         FIG. 22  shows a closing cap which covers the open top side of the pin after placing of the pin in the bone and after removal of the insertion aid; 
         FIG. 23  shows a highly schematic transparent side view of the leg of a patient with a broken bone, wherein a dash-dot line indicates the position in which the pin is inserted in a pre-drilled hole in the bone by means of the insertion aid; 
         FIG. 24A  shows a side view of a bolt with a reduced bending stiffness; 
         FIG. 24B  shows a side view of a bolt with reduced bending stiffness in another embodiment; 
         FIG. 25  shows a perspective exploded view of a screw coupling between two bodies with screw threads, the base surfaces of which are flat, i.e. have the shape of truncated cones with apex angles of 2×90°; 
         FIG. 26  shows a longitudinal section through the two bodies according to  FIG. 25  in a situation in which the bodies are moved toward each other such that the one screw thread is introduced into the other screw thread; and 
         FIG. 27  shows a longitudinal section corresponding to  FIG. 26  of the situation in which the bodies have been moved against each other, the screw threads have been introduced into each other and a rotation has taken place, whereby the bodies are coupled to each other. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a part of a rod  1  which is provided at its end with a known conical screw thread  2 . 
       FIG. 2  shows the detail II on enlarged scale. This shows that conical thread  2 , which in this embodiment has a pitch along the conical jacket surface of 2 mm, only makes contact with screw thread  3  of a complementary threaded hole at two points in the cross-section along the pitch length, compare for instance  FIG. 5 . 
       FIGS. 3 and 4  show views corresponding to  FIGS. 1 and 2  of a conical screw according to the teaching of the present invention. As shown in  FIG. 4 , conical thread  4  does not consist of circle segments as designated symbolically with R in  FIG. 2 , but of a sine function, therefore a continuous function, the first and second derivatives of which are also continuous. It must be understood that, in the case of the circle segments according to  FIG. 2 , the second derivative, roughly corresponding to the curve, undergoes an abrupt, discontinuous transition between a positive value R and a negative value −R. In a sine function there is no such abrupt transition. As a result it can be determined on a mathematical basis that the external screw thread  4  and internal screw thread  5  do not have line contact in common with each other, but surface contact. 
       FIG. 5  shows rod  1  with the conical thread  4  according to the invention, as well as a second rod  6  which is provided with a threaded hole  7  with a conical screw thread  8 . Conical thread  4  and conical thread  8  are the same and together form a screw coupling according to the invention. Situated in the drawn embodiment at the end of conical thread  4  is a locating pin  9  which can be received with some clearance in a recess  11  which, just as first rod  1 , conical thread  4 , locating pin  9 , second rod  6 , conical thread  8  and locating pin  9 , is placed coaxially relative to central axis  10 . 
     Situated in the transition zone between conical thread  4  and locating pin  9  is an annular recess  71  which defines the end of screw thread  4 . 
       FIG. 6  shows a screw coupling according to the invention substantially corresponding to  FIG. 5 . In this embodiment however, a friction ring  12 , for instance of a rubber-like material, connects to the annular widened lower surface  13  of first rod  1 , which in the structure according to  FIG. 5  can be placed into direct pressing co-action with the corresponding upper surface  14  of second rod  6 . When the screw coupling according to  FIG. 6  is effected, friction ring  12  is clamped with some force between said annular surfaces  13  and  14  at the end of the fastening turn. As a result of the thereby created tight friction coupling, a return relative rotation of first rod  1  and second rod  6  is at least more or less blocked and unintended and uncontrolled release of the screw coupling is at least substantially prevented. 
       FIG. 7  shows an embodiment in which mechanically controllable rotation locking means are arranged. 
     Situated for this purpose on the upper side of conical screw thread  4  is an expandable construction comprising a number of radially displaceable elements  15  which, in the non-active mode of the screw coupling drawn in  FIG. 7 , are retracted and lie inside the peripheral surface  16  of the part above conical thread  4 . In this situation the rounded inner zones  17  lie inside an annular recess  18  of a control rod  19  movable in axial direction  64 . In the situation shown in  FIG. 7  the external conical screw thread  4  can be brought into contact with the internal conical screw thread  8 . 
