Patent Publication Number: US-6336765-B1

Title: Twist lock for connecting containers

Description:
TECHNICAL FIELD 
     The present invention relates to a twist lock, to be disposed between two containers, for connecting the containers loaded or stored on top of each other in multiple tiers in a container yard or on a containership. 
     BACKGROUND ART 
     As illustrated in FIG. 11, containers Ct are generally loaded and unloaded between a container yard Yd and a containership Sh. 
     By way of example, for the loading of a container Ct, a container Ct stored at the container yard Yd is first loaded on a container vehicle V such as a trailer by means of a transfer crane Tc or a straddle carrier and then carried to the apron Ap. A container Ct at the apron Ap is lifted by a spreader Sp of a container crane Cr and rested on another container Ct on the deck of a containership Sh. 
     As for the unloading of a container Ct from the deck of a containership Sh, a container Ct is lifted and transferred to the apron Ap by means of the spreader Sp of the container crane Cr. The container Ct is then loaded on the container vehicle V, and carried from the apron Ap to the container yard Yd. 
     When containers Ct are loaded or stored in multiple tiers in a container yard Yd or on a S containership Sp, an upper container Ct and a lower container Ct are connected with each other to prevent them from falling off. As the connection means, container locks are disposed, via fitting parts, between the four bottom corners of the upper container and the four top corners of the lower container. 
     An example of such container locks is disclosed in International Publication No. WO92/05093 as twist lock. 
     The twist lock  100  is described, referring to FIGS. 12 to  15 . 
     The twist lock  100  comprises a housing  101  composed of a right housing component and a left housing component integrally joined by a bolt, a spindle  102  (FIG. 15) rotatably held in the housing  101 , and an upper twist lock cone  103  and a lower twist lock cone  104  which are integrally connected to the upper end and the lower end of the spindle  102 , respectively. 
     The housing  101  includes an upper raised part  101 U and a lower raised part  101 D integrally incorporated on the upper and lower portions. The upper and lower raised parts  101 U,  101 D are designed in a shape which matches a fitting hole Fa of a corner fitting F mounted on the container Ct (FIG.  18 ), and capable of fitting into a fitting hole Fa formed in a bottom corner fitting F of an upper container Ct and a fitting hole Fa formed in a top corner fitting F of a lower container Ct, respectively. 
     The housing  101  further includes a through-hole (not shown) which extends through the upper raised part  101 U and the lower raised part  101 D, and which supports the spindle  102 . 
     The corner fitting F having the fitting hole Fa to be mounted on a container Ct is not specifically illustrated herein. For their specification, reference can be made to JIS Z1616. 
     The upper twist lock cone  103  and the lower twist lock cone  104  are designed to match the shape of the fitting hole Fa in the corner fitting F of the container Ct, and engageable with the fitting hole Fa. In association with the rotation of the spindle  102 , the upper and lower twist lock cones  103 ,  104  rotate on the top surface of the upper raised part  101 U and the bottom surface of the lower raised part  101 D, respectively. When the upper twist lock cone  103  is detachable with respect to the fitting hole Fa in the bottom corner fitting F of the upper container Ct, the lower twist lock cone  104  is engaged with the fitting hole Fa in the top corner fitting F of the lower container Ct. On the other hand, when the lower twist lock cone  104  is detachable with respect to the fitting hole Fa in the top corner fitting F of the lower container Ct, the upper twist lock cone  103  integrally connected to the spindle  102  is engageable with the fitting hole Fa in the bottom corner fitting F of the upper container Ct, in which case a top view of the twist lock shows the spindle  103  crossing over the fitting hole Fa in the form of the letter X. 
     The lower twist lock cone  104  is shaped such that it is imparted with a rotating force when pressed against the fitting hole Fa in the corner fitting F, provided that the lower cone  104  is in an engaging position with respect to the fitting hole Fa. To be more specific, the lower twist lock cone  104  is engageable with the fitting hole Fa in the corner fitting F, when the spindle  102  stays at the first position A to be described below. Under this condition, by pressing the lower twist lock cone  104  against the edge of the fitting hole Fa in the corner fitting F, a pressurising force is imposed on the corner fitting F through the lower twist lock cone  104 , whereas the lower twist lock cone  104  receives a reactive force in return. Owing to the reactive force acting on the lower twist lock cone  104 , a properly shaped lower twist lock cone  104  is forced to rotate around the rotation axis of the spindle  102 . 
     As shown in FIG. 15, a cavity  101 X is formed inside the housing  101 , and provided with a first stop element  101   a  and a second stop element  101   b  which can be contacted by a stop arm  1021  securely integrated with the spindle  102 . In this structure, the spindle  102  is capable of rotating between the first position A and the third position C where the stop arm  1021  contacts the first stop element  101   a  and the second stop element  101   b  in the cavity  101 X, respectively. 
     Normally, the spindle  102  is urged by a torsion spring  105 , whereby the stop arm  1021  stays in contact with the first stop element  101   a  in the cavity  101 X. Further, a wire  106  is disposed along a groove  1022  formed in the circumference of the spindle  102 . One end of the wire  106  is looped around the stop arm  1021 , while the other end is drawn out through a nozzle  107  and fixed to a handle  108 . Of the nozzle  107 , the right and left edge portions are slidably fitted in a transverse slot  101 Y, and the upper and lower portions are provided with slots  107   a,    107   c,  respectively. Further, a stop element  1061  is provided in the vicinity of the handle-side end of the wire  106 . The stop element  1061  is capable of checking selectively at the slot  107   a  or  107   c.    
     The nozzle  107  is displaceable along the transverse slot  101 Y in which the right and left edge portions thereof are fitted. The nozzle  107  is usually urged into contact with one end of the transverse slot  101 Y by means of a spring  109  arranges within the housing  101 . 
     With the use of the above twist lock  100 , two vertically loaded containers Ct are joined in the following manner. First, a container Ct in a container yard Yd is loaded on a container vehicle V using a transfer crane Tc and carried to an apron Ap. Then, in the apron Ap, the container Ct is lifted by a spreader Sp of a container crane Cr and suspended approximately one meter above the ground, where the upper twist lock cone  103  of the twist lock  100  is mounted to every bottom corner fitting F of the container Ct (FIG.  16 ). Specifically, the handle  108  is pulled by hand in order to check the stop element  1061  of the wire  106  at the slot  107   a  of the nozzle  107 . At this first checking position, the spindle  102  is rotated to the third position C where the stop arm  1021  contacts the second stop element  101   b  in the cavity  101 X, while the upper twist lock cone  103  comes in agreement with the upper raised part  101 U of the housing  101 , as seen from above. Thereby, the upper twist lock cone  103  can be inserted together with the upper raised part  101 U into the fitting hole Fa in the bottom corner fitting F of the upper container Ct. Once the upper twist lock cone  103  is inserted into the fitting hole Fa, the handle  108  is pulled by hand again so as to release the stop element  1061  of the wire  106  from the slot  107   a  of the nozzle  107 . On the release of the stop element  1061 , the torsion spring  105  urges the spindle  102  back to the first position A where the stop arm  1021  contacts the first stop element  101   a  in the cavity  101 X. At the first position A, the upper and lower twist lock cones  103 ,  104  are both engaged with the fitting holes Fa in the corner fittings F, as mentioned above, whereby the twist lock  100  does not fall off or come out from the corner fitting F of the container Ct (FIG.  17 ). 
