1.Gear-wheels fitting together turn in opposite directions.
2.Gear-wheels fitted together turn in opposite directions.
Are there differences in meaning?
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The meaning in these two sentences is pretty much equivalent. Some words are implied:
Gear-wheels that are fitting together turn in opposite directions.
Gear-wheels that have been fitted together turn in opposite directions.
The use of active and passive is a matter of emphasis. The active voice emphasizes the subject, and the passive voice emphasizes the object. Consider these two sentences:
The technician installed the software.
The software was installed by the technician.
The first sentence places a bit more emphasis on the technician, while the second one places a bit more on the software.
In the case of your examples, the second one calls a bit of attention to the fact that someone fitted the gear wheels. The first sentence does not. That is the extent of the difference in meaning.
All that said, I would probably use this:
Gear-wheels that fit together turn in opposite directions.