6.0 RACKS
Gear racks (Figure 1.31) are important components in that they are a means of converting rotational motion into linear motion. In theory the rack is a gear with infinite pitch diameter, resulting in an involute profile that is essentially a straight line, and the tooth is of simple V form. Racks can be both spur and helical. A rack will mesh with all gears of the same pitch. Backlash is computed by the same formula as for gear pairs, equation 22. However, the pressure angle and the gears pitch radius remain constant regardless of changes in the relative position of the gear and rack. Only the pitch line shifts accordingly as the gear center is altered. See Figure 1.32.
7.0 INTERNAL GEARS
A special feature of spur and helical gears is their capability of being made in an internal form, in which an internal gear mates with an ordinary external gear. This offers considerable versatility in the design of planetary gear trains and miscellaneous instrument packages.
7.1 Development of the Internal Gear
The gears considered so far can be imagined as
equivalent pitch circle friction discs which roll on each other with external contact If
instead, one of the pitch circles rolls on the inside of the ether, it forms the basis of
internal gearing. In addition, the larger gear must have the material forming the teeth on
the convex side of the involute profile, such that the internal gear is an inverse of the
common external gear, see Figure 1.33a.
The base circles, line of action and development of
the involute profiles and action are shown in Figure 1.33b. As with spur gears there is a
taut generating string that winds and unwinds between the base circles. However, in this
case the string does not cross the line of centers, and actual contact and involute
development occurs on an extension of the common tangent. Otherwise, action parallels that
for external spur gears.
T58 Next Page