The rack is also divided into a straight rack and a helical rack, which are respectively paired with a spur gear and a helical gear;

The tooth profile of the rack is a straight line rather than an involute (planar to the tooth surface), which is equivalent to an infinite circle with a radius of infinity.

Main features of the rack

(1) Since the tooth profile of the rack is straight, each point on the tooth profile has the same pressure angle and is equal to the inclination angle of the tooth profile. This angle is called the tooth angle and the standard value is 20°.

(2) Any line parallel to the top line of the tooth has the same pitch and modulus.

(3) A line parallel to the top line of the tooth and having a tooth thickness equal to the width of the tooth groove is called a dividing line (middle line), which is a reference line for calculating the size of the rack.