     After the twist locks  100  are mounted on the bottom corner fittings F, the container Ct is lifted by a container crane Cr and transferred onto another container Ct on the deck of a containership Sh (FIG.  18 ). While the container Ct is being rested, the lower twist lock cone  104  of each twist lock  100  is forced to rotate along the edge of the fitting hole Fa in a corresponding top corner fitting F of the lower container Ct, against the urging force deriving from the torsion spring  105 . When the lower twist lock cone  104  is rotated into agreement with the lower raised part  101 D of the housing  101  as seen from above, the lower twist lock cone  104  and the lower raised part  101 D enter the fitting hole Fa in the top corner fitting F of the lower container Ct. Having entered the top corner fitting F, the lower twist lock cone  104  returns to the engaging position due to the urging force of the torsion spring  105  and engages with the fitting hole Fa. Upon the engagement, the upper and lower containers Ct are joined by the twist locks  100 , wherein the upper and lower twist lock cones  103 ,  104  are engaged with the bottom corner fittings F of the upper container Ct and the top corner fittings F of the lower container Ct, respectively (FIG.  19 ). 
     Now, the container Ct loaded on the deck of the containership Sh is unloaded in the following process. With the lower twist lock cone  104  locating at the engaging position, the handle  108  is pulled down or pushed down on the deck so as to check the stop element  1061  at the slot  107   c.  At this second checking position, the spindle  102  and the stop arm  1021  locate at the second position B between the first stop element  101   a  and the second stop element  101   b  in the cavity  101 X of the housing  101 . At the second position B, the lower twist lock cone  104  positionally corresponds to the lower raised part  101 D of the housing  101  as seen from above, and can be thus removed from the fitting hole Fa in the top corner fitting F of the lower container Ct (FIG.  20 ). 
     Following the release of the lower twist lock cone  104 , the container Ct is transferred by the spreader Sp of the container crane Cr, and then suspended approximately one meter above the ground at the apron Ap, where the upper twist lock cone  103  is removed from each bottom corner fitting F of the container Ct. Specifically, the handle  108  is pulled by hand to check the stop element  1061  of the wire  106  at the slot  107   a  of the nozzle  107 . At this first checking position, the spindle  102  is rotated to the third rotation position C where the stop arm  1021  contacts the second stop element  101   b  in the cavity  101 X, while the upper twist lock cone  103  comes in positional agreement with the upper raised part  101 U of the housing  101  as seen from above. Therefore, the twist lock  100  can be removed from the fitting hole Fa in the bottom corner fitting F of the container Ct (FIG.  21 ). Thereafter, the container Ct at the apron Ap is loaded onto the container vehicle V by the container crane Cr and transported to the container yard Yd. 
     DISCLOSURE OF THE INVENTION 
     The above-mentioned conventional twist lock has facilitated the loading and joining of containers. In the loading of containers, the twist lock is attached to a bottom corner fitting of a container to be loaded. By resting this container on another container, the lower twist lock cone enters a top corner fitting of the lower container, with rotating along the edge of the fitting hole in the corner fitting. Upon the entry, the lower cone automatically returns to the position engaging with the corner fitting as urged by a torsion spring. Eventually, the upper and lower containers are joined together. On the other hand, in the unloading of the containers, the spindle (i.e. the lower twist lock cone) needs to be rotated from the engaging position to the releasable position against the urging force of the torsion spring, in which case the rotation should be effected by manual operation of the handle. In order to check the stop element of the wire at the slot of the nozzle, a dockworker has to pull down or push down the handle with the use of a long actuator pole, on the top of the multiple tiers of containers, or on the deck (in a containership) or the ground (in a container yard) as shown in FIG.  20 . Such an unlocking process not only requires a long working time but also imposes a substantial physical burden to dockworkers. It is obviously difficult and exhausting to operate the handle with the tip of a long and heavy actuator pole. In addition, the twist lock of prior art is undesirable in the safety aspect, because operation on the top-tier containers inevitably involves the risks of falling off therefrom and other accidents. 
     In order to solve these problems, the present invention intends to provide a twist lock for connecting containers which enables the automatic connection and the automatic disconnection between an upper container and a lower container, without any high-place operations or the like. 
     According to the first embodiment of the present invention, a twist lock for connecting containers comprises a housing integrally including an upper fitting part and a lower fitting part each being capable of fitting into a fitting hole in a corner fitting of a container and being formed with a through-hole which extends through the upper and lower fitting parts, a spindle inserted in the through-hole in a rotatable and vertically displaceable manner, an upper lock connected to the top end of the spindle and being engageable with the fitting hole in the corner fitting of the container, and a lower lock being in positional agreement with and fixed securely to the lower fitting part of the housing and slidably fitted with a plurality of locking members which can engage with and disengage from the fitting hole in the corner fitting of the container. In this twist lock, the spindle is urged by a spring member in such a direction that the upper lock can engage with the fitting hole in the corner fitting of the container, while each of the locking members is urged by another spring member in such a direction that each locking member can retract into the lower lock. When the spindle locates at a lowered position, the locking members contact the spindle and project from the lower lock against the urging force of the spring means, thereby engaging with the fitting hole in the corner fitting of the container to prevent the release of the lower lock. When the spindle locates at a raised position, the locking members lose contact with the spindle and retract into the lower lock by the urging force of the spring means, thereby disengaging from the fitting hole in the corner fitting of the container to permit the release of the lower lock. 
     For the loading of a container, the twist lock of the first embodiment is attached to a container by mounting the upper lock into a fitting hole in a bottom corner fitting of the container. While the housing descends relative to the spindle due to its own weight, the locking members lose contact with the spindle and retract into the lower lock. As a result, the lower lock can be introduced into a fitting hole in a top corner fitting of another container. When the container is transferred onto the lower container, the spindle descends relative to the housing. As the spindle contacts and pushes out the locking members, they are engaged with the fitting hole in the top corner fitting of the lower container to connect the upper and lower containers. 
     Unloading of the container is performed simply by lifting the container. Thereby, the spindle is raised, relative to the housing, by the upper lock engaging with the fitting hole in the bottom corner fitting of the upper container. The locking members thus lose contact with the spindle and retract into the lower lock. Consequently, the locking members are disengaged from the fitting hole in the top corner fitting of the lower container, from which the lower lock can come out. 
     As described above, when a container mounted with the twist lock at the fitting hole in every bottom corner fitting is rested onto another container, the lower lock is fitted into the fitting hole in the top corner fitting of the lower container, and the locking members are engaged therewith. Likewise, simply by lifting the container, the locking members are disengaged from the fitting hole in the top corner fitting of the lower container. Thus, the twist lock of the present invention enables not only automatic connection between the upper and lower containers in loading, but also automatic disconnection between the twist lock and the lower container in unloading. This advantage eases the workload and provides safe working environments, as the dockworkers no longer have to work on the top of multiple-tier containers or to operate a working tool. 
