1 Celkový obraz O modulu Pomůcky a nástroje Pravidla a konvence 24.11.2009 Strana 2/18
2 Obsah 1. Celkový obraz 2 2. Obsah 3 3. Gears 4 3.2 Gears 4 4. Bevel gears 8 4.2 Bevel gears 8 5. Worm drive 11 5.2 Worm drive 11 6. Planetary gears 13 6.2 Planetary gears 13 7. Diferential 15 7.2 Differential 15 8. Rejstřík 18 24.11.2009 Strana 3/18
3 Gears 3.1 Gears Description A gear is a component used to transmit a force from one device to another. Unlike pulley, gears are wheels which have teeth (or cogs), which fit into to teeth of another wheel. Thus the rotational torque can be transferred without any slippage. Various gears can transmit forces from the power source at different speeds or in different directions. The cog wheels usually interlace with another cog wheel, but in fact any device with compatible teeth can be used, such as linear racks. The most important feature of gears is that various wheels with different diameters can be combined. Thus the rotational speed of the second cog wheel gets higher or lower. Colloquially the term "gear" is also used to refer to a particular arrangement of gears, and so we can hear collocations like "first gear" or "the car has five gears" etc. Types of gears External gear - its teeth are formed on the outer surface Internal gear - its teeth are formed on the inner surface 24.11.2009 Strana 4/18
Spur gears ("straight-cut gears") - the simplest and most common type in the form of a cylinder or a disk with the teeth projecting radially; the egde of each tooth is straight and parallel to the axis of rotation. Helical gears - the edges of the teeth are not parallel to the axis of rotation, but they are set in a certain angle and because the gear is curved, the tooth looks like a part of a helix. Helical gear work more smoothly and quietly than spur gears. Herringbone gears (also called double helical gears) - gears with two sets of teeth in the V shape; their advantage in comparison to helical gears is that they can withstand axial load Bevel gears - a type of gears, which is conically shaped; they are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles too. Epiclic gearing - a system consisting of one or more outer gears, (called planet gears), which revolve about a central gear (called the sun). Planet gears are usually mounted on a movable arm 24.11.2009 Strana 5/18
(called carrier) which also can rotate relative to the sun gear. Vocabulary gear - ozubený převod transmit - přenést force - síla device - zařízení; součástka pulley - řemenice cog - zub na ozubeném kole cog wheel - ozubené kolo torque - kroutivá síla, kroutivý moment slippage - prokluzování interlace - prolínat se rack - ozubnice diameter - poloměr rotational speed - rotační rychlost first gear - první rychlost, jednička (v automobilu) spur gear - čelní ozubený převod cylinder - válec radially - paprskovitě edge - okraj axis of rotation - osa rotace helical gear - šnekový převod herringbone gears - převod se šípovým ozubením bevel gear - kuželočelní převod conically - kuželově mount - namontovat shaft - hřídel angle - úhel epiclic gearing - planetová převodovka revolve - otáčet se movable arm - pohyblivé rameno relative to - vzhledem k 24.11.2009 Strana 6/18
Obrázky a fotografie použité v této kapitole pochází ze stránek {Wikimedia Commons XH http://commons.wikimedia.org}, kde byly zveřejněny pod licencí {GNU FDL XH http://commons.wikimedia.org/wiki/commons:gnu_free_documentation_license}. 24.11.2009 Strana 7/18
4 Bevel gears 4.1 Bevel gears Bevel gears Bevel gears are a type of gears, where the surfaces of cog-wheels are in the shape of a cone. The angle between the two wheels is usually 90 degrees, but they can be designed for other angles as well. Description Important terms to describe bevel gears are pitch surface and pitch angle. The pitch surface is an imaginary surface acquired by averaging the tops of the teeth (also called cogs) and the valleys between them. The angle between the pitch surface and the axis of the gear is called the pitch angle. Ordinary gears have the pitch surface in the shape of a cylinder, but pitch surfaces of bevel gears have to be conical. 24.11.2009 Strana 8/18
Types of bevel gears The pitch angles of the most common bevel gears are smaller than 90 degrees. In this case their teeth point outwards and therefore they are called external. If their pitch angles are more than 90 degrees, their teeth point inward and they are called internal. Occasionally the pitch angle can be exactly 90 degrees. Because the teeth are parallel with the axis of the gear and thus it resembles a crown, it is called a crown gear. Vocabulary bevel gears kuželové soukolí cog-wheel, gear ozubené kolo cone kužel conical kuželový shape tvar angle úhel degree stupeň surface plocha, povrch pitch surface roztečná plocha pitch angle úhel roztečného kužele imaginary myšlený average průměr, průměrovat 24.11.2009 Strana 9/18
