The uppermost long cylindrical head screw is used for locking in the keyway of the shaft and preventing the circumferential rotation effect. It can also be used in general unimportant situations when the key slides within the keyway of the shaft.
Tightening screws are generally harder, but if your top surface is hardened, consider the choice of tapered taper and female screws.
Multi-purpose recessed end screw for fixing the shaft GB/T80-2007. The flat end screw (GB/T77-2000) is suitable for the fixed contact surface is flat, the fixed end screw (GB/T80-2007) is suitable for the fixed contact surface is cylindrical, reason: avoid contact when fixing The face is point contact