1. A device for controlling speed of spur gears, the device comprising: multiple spur gears; a pressure measurer configured to collect parameters associated with a node pressure of the multiple spur gears; a gear stiffness calculator configured to calculate tooth surface contact stiffness K based on a rough surface of the multiple spur gears by: partitioning each tooth surface of spur gears using a hexagonal mesh partition manner such that an area of each square mesh is identical and is expressed as A 0 , calculating a contact pressure distribution of a smooth surface of the multiple spur gears to extract the node pressure of contact areas of the spur gears based on the collected parameters, the node pressure represented by P i , calculating normal contact stiffness K N and tangential contact stiffness K T of single square mesh based on a fractional theory using A 0 and P i , and calculating the tooth surface contact stiffness K based K N and K T ; and a reducer configured to modify the speed of the spur gears based on the tooth surface contact stiffness K of the spur gears.