     In a preferable structure of the above twist lock, the spindle is linked with a stop handle through a linking member, and the housing includes a stop for checking the stop handle. 
     According to this structure, the stop handle connected to the linking member is manually pulled and checked at the stop in the housing. Thereby, the spindle is rotated against the urging force of the spring means, allowing the upper lock fixed to the spindle to rotate to and remain at a position where the upper lock can be freely inserted into or removed from the fitting hole in the corner fitting of the container. Therefore, even if the twist lock may be broken, at least the upper container can be disconnected from the twist lock as an emergency measure. 
     In another preferable structure, a fitting groove is formed in the outer circumferential surface of the spindle, and at least one connecting bore is provided in the right and left components of the housing, respectively. Each connecting bore is formed in a predetermined length in the direction extending from a radius of the through-hole, and has an open end at a position facing the fitting groove in the spindle at the lowered position. At least one ball is rotatably accommodated in each connecting bore. 
     According to this structure, when the vertically connected containers are inclined, the ball rolls along one of the connecting bores corresponding to the inclined direction of the housing. Thus, the ball is to fit into the fitting groove in the spindle at the lowered position. Consequently, the spindle can bear the lift-up force resulting from the jump of the containers and transmitted via the upper lock. The connection between the upper and lower containers is thus maintained. 
     In still another preferable structure, a fitting groove is formed in the inner circumferential surface of the housing, and at least one connecting bore is provided in the right and left portions of the spindle, respectively. Each connecting bore extends in a radius direction of the through-hole and communicates with each other at the centre. Further, each connecting bore has an open end at a position facing the fitting groove in the housing, provided that the spindle locates at the lowered position and the upper lock is engaged with the fitting hole in the corner fitting of the container. At least one ball is rotatably accommodated in the connecting bores. 
     According to this structure, when the vertically connected containers are inclined, the ball rolls along one of the connecting bores corresponding to the inclined direction of the spindle at the lowered position. Thus, the ball is to fit into the fitting groove in the housing. Consequently, the spindle can bear the lift-up force resulting from the jump of the containers and transmitted via the upper lock. The connection between the upper and lower containers is thus maintained. 
     Preferably, each connecting bore in the above structures is inclined, by a predetermined downward gradient, from the open end facing the fitting groove formed in the spindle or the housing down to the direction in which the connecting bore extends. 
     Due to the gradient, when the containers stay level, each ball rolls down to the bottom end of the connecting bore and does not interfere with the fitting groove in the spindle. Therefore, this arrangement permits the upward movement of the spindle, except when the inclination of the containers exceeds the gradient of the connecting bore, while effectively preventing the accidental locking of the spindle in unloading the containers. 
     In a further preferable structure, a fitting groove is formed in the outer circumferential surface of the spindle, and at least one connecting bore is provided in the right and left components of the housing, respectively. Each connecting bore is formed in a predetermined length in the direction extending from a radius of the through-hole, and has an open end at a position facing the fitting groove in the spindle at the lowered position. In addition, each connecting bore accommodates a plurality of balls in a rotatable manner. The housing further includes a pair of fitting bores vertically formed on the right and left sides of the upper fitting part along a concentric circle of the through-hole, with the bottom end of each fitting bore communicating with the connecting bore. A guide is inserted in each fitting bore in a vertically slidable manner and urged by a spring means to project slowly from the top surface of the housing. Accordingly, an inner ball is allowed to fit in the fitting groove in the spindle when the guide locates at a lowered position, and allowed to roll out of the fitting groove when the guide locates at a raised position. 
     According to this structure, when the bottom corner fitting of the upper container is rested on the twist lock, the guides are pressed down against the urging force of the spring means. In connection with the descent of the guides, the inner balls are allowed to fit in the fitting groove in the spindle to block the upward movement of the spindle. On the other hand, when the upper container is lifted up, the guides project from the top surface of the housing by the urging force of the spring means. In connection with the rise of the guides, the inner balls roll out of the fitting groove in the spindle to permit the upward movement of the spindle. Even if the vertically connected containers are shaken violently due to high waves in the sea or the like, the rising rate of the guides is not affected by the sudden jump of the containers, so that the balls remain fitted in the fitting groove in the spindle for a certain period of time. As a result, even when the spindle is subjected to the lift-up force transmitted via the upper lock, this twist lock keeps the connection between the upper and lower containers, with preventing the upward movement of the spindle. 
     Preferably, the connecting bore in the above structure is inclined, by a predetermined downward gradient, from the open end facing the fitting groove in the spindle down to the other end communicating with the fitting bore. 
     Due to the gradient, when the containers stay level, each ball rolls down to the bottom end of the connecting bore and does not interfere with the fitting groove in the spindle. This arrangement permits the upward movement of the spindle, except when the inclination of the containers exceeds the gradient of the connecting bore, while effectively preventing the accidental locking of the spindle in unloading the containers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an entire perspective view of a twist lock according to the first embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of the twist lock of FIG. 1 in the interconnecting state with containers. 
     FIG. 3 is a cross-sectional view of the twist lock of FIG. 1 in the removable state with containers. 
     FIG. 4 is a longitudinal cross-sectional view of a twist lock according to the second embodiment of the present invention, vertically taken along the axial centres of the connecting bores. 
     FIG. 5 is a longitudinal cross-sectional view of a twist lock according to a modification of the second embodiment of the present invention, vertically taken along the axial centres of the connecting bores. 
     FIG. 6 is an entire perspective view of a twist lock according to the third embodiment of the present invention. 
     FIG. 7 is a longitudinal cross-sectional view of the twist lock of FIG. 6, vertically taken along a straight line linking the centres of the guides. 
     FIG. 8 is an enlarged view showing the safety unit and the operation control unit in the twist lock of FIG.  7 . 
     FIG. 9 is a cross-sectional view of the twist lock of FIG. 6, as mounted in the fitting hole in a bottom corner fitting of a container. 
     FIG. 10 is a cross-sectional view of the twist lock of FIG. 6 in the interconnecting state with containers. 
     FIG. 11 is a schematic diagram showing transfer of a container between the container yard and the apron by means of a container vehicle, and loading and unloading of a container between the container vehicle and the containership by means of the container crane. 
     FIG. 12 is a perspective view of a conventional twist lock for joining containers. 
     FIG. 13 is a plan view of the twist lock of FIG.  12 . 
     FIG. 14 is a bottom view of the twist lock of FIG.  12 . 
     FIG. 15 is a transverse cross-sectional view of the twist lock of FIG.  12 . 
     FIG. 16 is a schematic diagram showing the attachment of the twist lock of FIG.  12  and the loading of the container. 
     FIG. 17 is a schematic diagram showing the attachment of the twist lock of FIG.  12  and the loading of the container. 
     FIG. 18 is a schematic diagram showing the attachment of the twist lock of FIG.  12  and the loading of the container. 