cog zub (na ozubeném kole) axis osa cylinder válec point směřovat outwards směrem ven inwards směrem dovnitř external vnější internal vnitřní occasionally příležitostně parallel rovnoběžný crown gear korunové kolo Obrázky a fotografie použité v této kapitole pochází ze stránek Wikimedia Commons, kde byly zveřejněny pod licencí GNU FDL. 24.11.2009 Strana 10/18
5 Worm drive 5.1 Worm drive Worm drive is a gearing system consisting of a worm and some other type of a gear. Description The worms are a type of helical gears resembling a screw, which are usually quite long in the axial direction and have quite a large helix angle (often just little less than 90 degrees). The worm intermeshes with another gear, which is usually an ordinary gear in the shape of a disk. This combination allows to reach high gear ratio in a small space while typical helical gears are usually limited to gear ratio maximum up to 10:1, worm drives can have the ratio much higher (usually up to 100:1, occasionally even 500:1). The main disadvantage with large helix angles is the sliding action, which causes that the efficiency of the drive is smaller than 90 per cent, sometimes even than 50 per cent. The worms can differ according to the direction of the teeth twist. If the twist goes along the axis clockwise, it is called a right hand worm, if it goes anticlockwise, it is a left hand worm. The terminology is the same as with screws. Direction of transmission One of the differences between worm drive and most other types of gearing systems is that the direction of transmission is not reversible. This means that the worm can drive the gear, but the gear cannot drive the worm, because the friction in this direction is too high. This can be useful when we need to eliminate the reverse action. Such gear systems are called self-locking. The self-locking properties appear when the tangent of the helix angle is less than the coefficient of friction. Vocabulary worm drive šnekový převod worm šnek gearing system převodový systém gear ozubené kolo helical gear kolo se spirálovým ozubením resemble podobat se screw vrut 24.11.2009 Strana 11/18
in the axial direction ve směru osy helix angle úhel stoupání šroubovice intermesh proplétat se gear ratio převodový poměr disadvantage nevýhoda sliding prokluz efficiency výkon drive pohon differ lišit se teeth twist zakřivení zubů axis osa clockwise ve směru hodinových ručiček counterclockwise proti směru hodinových ručiček right hand worm pravochodý šnek left hand worm levochodý šnek friction tření reverse action opačný chod self-locking samosvorný, samozajišťovací property vlastnost tangent tangens coefficient součinitel Obrázky a fotografie použité v této kapitole pochází ze stránek {Wikimedia Commons XH http://commons.wikimedia.org}, kde byly zveřejněny pod licencí {GNU FDL XH http://commons.wikimedia.org/wiki/commons:gnu_free_documentation_license}. 24.11.2009 Strana 12/18
6 Planetary gears 6.1 Planetary gears Planetary gearing, also called epicyclic gearing, is a system, where one or more outer gears revolve around one central gear. Description The planetary gearing system consists of two or more gears. The central gear is called the sun, while the outer ones are called planet gears. Planet gears rotate around the sun gear on an arm, which can be stationary or movable as well. Sometimes the system is placed inside a ring gear, called annulus. Although the axes of the gears are usually parallel, some exceptions may occur. For example the gears used in some pencil sharpeners are placed at angle. Gear ratio Usually, one of the components (sun, planetary carrier or annulus) is stationary, one of them is used as an input and the third one as an output. If the carrier is stationary, than the sun gear works as an input and planetary gears rotate around their axes. The gear ratio depends on the number of teeth of individual gears and can be calculated as gr = s/p, where s is the number of the sun gear teeth and p of the planet gear teeth. As an example we can take 24-tooth sun gear and 16-tooth planet gears. Then gr = 24/16 = 3/2. The minus sign means a change in the direction. In this case one three clockwise turns of the sun gear produce 2 counterclockwise turns of the planet gears. The rotation of the planet gears drives the annulus. Similarly, the gear ratio can be calculated gr = p/a, where a is the number of the teeth of the annulus. Advantages and disadvantages The main