Patent ID: 8302438

Claim:
A driving device for an electric lock latch, comprising: a) a housing; b) a motor positioned within the housing and having a power output shaft connected to a drive shaft with an external thread on a part thereof, a diameter (d 2 ) of a root of the external thread being larger than a diameter (d 1 ) of the drive shaft; c) an unequal diameter coil spring having a cylindrical spiral in a middle section thereof, the unequal diameter coil spring corresponding in a spiral direction with the external thread, an axial inner side of the cylindrical spiral outwardly enlarged and forming a first conical spiral, and an opposite side of the cylindrical spiral outwardly enlarged and forming a second conical spiral, such that the unequal diameter coil spring being a shape with small section in the middle and the enlarged conical spiral on both ends, an inner diameter (D 1 ) of the cylindrical spiral being larger than a diameter (d 2 ) of the root of the external thread and being smaller than an outer diameter (d 3 ) of the crest of the external thread; the first conical spiral mounted on an external peripheral of the drive shaft, and fixed in the housing or at the motor; when the unequal diameter coil spring is in a free length, there is a screwed relation between a least a part of the cylindrical spiral and the external thread of the drive shaft, and there is not a screwed relation between both the first conical spiral and the second conical spiral and the external thread of the drive shaft, the unequal diameter coil spring positioned within the housing for telescoping but not being rotated by the drive shaft; and d) a lock latch secured to an outer end of the second conical spiral and telescopically moved with the second conical spiral; whereby when the motor drives the drive shaft into a forward rotation or reverse rotation, the external thread of the drive shaft passes the rotation power to the cylindrical spiral and then a rotary motion of the drive shaft is converted into a linear motion of the unequal diameter coil spring for changing an elongation or compression of the unequal diameter coil spring, such that the lock latch is moved by the second conical spiral; when the unequal diameter coil spring is stretched or compressed to a predetermined position, the external thread is idling without driving the unequal diameter coil spring even if the drive shaft is still rotating; when the drive shaft stops rotating and the unequal diameter coil spring is compressed, the cylindrical spiral is fixed by an outer side thereof being against an inner side of the external thread, compressing the lock latch for being in a compression state; when the drive shaft stops rotating and the unequal diameter coil spring is stretched, the cylindrical spiral is fixed by an inner side thereof being against an outer side of the external thread, forming an axial supporting force for the lock latch being in an elongation state.