Traditional alarm clocks use an oscillating wheel with two separate springs attached to the gear train; one spring drives the time-counting mechanism, while a second spring activates the alarm after a certain number of ticks. The user primes the second spring and thereby sets the alarm by using winding knobs on the back of the clock.
The alarm segment of a traditional alarm clock has its own gear train and escapement, according to How Stuff Works. This allows the clock's alarm segment to operate independently of the timekeeping mechanism. A small alarm clock features about 12 different moving parts, most of which are gears. Four gears work in unison to operate the hour hand, minute hand and alarm hand, while another two gears operate the alarm hammer. One of the alarm hammer's gears also functions as an escapement wheel for the alarm mechanism.
Alarm clocks are similar to table clocks and wrist watches in that they use an oscillating wheel rather than a pendulum to keep time. The gears and springs inside the clock provide force that keeps the wheel oscillating in time. Alarm clock builders time the action of these gears and springs to make the wheel oscillate in a predictable, accurate manner.