Construction and Working of Gears Prof. H. D. Mhatre Construction and Working of Gears Prof. H. D. Mhatre Prof. H. D. Mhatre

Content Introduction CLASSIFICATION OF GEARS 1) Spur Gear 2) Helical Gear 3) Herringbone gear 4) Worm Gear 5) Bevel Gear 6) Internal Spur Gear Gear Parameters LAW OF GEARING Spur Gear Nomenclature DESIGN OF GEAR BLANKS Prof. H. D. Mhatre

Introduction Gears are defined as toothed wheels or multilobed cams, which transmit power and motion from one shaft to another by means of successive engagement of teeth. They can be applied between two shafts which are „ Parallel Collinear „ Perpendicular and intersecting „ Perpendicular and nonintersecting „ Inclined at any arbitrary angle Gears are made to high precision Purchased from gear manufacturers rather than made in house „ However it is necessary to design for a specific application so that proper selection can be application so that proper selection can be made Prof. H. D. Mhatre

CLASSIFICATION OF GEARS 1. Spur Gears 2. Helical gears Prof. H. D. Mhatre

3. herringbone gear 4. Warm Gears Prof. H. D. Mhatre

5. Bevel Gears 6. Internal spur gear Prof. H. D. Mhatre

Gear Parameters Number of teeth „ Form of teeth „ Size of teeth „ Face Width of teeth „ Style and dimensions of gear blank „ Design of the hub of the gear „ Degree of precision required „ Means of attaching the gear to the shaft „ Means of locating the gear axially on the shaft Prof. H. D. Mhatre

LAW OF GEARING The fundamental law of gearing states ‘The common normal to the tooth profile at the point of contact should always pass through a fixed point, called the pitch point, in order to obtain a constant velocity ratio’. Prof. H. D. Mhatre

Spur Gear Nomenclature Prof. H. D. Mhatre

DESIGN OF GEAR BLANKS Fig 1. Design of gear blanks for small diameter gears d < 200 mm Prof. H. D. Mhatre

Fig 2. Cluster and sliding gears The design of gear blank depends on the size, load carrying capacity, speed of operation, space limitations and application. Small gears up to a pitch diameter of 200 mm are normally made of solid blanks as in Fig. 1 In multi-speed gear boxes to make the arrangement compact, cluster and sliding gears as shown in Fig 2.(a) to (e) are used. The gaps between the gears should be adequate to relieve the gear cutter. Fig 2. Cluster and sliding gears Prof. H. D. Mhatre

Many a times, the tool relief does not result in compact drive Many a times, the tool relief does not result in compact drive. Hence, to further make it compact, glue jointed or shrink fitted composite gears shown in Fig. 3 (a) and (b) are often used. Fig. 3 Glue-jointed / shrink fitted composite gears In the case of medium sized gears normally forging process is used. The wheels are made solid or cored as shown in Fig. 4 (a). Cast wheel with crossed I shaped spokes shown in Fig.4 (b) are used when the diameter is <1000 mm and f<200 mm. Prof. H. D. Mhatre

Fig. 4 Design of medium size gear blanks Prof. H. D. Mhatre

Fig. 5 Design of large gear blanks Large gears are normally cast as in Fig.5 (a) to (d) with web straight or inclined. To reduce the weight of the gears, non stressed portion is made hollow by keeping cores. When a small number of high quality gears are required, the gears made with rimmed, bolted or welded construction as shown in Fig.4 (e) to (g). Prof. H. D. Mhatre

Fig. 6 Gear wheel proportions (a) spur gear (b) bevel gear The empirical formulae for finding proportions of spur gear wheel are given in Fig.6 (a) and that for a bevel gear wheel is given in Fig. 6 (b) Fig. 6 Gear wheel proportions (a) spur gear (b) bevel gear Prof. H. D. Mhatre

Fig. 7 Spur gear proportions for cast wheels. The empirical formulae for finding proportions of wheel elements of cast spur and helical gears are given in Fig. 7 Fig. 7 Spur gear proportions for cast wheels. Prof. H. D. Mhatre