1) How do you identify a planetary gear? A gear whose axis itself is revolving is called a planetary gear. In some epicyclic gear trains, we may find two or more. described a number of gear applications, typical ones are shown in Figs. used in high speed and high load application in all types of trains and a wide range. The velocity ratio of an epicyclic gear train is determined by the following methods: (a) Tabulation method; (b) Formula method; and (c) Instant centre method or.
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The limit on velocity ratio depends on gear pair for example:. Helical gearing, in which the teeth are cut at an angle with the axis of rotation, was developed after spur gearing and has the advantages that has the smoother action and tends to be quieter Fig. Following empirical formulae are used to estimate effective temperature T F. There is no limit to the speed reduction ratio that can be achieved using gearing; but larger ratio must be obtained using multi-stage reduction. If sufficient backlash has not been provided to take care of the differential thermal expansion, the teeth will bind, with disastrous results.
A worst situation from tribology point of view i. To design gear, we need to estimate W n Fig. But for home applications, we use N-N gear pair, where rate of heat generation is relatively low.
In simple gear mesh, a maximum ratio in order of 7: An efficient method of achieving high reduction ratios in minimum space is the use of planetary gearing. Nptle adding teeth of the proper shape on disk, power can be transmitted without slip at uniform rate.
Gear surfaces coated with boundary additive or antifriction coating. Surface failure of gear tooth occurs due to very high local contact stresses. Typical values of gear efficiencies are listed in Table 6. In addition, helical gears causes axial thrust force and impose load on bearings. Friction between gear pair occurs due to sliding between meshed teeth and churning of lubricant. The sliding friction losses are related to the coefficient of friction, normal load and sliding velocity on the contact surfaces, while the rolling trainx losses occur due to the formation of an elastohydrodynamic EHL film.
Due to sliding, power loss occurs and transmission efficiency decreases. This happens at pitch point.
In a geared system, the total power loss is comprised of two groups of losses: For high speed reduction, two stage or three stage construction are preferred, otherwise gear wheel size increases, which increases the gearbox size. Maximum contact stress is equal to p max. Normal load on tooth. But sometime failure of lubrication grar failure, filter chocking, excessive leakage occurs and gear materials must be able to handle such extreme situations.
In absence of lubricant additives and antifriction coating, gears will be subjected to direct friction. In this year, friction and wear are influenced by the ability of lubricant to create protective boundary films on gear tooth with chemical and physical reactions, and by its viscous characteristics.
Rolling and sliding in gears: Bore diameter of pinion is 17 mm, and bore dia of gear is 20 mm. In manufacturing point of view also, external gears are simpler than internal gears. To reduce the value of T feffective lubrication that maintains low friction coefficient is year.
On changing center distance, line of action still remains tangent to both base circles but slope changes. Sliding action in spur gears. At all other contact points as shown in Fig.
This is hypothetical situation which occurs rarely in extreme conditions. Friction losses can be divided into two major categories: Generally, a gear pair acts as a speed reducer aiming torque amplification at output shaft. Concept of involute profile. Internal gears are generally more efficient since the sliding velocity along the profile is lower than equivalent external gears.
Hypoid gears are widely used nptep many power trains to transfer power between two non-intersecting crossed axes.
Therefore, contact stress is. Gears in action . Composite roughness depends on gear manufaturing process as given in Table 6. Tooth curves of the mating teeth need to be tangent to each other as shown in Fig. Nominal pressure angle is given by: Higher entraining velocity increase “spin losses”. In helical angle greater than 15 degrees, the tooth bending capacity generally begins to drop off due to the fact that the tooth thickness decreases rapidly.
The total mechanical loss is then given as the sum of losses from all nptep meshes and bearings. Lack of lubricant, may initiate scuffing failure.
Gear life depends on effective lubrication, which can be quatified by minimum film thickness to R compositeas shown in Fig. It is interesting to note that oil sample collected after 3 hours of operating gear at rpm no load conition show wear Fig. Long service life, free from wear problems depends on lubrication system; its ability to keep gear cool, and to deliver lubricant free from hard particles filter with 5 micron rating.
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