     FIG. 19 is a schematic diagram showing the attachment of the twist lock of FIG.  12  and the loading of the container. 
     FIG. 20 is a schematic diagram showing the unloading of a container connected by the twist lock of FIG. 12 from the containership. 
     FIG. 21 is a schematic diagram showing the unloading of a container connected by the twist lock of FIG. 12 from the containership. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to the attached drawings, the embodiments of the present invention are hereinafter disclosed. 
     FIGS. 1 to  3  illustrate a twist lock according to the first embodiment of the present invention. 
     The twist lock  1  for connecting containers comprises a housing  2  composed of right and left housing components integrally joined by a bolt, a spindle  3  rotatably held in the housing  2  (FIGS.  2  and  3 ), an upper lock  4  integrally joined at the top end of the spindle  3 , and a lower lock  5  securely fixed to the housing  2 . 
     The housing  2  includes an upper fitting part  21  and a lower fitting part  22  integrally formed on the upper and lower portions thereof. The upper and lower fitting parts  21 ,  22  are shaped in agreement with the shape of a fitting hole Fa of a corner fitting F of a container Ct, and are capable of fitting into a fitting hole Fa in a bottom corner fitting F of an upper container Ct and a fitting hole Fa in a top corner fitting of a lower container Ct, respectively. At one end of the housing  2 , there are formed a stop  23  for checking a stop handle S to be mentioned below, and a storage space  24  for accommodating the stop handle S. 
     Further, the housing  2  includes a through-hole  25  which extends through the upper and lower fitting parts  21 ,  22 , and a cavity  26  which has a fixed height and a larger diameter than the through-hole  25 . The spindle  3  is rotatably inserted in the through-hole  25  and vertically displaceable within the height of the cavity  26 . The spindle  3  comprises a small-diameter spindle part  31  corresponding to the diameter of the through-hole  25 , and a large-diameter spindle part  32  corresponding to the diameter of the cavity  26  and located in the near-middle portion of the small-diameter spindle part  31 , wherein the large-diameter spindle part  32  is smaller in height than the cavity  26  in the housing  2 . According to this structure, the spindle  3  normally stays at the lowered position due to its own weight, where the bottom surface of the large-diameter spindle part  32  rests on the bottom step defined by the through-hole  25  and the cavity  26 . The total height of the spindle  3  is such that, at the lowered position, there remains a slight clearance between the upper lock  4  and the upper fitting part  21  of the housing  2  for allowing the rotation of the upper lock  4 , and also that the bottom end of the spindle  3  should enter a guide hole  51  of the lower lock  5  to be described below. In addition, the upward movement of the spindle  3  is limited to the raised position, where the top surface of the large-diameter spindle part  32  contacts the top step defined by the through-hole  25  and the cavity  26 . 
     The ascent/descent stroke of the spindle  3  is set smaller than the height of the upper fitting part  21  of the housing  2 . Thereupon, the upper fitting part  21  does not drop off from the fitting hole Fa in the corner fitting F of the container Ct, when the housing  2  hangs from the corner fitting F and descends relative to the spindle  3 , with the upper lock  4  being engaged with the fitting hole Fa. Thus, the position of the twist lock  1  is maintained (FIG.  3 ). 
     In connection with the spindle  3 , although not shown in detail, an outwardly projecting arm is formed on the large-diameter spindle part  32  of the spindle  3 , and a contact member for stopping the arm is formed in the cavity  26  in the housing  2 . With the arm being in contact with the contact member, the spindle  3  cannot rotate any further. Besides, a spring means such as a torsion spring  6  is disposed around the lower part of the spindle  3 , with one end of the spring  6  being linked to the spindle  3  and the other end being linked to the housing  2 . The torsion spring  6  usually urges the spindle  3  such that the arm (not shown) contacts the contact member (not shown) formed in the cavity  26  of the housing  2 . In this state, the position of the upper lock  4  is in engagement with the fitting hole Fa in the bottom corner fitting of the container Ct, as shown in FIG.  1 . 
     Incidentally, one end of the torsion spring  6  is fitted into a hole (not shown) vertically formed in the large-diameter spindle part  32  of the spindle  3 . Therefore, when the spindle  3  moves upwardly within the height of the cavity  26  in the housing  2 , the torsion spring  6  does not come off from the spindle  3  and keeps the linkage therewith. 
     As for the spindle  3  which is supported in the through-hole  25  in the housing  2 , the bottom end thereof is shaped approximately in the form of a cone so as to be in smooth contact with the base ends of locking members  53  disposed in the lower lock  5  to be described below. The spindle  3  further includes a groove  32   a  formed around the circumference of the large-diameter spindle part  32  for guiding a linking member such as a wire  7 . One end of the wire  7  is secured to the spindle  3 , while the other end is drawn out through a wire-drawing passage  24   a  formed in the housing  2  and secured to a stop handle S in the storage space  24 . The stop handle S is usually accommodated in the storage space  24  in the housing  2  in such a manner that the wire  7  has some sag. Therefore, when the stop handle S connected with the wire  7  is pulled by hand, the spindle  3  (i.e. the upper lock  4 ) rotates against the urging force of the torsion spring  6 . The stop handle S is then checked at the stop  23  of the housing  2 , as shown in FIG. 2 by an imaginary line. In this checking state, the position of the upper lock  4  should agree to that of the upper fitting part  21  of the housing  2  as seen from above. Such positional relationship is achieved by judiciously designing the wire length and the positions to secure the wire  7 . 
     When the spindle  3  yields to its own weight and lies at the lowered position on the bottom step defined by the through-hole  25  and the cavity  26  in the housing  2 , the groove  32   a  in the spindle  3  is in straight communication with the wire-drawing passage  24   a  in the housing  2 , as seen from the side (FIG.  2 ). Further, the inner end of the wire-drawing passage  24   a  opens to the cavity  26  in the form of a bell, while the groove  32   a  is gradually widened downwards at a position facing the wire-drawing passage  24   a.  These configurations are effective when the spindle  3  is upwardly displaced to the raised position, while the upper lock  4  is urged by the torsion spring  6  to the position engageable with the fitting hole Fa in the bottom corner fitting F of the container Ct. As a result, the wire  7  runs flexibly between the groove  32   a  and the wire-drawing passage  24   a,  irrespective of the height difference resulting from the upward movement of the spindle  3  (FIG.  3 ). The stretch of the wire  7  is compensated by the sag thereof drawn out in the storage space  24 . 
     The lower lock  5  is formed in a shape matching to the fitting hole Fa in the corner fitting F of the container Ct. As seen from above, the lower lock  5  is positioned in agreement with the lower fitting part  22  of the housing  2  and securely fixed on the bottom surface thereof. The lower lock  5  includes a guide hole  51  having a circular cross-section and vertically communicating with the through-hole  25  in the housing  2 , and a pair of passages  52  extending through the lower lock  5  in the forward/backward direction (the right/left direction in FIG.  2 ), with one end of each passage  52  opening to the guide hole  51 . Each of the passages  52  is slidably fitted with a locking member  53  having a chamfered base end and provided with a flange  531  in the vicinity of the base end. A spring means (e.g. a spring  54 ) is disposed between the flange  531  of the locking member  53  and the step portion on the extreme end side defined by the small-diameter part and the large-diameter part of the passage  52 . Due to the urging force of the spring  54 , each locking member  53  is urged such that its base end can project into the guide hole  51 . 
