This is the eighteenth installment of the Fortnightly Topic Challenge described here, with topics suggested and voted on here. This fortnight's topic is mechanical-puzzles (suggested by Emrakul), and will span from the 17th of October to the 30th of October. During this period, we will compile the list of questions with this tag and post it as an answer to this question.
In the meantime, please go and propose and vote on future challenges!
Everyone have fun, and happy puzzling!
Link to other Fortnightly Topics.