Parallel Keys are the most widely used. They have a square or rectangular cross-section. Square keys are used for smaller shafts and rectangular faced keys are used for shaft diameters over 6.5 in (170 mm) or when the wall thickness of the mating hub is an issue. Set screws often accompany parallel keys to lock the mating parts into place. The keyway is a longitudinal slot in both the shaft and mating part.
Parallel Keys are generally used for Automobiles, gearboxes, starters, alternators, fuel injection pumps, oil pumps, water pumps and brake compressors, Two wheelers, Electric motors, alternators and switch gears, Machine tools and all types of pumps Textile machineries, Pharmasitical Machinery, Heavy Duty Machinery, Earth moving Machinery, Automotive Cran etc.