A common U-shaped lock, shown in FIG. 6, includes a U-shaped shackle 1 with an annular groove 11 formed in each of the shackle's ends. Each of the grooves 11 is locked by a dead bolt of a locking unit disposed in a housing 2. The locking principle of the locking unit of this conventional lock is shown in FIG. 7, wherein a key is inserted in a lock block 3 to rotate the block 3 and an actuating block 31. The actuating block 31 pushes a right and a left pusher 321,331 of a right and a left slide block 32, 33, forcing two dead bolts 322,332 to move inward and thereby free the U-shaped shackle 1 from the housing 2. Then in locking it, the two ends of the U-shaped shackle i are moved into the housing 2, and the key is pulled out of the locking block 3. Removal of the key allow the springs 34, 35 to push the right and left slide block 32, 33 to the respective right and left sides, forcing the dead bolts 322,332 to engage the two grooves 11 of the shackle 1. In this position, the shackle 1 is locked. If this lock is needed to be unlocked, the key is again used to rotate the locking block 3 for unlocking.
Nevertheless, this conventional lock has undesirable disadvantages as follows:
1. Though the U-shaped shackle is easy to make, the housing is made by a molding process, by means of molds of rather high cost.
2. The structure of the locking unit is rather complicated, inconvenient for assemblage, which results in a high cost.
3. Two springs are used for locking, but they are subject to a loss of elasticity by incessant compression in locking, and in addition, incorrect assemblage of the springs can cause a malfunction of the lock.