     To be specific, when the spindle  3  rests at the lowered position on the bottom step defined by the through-hole  25  and the cavity  26  in the housing  2 , the bottom end of the spindle  3  is inserted into the guide hole  51  in the lower lock  5 , with the outer circumferential surface thereof being in contact with the base ends of the locking members  53 . As a result, the locking members  53  are pushed outwardly against the urging force of the springs  54 , and the extreme ends thereof project from the surface of the lower lock  5  (FIG.  2 ). 
     On the other hand, when the spindle  3  is pulled up to the raised position and contacts the top step defined by the through-hole  25  and the cavity  26  in the housing  2 , the base ends of the locking members  53  no longer contact the outer circumferential surface of the spindle  3 . As a result, the locking members  53 , urged by the springs  54 , retract the extreme ends into the lower lock  5  (FIG.  3 ). In the course of projection, the flanges  531  of the locking members  53  come in contact with the step portions on the base end side defined by the small-diameter parts and the large-diameter parts of the passages  52 , whereby the locking members  53  cannot project in the direction of the guide hole  51  any further (FIG.  3 ). 
     It is worth noting that the bottom end of the spindle  3  is shaped approximately in the form of a cone, and that the base ends of the locking members  53  are chamfered. This combination allows smooth displacement of the locking members  53  in association with the vertical movement of the spindle  3 . 
     Next, description is made of the operation of the first embodiment of the twist lock  1 . 
     FIG. 1 shows the initial state of the twist lock  1 , wherein the upper lock  4  is urged by the torsion spring  6  to be in engagement with the fitting hole Fa in the corner fitting F of the container Ct. In order to mount the twist lock  1  to the fitting hole Fa in the corner fitting F, the upper lock  4  is twisted against the urging force of the torsion spring  6  to the position in agreement with the upper fitting part  21  of the housing  2  as seen from above, and, thereafter, the upper lock  4  is inserted together with the upper fitting part  21  into the fitting hole Fa in the bottom corner fitting F of the container Ct. 
     Once the upper lock  4  enters the fitting hole Fa in the bottom corner fitting F, the spindle  3  is urged back to the original position by the torsion spring  6 , thus bringing the upper lock  4  into engagement with the fitting hole Fa in the bottom corner fitting F. With the upper lock  4  engaging with the fitting hole Fa, the housing  2  descends relative to the upper lock  4  (i.e. the spindle  3 ). Thus, the twist lock  1  hangs down by itself even if it is no longer supported by hand. From another point of view, the spindle  3  rises relative to the housing  2 , whereby the base ends of the locking members  53  are relieved from the contact with the outer circumferential surface of the spindle  3  which stops the projection of the locking members  53 . Consequently, the locking members  53  project into the guide hole  51  by the urging force of the springs  54 , whereas the extreme ends thereof retract into the passages  52  in the lower lock  5  (FIG.  3 ). In the meantime, the projection of the locking members  53  is terminated when the flanges  531  contact the step portions on the base end side defined by the large-diameter parts and the small-diameter parts of the passages  52 . After the twist lock  1  is attached to the fitting hole Fa in every bottom corner fitting F, the container Ct is transferred onto another container Ct. As mentioned above, while the twist lock  1  hangs from the bottom corner fitting F, the extreme ends of the locking members  53  retract into the passages  52  in the lower lock  5  which is positioned in agreement with and securely fixed to the lower fitting part  22  of the housing  2 , as seen from above. Therefore, the lower lock  5  and the lower fitting part  22  can be accepted into the fitting hole Fa in the top corner fitting F of a lower container Ct. While the container Ct is being rested on another container, the upper lock  4  engaged with the fitting hole Fa (i.e. the spindle  3 ) comes down because of its own weight. At this stage, the bottom end of the spindle  3  comes in contact with the base ends of the locking members  53  which project into the guide hole  51 , and outwardly forces the locking members  53  against the urging force of the springs  54 , until the extreme ends thereof project from the surface of the lower lock  5 . The locking members  53  thus engage with the fitting hole Fa in the top corner fitting F of the lower container Ct and prevent the release of the lower lock  5 . In the end, the upper and lower containers Ct are connected by means of the twist locks  1 , wherein the upper lock  4  is engaged with the fitting hole Fa in every bottom corner fitting F of the upper container Ct and the locking members  53  in the lower lock  5  are engaged with the fitting hole Fa in every top corner fitting F of the lower container Ct. For the completion of the loading, the upper container Ct is lowered until the bottom corner fittings F are rested on the housing  2 . 
     The thus loaded containers Ct remain in the connected state, where the upper lock  4  is engaged with the fitting hole Fa in the bottom corner fitting F of the upper container Ct and the locking members  53  of the lower lock  5  are engaged with the fitting hole Fa in the top corner fitting F of the lower container Ct. In the unloading process, disconnection of the two containers is effected simply by lifting the upper container Ct in the connected state. When the upper container Ct is lifted up, the spindle  3  is raised together with the upper lock  4 . Accordingly, the locking members  53  lose contact with the spindle  3  and retract their extreme ends into the passages  52  as urged by the springs  54 . The lower lock  5  can be thus released from the fitting hole Fa in the top corner fitting F of the lower container Ct. 
     The container Ct is then transferred and suspended approximately one meter above the ground, where the upper lock  4  of the twist lock  1  is removed from the fitting hole Fa in every bottom corner fitting F of the container Ct. Release of the upper lock  4  is effected by manually pulling the stop handle S connected to the wire  7 . With checking the stop handle S at the stop  23  in the housing  2 , the upper lock  4  is rotated against the urging force of the torsion spring  6  to the position corresponding to the upper fitting part  21  of the housing  2 , as seen from above. Thus, the twist lock  1  is made releasable from the fitting hole Fa in the bottom corner fitting F of the container Ct. 
     In some cases, the twist lock  1  may not be removed from the fitting hole Fa in the top corner fitting F of the lower container Ct, because of breakage or other unexpected accidents. In such emergency, the stop handle S connected to the wire  7  is manually pulled and checked at the stop  23  in the housing  2 , so that the upper lock  4  rotates against the urging force of the torsion spring  6  and positionally agrees with the upper fitting part  21  of the housing  2 , as seen from above. Finally, the upper container Ct can be lifted up, leaving the twist lock  1  on the lower container Ct. As the upper half of the twist lock  1  is exposed after the removal of the upper container Ct, it can be later removed from the fitting hole Fa in the top corner fitting F of the container Ct. 
