Bevel, Worm And Cross Helical Gears

Common terms used:

1. Pitch cone. It is a cone containing the pitch elements of the teeth.

2. Cone centre. It is the apex of the pitch cone. It may be defined as that point where the axes of two mating gears intersect each other.

3. Pitch angle. It is the angle made by the pitch line with the axis of the shaft. It is denoted by ‘θP’.

4. Cone distance. It is the length of the pitch cone element. It is also called as a pitch cone radius. It is denoted by ‘OP’. Mathematically, cone distance or pitch cone radius,

5. Addendum angle. It is the angle subtended by the addendum of the tooth at the cone centre. It is denoted by ‘α’ Mathematically, addendum angle,

where a = Addendum, and OP = Cone distance.

6. Dedendum angle. It is the angle subtended by the dedendum of the tooth at the cone centre. It is denoted by ‘β’. Mathematically, dedendum angle,

where d = Dedendum, and OP = Cone distance.

7. Face angle. It is the angle subtended by the face of the tooth at the cone centre. It is denoted by ‘φ’. The face angle is equal to the pitch angle plus addendum angle.

8. Root angle. It is the angle subtended by the root of the tooth at the cone centre. It is denoted by ‘θR’. It is equal to the pitch angle minus dedendum angle.

9. Back (or normal) cone. It is an imaginary cone, perpendicular to the pitch cone at the end of the tooth.

10. Back cone distance. It is the length of the back cone. It is denoted by ‘RB’. It is also called back cone radius.

11. Backing. It is the distance of the pitch point (P) from the back of the boss, parallel to the pitch point of the gear. It is denoted by ‘B’.

12. Crown height. It is the distance of the crown point (C) from the cone centre (O), parallel to the axis of the gear. It is denoted by ‘HC’.

13. Mounting height. It is the distance of the back of the boss from the cone centre. It is denoted by ‘HM’.

14. Pitch diameter. It is the diameter of the largest pitch circle.

15. Outside or addendum cone diameter. It is the maximum diameter of the teeth of the gear. It is equal to the diameter of the blank from which the gear can be cut. Mathematically, outside diameter,

16. Inside or dedendum cone diameter. The inside or the dedendum cone diameter is given by

                              Dd = DP – 2d cos θP

where Dd = Inside diameter, and

            d = Dedendum

Design procedure for Bevel Gear

17. Calculation of gear ratio (i) and pitch angle: