Gears Flra Hajdu B 406 hajdflsze hu Content

Gears Flóra Hajdu B 406 hajdfl@sze. hu

Content • Gears • Technical representation 2021. 05. 24. Hajdu Flóra

Gear drives • The function of a gear is to transmit motion from one machine part to another and where necessary to reduce or increase the revolutions of a shaft • Gears are the most durable and rugged of all mechanical drives - gears rather than belts or chains are used in automotive transmissions and most heavyduty machine drives. • may be grouped according to the position of the shafts that they connect • • Spur gears connect parallel shafts bevel gears connect shafts whose axes intersect worm gears connect shaft whose axes do not intersect. A spur gear with a rack converts rotary motion to reciprocating or linear motion. • Gear design is very complicated, dealing with problems as strength, wear, and material selection - Normally a gears are selected from a catalog. • Most gears are made of cast iron or steel, but brass, bronze, and plastic are used when factors such as wear or noise must be considered. 2021. 05. 24. Hajdu Flóra

Gear drives 2021. 05. 24. Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design Hajdu Flóra

Spur gears • Gears are used to transmit motion and power at constant angular velocity. • The specific form of the gear that best produces this constant angular velocity is the involute is curve traced by a point on a taut string unwinding from a circle (base circle). • Every involute gear has only one base circle from which all the involute surfaces of the gear teeth are generated. • This base circle is not a physical part of the gear and cannot be measured directly. Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design 2021. 05. 24. Hajdu Flóra

Spur gear definitions 2021. 05. 24. Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design Hajdu Flóra

Spur gears – schematics representation • 2021. 05. 24. Hajdu Flóra

Spur gears – schematics representation 2021. 05. 24. Hajdu Flóra

Gear pair – schematics representation 2021. 05. 24. Hajdu Flóra

Rack and pinion • A rack is a straight bar having teeth that engage the teeth on a spur gear. • In theory, it is a spur gear having an infinite pitch diameter -> all circular dimensions become linear. • The addendum, dedendum, and tooth thickness are the same as Source: C. Jensen, J. D. Helsel, D. R. Short: those of the mating spur gear. Engineering Drawing&Design 2021. 05. 24. Hajdu Flóra

Rack – schematics representation 2021. 05. 24. Hajdu Flóra

Rack and pinion – schematics representation 2021. 05. 24. Hajdu Flóra

Bevel gears Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design • Bevel gears are used to transmit power between two shafts whose axes intersect. • The axes may intersect at any angle, but the most common is 90°. • The teeth are the same shape as spur gear teeth but taper toward the cone apex. • Cast iron is normally used for gears that are not subject to heavy duty. • Often a gear (larger diameter) and pinion (smaller diameter) are made of different materials for efficiency and durability. • The pinion is made of a stronger material - the teeth on the pinion come into contact more times than the teeth on the gear. • Common combinations are steel and cast iron, and steel and bronze. 2021. 05. 24. Hajdu Flóra

Bevel gear – schematics representation 2021. 05. 24. Hajdu Flóra

Bevel gear pair – schematics representation 2021. 05. 24. Hajdu Flóra

Worm and worm gears • Worm gears are used to transmit power between two shafts that are at right angles to each other and are nonintersecting. • The teeth on the worm are similar to the teeth on the rack, and the teeth on the worm gear are curved to conform with the teeth on the worm. • Thread terms (pitch, lead) are used on the worm • Since a single-thread worm in one revolution advances the worm gear only one tooth and space, a large reduction in velocity is obtained. • The ratio of worm gear speed to the worm speed is the ratio between the number of teeth on the worm gear and the number of threads on the worm. A worm gear with 33 teeth and a worm with a multiple thread of three has a ratio of 11: 1. • a single-thread worm has a low lead (or helix) angle, it is inefficient and consequently not used to transmit power - multi-thread worms are used. 2021. 05. 24. Hajdu Flóra

Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design Worm and worm gears 2021. 05. 24. Hajdu Flóra

Worm – schematics representation 2021. 05. 24. Hajdu Flóra

Worm gear – schematics representation 2021. 05. 24. Hajdu Flóra

Worm -worm gear pair – schematics representation 2021. 05. 24. Hajdu Flóra

Gears– schematics representation • Helical gear • Curved gear • Herringbone gear 2021. 05. 24. Hajdu Flóra

Comparison of chain and gear drives • Advantages of chains • Shaft center distances for chain drives are relatively unrestricted, whereas with gears, the center distance must be such that the pitch surfaces of the gears are tangent -simpler, less costly, and more practical design. • Chains are easily installed. Assembly tolerances for chain drives are not as restricted as those for gears - Savings in the time of installation. • The ease of chain installation is a definite advantage when later changes in design, such as speed ratio, capacity, and centers, are anticipated. 2021. 05. 24. Hajdu Flóra

Comparison of chain and gear drives • Advantages of gears • When space limitations require the shortest possible distance between shaft centers, a gear drive is usually preferable to a chain drive • The maximum speed ratio for satisfactory operation of a gear drive is usually greater than that for a chain drive. • Gears can be operated at higher rotative speeds than chain drives. 2021. 05. 24. Hajdu Flóra

Summary • Gear types • Gear assemblies • Next week: Geometric tolerances

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