     As described above, the first embodiment of the twist lock  1  works sufficiently in connecting two containers stored in a container yard Yd or the like, where no lift-up force acts on the twist lock  1 . However, when this twist lock  1  is employed to connect upper and lower containers Ct loaded in a containership Sh, the influence of a lift-up force should be taken into consideration. As the containership Sh pitches or rolls owing to high waves in the sea, the containers Ct may jump up, imposing a lift-up force on the upper lock  4  engaged with the fitting hole Fa in the bottom corner fitting F of the upper container Ct. Such lift-up force pulls up the spindle  3 , and may eventually cause disengagement of the locking members  53  in the lower lock  5  from the fitting hole Fa in the top corner fitting F of the lower container Ct. 
     FIG. 4 shows a twist lock  1  according to the second embodiment, which endures a lift-up force acting on the upper lock  4  and retains the connection between the upper and lower containers Ct, provided the inclination of the containers Ct exceeds a predetermined degree. 
     In the following description of the twist lock  1  according to the second embodiment, it should be understood that the description is focused on the difference from the first embodiment, and that the same constitutive elements are indicated by the same reference signs without giving any details. 
     The twist lock  1  of the second embodiment includes a safety unit  8  within the housing  2 . The safety unit  8  comprises a pair of connecting bores  27  each being formed in a predetermined length and having an open end which faces the cavity  26  in the housing  2 , and at least one ball  81  rotatably accommodated in each of the connecting bores  27 . The connecting bores  27  are, as seen from above, formed in the direction extending from a diameter of the through-hole  25  and positionally arranged at a predetermined angle (e.g. 45 degrees) in the forward/backward direction. Further, the connecting bores  27  have a downward gradient of a predetermined degree (e.g. 5 degrees), descending from the open ends facing the cavity  26  to the terminal ends. 
     Additionally, a fitting groove  32   b  is formed on the outer circumferential surface of the large-diameter spindle part  32  of the spindle  3 , at a position that does not interfere with the groove  32   a.  The fitting groove  32   b  faces the open ends of the connecting bores  27  which open to the cavity  26 , when the spindle  3  stays at the lowered position on the bottom step defined by the through-hole  25  and the cavity  26 . The configuration of the fitting groove  32   b  is designed in correspondence with the diameter of the ball  81 . 
     When the containers Ct are level, the balls  81  roll down along the gradient of the connecting bores  27  to the terminal ends. The length of the connecting bores  27  is arranged such that the balls  81  at the terminal ends stay away from the fitting groove  32   b  of the spindle  3 . Likewise, if a plurality of balls  81  are to be accommodated in each connecting bore  27 , the inner balls  81  should stay away from the fitting groove  32   b  and be entirely accommodated in the connecting bores  27 , when the outer balls  81  reach the terminal ends of the connecting bores  27 . 
     Now, the operation of the twist lock  1  of the second embodiment is explained. 
     Referring to FIG. 4 which shows the initial state of the twist lock  1 , the upper lock  4  is urged by the torsion spring  6  to be engaged with the fitting hole Fa in the corner fitting F of the container Ct. The balls  81  have rolled down along the gradient of the connecting bores  27  to the terminal ends. As the balls  81  stay away from the fitting groove  32   b,  the spindle  3  can move upwards. 
     The twist lock  1  in the initial state is attached to the fitting hole Fa in the corner fitting F of the container Ct in the same manner as in the first embodiment. The container Ct is then transferred onto another container Ct in such a manner that the lower locks  5  can enter the fitting hole Fa of the top corner fittings F of the lower container Ct. The upper and lower containers Ct are eventually connected by the upper lock  4  and the locking members  53  in the lower lock  5 , wherein the upper lock  4  is engaged with the fitting hole Fa in every bottom corner fitting F of the upper container Ct and the locking members  53  are engaged with the fitting hole Fa in every top corner fitting F of the lower container Ct. 
     Due to high waves or the like, the inclination of the connected containers Ct may exceed the gradient of the connecting bores  27 . In this case, the upward movement of the spindle  3  is blocked by at least one of the balls  81 , which rolls up along the connecting bore  27  to be half fitted into the fitting groove  32 b. If the containers Ct jump up to impose a lift-up force on the upper lock  4 , the spindle  3  cannot move upwards in the locked state. Meanwhile, the locking members  53  of the lower lock  5  are kept engaged with the fitting hole Fa in the top corner fitting F of the lower container Ct, preventing the release of the lower lock  5 . Consequently, the connection between the upper and lower containers Ct is maintained. 
     It should be noted that the connecting bores  27  are located at a predetermined angle (45 degrees in this embodiment) in the forward/backward direction of the twist lock  1 . Owing to this arrangement, the safety unit  8  can respond to the inclination in every direction, whether the containers Ct are shaken in the right/left or forward/backward direction or in a composite direction thereof. As far as the degree of such inclination exceeds the gradient of the connection bores  27  in the axial direction thereof, the safety unit  8  allows at least one of the balls  81  to fit into the fitting groove  32   b  without fail. Since the upward movement of the spindle  3  is blocked in this manner, the upper and lower containers Ct are kept connected. 
     Moreover, when the containers Ct stay level, the balls  81  roll down along the connecting bores  27  to the terminal ends and stay away from the fitting groove  32   b  of the spindle  3 . Therefore, in unloading the containers Ct, the upward movement of the spindle  3  is not hindered by an accidental operation of the safety unit  8 . 
     Thereafter, the container Ct can be unloaded simply by lifting the container Ct in the same manner as in the first embodiment. As described above, the balls  81  stay away from the fitting groove  32   b  to permit the upward movement of the spindle  3 . When the container Ct is lifted up, the upper lock  4  (i.e. the spindle  3 ) is raised by the bottom corner fitting F of the upper container Ct. On the arrival of the spindle  3  to the raised position, the locking members  53  retract into the passages  52  in the lower lock  5 . As a result, the lower lock  5  becomes removable from the fitting hole Fa in the top corner fitting F of the lower container Ct. 
     Then, the container Ct is transferred and suspended approximately one meter above the ground, where the upper lock  4  of the twist lock  1  is removed from the fitting hole Fa in the bottom corner fitting F of the container Ct. 
     As has been disclosed with reference to FIG. 4, the twist lock  1  of the second embodiment is provided with the safety unit  8  comprising a pair of connecting bores  27  formed within the housing  2 , and the balls  81  rotatably accommodated in the connecting bores  27 . When the inclination of the containers Ct exceeds a predetermined degree, at least one of the balls  81  fits into the fitting groove  32   b  formed in the spindle  3  to cope with the lift-up force imposed on the upper lock  4 . Alternatively, the safety unit  8  may situate in the spindle  3 , while the fitting groove is formed in the housing  2 . 
     As shown in FIG. 5, an alternative safety unit  8  is provided in the spindle  3 , and comprises a pair of connecting bores  321  formed in a diameter direction of the large-diameter spindle part  32  and a ball  81  rotatably accommodated in the connecting bores  321 . A fitting groove  26   a  is formed in the inner circumferential surface of the cavity  26  of the housing  2 , wherein the fitting groove  26   a  faces the open ends of the connecting bores  321  in the spindle  3  at the lowered position, and shaped in correspondence with the diameter of the ball  81 . When the upper lock  4  is engaged with the engaging hole Fa in the bottom corner fitting F of the container Ct, the connecting bores  321  are positioned at a predetermined angle (e.g. 45 degrees) in the forward/backward direction as seen from the top, with a downward gradient of a predetermined degree (e.g. 5 degrees) from both open ends to the centre. 