     After fully realizing the coupling by performing the rotation serving for this purpose, for instance through a half-turn or about 180°, the rotation locking with the radially displaceable elements  15 , for instance pins or segments, can take place by moving control rod  19  downward as according to an arrow  20  to the bottom of the cylindrical coaxial cavity  21  intended for this purpose in first rod  1  in the region of conical screw thread  4 . 
     As shown in  FIG. 8A , annular recess  18  hereby passes over the rounded inner zones of the displaceable rotation locking elements  15 , which are thereby urged to move outward as according to arrows  22  and in this situation can engage in correspondingly formed cavities  23 , for instance one annular cavity, such that in the expanded situation according to  FIG. 8  the locking elements  15  engage in cavities  23 . These cavities can take an individual form such that, even in the case there is no clamping force, elements  15  provide for a rotation locking. It can also be the clamping force caused by enclosing of the elements  15  between control rod  19  and the bottom of cavities  23  which brings about the rotation locking. 
       FIG. 8B  shows a variant in which a control rod  72  does not extend through first rod  1  as in the embodiment according to  FIG. 8A , but through second rod  6 . 
     It is here noted that, although rods  1  and  6  in  FIGS. 8A and 8B  are not identical, they are nevertheless designated with the same reference numerals because their ultimate functionalities are the same. 
     Control rod  72  has a roughly conical leading zone  73  with rounded tip, functionally analogous to annular recess  18  of control rod  19 . As control rod  72  is displaced upward according to arrow  74 , the displaceable elements  15  move outward according to arrows  22  and thus take up their locking position. It is noted that displaceable elements  15  need only provide a locking against axial displacement. This is because, as a result of the screw structure, a rotation locking is hereby also realized, which may for instance be necessary in the case of driving in two directions. 
       FIG. 8C  shows a rotation locking of a different type. In this embodiment locating pin  9  has a central non-round hole  75  into which a rod  76  of complementary form and with at least a non-round end can be inserted. The rod extends through second rod  6 . Locking rod  76  is coupled to second rod  6  for rotation locking by means of means which are not shown. This ensures the rotation locking between the first rod and second rod  6 . The direction of displacement between the non-operative position and the operative position of locking rod  76  is indicated with arrow  74 . 
       FIG. 8D  shows that locking rod  76  has a square cross-sectional shape. 
       FIG. 9  shows a screw  24  with a conical screw thread  25  according to the invention, which thread  25  has four grooves  26 ,  27 ,  28 ,  29  for passage of medium, for instance cooling medium. 
     As shown clearly in  FIG. 10 , the four grooves are arranged in angularly equidistant manner at angles of 90°, and grooves  26 - 29  extend in straight lines along the general jacket surface or base surface of conical screw thread  25 . 
     During use of the screw coupling of the invention for coupling rotating elements, for instance the output or driven shaft of a motor and a rotor driven thereby, it must be ensured that the axis of inertia coincides with the rotation axis. 
       FIG. 11  shows a pipe coupling between two pipes  30  and  31 . These latter are provided with respectively an internal conical screw thread  32  and an external conical screw thread  33  according to the invention. A rotation locking ring  35  axially slidable as according to an arrow  34  can provide for a rotation locking after the screw coupling between pipes  30 ,  31  has been realized. 
       FIG. 12  shows an at least partly transparent top view of the locked situation. In the coupled position in  FIG. 12  corresponding flat surfaces  36 ,  37  on pipe  30  and  31  respectively lie in a shared imaginary plane and the ring provided with a flat side  38  is in contact with both flat surfaces  36  and  37  and thus blocks the relative rotation of pipes  30  and  31 . 
       FIG. 14  shows a coupling sleeve  39  which is coupled by means of two screw couplings  40 ,  41  according to the invention to respective pipes  42  and  43 . Rotation locking is ensured by means of socket screws  44 ,  45 , which engage with pressing force on the respective outer surfaces of pipes  42 ,  43 . 
