Meshing of involute gears | line of action | contact ratio | pitch point | center distance

In this video, we look at the meshing of involute gears. When meshing, the teeth always exert a force along the so-called line of action. The line of action corresponds to the tangent to the base circles. The transmission ratio is largely independent of the center distance. This is the great advantage of involute gears.
The distance actually covered on the line of action is called the line of contact. The line of contact shortens as the center distance increases.
For continuous power transmission between the teeth, at least one pair of teeth must always mesh on the line of contact. The distance between two involute teeth on the line of contact corresponds to the base pitch pb on the base circle (also called the meshing pitch). The ratio of the length of the line of contact to the base pitch is called the contact ratio and must always be greater than 1. The contact ratio indicates how many teeth are simultaneously meshing on the line of contact.
The pitch point, where the circumferential speeds of the two gears are identical, corresponds to the intersection of the line of action and the center line of the two gear axes. At the pitch point, there is pure rolling without relative movement between the teeth. For a constant transmission ratio, the direction of the tooth force must always pass through the pitch point! This is called the law of gearing.
The angle between the line of action and the normal of the center line gives the so-called pressure angle.
00:00 Construction of an involute
01:10 Line of action
02:14 Line of contact
02:38 Pitch point
03:14 Relative speeds
04:02 Standard pressure angle
04:40 Operating pressure angle
05:25 Base pitch and contact ratio
07:07 Operating pitch circle
08:54 Transmission ratio when changing the center distance
10:56 Cycloidal gears
12:05 Law of gearing
14:05 Explanation fallacy

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I'm surprised about how little views this has. It's really well explained.