The rack is also divided into a straight rack and a helical rack, which are used in pairs with a straight cylindrical gear and a helical cylindrical gear respectively. 

       The tooth profile of the rack is a straight line rather than an involute (a plane for the tooth surface), which is equivalent to a cylindrical gear with an infinite dividing circle radius

The main feature of rack

(1) since the rack tooth profile is a straight line, all points on the tooth profile have the same pressure Angle, which is equal to the inclination Angle of the tooth profile. This Angle is called the tooth profile Angle, and the standard value is 20°. 

(2) any straight line parallel to the addendum line has the same pitch and modulus. 

(3) the straight line parallel to the addendum line and with the tooth thickness equal to the width of the tooth groove is called the dividing line (middle line), which is the reference line for calculating the size of the rack. 

The main parameters of rack: tooth groove width, addendum height, tooth root height, tooth height, tooth thickness, tooth root circle radius, etc