     This alternative embodiment operates in the same manner as the above embodiment. When the inclination of the containers Ct exceeds the gradient of the connecting bores  321 , the ball  81  accommodated therein rolls along either of the connecting bores  321  to fit into the fitting groove  26   a  in the housing  2 . By engaging with the fitting groove  26   a  as well as the connecting bore  321 , the ball  81  blocks the upward movement of the spindle  3 . 
     In the twist locks  1  of the second embodiments shown by FIGS. 4 and 5, as seen from the top, the connecting bores  27  run in the direction extending from a diameter of the through-hole  25 , while the connecting bores  321  run in a diameter direction thereof. Besides, the connecting bores  27 ,  321  are positioned at an angle of 45 degrees in the forward/backward direction. Apparently, the direction of the connecting bores  27 ,  321  should not be limited to a diameter direction of the through-hole  25  or the direction extending from a diameter thereof, wherein the open ends of the connecting bores  27 ,  321  being situated 180 degrees apart from each other. Moreover, the connecting bores  27 ,  321  are not necessarily positioned at an angle of 45 degrees in the forward/backward direction. In addition, the connecting bores  27 ,  321  may provided in one or plurality, and the gradient thereof may be other than 5 degrees. 
     In other words, as the connecting bores  27 ,  321 , at least one bore is formed respectively in the right component and the left component of the housing  2  and optionally angled in the forward/backward direction, assuming that the upper lock  4  engages with the fitting hole Fa in the top corner fitting F of the container Ct and also that the spindle  3  stays at the lowered position. However, if the connecting bores  27 ,  321  are formed in the right/left direction, which is perpendicular to the forward/backward direction, the safety unit  8  can operate against a lift-up force in the right/left direction deriving from the inclination of containers Ct, but may fail to operate against a lift-up force in the forward/backward direction. In this case, an additional pair of connecting bores  27 ,  321  should be formed in the forward/backward direction to compensate for the disadvantage. 
     In the twist locks  1  according to the second embodiment and its alternative embodiment, the ball  81  fits into the fitting groove  32   b  and locks the spindle  3  against the lift-up force imposed thereon, when the inclination of the containers Ct exceeds the gradient of the connecting bores  27 ,  321 . However, if the inclination of the containers Ct is less than the gradient of the connecting bores  27 ,  321 , the spindle  3  may yield to the lift-up force and come off. 
     FIGS. 6 and 7 illustrate a twist lock  1  according to the third embodiment, which endures a lift-up force acting on the upper lock  4  and retains the connection between the upper and lower containers Ct, even when the inclination of the containers Ct does not exceed the gradient of the connecting bores  27 . 
     In the following description of the twist lock  1  according to the third embodiment, it should be understood that the description is focused on the difference from the preceding embodiments, and that the same constitutive elements are indicated by the same reference signs without giving any details. 
     The twist lock  1  of the third embodiment further includes an operation control unit  9  formed in the housing  2  for controlling the displacement of the balls  81  in the safety unit  8 . The operation control unit  9  comprises a pair of fitting bores  28  vertically formed on the right and left sides of the upper fitting part  21  and positioned 180 degrees apart from each other along a concentric circle of the through-hole  25 , with the bottom ends of the fitting bores  28  communicating with the connecting bores  27 , guides  91  fitted in the fitting bores  28  in a vertically slidable manner, spring means (e.g. springs  92 ) disposed between the guides  91  and the fitting bores  28 , stoppers  93  provided at the top ends of the fitting bores  28  for preventing the detachment of the guides  91 , and sleeves  94  for housing the springs  92 . Each guide  91  has a conical bottom surface and is slidably inserted in the fitting bores  28 . The guide  91  comprises a guide body  911 , a holding bore  911   a  formed therein, and a pin  912  upwardly projecting from the top surface of the guide body  911 . The guide  91  is capable of sliding vertically along the fitting bore  28 , with accommodating the sleeve  94  in the holding bore  911   a  of the guide body  911 . The pin  912  is slidably fitted in an insertion hole  93   a  formed in the stopper  93 , and usually projects beyond the top surface of the housing  2  by the urging force of the spring  92 . 
     As shown in FIG. 8 in detail, O-rings  95 ,  96  are arranged around the outer circumferential surface of the guide body  911  of the guide  91  and the inner circumferential surface of the insertion hole  93   a  of the stopper  93 , respectively. The stopper  93  is formed with a choke  93   b  running in the vertical direction. This structure provides an air damper effect to the guide  91 . Specifically speaking, when the guide  91  slides upwards as urged by the spring  92 , the air in a space enclosed by the stopper  93 , the fitting bore  28  and the guide  91  is compressed through the choke  93   b.  Accordingly, the guide  91  functions as an air damper and rises slowly. As will be described later, in the twist lock  1 , the guides  91  are pressed down to the lowered position by the bottom corner fitting F of the container Ct which rests on the housing  2 . The containers Ct in the resting state may suddenly jump or shake, whereas the guides  91  rise slowly and do not follow the sudden jump of the containers Ct. 
     In the safety unit  8  of the third embodiment, each connecting bore  27  accommodates a plurality of balls  81 . In FIGS. 7 and 9, the guides  91  are raised by the urging force of the springs  92  and contact the stoppers  93 . In this case, the outer balls  81  roll down along the connecting bores  27  and stop on contact with the sleeves  94 , whereby the inner balls  81  roll out of the fitting groove  32   b  to be accommodated in the connecting bores  27 . In FIG. 10, on the other hand, the guides  91  are pressed down against the urging force of the springs  92 . In this case, the outer balls  81  are pushed up along the connecting bores  27  and contact the outer circumferential surfaces of the guides  91 , whereby the inner balls  81  fit in the fitting groove  32   b.  Since the inner balls  81  are engaged with the connecting bores  27  as well as the fitting groove  32   b,  the spindle  3  cannot move upwards. The length of the connecting bores  27  is decided to achieve the above results. 
     In this twist lock  1 , the height of the upper fitting part  21  and the degree of the projection of the pins  912  which project from the top surface of the housing  2  depend on the thickness of the corner fitting F of the container Ct. Such dimensions should be determined to effect the following operations. In the first place, when the spindle  3  stays at the lowered position, the projections of the pins  912 , which project from the top surface of the housing  2  by the urging force of the springs  92 , should not interfere with the insertion and engagement of the upper lock  4 , so that the upper lock  4  can be inserted into the fitting hole Fa in the corner fitting F of the container Ct and engaged therewith as urged by the torsion spring  6 . Secondly, when the pins  912  are pressed against the urging force of the springs  92  by the bottom fitting part F of the upper container Ct which rests on the housing  2 , the gap between the bottom surface of the upper lock  4  and the opposite inner surface of the bottom corner fitting F is greater than the projections of the pins  912 . 
