Have you been amazed at how easily you can raise and lower your car windows by just pressing a button? The mechanism inside your car window is so compact and smart that the window always moves straight and smooth. The power window has replaced the traditional manual window system. Also, you might have noticed that you’re not able to move the window even if you try to push it down or pull it up manually. In this article I will explain this tricky mechanism that controls your car window.
Before understanding the power window mechanism let me explain the basic mechanism of a car window. You can imagine how a car window goes up and down by just rotating the crank. Now, obviously, some mechanism in the door has to push or pull the windows. These motions can be accomplished by a simple crank and gear arrangement as shown in fig 1. Observe how the different parts of the mechanism are connected and imagine how the motion will be transferred from crank to window. When I rotate the crank, the parts of the window move and the end motion will either push or pull (open or close) the window.
As you can see in fig 1, the glass is supported at a single point at all times (marked in orange colour). Therefore, a small difference in friction could cause an imbalance, and the window may not move straight. I have to provide the horizontal support link to the glass at two points, it can solve this problem. This design suggestion can be easily understood from this weightlifter analogy as shown in fig 2a. Inspiring from this analogy, I have added two more linkages, as shown in fig 2b. One end of these newly introduced linkages are pivoted in the middle, and the other ends are connected to the sliders. The mechanism I just developed is known as a scissor-type mechanism.
The two supports which I gave to the glass makes the mechanism quite robust. Interestingly the scissor mechanism is getting used in many weight lifting equipment, folding gates and even in your computer keyboard. This cool mechanism was in the mainstream of car windows till the 1990s and some classic cars still use them. The way it works is absolutely perfect for the car windows movement. The mechanism’s top portion always moves perfectly straight.
This arrangement of car windows has one main drawback. It will make the window fall abruptly at the bottom. To avoid this issue, let me place a spiral spring on a pivot point as shown in fig 3a. This spring unwinds during the window’s upward movement and compresses for its downward movement. Now, the glass doesn’t fall suddenly, and this new arrangement also reduces the effort needed for the upward movement of the glass.
Another issue is that anyone can operate this mechanism even by pushing or pulling the glass from outside of the car. This means that theft is quite easy if car windows are equipped with this design. So how can we solve this issue? The worm gear arrangement can solve this issue effectively as shown in fig 3b. The worm can rotate the gear, but the gear can’t rotate the worm. This means only if I provide input motion to the worm, then the mechanism will work. The crank is connected to the worm. So the passenger can easily operate the mechanism. However, when somebody tries to move the glass, the input motion goes to the worm gear and everything gets stuck. Nobody can move the window glass up or down forcibly. Moreover, the worm and gear arrangement also works as a torque multiplier.
Simply by connecting a DC motor to the worm, I can convert the manual window to a power window as shown in fig 4a. The major concern with this linkage mechanism is that it has many sliding contacts. Frictional losses will be high due to this.
This design is improved by using a rope-and-pulley system. In this mechanism, two plastic sliders are attached (refer fig 4b). They are connected with the rope. These sliders support the glass. The way power and motion generated by the motor reach the rope is shown in fig 4b. For smooth operation, two vertical guides are used. This mechanism is smoother than the previously-seen linkage mechanism since it has very few metal-to-metal contacts.
Some of the rope-and-pulley mechanism’s known issues are; extension of rope and high maintenance cost.
That's all in this article. I hope you have learned how car window mechanisms work.
Thanks for reading!