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DESIGN DATA---PSG TECH 7.115

Q1]Define contact ratio ,effect on gears ,how can it be improved.

def as the length of the arc of contact to the circular pitch..

length of arc of contact is the ration of length of path of contact to cos alpha where alpha is the pressure angle and

length of path of contact is the sum of length of approach and length of path of recess .contact ratio =length of arc of contact / Pc

larger the contact ratio ,larger will be the overlap and smother will be the teeth action therefore larger contact ratio isdesired. by increasing the length of path of contact ,reducing module and increasing the pressure angle one canincrease contact ratio.

Q2]advantages of belt drive, v-belt

-power and motion can be transmitted over considerable shaft distances .

-cheaper for installation maintenance

-smother and silent operations

-moderate shocks can be absorbed also vibrations are damped up to a certain limit

-in case of accidental overloads belt slips protecting further transmissions system

-variable speed drive is possible using stepped or cone pulleys-reliable drive

Disadvantages of belt drive

-it is not a positive drive

-slip and creep are main problems of drive-dirt ,dust ,humidity affects performance of drive

-moderate power capacity

-higher overall dimensions

Q3]why are brakes generally shoe type whereas clutches disk type?

For cluthches area available for heat dissipation is small as compared to brakes because brakes are generally at outer

side of vehicle near the wheels therefore brakes can be of shoe type .it is wear to replace worn shoe brake. where asit is difficult to replace worn friction member of clutch hence clutch is generally of disk type to have ample area offriction member. for uniform wear the pressure should be uniform with shoe practically it is difficult to haveuniform operating pressure hence for clutch instead of shoe disk is preferred .but for brake if the pressure and wear

is not uniform still that does not matter a lot therefore brakes can have suitably a shoe an compared to a clutch.

Q4]short note on gear teeth failureTooth is considered as failed when it stops functioning properly it happens by two modes.

Tooth breakage ,surface destruction .

Corrosion chemical attack of lubricating oil causes corrosion of surface .

Abrasion dirt ,rust ,abrasion particles in the lubricating oil produce scratches on tooth surface causing abrasion of

surface.Initial pitting it is effect of shearing of high spot in intial stages of operations of anew gear .while shear high spot a

pit is produced on tooth surfacePitting-during service if the load cause the stress to cross surface endurance strength then fatigue cracks are

developed. On surface causing destructive pitting of the surface

Scoring-development of excessive heat causes stick slip phenomenon which is nothing but scoring.

Q] state assumptions in lewis eqn

- gear tooth is assumed to be quivalent to cantilever beam loaded at its free end.-the number of teeth in contact is assumed as one where as in reality contact ration is generaaly graeter than actual

applied load .

-effect of radial load Pr is neglected assuming higher factor of safety tangential load is assumed to be uniform

distributed over entire face width of gear tooth which is possible only when the tooth is perfectly machined and itshighly rigid.

Q] short note on materials used for sliding contact bearing.

-less coefficient of friction

-higher compressive endurance strength

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