| 摘要 |
An encoder includes a casing having an open end and a base which fits in the open end. Trapezoidal cutouts in the casing are arranged to receive partly trapezoidal projections on the base. The casing contains an optical sensor, a circuit board for operation of the sensor, and a timing disc mounted on a rotary hub. The hub has a passage for a drive shaft of a motor, and the passage is in register with an opening in the base. During assembly, the drive shaft is passed through the opening in the base and the motor and casing are urged towards one another to push the base into the casing. When the base enters the casing, the projections are received in the cutouts and a guide surface in the casing engages a centering surface on the hub. Once the projections are in the cutouts, the casing and the base are rotated relative to each other to secure the base to the casing. Due to the trapezoidal shape of the cutouts and the partly trapezoidal shape of the projections, the casing moves in a direction away from the base during rotation. This causes the guide surface of the casing to disengage from the centering surface of the hub.
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