Spur gears will be the most easily visualized common gears that transmit movement between two parallel shafts. Because of their shape, they are classified as a kind of cylindrical gears. Since the tooth areas of the gears are parallel to the axes of the installed shafts, there is absolutely no thrust force produced in the axial path. Also, due to the simple production, these gears can be made to a high degree of precision. However, spur gears have a disadvantage in that they quickly make noise. Generally Rotary Air Compressor speaking, when two spur gears are in mesh, the apparatus with more teeth is named the “equipment” and the main one with small number of tooth is named the “pinion”.
The unit to point the sizes of spur gears is commonly stated, as specified by ISO, to be “module”. Recently, it is usual to set the pressure angle to 20 degrees. In commercial machinery, it is most common to employ 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 gear teeth. They are made by adjusting the distance between the gear cutting device called the hobbing tool and the apparatus in the creation stage. When the change is certainly positive, the bending power of the apparatus increases, while a poor shift somewhat reduces the center distance. The backlash may be the play between the tooth when two gears are meshed and is necessary for the even rotation of gears. When the backlash is certainly too big, it leads to increased vibration and noise as the backlash that is too small leads to tooth failure because of the lack of lubrication.
All KHK spur gears have an involute tooth shape. Basically, they are involute gears using part of the involute curve as their tooth forms. Looking generally, the involute form may be the most wide-spread equipment tooth form due to, among other factors, the ability to absorb small middle distance errors, quickly made production equipment simplify 
manufacturing, heavy roots of one’s teeth make it solid, etc. Tooth shape is often referred to as a specification in drawing of a spur equipment as indicated by the elevation of teeth. In addition to standard full depth teeth, prolonged addendum and stub tooth profiles can be found.