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 a tooth surface contact stiffness K based on a rough surface of the multiple spur gears by: partitioning each tooth surface of the multiple spur gears using a hexagonal mesh partition manner to provide a plurality of square meshes such that an area of each square mesh of the plurality of square meshes is identical and is expressed as A o , calculating a contact pressure distribution of a smooth surface of the multiple spur gears to extract the node pressure of contact areas of the multiple spur gears based on the collected parameters, the node pressure represented by P i , calculating a normal contact stiffness K N and a tangential contact stiffness K T of a single square mesh of the plurality of square mesh based on a fractional theory using A o and P i , and calculating a tooth surface contact stiffness K based on the K N and the K T ; and a reducer configured to modify a speed of the multiple spur gears based on the calculated tooth surface contact stiffness K of the multiple spur gears.