The rack is an involute gear whose base circle radius is infinite, and its contour has changed from involute to straight line, so the rack has the characteristics of involute gear. Because the rack has the particularity of constant pressure angle, modulus and pitch, the rack also has special meshing. No matter how the installation position of the rack relative to the gear changes, the meshing angle is constant and always equal to the pressure angle. The gear indexing circle always coincides with its pitch circle, that is, the pitch line of the rack is always tangent to the indexing circle of the gear. When the number of teeth of the standard external gear increases to infinity, the base circle and other circles of the gear become parallel straight lines, and the involute tooth profile on the same side also becomes parallel oblique straight line tooth profile, so as to form the standard rack. The main features of the rack are:
(1) Each point on the tooth profile has the same pressure angle and is equal to the inclination angle of the tooth profile. The standard value is 20 °.
(2) Any straight line parallel to the tooth top line has the same tooth pitch and modulus, and P = π M.
(3) The straight line parallel to the tooth top line and the tooth thickness is equal to the tooth slot width is called the dividing line, which is the datum line to determine the size of the rack.
| Name | Symbol | Calculation formula |
| Modulus | M | |
| Number of teeth | Z | |
| Pressure angle | A | |
| Helix angle | Beta | |
| Tooth width | B | |
| Addendum height coefficient | Ha | |
| Tooth top height | Hk | Hk=M*Ha |
| Tooth root height | Hf | Hf=M*(Ha+C) |
| Full height of tooth | H | H=Hk+Hf |
The basic parameters of the rack of automobile rack and pinion steering gear include the number of teeth, modulus, pressure angle, tooth width and other basic parameters. According to gb1356-88, the standard value is: Ha = 1, C = 0.25. Through these parameters, the rack of the automobile rack and pinion steering gear can be drawn. The main parameters used in the rack modeling process of the automobile rack and pinion steering gear are shown in the table.