       FIG. 15  shows a coupling sleeve  45  with an external conical thread  46 ,  47  on either end. A respective hose  48 ,  49  is pushed thereover with some elastic deformation. A respective screw ring  50 ,  51  with internal thread  52 ,  53  respectively is arranged over this coupling zone by rotation in screwing and thus clamping manner. Applying the teaching according to the invention, a protruding sealing and pressure-resistant, i.e. tensively strong, coupling is in this way realized between hoses  48 ,  49  via the elastically and possibly also plastically deformed end zone of the relevant hose  48 ,  49 . 
       FIG. 16  shows a technically corresponding structure, although for thick-walled rubber hoses. The various components corresponding to  FIG. 5  are designated in  FIG. 16  with the same reference numerals, here however with the addition of an accent. 
     Depending on the intended application, diverse materials can be considered for sleeve  45  and screw rings  50 ,  51 . It is for instance possible to envisage a metal, although plastics with a sufficient mechanical strength, thermal resistance, chemical resistance and other desirable properties can also be applied. A plastic reinforced with fibres, such as glass fibres, is also suitable. Suitable plastics could be ABS and POM. 
     In the structures according to  FIGS. 11 ,  12 ,  13 ,  14 ,  15  and  16  the mutually adjacent inner surfaces connect to each other without any substantial narrowing, obstruction or interruption. This greatly enhances the uninterrupted flow of throughflow medium. 
       FIG. 17  shows a coupling plate  54  for coupling adjacent parts  77 ,  78  of a bone  65 . Coupling plate  54  has a number of inserts, all designated  55  and each provided with an internal conical thread  56 . This thread  56  can co-act with the corresponding conical thread  57  of a screw head  58  such that threads  56  and  57  form a screw coupling according to the invention between insert  55  and a screw  59  of which head  58  forms part. 
     Coupling plate  54  extends over the bone break  79  and fixes bone parts  77 ,  78  relative to each other in the manner indicated in  FIG. 17 , such that these parts can grow against each other so that the bone will once again heal. 
     As  FIG. 18  shows, screw  59  is provided with a self-drilling screw tip  60  and cylindrical shank  61  does not have a continuous screw thread as is usual for most screws, but a discrete screw thread which comprises a number of more or less sphere segment-shaped, at least rounded protrusions  62  disposed on a helical line around shank  61 . Such a screw is highly suitable for bone surgery in particular. 
       FIG. 19  shows an insert piece  55 ′, the conical screw thread  56 ′ of which extends at an angle of about 15° relative to the normal. This angle corresponds to the position of screw  59 ′ drawn in  FIG. 17 , which is drawn with broken lines, in addition to a screw  59  serving as reference and placed at normal position, which is drawn in full lines. 
       FIG. 20  shows an insert piece  50 ″ with an internal conical thread  56 ″ extending at about 30° relative to the normal in accordance with screw  59 ″ drawn with broken lines in  FIG. 17 . 
     Finally,  FIG. 21  shows a pin  63  which is applied in surgery in the case of a broken leg or ankle, with a number of screw couplings according to the invention. 
     Pin  63  has a coaxial continuous hole  80 . This is important because the pin, which must be arranged in a pre-drilled hole through the relevant bone, must be provided with a pressure equalization in respect of displacement of material, such as moisture, bone marrow, bore dust and the like present in the bone. Pin  63  has a rounded form on the underside. On its top side pin  63  has an internal screw thread  82  of the conical type according to the invention. This is intended for co-action with external screw thread  83  of a rod-like insertion aid  84  which is intended for pressing pin  63  with force into the hole drilled in the bone. Once this pin has been placed at its intended position, which the surgeon can determine on the basis of x-rays or other imaging techniques, aid  84  can be uncoupled from pin  63  by a simple return rotation, retracted and removed from the wound. 
     A closing cap  85  as according to  FIG. 22  is then placed on pin  63 . This closing cap is also provided with an external conical screw thread  83 . The closing cap closes the end zone and hole  80  of pin  63  by means of an annular protruding head  86 . 
     Analogously to the situation shown in  FIG. 17 , pin  63  has two coupling zones with screw connections according to the invention. These screw connections are designated  87  and  88 . 
     Pin  63  has an internal screw thread  90  for co-action with the corresponding external thread  91  of a screw  92  with smooth shank and a head  93 . Threads  90  and  91  form a screw connection according to the invention. The length of the smooth shank of screw  92  is preselected on the basis of the local geometry, such that annular outer surface  94  of head  93  can rest against the outer surface of the bone. 
