GGears fail for a number of reasons, pick up any machine design text book and there will be a long list of failure modes listed. ISO 10825 lists a number of these failures by type and sub-types. While in systems where the gears are not enclosed, such as a bicycle, the small chip or broken teeth falls away from the gears and no further damage is done. Even in gears which are enclosed but without any cooling fluids such as those in wall-clocks the chip and debris will only collect at the bottom of the case.
HHowever, things are NOT so pretty in a sealed system with a cooling fluid in it. The broken chips and debris has no where to go in a sealed system add fluid which serves as the transport mechanism for the chip & debris to jump from one set of moving components to another and sure enough the entire system will be in pieces fairly quickly! The typical process for a transmission destruction goes something like this:
1.As Transmission fluid gets old it starts to loose out on a number of its mechanical and chemical properties especially cooling characteristics.
2.On days where the transmission is stressed (e.g. Extreme hot / Cold temperatures, Lead footed acceleration)the metal gears heat up resulting in stress fractures and chips falling off.
3.On other days, improper gear changes (especially for Manumatic/triptronic style systems) or the good ol’ trying to engage the Reverse gear while driving also damages the metal gears.
4.As the debris/chips mix with the transmission fluid, not only does it get trapped between other moving parts causing more debris to be created, but it also alter’s the fluids characteristics for worse.
5.Each time the pieces that come off are bigger, causing even more damage then the previous ones. This process takes the shape of a cycle, which just repeats itself until the every gear inside the transmission becomes mush.
Some drivers pick-up on violent noises coming form their transmissions; others ignore them. However, if a large enough chunk comes off one of the gears, the transmission may be completely gone in less then a few hundred meters of driving. Moral of the story is to at the very least meet the manufacturer’s recommendations for the frequency of having transmission fluid replaced, if not exceeding it. Below are a few pictures of failed gears:
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