Gear metrology

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Gear metrology

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<ul><li> 1. Gear Metrology </li> <li> 2. Introduction Gears are mainly used for transmission of power or motion from one shaft to another, without slip. Thus they are positive in action and provide constant velocity ratio. Terminology of Gear tooth Addendum:- This is the radial distance from the pitch circle to the tip of the tooth. Its value is equal to one module. Clearance:- This is the radial distance from the tip of a tooth to the bottom of a mating tooth space when the teeth are symmetrically engaged. Its standard value is 0.157 m. Dedendum:- This is the radial distance from the pitch circle to the bottom of the tooth space. Dedendum = Addendum + Clearance = m+0.157 m=1.153m. </li> <li> 3. Face of Tooth:- It is that part of the tooth surface which is above the pitch surface. Flank of Tooth:- It is that part of the tooth surface which is lying below the pitch surface. Tooth thickness:- The thickness of tooth, measured along the pitch circle. Whole depth= Addendum + Dedendum Working depth= Addendum + Dedendum - Clearance Pitch circle:- In every pair of gears in mesh, the two plain wheels in contact are assumed to exist; which are known as pitch circle. Pitch point:- It is the point of contact between the pitch circle of two gears in mesh. </li> <li> 4. Circular pitch:- It is the arc distance measured around the pitch circle from a point on one tooth to a corresponding point on the adjacent tooth. Circular pitch = d/T = m d = pitch circle diameter T = no. of teeth m = module = d/T Diametral pitch = Number of teethPitch circle diameter = Td Module = (1/ Diametral pitch) Back lash = The diameter through which a gear can be rotated to bring its non-working flank in contact with the teeth of mating gear. </li> <li> 5. Pressure angle or angle of obliquity:- It is the angle which common normal to the two teeth at the point of contact makes with common tangent to the two pitch circles at the pitch point. </li> <li> 6. Tooth thickness measurement:- Since tooth thickness is defined as length of an arc, it is not possible to measure it directly. There are various methods of measuring the gear tooth thickness. (i)Measurement of tooth thickness by gear tooth vernier calliper (Chordal thickness method) (ii)Constant chord method (iii)Base tangent method (iv)Measurement by dimension over pins. </li> <li> 7. Question :- Calculate the setting of vernier for a straight spur gear having 40 teeth of module 3. </li> </ul>

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