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: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to blinds and, more specifically, to a lift lock used in a blind to lock the lift rod. 
     2. Description of the Related Art 
     A conventional Venetian blinds uses a lift cord to control the extending status, and bladder tapes to share the weight of the blind slats. When receiving the blind, the lift cord starts to bear the weight of the blind slats. When the blind slats received in a stack between the headrail and the bottom rail, the lift cords bears the total weight of the blind slats. When lifting or lowering the blind slats, the user needs only to pull the suspending part of the lift cord outside the headrail. However, because the suspended part of the lift cord is exposed to the outside of the headrail and easily accessible by a child, the suspending part of the lift cord may be hung on a child&#39;s head accidentally. In order to eliminate this problem, blinds with hidden lift cord are disclosed. A blind with hidden lift cord comprises a lift rod fastened pivotally with the inside of the headrail, and a spring mechanism mounted inside the headrail and coupled to the lift rod. The lift rod can be rotated to roll up or let off the lift cord, so as to further lift or lower the bottom rail of the blind. The spring power of the spring mechanism bears the weight of the bottom rail as well as the blind slats and is maintained in balance with the torque of the lift rod, enabling the blind to be positioned in the desired extending position. During operation, the user needs only to impart an upward or downward pressure to break the balance, i.e., when the user lowering or lifting the bottom rail and then releasing the hand from the bottom rail, the reversing force of the spring mechanism balances the torque of the lift rod, thereby keeping the blind in position. In order to keep the spring force of the spring mechanism in balance with the torque of the lift rod at different elevations, a variable adjusting means is provided in the spring mechanism to automatically regulate the reversing force of the spring mechanism subject to the elevation of the blind. Alternatively, the bottom rail may be made relatively heavier and the blind slats relatively lighter to control the variation of load within 15%. However, the aforesaid conventional designs cannot accurately lock the blind in position. When the spring power of the spring mechanism designed to be excessively high, the blind tends to be lifted slightly after pulled to the desired elevation, and cannot be set in the fully extended position. When the spring power of the spring mechanism designed to be insufficient or when the spring mechanism started to wear, the blind tends to be lowered slightly after pulled to the desired elevation, and cannot be fully received in the upper limit position. 
     Therefore, it is desirable to provide a lift control for blind that eliminates the aforesaid drawbacks. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a lift lock for blind, which is made in the form of an independent module. 
     It is another object of the present invention to provide a lift lock for blind, which achieves accurate positioning of the lift rod of the blind. 
     It is still another object of the present invention to provide a lift lock for blind, which is inexpensive to manufacture and easy to install. 
     It is still another object of the present invention to provide a lift lock for blind, which has a compact and simple structure that requires less installation space. 
     To achieve these objects of the present invention, the lift lock for locking position of a lift rod of a blind when a rotary driving force applied to the lift rod is dropped below a predetermined value comprises a shaft, a guide wheel, balls, a casing, a spring member and a retainer. The shaft is adapted to receive the lift rod of the blind. The balls are respectively received in respective sliding grooves around one side of the guide wheel which is sleeved onto the shaft. The retainer is coupled to the shaft. The spring member is sleeved onto the shaft for forcing an engagement portion of the shaft into engagement with a positioning opening of the casing. When the lift rod rotated by an external rotary driving force that surpasses the friction resistance between the shaft and the casing, the shaft is rotated with the lift rod. When the external rotary driving force dropped below the friction resistance between the shaft and the casing, the shaft is stopped to hold down the lift rod in position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the installation of a lift lock in a blind according to the present invention. 
     FIG. 2 is an exploded view of the lift lock according to the present invention. 
     FIG. 3 is a right side view of the lift lock according to the present invention. 
     FIG. 4 is a sectional view of the lift lock according to the present invention, showing one positioning status of the balls in the sliding grooves of the guide wheel. 
     FIG. 5 is similar to FIG. 4 but showing another positioning status of the balls in the sliding grooves of the guide wheel. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIGS. 1 and 2, a lift lock  1  is installed in a blind A and coupled to the rectangular lift rod A 1  of the blind A, and adapted to lock the lift rod A 1 . The lift lock  1  is comprised of a shaft  10 , a guide wheel  20 , balls  30 , a casing  40 , a cushion  50 , a spring member  60 , a retainer  70 , and a cap  80 . 
