The second Video Lego under X-ray, about the small wind up motor #42073.


Wind up motor always fascinated me, how does it work and are made ? There are two type in lego. The first one is the wind up motor, which is wound by an input and give power by a other output, often slow but constant, used in machine or play sports. The second type has the same input and output also called Pull-back motor, classical used in small pull back car. We will see this type of motor in a next video.


The wind up motor today is the 42073 which is a small box with a input and output. Here is the X-ray analysis :

You can see many design point here, the first is how does it made. It is made by using a spiral spring to bring the power and then restitutes. This spring is loaded by the input using plenty of gears but especially a ratchet mechanism to prevent loaded the motor in incorrect way.

The second point is about the speed of this motor, it is in fact pretty slow and constant to compare to the pull back motor which has a speed and torque depending of how it is loaded. This slow and constant speed is used by a geared up inertia wheel, in order to give enough intertia and thus energy to drive the output slowly and constantly (that is why it is used in machine of lego soccer and football sport).


The last design point is a bout a special function in order to keep the output running even the spring is totally unload. On a pull back motor, it is important to let the car or the mechanism running when the spring is unloaded. But it is also useful for small wind up motor, especially if it drives inertia model or machine. This function is made by an axle of a gear which can move inside the casing. In the first way of rotation (loaded the spring or running the mechanism), the axle is locked in the casing by the force on the gear, enables the running and loading of the spring. But in other way of rotation, the axle is free to move, resulting to not engage the gearing resulting a free rotation of the output after stop of the spring.


Note that the X-ray scan has a lot of metal artefact causes by the metal in the motor (especially the spiral spring) but it doesn’t really affect the understanding of how the lego parts works.

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