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

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The present invention is a helical locking mechanism for doors. 
     BACKGROUND OF THE INVENTION 
     Helix-driven door mechanisms are widely used. Such mechanisms are used, for example, in vehicle doors, shielding doors, and civil doors. The helix-driven door mechanisms usually have problems on locking and unlocking of the door. At present, both home and abroad, helix-driven door mechanisms usually adopt various locks formed by brakes and clutches or the locks with electromagnetic, hydraulic and pneumatic driving modes for locking and unlocking. Most door locking mechanisms mentioned above have disadvantages of complicated mechanism and low reliability, and that their unlocking usually requires additional power sources. 
     SUMMARY OF THE INVENTION 
     The present invention is aimed to solve the defects mentioned above, to put forward a simple and reliable helical locking mechanism for doors, and to realize the locking and powerless self-unlocking of helix-driven door mechanism. 
     The present invention provides a powerless helical locking mechanism for door, comprised of a screw with variable lead angle, and a self-adaptive nut. 
     The screw is connected with a power source, and the self-adaptive nut is connected with the door. The screw slot is divided into three sections: a working section with the lead angle more than the friction angle, a locking section with the lead angle less than the friction angle, and a transition section therebetween. The power source can drive the screw to rotate bidirectionally. The self-adaptive nut comprises a connected shaft sleeve and pin shaft. The self-adaptive nut is assembled with the screw to form a screw kinematic pair. 
     The pin shaft in the self-adaptive nut is kept deep in the screw slot and realizes linear contact with the screw slot so that the pin shaft and a screw slot form a matched screw pair to realize power and motion transfer from the power source to the self-adaptive nut. 
     The inventive mechanism is powerless in that both the locking and unlocking of machine does not require an additional power source. 
     The inventive mechanism offers high reliability in that the locking section of the screw, with a lead angle of screw pair being less than the friction angle causes self-locking and thus lets the screw with variable lead angle lockup the self-adaptive nut; that is, securely lock the door. No unlocking problems are caused by vibration, etc. While the power source drives the clockwise (CW) and counter-clockwise (CCW) rotations of the screw with variable lead angle, it also drives the self-adaptive nut and door to move synchronously in parallel with the axis of the screw, with the self-adaptive nut entering and exiting the locking section of the screw to realize the locking and powerless self-unlocking of door. 
     The inventive door lock mechanism has less parts and a simple structure as compared to the prior art. The present invention is suitable for various helix-driven door locks. 
     Working Principles of the present invention are explained below. 
     When the power source closes the door, the screw with variable lead angle makes the clockwise (CW) rotation and drives the self-adaptive nut to move from a working section to a locking section of the screw. Once the self-adaptive nut enters the locking section of the screw, the closing of the door is realized, and then the automatic locking of the door is realized. 
     When the power source opens the door, the screw with variable lead angle makes the counter-clockwise (CCW) rotation and drives the self-adaptive nut to move from the locking section to the working section of the screw. Once the self-adaptive nut withdraws from the locking section of the screw, the automatic unlocking of door is realized, and then the opening of the door is realized. 
     When closing the door with hands, the difference from closing the door with power source is that the self-adaptive nut may drive the screw to rotate and let the self-adaptive nut enter the locking section of the screw to realize the automatic locking of the door and fulfill the closing of the door. 
     When opening the door with hands, with a device to let the screw make the counter-clockwise (CCW) rotation of a specific angle, the self-adaptive nut withdraws from the locking section of the screw and unlocking is realized. Then, the opening of the door is realized by the counter-clockwise (CCW) motion of the self-adaptive nut. A shift lever, a gear, a clutch unlocking device, and many other devices may be applied for this purpose. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a working principle drawing of the present invention. 
         FIG. 2  is a partial enlargement view of a typical section of the screw  1 . 
         FIG. 3  is the perspective cross-sectional view of a pin shaft of a self-adaptive nut at the working section of the screw. 
         FIG. 4  is the perspective cross-sectional view of the pin shaft of the self-adaptive nut at the locking section of the screw. 
         FIG. 5  is the working principle schematic diagram of a manual unlocking device. 
         FIG. 6  is the 3D illustration of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Identification of elements illustrated in  FIG. 1-6 :
       1 —screw with variable lead angle,     2 —nut,     3 —retainer ring,     4 —torsion spring,     5 —pin shaft,     6 —rolling bearing,     7 —spindle sleeve,     8 —bearing cap,     9 —nut sleeve,     10 —door,     11 —power source,     12 —pull-wire wheel,     13 —left shift lever,     14 —right shift lever,     15 —right connecting plate,     16 —pull-wire,     17 —torsion spring,     18 —middle strut,     19 —self-adaptive nut,     20 —screw slot   

