Spur gears will be the most easily visualized common gears that transmit motion between two parallel shafts. Because of the shape, they are categorized as a kind of cylindrical gears. Because the tooth areas of the gears are parallel to the axes of the installed shafts, there is absolutely no thrust force generated in the axial direction. Also, due to the ease of production, these gears can be made to a higher degree of precision. On the other hand, spur gears possess a disadvantage in that they very easily make noise. In most cases, when two spur gears are in mesh, the apparatus with more tooth is called the “equipment” and the main one with the smaller number of the teeth is called the “pinion”.
The unit to indicate the sizes of spur gears is commonly stated, as specified by ISO, to be “module”. In recent years, it is normal to create the pressure angle to 20 degrees. In commercial machinery, it is most common to use a part of an involute curve as the tooth profile.
Even though not limited to spur gears, profile shifted gears are used when it is required to adjust the center distance slightly or to strengthen the equipment teeth. They are produced by adjusting the range between your gear cutting device called the hobbing device and the apparatus in the creation stage. When the change can be positive, the bending strength of the apparatus increases, while a poor shift somewhat reduces the guts distance. The backlash may be the play between the the teeth when two gears are meshed and is needed for the Coupling simple rotation of gears. When the backlash is definitely too large, it leads to increased vibration and noise as the backlash 
that is too small leads to tooth failing due to the lack of lubrication.
All KHK spur gears have an involute tooth form. Basically, they are involute gears using area of the involute curve as their tooth forms. Looking generally, the involute form is the most wide-spread equipment tooth form because of, among other factors, the capability to absorb small middle distance errors, quickly made production tools simplify manufacturing, heavy roots of one’s teeth make it solid, etc. Tooth shape is often described as a specification in drawing of a spur equipment as indicated by the elevation of teeth. In addition to standard complete depth teeth, extended addendum and stub tooth profiles can be found.