# TERMINOLOGY OF SPUR GEAR

The terminology of gear includes number of terms peculiar to gear and it forms the basic of gear language .The terminology applied to spur gear is illustrated .
gear terminology consist following terms ;

1. Pinion - A pinion is smaller of the two mating gears .

2. Gear - A gear is larger of the two mating gears .

3. Velocity ratio - Velocity ratio is the ratio of angular velocity of the driving gear to the
angular velocity of the driven gear . It is also called the speed ratio .

4. Transmission ratio - The transmission ratio is the ratio of the angular speed of first driving gear to the angular speed of the last driven gear in a gear train .

5. Pitch Surface - The pitch surface of the gears are imaginary planes cylinders or cones that roll together without slipping .

6. Pitch circle - The pitch circle in the curve of intersection of the pitch surface of revolution and the plane of rotation . It is an imaginary circle that rolls without slipping with the pitch circle of a mating gear . The pitch circles of a pair of mating gears are tangent to each other .

7. Pitch circle diameter - The pitch circle diameter is the diameter of pitch circle .The size of the gear is usually specified by pitch circle diameter . It is also called pitch diameter .The pitch circle diameter is denoted by 'd' .

8. Pitch point  -  The pitch point is a point on line on line of centers of two gears at which two pitch circles of mating gears are tangent to each other .

9. Top land - The top land is the surface of the top of the gear tooth ..

10. Bottom land - The bottom land is the surface of the gera between the flanks of adjacent teeth .

11. Involute - An involute is a curve traced by a point on a line rolls without slipping on a circle .

12. Base circle - The base circle is an imaginary circle form which the involute curve of the tooth profile is generated .the base circles of two mating gears are tangent to the pressure line .

13. Addendum circle - The addendum circle is an imaginary circle that borders the tops of gear teeth in the cross-section .

15. Dedendum circle - The dedendum circle is an imaginary circle that borders the bottom of  spaces between teeth in the cross-section . It is called root circle .

16. Dedendum - The dedendum is the radial distance between pitch and the dedendum circles . The dedendum indicates the depth of the tooth below the pitch circle .

17. Clearance -  The clearance is the amount by which the dedendum of a given gear exceed the addendum of its mating tooth .

18. Face of tooth  - The surface of the gear tooth between the pitch cylinder and the addendum cylinder in called face of tooth .

19. Flank of tooth - The surface of the gear tooth between the pitch cylinder and the root cylinder in called flank of tooth .

20. Face width - Face width is width of the tooth measured parallel of the axis .

21. Fillet radius - The radius that connects the root circle to the profile of the tooth is called fillet radius .

22. Circular tooth thickness - The length of  the arc on pitch circle subtending a single gear tooth is called circular tooth thickness . Theoretically , circular tooth thickness is half of circular pitch .

23. Tooth space -  The width of the space between two adjacent teeth measured along the pitch circle is called the tooth space . theoretically tooth space is equal to circular tooth thickness or half of circular pitch .

24. Working Depth - The working depth is the depth of engagement of two gear teeth , that is the sum of their addendums .

25. Whole depth  - The whole depth is the total depth of the tooth space that is the sum of addendum and dedendum . Whole depth is also equal to working depth plus clearance .

26. Center Distance - The center distance is the distance between centers of pitch circles of mating gears . It is also the distance between centers of base center of mating gears .

27. Pressure Angle - The pressure angle is the angle that the line of action makes with the common tangent to the pitch circles . The pressure angle is also called the angle of obliquity ,It is denoted by α .

28. Line of action - The line of action is the common tangent to the base circles of mating gears . The contact between the involve surface of mating teeth must be  on this line to give smooth operation . The force is transmitted from the the driving gear to the driven gear on this line .

29. Arc of  contact - The arc of contact is the arc of the pitch circle through which a tooth moves form the beginning of contact until the point of contact arrives at the pitch circle .

30. Arc of approach -The arc of approach is the arc of the pitch circle through which a tooth moves form its beginning of contact until the point of contact arrives at the pitch point .

31. Arc of Recess - The arc of recess is the arc of the pitch circle through which a tooth moves form the contact at the pitch point until the contact ends .

32. Contact ratio - The number of pairs of teeth that are simultaneously engaged is called contact ratio .

33. Circular pitch - The circular pitch (p) is the distance measured along the pitch circle between two similar points on teeth .

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