TAILIEUCHUNG - Ebook Theory of machines and mechanism: Part 2

(BQ) Part 3 includes essential topics on the dynamics of machines such as vibration analysis, balancing, cam dynamics, gyroscopes, dynamics of reciprocating engines, flywheels, planar and spatial dynamic force analysis, and static force analysis. The exercises and the solved problems allow the students to practice their knowledge better. | CHAPTER EIGHT HELICAL WORM AND BEVEL GEARS Most engineers prefer to use spur gears when power is to be transferred between parallel shafts because they are easier to design and often more ị economical to manufacture but sometimes the design requirements are such that helical gears are a better choice. This is especially true when loads are heavy speeds are high or the noise level must be kept low. When motion is to be transmitted between shafts which are not parallel J the spur gear cannot be used the designer must then choose between crossed-helical worm bevel or hypoid gears. Bevel gears have straight teeth line contact and high efficiencies. Crossed-helical and worm gears have a much lower efficiency because of the increased sliding action however if good engineering is used crossed-helical and worm gears can be designed with quite acceptable values of efficiency. Hypoid and bevel gears are used for similar applications and although hypoid gears have inherently stronger teeth the efficiency is often much less. Worm gears are used when very high 1 velocity ratios are required. 8-1 PARALLEL-AXIS HELICAL GEARS Helical gears are used for the transmission of motion between nonparallel and parallel shafts. When they are used with nonparallel shafts they are called crossed-helical gears these are considered in Sec. 8-6. The shape of the tooth of a helical gear is an involute helicoid as illustrated in Fig. 8-1. If a piece of paper is cut into the shape of a parallelogram and wrapped around a cylinder the angular edge o the paper becomes a helix. If the paper is then unwound each point on the angular edge 282 HELICAL. WORM. AND BEVEL GEARS 283 Figure 8-1 An involute helicoid. generates an involute curve. The surface obtained when every point on the edge generates an involute is called an involute helicoid. The initial contact of spur-gear teeth is a line extending all the way across the face of the tooth. The initial contact of helical-gear teeth is a point which .

• Cơ khí - Chế tạo máy
• Theory of machines and mechanism
• The dynamics of machines
• Vibration analysis
• Dynamics of reciprocating engines
• Static force analysis
• Spatial dynamic force analysis
• The geometry of motion
• Position and displacement
• Numerical methods in kinematic analysis