standard interchangeable tooth profiles (systems of gear teeth)

STANDARD INTERCHANGEABLE TOOTH PROFILES 

(Systems of Gear Teeth)

• A set of gears is called interchangeable or standard gears when any two gears of the set will mesh satisfying the law of gearing.

• The advantages of using interchangeable (i.e., standard) gears include easier design, quick and economical production of gears, easy availability of production tools, better quality assurance, and easier replacement of gears.

• The six types of tooth profiles (standard tooth systems) used for interchangeability are: 

1. 141⁄2 composite system,

2. 14 ½° full depth involute system,

3. 20° full depth involute system,

4. 20° stud involute system,,

5. 25° full depth involute system, and

6. 25° stub involute system.

• Standard proportions of gear systems: Table 4.3 presents the standart proportions for the four gear systems in module (m).

Table 4.3. Standard tooth proportions

• The standard tooth proportions for the four types of tooth profiles are diagrammatically illustrated in Fig.4.21.

• 14 ½°  composite Vs 14 ½°  full depth involute system:

• As could be seen from Fig.4.21(a) & (b), the 141⁄2 composite tooth profile has cycloidal curves at top and bottom portions, and involute curve at the middle portion, whereas the 14 ½° full depth tooth profile has involute straight portion throughout. 

• Both 14 ½° composite and 14 ½° full depth involute tooth profiles are becoming obsolete and are used mainly as replacement gears (for replacing 14 ½° gears that were used in older gear sets).

• 20° full depth Vs 20° stub involute system:

• The 20° full depth tooth profile is very similar to 14 ½° full depth tooth profile except the pressure angle, as shown in Fig.4.21(c).

• The increased pressure angle makes tooth with a much larger base, which results in a stronger tooth.

• The 20° stud tooth profile has a working depth smaller than that of a 20° full depth tooth profile, as shown in Fig.4.21(d). This is obtained by cutting short both addendum and dedendum.

• The 20° stud tooth profile minimizes the interference problem arised in 20° full depth tooth profile. Also this type of tooth profile can be used for heavy loads.

• The 25° full depth tooth profile and 25° stub tooth profile are very similar to 20° full depth tooth profile and 20° stub tooth profile respectively, except the pressure angle.

• The 25°stub tooth system has become obsolete now-a-days.