CONTENTS CONTENTS 1066 C H A P T E R n A Textbook of Machine Design 29 Helical Gears 1. 2. 3. 4. Introduction. Terms used in Helical Gears. Face Width of Helical Gears. Formative or Equivalent Number of Teeth for Helical Gears. 5. Proportions for Helical Gears. 6. Strength of Helical Gears. Introduction Introduction A helical gear has teeth in form of helix around the gear. Two such gears may be used to connect two parallel shafts in place of spur gears. The helixes may be right handed on one gear and left handed on the other. The pitch surfaces are cylindrical as in spur gearing, but. | CONTENTS C H A P T E R 29 Helical Gears 1. Introduction. 2. Terms used in Helical Gears. 3. Face Width of Helical Gears. 4. Formative or Equivalent Number of Teeth for Helical Gears. 5. Proportions for Helical Gears. 6. Strength of Helical Gears. Introduction A helical gear has teeth in form of helix around the gear. Two such gears may be used to connect two parallel shafts in place of spur gears. The helixes may be right handed on one gear and left handed on the other. The pitch surfaces are cylindrical as in spur gearing but the teeth instead of being parallel to the axis wind around the cylinders helically like screw threads. The teeth of helical gears with parallel axis have line contact as in spur gearing. This provides gradual engagement and continuous contact of the engaging teeth. Hence helical gears give smooth drive with a high efficiency of transmission. We have already discussed in Art. that the helical gears may be of single helical type or double helical type. In case of single helical gears there is some axial thrust between the teeth which is a disadvantage. In order to eliminate this axial thrust double helical gears . 1066 CONTENTS Hellical Gears 1067 herringbone gears are used. It is equivalent to two single helical gears in which equal and opposite thrusts are provided on each gear and the resulting axial thrust is zero. Terms used in Helical Gears The following terms in connection with helical gears as shown in Fig. are important from the subject point of view. 1. Helix angle. It is a constant angle made by the helices with the axis of rotation. 2. Axial pitch. It is the distance parallel to the axis between similar faces of adjacent teeth. It is the same as circular pitch and is therefore denoted by pc. The axial pitch may also be defined as the circular pitch in the plane of rotation or the diametral plane. 3. Normal pitch. It is the distance between similar faces of adjacent teeth along a helix on the pitch cylinders .