advantage of planetary gearing is its small size, especially in comparison with quite big gear ratios. It is also very efficient with a very small loss of energy inside the gearbox. Its operating life is usually longer than the lifetime of other gear systems. Coaxial shafting can also be considered an advantage. The main disadvantage is quite complex design, which makes it accessible only with difficulties. Its production is also much more expensive than the production of other types of gearing systems. Vocabulary planetary gearing, epicyclic gearing planetový převod outer gear, planet gear vnější ozubené kolo central gear, the sun vnitřní ozubené kolo rotate rotovat stationary nehybný movable pohyblivý ring gear, annulus ozubený věnec 24.11.2009 Strana 13/18
axe osa pencil sharpener ořezávátko na tužky angle úhel carrier nosič input přívod output výstup gear ratio převodový poměr, poměr převodových stupňů ring gear ozubený věnec gear ratio převodový poměr clockwise turn otáčka ve směru hodinových ručiček counterclockwise turn otáčka proti směru hodinových ručiček drive pohánět advantage výhoda disadvantage nevýhoda efficient výkonný loss of energy ztráta energie gearbox převodovka operating life, lifetime životnost coaxial souosý shafting hřídelové vedení accessible přístupný production výroba Obrázky a fotografie použité v této kapitole pochází ze stránek {Wikimedia Commons XH http://commons.wikimedia.org}, kde byly zveřejněny pod licencí {GNU FDL XH http://commons.wikimedia.org/wiki/commons:gnu_free_documentation_license}. 24.11.2009 Strana 14/18
7 Diferential 7.1 Differential A differential is a device, which is capable to transmit torque and rotation through three shafts. This is usually achieved by the means of gears. It receives one input which results into two outputs. Sometime differentials work also the other way: two inputs are combined into one output, which is their difference. Application Differentials are usually applied in automobiles and other vehicles with the need to rotate their wheels at different speeds. This situation occurs especially when turning the inner wheel travels a shorter distance and needs to be slower then the outer wheel, which travels a longer distance. Vehicles, such as karts, which have no differential, have to drag the outer wheel on the road surface when cornering, which makes their handling difficult, damages their wheels or tyres, powertrain, as well as roads. Way of operation The differential is placed on the propeller shaft (also called the drive shaft), which receives the torque from the engine. A spiral bevel gear is driven by the end of the propeller shaft and meshes with a large crown gear. The crown gear is attached to a carrier, containing a small planetary gearing system. In this system, two sun gears sharing the same axis with the crown gear, drive the half shafts with vehicles driven wheels. The sun gears also mesh with a planet gear on a perpendicular axis and changes orientation as the ring gear rotates. A lot of differentials have two planet gears opposite to each other (or even more). If the resistance at both wheels of the vehicle is equal, the planet gear does not rotate, and both wheels run at the same speed. But if the resistance of one of the wheels is bigger, than part of the torque is transmitted onto the planet gear, which starts rotating and slows down the rotation of the particular half shaft. Vocabulary differential diferenciál device zařízení capable schopný transmit přenést torque točivý moment, kroutivá síla shaft hřídel by the means of prostřednictvím gear ozubené kolo input vstup output výstup 24.11.2009 Strana 15/18
difference rozdíl turn corner turning, cornering zatáčení inner vnítřní outer vnější distance vzdálenost application použití apply použít vehicle vozidlo kart vozík drag táhnout surface povrch handling zacházení damage poškodit tyre pneumatika powertrain hnací ústrojí propeller shaft (drive shaft) hnací hřídel spiral bevel gear kuželové ozubené kolo se zakřivenými zuby drive (drove, driven) hnát, pohánět crown gear věncové ozubené kolo planetary gearing system planetární převodový systém sun gear centrální kolo planet gear planetové kolo axis osa half shaft hnací hřídel polonápravy, hnací poloosa mesh proplétat se perpendicular kolmý resistance odpor equal stejný speed rychlost slow down zpomalit Obrázky a fotografie použité v této kapitole pochází ze stránek {Wikimedia Commons XH http://commons.wikimedia.org}, kde byly zveřejněny pod licencí {GNU FDL XH 24.11.2009 Strana 16/18
http://commons.wikimedia.org/wiki/commons:gnu_free_documentation_license}. 24.11.2009 Strana 17/18
8 Rejstřík 24.11.2009 Strana 18/18