The most difficult me­chan­ics riddles

Some gas fittings use reverse threads to prevent disastrous misconnections. For example, oxygen fittings are right-handed, but acetylene and other flammable gases use left-handed fittings.

A balance wheel, or balance, is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral torsion spring, the balance spring or hairspring. It is driven by the escapement, which transforms the rotating motion of the watch gear train into impulses delivered to the balance wheel.

A stepper motor is used in devices that need precise positioning and speed control. Because it moves in precise repeatable steps, the stepper motor is used in devices like 3D printers, camera platforms, plotters, scanners, analog quartz watches. etc.

For bicycle pedals, the screw on the right must be reverse-threaded, or the pedal will unscrew during use, being subject to counterclockwise torque (which would work to undo a right-hand thread).

Nut is a rolling cylindrical pawl. It has a perpendicular centre slot for the bolt, and an intersecting axial slot for the string, along with a lower face or slot against which the internal trigger sits.