Engineering Design Gears & Springs
To mesh or not to mesh - spur, helical and worm drive gear examples 
Gears - An ancient concept, gears (above, a spur gear, LH/RH helical gear and a worm drive) are a very effiective way of both transferrring torque and varying the applied torque (increasing/decreasing the 'gear ratio') from a drive input unit to a final output unit. Gears may be plastic or metallic (ferrous or non-ferrous). Most ferrous gears are produced by forging, casting or sintering, producing a part with a dense grain structure and superior strength.
Springs - Usually used in an assmebly/combination to provide a reaction 'force' via 'storing' 'spring' engergy (via displacement) whilst compressed (as above). A combination of materials may be used depending on application.