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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This favorable behavior is utilized by intentionally creating protective surface coating. Helical crossed axis gears. This means mild wear is bound to occur in gear operations.

Epicyclic gear train nptel pdf merge

Friction losses can be divided into two major categories: Usually, following two types of lubrication mechanisms are commonly used for gear lubrication. Often gears are treated as pitch cylinder, which roll together without slip as shown in Fig. Higher value of T f cause scuffing failure of gears. In this topic, we shall discuss about the geometry of gears and then consider three different aspects of gearing namely elastohydrodynamic lubrication, tooth contact phenomena, and wear of gears.

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.

For rolling action, tangential velocities at point of contact must be equal to make sliding zero. But for home applications, we use N-N gear pair, where rate of heat generation is relatively low.


Following empirical formulae are used to estimate effective temperature T F. Difference between tooth space and tooth thickness is known as backlash.

At all other contact points as shown in Fig. Recommended oil viscosity [2].

NPTEL Phase II :: Tribology

Concept of involute profile. For high speed reduction, two stage or three stage construction are preferred, otherwise gear wheel size increases, which increases the gearbox size.

Gear surfaces coated with boundary additive or antifriction coating.

Long service life, free from wear problems depends on lubrication system; its geaf to keep gear cool, and to deliver lubricant free from hard particles filter with 5 micron rating.

On changing center distance, line of action still remains tangent to both base circles but slope changes. The internal gears eliminates the use of an idler gear, where it is necessary to have two parallel shafts rotate in the same direction.

Sliding and rolling friction losses at the loaded gear meshes and at the bearings largely define the load-dependent mechanical power losses. Insufficient backlash is sometimes the cause of excessive heat and wear. This friction process is characterized by solid-body friction as well as by fluid friction Fig.

In practice, sliding comes along rolling action. Friction between gear pair occurs due to sliding between meshed teeth and gar of lubricant. Most gears are in the hardness range of approximately Rc 30 to Rc 38 or Rc 55 to Rc Internal gears are generally more efficient since the sliding velocity along the profile is lower than equivalent external gears.

Typical values of gear efficiencies are listed in Table 6. The tooth load is simply the torque on a given gear divided by the gear pitch radius. Using AGMA pitting equation formula, determine the maximum contact stress. Gear life depends on effective lubrication, which can gesr quatified by minimum film thickness to R compositeas shown in Fig.


But sometime failure of lubrication pump failure, filter chocking, excessive leakage occurs and gear materials must be able to handle such extreme situations.

Ideally, rolling gears are required. Normal gear load W n for a given application depends on pitch diameter and face width. In this regime, 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.

Lack of lubricant, may initiate scuffing failure.

These dimensions are determined on the basis of tooth stresses, which are imposed by the transmitted tooth load. The limit on velocity ratio yrains on gear pair for example:. Composite roughness depends on gear manufaturing process as given in Table 6. Effect of Brinell hardness on allowable traind stress for through-hardness steel [2]. Maximum contact pressure at the contact point between two cylinders is given by:.

Therefore, contact stress is. To design gear, we need to estimate W n Fig.

Epicyclic gear train nptel pdf merge | sijthre

Sliding at point I. In addition, lower loads are imposed on the support bearings because of a decrease radial load component. This friction process is aided by the presence of small quantities of lubricant, at the point of friction.

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.