# Gears

Topics: Gear, Pinion, Gears Pages: 12 (2437 words) Published: January 15, 2013
Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Module 2- GEARS Lecture – 11 HELICAL GEARS Contents
11.1 Helical gears – an introduction 11.2 Helical gears – Kinematics 11.3 Helical gears – geometry and nomenclature 11.4 Helical gears – force analysis 11.5 Helical gears – bending stress 11.6 Helical gears – contact stress 11.7 Crossed helical gears 11.7.1 Tips for crossed helical gear design 11.1 HELICAL GEARS – an introduction In spur gears Fig.11.1 dealt earlier, the teeth are parallel to the axis whereas in helical gears Fig.11.2 the teeth are inclined to the axis. Both the gears are transmitting power between two parallel shafts.

Fig.11.1 Spur gear

Fig.11.2 Helical gear

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Helical gear can be thought of as an ordinary spur gear machined from a stack of thin shim stock, each limitation of which is rotated slightly with respect to its neighbours as in Fig.11.3. When power is transmitted both shafts are subjected to thrust load on the shaft.

Fig.11.3 Illustration of concept of helical gear

Fig.11.4 Double helical gear or herringbone gear

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Herringbone or double helical gear shown in Fig. 11.4 can be two helical gears with opposing helix angle stacked together. As a result, two opposing thrust loads cancel and the shafts are not acted upon by any thrust load. The advantages of elimination of thrust load in Herringbone gears, is obliterated by considerably higher machining and mounting costs. This limits their applications to very heavy power transmission.

Fig.11.5 Double helical gear of a cement mill rotary gear drive

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Fig. 11.6 Crossed helical gears. Crossed helical gears As in Fig. 11.6 are used for transmitting power between two nonparallel, non-intersecting shafts. Common application is distributor and pump drive from cam shafts in automotive engines. 11.2 HELICAL GEARS- KINEMATICS

Fig.11.7 Helical gear When two helical gears are engaged as in the Fig. 11.7, the helix angle has to be the same on each gear, but one gear must have a right-hand helix and the other a left-hand helix.

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Fig.11.8 Illustration of helical gear tooth formation The shape of the tooth is an involute helicoid as illustrated in the Fig. 11.8. If a paper piece of the shape of a parallelogram is wrapped around a cylinder, the angular edge of the paper becomes the helix. If the paper is unwound, each point on the angular edge generates an involute curve. The surface got when every point on the edge generates an involute is called involute helicoid. In spur gear, the initial contact line extends all the way across the tooth face. The initial contact of helical gear teeth is point which changes into a line as the teeth come into more engagement. In spur gears the line contact is parallel to the axis of rotation; in helical gear the line is diagonal across the face of the tooth. Hence gradual engagement of the teeth and the smooth transfer of load from one tooth to another occur. This gradual engagement makes the gear operation smoother and quieter than with spur gears and results in a lower dynamic factor, Kv. Thus, it can transmit heavy loads at high speeds. Typical usage is automotive transmission for compact and quiet drive. 11.3 HELICAL GEARS – GEOMETRY AND NOMENCLATURE The helix angle ψ, is always measured on the cylindrical pitch surface Fig. 11.8. ψ value is not standardized. It ranges between 15o and 45o. Commonly used values are 15, 23, 30 or 45o. Lower values give less end thrust. Higher values result in smoother operation and more end thrust. Above 45o is not recommended.

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