     Screw connection  88  comprises a first screw  95  of a relatively large diameter, which co-acts with an internal conical screw thread of pin  63  by means of an external conical screw thread. Screw  95  itself likewise has on its free outer end an internal screw thread with which a second screw  96  can co-act. In this way a double screw connection according to the invention can be realized, wherein the smooth shanks of screws  95 ,  96  have the same central axes. 
     Head  86  of closing cap  85 , head  93  of screw  92 , head  97  of first screw  95  and head  98  of second screw  96  are all of the type which can be engaged by means of a suitable tool for the purpose of exerting a rotation force. Known engaging forms are suitable, such as a single elongate recess, a cross shape, a non-round recess or the like. 
     In  FIG. 21  the angle  89  indicates highly schematically that the screw connection in question, just as all screw connections shown and described in this specification, is to a very great extent self-locating. This is a highly practical property in for instance the described application in the context of bone surgery. A surgeon can realize the screw coupling substantially “blindly”. As soon as the tip of the external screw thread  83  comes into contact with the inner side of the part with internal screw thread  82 , the locating function is in fact hereby completed, and the definitive screw coupling can be effected with some force by performing a continued axial displacement in combination with a rotation. This is a very important property of all screw couplings of the type according to the invention. 
     Drawn in  FIG. 21  is an angle  89  which defines a general cone shape, the jacket surface of which roughly marks the limits over which the insertion aid can be inserted into internal screw thread  82 . It will be apparent that in accordance with the teaching of the invention a very high degree of tolerance can be allowed in the necessary positioning accuracy. This makes relatively simple the preparatory work of the surgeon for removal of the pin after the relevant bone or joint has healed. 
       FIG. 23  shows a leg  98  of a patient. A dash-dot line  99  indicates that a hole is drilled in the bone as according to an arrow  100  for the purpose of inserting a pin  63 . 
       FIG. 24A  shows an alternative screw  101  which differs from screw  74  to the extent that its smooth shank has an annular recess  102 , whereby the screw possesses a certain measure of flexibility. This is an important option for the intended application in the fixing of bone parts. In order to nevertheless ensure a smooth shank and to ensure that bone does not begin to grow into annular recess  102 , whereby it would be difficult or impossible to remove the screw, annular recess  102  is filled with a suitable polymer material  103 , such as polytetrafluoroethylene (PTFE). 
       FIG. 24B  shows a variant. A screw  104  according to  FIG. 24B  does not comprise a peripheral recess  102  but two pairs of recesses  105 ,  106  and  107 ,  108  respectively mutually offset through 90°. 
     Both screw  101  and screw  104  thus have a substantially isotropic additional flexibility. 
       FIG. 25  shows two bodies  110 ,  111  placed at a mutual distance and provided with schematically represented screw threads  112 ,  113  respectively of the type according to the invention. In this embodiment the base surfaces are flat or, in other words, the base surfaces take the form of truncated cones with apex angles of 2×90°. The shared rotation axis of the screw coupling is designated with reference numeral  114 . 
       FIG. 26  shows with arrows  115  that bodies  110 ,  111  are first moved toward each other such that they mutually engage and the one screw thread is introduced into the other. Once this situation has been reached, the two bodies  110 ,  111  placed against each other are rotated relatively as according to an arrow  116 , either in the one direction or in the other, depending on the forms of the screw threads. Particularly in the case of at least more or less symmetrical screw threads the rotation direction is immaterial. The coupling is brought about by the rotation. The coupled situation in question is shown in  FIG. 27 . Only by performing a subsequent rotation counter to the direction of the performed rotation can the coupling between the two bodies be undone. The coupling is often so strong that an impulsive force, for instance a tap, a blow or an impact, must be applied to be able to cause the bodies to perform the desired relative displacement. 
     The profile depth of the screw threads need often only be small, for instance several tenths of a millimeter. Other than suggested in the schematic  FIGS. 25 ,  26  and  27 , such screw threads are often barely visible to the naked eye. 
     Finally, it is noted generally that the materials applied for surgical purposes are of a biocompatible type, for instance surgical steel, vitallium or the like.