     The invention provides a helical locking mechanism for a door. The locking mechanism comprises a screw  1  with a variable lead angle ( FIG. 2 ) and a self-adaptive nut  19 . The screw  1  is connected with a power source  11 . The power source  11  can drive the screw to rotate bi-directionally. 
     The self-adaptive nut  19  is connected with the door  10  so that the self-adaptive nut  19  and the door  10  move synchronously. 
     With reference to  FIG. 2 , the slot  20  of the screw  1  is divided into three sections: i) a working section C with the lead angle more than the friction angle, ii) a locking section A with the lead angle less than the friction angle, and iii) a transition section B located between the working section C and the locking section A. 
     The screw slot  20  has rectangle or trapezoid threaded end face. The screw slot  20  may have a single head or multiple heads. 
     With reference to  FIG. 1 , the self-adaptive nut  19  comprises a spindle sleeve  7 , a pin shaft  5 , a nut sleeve  9 , a nut  2 , a rolling bearing  6  with a bearing cap  8 , a retainer ring  3 , and a torsion spring  4 . 
     The nut  2  and the nut sleeve  9  have a circumference rotary connection, and have a rigid connection through the retainer ring  3  in an axis of the screw  1 . One end of the torsion spring  4  is connected with the nut sleeve  9 . The other end of the torsion spring  4  is connected with the nut  2 . 
     The pin shaft  5  and the spindle sleeve  7  are connected in rigid connection or rotary connection. When the pin shaft  5  and the spindle sleeve  7  are in rigid connection, a screw pair in sliding friction is form. When the pin shaft  5  and the spindle sleeve  7  are in rotary connection, a screw pair is in rolling friction is formed. 
     When the power source  11  closes the door, the screw  1  makes a clockwise (CW) rotation to drive the self-adaptive nut  19  to move from the working section C of the screw to the locking section A of the screw, until the self-adaptive nut  19  enters the locking section A and the door is locked. 
     When the power source  11  opens the door, the screw  1  makes a counter-clockwise (CCW) rotation to drive the self-adaptive nut  19  to leave the locking section A and move reversely to open the door. 
     When manually closing the door, the movement of self-adaptive nut  19  drives the screw  1  to make the clockwise (CW) rotation. This clockwise (CW) rotation lets the self-adaptive nut  19  enter the locking section A of the screw  1  to manually close the door and lock the door. 
     The manual opening mechanism of the door is shown in  FIG. 5 . 
     The right shift lever  14  is connected with the nut  2  of the self-adaptive nut  19  through the right connecting plate  15 . The left shift lever  13  is connected with the pull wire wheel  12 . The pull wire wheel  12  is idly set on the screw  1 . The pull wire  16  is connected with the pull wire wheel  12 . One end of the torsion spring  17  is connected with the pull wire  16 . The other end of the torsion spring  17  is connected with the middle strut  18 . 
     The pull wire  16  drives the pull wire wheel  12  and the left shift lever  13  to rotate. Through the right shift lever  14 , the right connecting plate  15  drives the nut  2  to rotate to thereby realize the rotation of the screw  1  to a specific angle. After the manual unlock is completed, the door may be opened by hands with the counter-clockwise (CCW) rotation of the self-adaptive nut  19 . After unlocking, under the torsion of the torsion spring  17 , the pull wire wheel  12  and the pull wire  16  reset to be ready for the next manual unlocking. 
       FIG. 2  is a partial enlarged view of a typical section of the screw slot  20 . Part A is the locking section, with the lead angle less than the friction angle. Part C is the working section, with the lead angle more than the friction angle. Part B is the transition section located between parts A and C. In part B the lead angle varies continuously. 
       FIG. 3  is an illustration of the pin shaft  5  of the self-adaptive nut  19  at the working section C of the screw  1 . The self-adaptive nut  19  and the screw  1  are assembled into a screw kinematic pair. The pin shaft  5  is deep in the screw slot  20  and is in linear contact with the screw slot  20 . The pin shaft  5  and the screw slot  20 , with any lead angles, can form the matched screw pair to transfer power and motion, to realize opening and closing of the door. 
       FIG. 4  is an illustration of the pin shaft  5  of the self-adaptive nut  19  at the locking section A of the screw  1 . The self-locking is caused by the lead angle of the screw pair being less than the friction angle. The screw slot  20  can lockup the pin shaft  5  so that the self-adaptive nut  19  is unable to move. This reliably locks the door.

Summary:
A powerless helical locking mechanism for a door includes a screw with a variable lead angle connected with a power source, and a self-adaptive nut connected to the door. The helical slot of the screw is divided into a working segment with the helical lead angle greater than the friction angle, a closing segment with the helical lead angle smaller than the friction angle, and a transition segment between the closing and working segments. The power source actuates the screw to rotate bidirectionally.