     Now, the operation of the twist lock  1  of the third embodiment is explained. 
     FIGS. 6 and 7 illustrate the initial state of the twist lock  1 . The upper lock  4  is urged by the torsion spring  6  to a position engaging with the fitting hole Fa of the corner fitting F of the container Ct. The guides  91  are raised to contact the stoppers  93  by the urging force of the springs  92 , with the pins  912  projecting from the top surface of the housing  2 . The balls  81  in the safety unit  8  have rolled down along the connecting bores  27 , whereby the inner balls  81  roll out of the fitting groove  32   b  to be accommodated in the connecting bores  27 . As the inner balls  81  do not interfere with the spindle  3 , the spindle  3  is upwardly movable in the initial state. This twist lock  1  can be attached to the fitting hole Fa in the corner fitting F of the container Ct in the same manner as the above embodiments. Firstly, the upper lock  4  is twisted to the position corresponding to that of the upper fitting part  21  of the housing  2  as seen from above, and then the upper lock  4  and the upper fitting part  21  are inserted together into the fitting hole Fa in the bottom corner fitting F of the container Ct (FIG.  7 ). 
     Once the upper lock  4  enters the fitting hole Fa, the spindle  3  is urged back to the original position by the torsion spring  6 , effecting the engagement of the upper lock  4  with the fitting hole Fa in the bottom corner fitting F. The twist lock  1  now hangs down by itself if it is no longer supported by hand, in which case the housing  2  descends relative to the upper lock  4  (i.e. the spindle  3 ) (FIG.  9 ). Prior to this relative displacement, the outer circumferential surface of the spindle  3  contacts the base ends of the locking members  53  and blocks their projection. As a result of the relative displacement, the locking members  53  project into the guide hole  51  as urged by the springs  54 , with retracting the extreme ends into the passages  52  in the lower lock  5 . 
     After the twist locks  1  are attached to the fitting hole Fa of the bottom corner fittings F of the container Ct, the container Ct is lifted and transferred onto another container Ct. As mentioned above, the extreme ends of the locking members  53  retract into the passages  52  in the lower lock  5  which is in positional agreement with and fixed to the lower fitting part  22  of the housing  2 , as seen from above. Therefore, the lower lock  5  and the lower fitting part  22  can be accepted into the fitting hole Fa in the top corner fitting F of the lower container Ct. 
     As the upper container Ct descends further, the upper lock  4  (i.e. the spindle  3 ) yields to its own weight and reaches the lowered position, at which position the bottom end of the spindle  3  comes in contact with the base ends of the locking members  53  projecting into the guide hole  51 . Thus, the locking members  53  are pushed outwardly against the urging force of the springs  54 , until the extreme ends project from the surface of the lower lock  5 . While the container Ct is lowered still further, the bottom corner fitting F of the upper container Ct presses down the pins  912  together with the guides  91  against the urging force of the springs  92  and finally rests on the housing  2 . In association with the descent of the guides  91 , the bottom ends of the guide bodies  911  push the outer balls  81  into the connecting bores  27 , and the outer balls  81  push the inner balls  81  into the fitting groove  32   b.  Each of the inner balls  81  is half fitted in the fitting groove  32   b  of the spindle  3  and half fitted in the connecting bore  27 , thereby blocking the upward movement of the spindle  3  (FIG.  10 ). In this state, the locking members  53  are engaged with the fitting hole Fa in the top corner fitting F of the lower container Ct to prevent the release of the lower lock  5 . As a result, the upper and lower containers Ct are connected by the upper lock  4  and the locking members  53  of the lower lock  5 , while the upward movement of the spindle  3  is blocked, wherein the upper lock  4  is engaged with the fitting hole Fa in every bottom corner fitting F of the upper container Ct and the locking members  53  are engaged with the fitting hole Fa in every top corner fitting of the lower container Ct. 
     Incidentally, the containers Ct connected by the twist locks  1  may be shaken due to high waves in the sea or the like. When the inclination of the containers Ct exceeds the gradient of the connecting bores  27  in the axial direction, the safety unit  8  operates as described in the second embodiments. That is, one of the balls  81  fits in the fitting groove  32   b  to block the upward movement of the spindle  3 . Therefore, the locking members  53  of the lower lock  5  remain engaged with the fitting hole Fa in the top corner fitting F of the lower container Ct, thereby maintaining the connection between the upper and lower containers Ct. 
     On the other hand, the inclination of the containers Ct may not exceed the gradient of the connecting bores  27 . In this case, the guides  91  do not follow the sudden jump or shake of the containers Ct, and instead rise slowly according to the air damper effect based on the combination of the guides  91 , the stoppers  93  and the fitting bores  28 . Thus, irrespective of the sudden jump or shake of the containers Ct, the balls  81  remain fitted in the fitting groove  32   b  of the spindle  3  for a certain period of time, with blocking the upward movement of the spindle  3 . As a result, the two containers Ct are kept connected. 
     It should be understood that the container Ct jumps up only for a moment. This means that the container Ct comes down to press the pins  912  of the guides  91  again, before the slowly rising guides  91  contact the stoppers  93 , or, in other words, before the inner balls  81  roll out of the fitting groove  32   b  of the spindle  3 . Therefore, the inner balls  81  remain in the fitting groove  32   b.    
     Thereafter, unloading is performed by lifting the container Ct. First, the bottom corner fitting F of the upper container Ct leaves the top surface of the housing  2 . At the same time, the guides  91  rise slowly as urged by the springs  92  and come in contact with the stoppers  93 . Upon this contact, the outer balls  81  roll down along the connecting bores  27  and contact the sleeves  94 , while the inner balls  81  roll out of the fitting groove  32   b,  thereby permitting free upward movement of the spindle  3 . In this embodiment, the container Ct is to be lifted at a rate corresponding to the controlled rise of the guides  91 . As the upper container Ct is further lifted up, the upper lock  4  is raised by the bottom corner fitting F of the upper container Ct to pull up the spindle  3  to the raised position. At this stage, the locking members  53  lose contact with the outer circumferential surface of the spindle  3 , and retract the extreme ends into the passages  52  in the lower lock  5  as urged by the springs  54 . Consequently, the locking members  53  of the lower lock  5  are disengaged from the fitting hole Fa in the top corner fitting F of the lower container Ct. By lifting the upper container Ct, the lower lock  5  comes out of the fitting hole Fa in the top corner fitting F of the lower container Ct. 
     Then, the container Ct is transferred and suspended at a height of approximately one meter above the ground, where the upper lock  4  of the twist lock  1  is removed from the fitting hole Fa in every bottom corner fitting F of the container Ct. 
     It should be borne in mind that the mode of the third embodiment should not be limited to the above disclosure. Apparently, the direction of the connecting bores  27  is not limited to the direction extending from a diameter of the through-hole  25 , wherein the open ends of the connecting bores  27  being situated 180 degrees apart from each other. Moreover, the connecting bores  27  are not necessarily positioned at 45 degrees in the forward/backward direction. In addition, the connecting bores  27  may be provided in one or plurality, and the gradient thereof may be other than 5 degrees.