     The shaft  10  has a first end  104  of relatively bigger outer diameter, a second end  105  of relatively smaller outer diameter, a rectangular center through hole  101  axially extended through the center of the first end  104  and the second end  105 , a cone-like engagement portion  102  at the first end  104  around the rectangular center through hole  101 , and a locating portion, for example, an annular locating groove  103  extended around the periphery of the second end  105 . 
     The guide wheel  20  is an annular member sleeved onto the second end  105  of the shaft  10  adjacent the cone-like engagement portion  102 , having symmetrical pairs, for example, three pairs of sliding grooves  201  adapted to receive the balls  30 . Each pair of sliding grooves  201  includes a first sliding groove  202  and a second sliding groove  203 . The first sliding grooves  202  and second sliding grooves  203  of the pairs of sliding grooves  201  are alternatively equiangularly spaced around the center of the guide wheel  20 . As illustrated in FIG. 3, each first sliding groove  202  has an arched shallow projecting end  202   a  and a flat deep receiving end  202   b . The arched shallow projecting ends  202   a  and flat deep receiving ends  202   b  of the first sliding grooves  202  are alternatively arranged in directly along the periphery of the guide wheel  20 . When the ball  30  stopped at the arched shallow projecting end  202   a  of the corresponding first sliding groove  202 , it protrudes over the periphery of the guide wheel  20  at a distance. On the contrary, when the ball  30  stopped at the flat deep receiving end  202   b  of the corresponding first sliding groove  202 , it is received inside the guide wheel  20 . The connection area between the arched shallow projecting end  202   a  and flat deep receiving end  202   b  of each first sliding groove  202  is made gradually deeper in direction from the arched shallow projecting end  202   a  toward the flat deep receiving end  202   b . Each second sliding groove  203  has an arched shallow projecting end  203   a  and a flat deep receiving end  203   b . The arched shallow projecting ends  203   a  and flat deep receiving ends  203   b  of the second sliding grooves  203  are alternatively arranged in directly along the periphery of the guide wheel  20  and reversed to the arched shallow projecting ends  202   a  and flat deep receiving ends  202   b  of the first sliding grooves  202 . The number of the balls  30  is equal to the number of the sliding grooves  201  of the guide wheel  20 , for enabling the balls  30  to be respectively received in the sliding grooves  201 . 
     The casing  40  comprises a hollow body  401  defining an axially extended stepped through hole formed of a positioning opening  403  and a first receiving chamber  402  in one end and a second receiving chamber  404  and a third receiving chamber  405  in the other end. The hollow body  401  is fixedly fastened to the blind A to receive the shaft  10  and the guide wheel  20 . The guide wheel  20  is received in the second first receiving chamber  402 , keeping the sliding grooves  201  facing the cone-like engagement portion  102  of the shaft  10 . The shaft  10  is inserted through the hollow body  401  of the casing  40 , keeping the cone-like engagement portion  102  received in the positioning opening  403 . The positioning opening  403  is a tapered opening fitting the cone-like engagement portion  102 . 
     The cushion  50  is sleeved onto the shaft  10  and firmly received in the second receiving chamber  404  of the hollow body  401  of the casing  40 . The spring member  60  is sleeved onto the shaft  10  and supported on the cushion  50 . The retainer  70  is a C-shaped retaining ring fastened to the annular locating groove  103  of the shaft  10 . The cap  80  is a hollow cap axially slidably mounted on the shaft  10  in the third receiving chamber  405  of the casing  40  and supported between the spring member  60  and the retainer  70 . The spring member  60  imparts an outward pressure to the retainer  70  (because the cushion  50  is firmly stopped in the second receiving chamber  404  of the casing  40 ), thereby causing the cone-like engagement portion  102  to be closely received in the positioning opening  403  of the casing  40 . 
     The operation of the lift lock is outlined hereinafter with reference to FIGS. 4 and 5 and FIGS.  1 ˜ 3  again. When the tilt rod A 1  biased counter-clockwise by an external rotary force that surpasses the friction resistance between the shaft  10  and the casing  40 , the shaft  10  is rotated with the tilt rod A 1 . At this time, the balls  30  in the first sliding grooves  202 , namely, the first balls  301  are respectively forced to rotate in direction from the respective flat deep receiving ends  202   b  toward the respective arched shallow projecting ends  202   a , and the balls  30  in the second sliding grooves  203 , namely, the second balls  302  are respectively forced to rotate in direction from the respective arched shallow projecting ends  203   a  toward the respective flat deep receiving ends  203   b . During movement of the first balls  301  and the second balls  302 , the guide wheel  20  is forced to bias slightly. However, because the speed and angle of rotation of the shaft  10  are greater than the guide wheel  20 , the second balls  302  are maintained in the respective flat deep receiving ends  203   b , and the first balls  301  are moved to the respective arched shallow projecting ends  202   a . When the first balls  301  moved to the respective arched shallow projecting ends  202   a , they protrude over the periphery of the guide wheel  20  and are stopped against the cone-like engagement portion  102  of the shaft  10  to force the cone-like engagement portion  102  away from the positioning opening  403  of the casing  40  (see FIG.  4 ), enabling the shaft  10  to be synchronously rotated with the lift rod A 1 . 
