Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9-1 Nomenclature of Spur Gear Teeth = (tooth spacing) driven gear – (tooth thickness) driver, measured on the pitch circle. Backlash Pitch circle gear diam. Fillet radius Clearance Base Circle

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9-5 Gear Details(Spur) The circle marked in red shows the outer limit of the teeth whilst the green circles are known as the pitch circles. The pitch circle of a gear is very important as it is used by engineers to determine the shape of the teeth and the ratio between gears (ratios will be explained later). The pitch of a gear is the distance between any point on one tooth and the same point on the next tooth. The root is the bottom part of a gear wheel. The pitch point is the point where gear teeth actually make contact with each other as they rotate.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9-7 NOMENCLATURE…. Diametral pitch (Pd): The number of teeth of a gear unit pitch diameter. The diametral pitch is, by definition, the number of teeth divided by the pitch diameter. That is, Where Pd = diametral pitch N = number of teeth D = pitch diameter Module (m): Pitch diameter divided by number of teeth. The pitch diameter is usually specified in inches or millimeters; in the former case the module is the inverse of diametral pitch. m = D/N Circular pitch (Pc) : The width of a tooth and a space, measured on the pitch circle.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display SPUR GEAR NUMENCLATURE NomenclatureSymbolFormulae Module Diametral Pitch Pitch Number of teeth Pitch Circle Diameter Tooth height Addendum Dedendum Root circle diameter Outside diameter Distance between the axis of the two mating gears t 1 & t2 Pressure angle m D.P. P t D h h’ h” D1 Do A α m=p/Π = D/t t/D = Π/p = 1/m P= Π.m = 3.14m t = D/m D = t.m h = 2.2 m h’ = m h” = 1.2 m D1 = Do – 4.4m Do = m (t+2) A = (t1 + t2) m/2 20˚(usually) or 14½°

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Tooth Thickness Measurement Tooth Thickness = S x Tooth Depth = H x sxsx HxHx Module (m) Hx= m Sx = m

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Fundamental Law of Gearing The angular velocity ratio between the gears of a gearset remains constant throughout the mesh. The angular velocity ratio (m V ) is equal to the ratio of the radius of the input gear to that of the output gear The torque ratio (m T )

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Contact Ratio If m p is 1, then one tooth is leaving contact just as the next is beginning contact.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Standard tooth systems for spur gears