Such information is critical to enable suitable bearings to be selected for both shafts. see Dudás, 2005 Litvin and Kin, 1992) and this is outlined in Section 11.2. Cast iron and polymers are sometimes used for lightly loaded, low speed applications.Īn analysis of the forces associated a worm and wheel gear-set can be undertaken readily (e.g. Sand-cast or forged bronze is commonly used. The worm gear needs to be of softer material that is compliant enough to run-in and conform to the worm under the high sliding running conditions. They are typically ground or polished to a roughness of R a = 0.4 μm. The worm is subject to high stresses and is normally made using a hardened steel such as AISI 1020, 1117, 8620, 4320 hardened to HRC 58-62 or a medium carbon steel such as AISI 4140 or 4150 induction or flame hardened to a case of HRC 58-62 ( Norton, 2006). The tooth forms for worm and wheel gear sets are not involutes. The various terms are illustrated in Fig. 31.15. The fillet is the rounded portion at the bottom of the tooth space. The pressure angle is the angle formed between the common tangent and the line of action. The line of action is the common tangent to the base circles, and the path of contact is that part of the line of action where contact takes place between the teeth. the number of teeth divided by the pitch-circle diameter. The diametral pitch is the reciprocal of the module, i.e. The module is the pitch-circle diameter divided by the number of teeth. The tooth width is the length of arc from one side of the tooth to the other, measured round the pitch-circle circumference. The circular pitch is the distance from a point on one tooth to the corresponding point on the next tooth, measured round the pitch-circle circumference. The whole depth of the tooth is the sum of the addendum and the dedendum. The clearance is the algebraic difference between the addendum and the dedendum. The dedendum is the radial depth of the tooth from the pitch circle to the root of the tooth. The addendum is the radial depth of the tooth from the pitch circle to the tooth tip. The center distance is the sum of the pitch-circle radii of the two gears in mesh. The root diameter is the diameter at the base of the tooth. The base circle is the circle from which the involute is generated. The pitch circles are imaginary friction discs, and they touch at the pitch point. The pitch-circle diameters of a pair of gears are the diameters of cylinders co-axial with the gears which will roll together without slip.
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