     When the input counter-clockwise torque of the lift rod A 1  became smaller than the friction resistance between the shaft  10  and the casing  40 , the shaft  10  is forced by the spring force of the spring member  60  to move axially relative to the casing  40 , thereby causing the cone-like engagement portion  102  to be fitted into the positioning opening  403 . At the same time, the first balls  301  are forced by the periphery of the cone-like engagement portion  102  to move to the flat deep receiving ends  202   b  of the respective first sliding grooves  202 . Therefore, the cone-like engagement portion  102  is maintained in close contact with the periphery of the positioning opening  403  of the casing  40  to stop the shaft  10  from rotation (see FIG.  5 ). 
     On the contrary, when the tilt rod A 1  biased clockwise by an external rotary force that surpasses the friction resistance between the shaft  10  and the casing  40 , the cone-like engagement portion  102  of the shaft  10  is forced outwards from the positioning portion  403  of the casing  10 . At this time, the first balls  301  are respectively forced by the rotating shaft  10  to the respective flat deep receiving ends  202   b , and the second balls  302  are respectively moved to the respective arched shallow projecting ends  203   a . When the second balls  302  moved to the respective arched shallow projecting ends  203   a , they protrude over the periphery of the guide wheel  20  and are stopped against the cone-like engagement portion  102  of the shaft  10  to force the cone-like engagement portion  102  of the shaft  10  away from the positioning opening  403  of the casing  40 , enabling the shaft  10  to be synchronously rotated with the lift rod A 1 . 
     When the input clockwise torque of the lift rod A 1  became smaller than the friction resistance between the shaft  10  and the casing  40 , the shaft  10  is forced by the spring force of the spring member  60  to move axially relative to the casing  40 , thereby causing the cone-like engagement portion  102  to be fitted into the positioning opening  403 . At the same time, the second balls  302  are forced by the periphery of the cone-like engagement portion  102  to move to the flat deep receiving ends  203   b  of the respective second sliding grooves  203 . Therefore, the cone-like engagement portion  102  is maintained in close contact with the periphery of the positioning opening  403  of the casing  40  to stop the shaft  10  from rotation. 
     In general, the invention provides the following advantages: 
     1. The friction design between the casing and the shaft and the arrangement of the symmetrical pairs of sliding grooves in the guide wheel and the balls in the sliding grooves enable the lift rod to be accurately locked in the desired angular position after each forward or backward adjustment. 
     2. The design of the three symmetrical pairs of sliding grooves causes the radial components of force produced from the balls to compensate one another, enabling the balls to accurately provide the desired axial push force. 
     3. The lift lock is an independent module that can be installed in any part of the lift rod. During installation, the shaft of the lift lock is sleeved onto the lift rod and moved along the lift rod to the desired location, and then the casing is fixedly fastened to the headrail of the blind. 
     4. Because the engagement portion of the shaft is shaped like a cone, the shaft produces a high friction resistance when engaged into the positioning opening of the casing and, the dimension of the whole assembly is minimized. 
     5. Because the casing has a second receiving chamber and a third receiving chamber for receiving the cushion, the spring member and the casing, the outer appearance of the lift lock looks in unity. 
     6. Because the whole structure of the lift lock is simple, the manufacturing cost of the lift lock is low.

Summary:
A lift lock for a blind is disclosed to include a casing holding a guide wheel, balls respectively received in respective sliding grooves around one side of the guide wheel, a shaft inserted through the casing and coupled to a lift rod of the blind, and a spring member sleeved onto the shaft for forcing an engagement portion of the shaft into engagement with a positioning opening of the casing. When the torque inputted into the lift rod surpassed the friction resistance between the casing and the shaft, the balls force the shaft to disengage the engagement portion from the positioning opening of the casing for enabling the lift rod to be freely rotated. When the input torque of the lift rod dropped, the spring member returns the shaft into engagement with the positioning opening of the casing to stop the lift rod in position.