Let's start with the lifting mechanism. This cool device is the heart of a power-window system.
The window lift on most cars uses a really neat linkage to lift the window glass while keeping it level. A small electric motor is attached to a worm gear and several other spur gears to create a large gear reduction, giving it enough torque to lift the window.
An important feature of power windows is that they cannot be forced open -- the worm gear in the drive mechanism takes care of this. Many worm gears have a self-locking feature because of the angle of contact between the worm and the gear. The worm can spin the gear, but the gear cannot spin the worm -- friction between the teeth causes the gears to bind.
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Animation of window lifting mechanism at work,
with inset of motor and gear reduction
The linkage has a long arm, which attaches to a bar that holds the bottom of the window. The end of the arm can slide in a groove in the bar as the window rises. On the other end of the bar is a large plate that has gear teeth cut into it, and the motor turns a gear that engages these teeth.
The same linkage is often used on cars with manual windows, but instead of a motor turning the gear, the crank handle turns it. In the next section we'll learn about some of the neat features some power windows have, including the child lockout and automatic-up.