I have been trying to research how the gears of the crank mechanism of a car window works, either manual or electric. I want to know why it doesn't fall down from gravity. Can anyone link me to an animation or images that explain why it remains stable?


3 Answers 3


Wormdrive or friction.


Worm drive is a non-reversible mechanism. This means that turning the input (worm) will turn the output (gear), but when you try to turn the gear, it won't move. This solution is most commonly encountered with powered windows, because worm drive inherently has very high ratio: turning the worm one full rotation will turn the gear only one spur. This is advantageous when you have a small electric motor which usually have high rpm but low torque. Worm gear serves here as both gearing and a lock.


As simple as that. The whole mechanism is merely "stiff" enough to stay where you put it. This solution is most commonly encountered with manually operated windows. There is no worm drive here because contrary to electric motor, human hand turning a crank provides low rpm but relatively high torque, so there is neither need nor room for such high ratio gears. In neglected old cars, when the window mechanism gets worn out and loose, it may reach a state where partially rolled down window will fall all the way after you let the crank go.

The "partially rolled down" is important because without worm gear, an additional feature is required: in the very top position an additional locking mechanism engages. So in that case the weight of the glass pane is supported by the lock. Without the lock it would be always possible to force the window down, as it's possible to do it when it's cracked open.

  • Yet a worm drive is reversible. How would the window roll down and then back up otherwise? Maybe you'd like to reword what you've said to say it more accurately? Jul 15, 2016 at 20:30
  • Thanks for your effort guys. While I assume that inside the window motor from the image above there is a worm drive, I would love to see proof that it actually is one somewhere on the internet. Better if its a manual rather than an electric window so that it can be clearly exposed.
    – paul
    Jul 17, 2016 at 14:28
  • auto.howstuffworks.com/power-window1.htm Confirmed. Thanks everyone. You revealed the truth and its really disappointing that worm gears are required to hold loads, given that they can never be 1:1. Also, Google sucks because you need to know the answer before you can look things up. For example, without typing 'worm gear' or 'worm drive', try to find the answer to my question on Google. If someone can do this, I'll be VERY impressed. PS. Don't link to this page you're reading right now ;)
    – paul
    Jul 17, 2016 at 14:32
  • @Pᴀᴜʟsᴛᴇʀ2 I meant reversible like in thermodynamic process. Eg combustion engine is not reversible because you can't turn the crank to produce gas, while some electric motors are reversible because they can work as generators as well. Perhaps there is a better English word for that, but I don't know it.
    – Agent_L
    Jul 20, 2016 at 9:16
  • Basically the worm can drive a gear either direction but the gear cannot drive the worm
    – Techlord
    May 18, 2020 at 22:13

Every window has what's called a window regulator (here's a representative image):

enter image description here

The window itself attaches to the window track. This keeps it steady and level while moving up and down. As you can see, there are gears at the bottom which connect to a motor (or a crank if a manual), which prevent the whole assembly from going down all at once. This is why it's called a regulator, as it regulates the amount of distance the window can travel. The motor has a limiting switch which only allows it to move so far in one direction.

Many newer vehicles use cables to provide the regulation of the window, but the function is pretty much the same. Here is a representative image for a cable type one:

enter image description here

Mind you, every manufacturer is going to do their own thing and they will all look slightly different, but the function is pretty much the same.

  • 6
    I think that one of the key mechanisms that stop gravity making the motor turn and then allowing the window to drop will be the use of a worm gear in the electric motor gearbox.
    – HandyHowie
    Jul 11, 2016 at 11:30
  • @HandyHowie Manual mechanisms use no worm gear, yet they don't fall down either. (ok, sometimes they do, when the car is old and the road is bumpy : )
    – Agent_L
    Jul 12, 2016 at 12:50
  • @Agent_L - I answered the OPs question and then some. Even what you are suggesting can be easily implied. The regulator regulates window travel ... if that doesn't cover it not falling to the bottom, I don't know what will. With that in mind, I'm not sure what you're really getting at? Jul 12, 2016 at 21:21
  • @Agent_L - Really, you should read my answer again as I explain about the gears, etc. No worries. It's just an answer. Jul 13, 2016 at 10:46
  • @Agent_L - I'm sorry this isn't intuitive to you. Maybe we can continue this discussion in The Pitstop, our chatroom. Jul 15, 2016 at 11:42

The worm drive in the motors or manual crank mechanism has a defined "input" and "output" shaft; if you try to move the "output" side of the drive, nothing happens. Check out the examples at https://en.wikipedia.org/wiki/Worm_drive, it should be intuitively clear why it cannot be otherwise.

  • This answer could be improved with a description of how the worm drive applies to the specific case.
    – Gusdor
    Jul 11, 2016 at 15:11
  • How so, Gusdor? I think it's pretty clear?
    – AnoE
    Jul 12, 2016 at 6:22
  • Thanks AnoE, and Gusdor. If I did not already know what a worm drive was and that it had about a 14:1 speed reduction to allow for torque and hold then I probably would have required more information as to where it is located in the above images as Gusdor says. As far as I can tell, we are all guessing that the window motor contains a worm drive. Is there a link that shows this somewhere on the internet? I mean, that proves that car window motors or crank handles use a worm drive system?
    – paul
    Jul 17, 2016 at 14:25

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