interchangeability.pptx

7/29/2019 Interchangeability.pptx

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Machined parts

I.C engine


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Fig. Coupling

Fig. Turbine


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Fig. Lathe machine


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if a shaft is to rotate in a hole, there must be enough

clearance between the shaft and hole to allow the oil film

to be maintained for lubrication.

If the clearance is too small, excessive force would be

required in rotation of shaft.

If clearance is too wide, there would be vibrations and

rapid wearand ultimate failure


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Interchangeability

This term is normally employed for the mass production of

identical items.

The sizes of parts are within a close degree of accuracy.

But even then there will be small variations.

If the variations are within certain limits, all parts of

equivalent size will be equally fit for operating in machines

and mechanisms.


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Generally in engineering, any component manufactured is required to fit

or match with some other component.

The correct and prolonged functioning of the two components in match

depends upon the correct size relationships between the two, i.e., theparts must fit with each other in a desired way

Limit and Fit

Fig. Machined parts and assembly


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Nominal size: Size of a part specified in the drawing as a matter of

convience.

Limits of sizes: There are two extreme permissable sizes for a

dimensions of the part as shown in Fig.

The largest permissable size for a dimesnsion of the part is called

upper limit, where as smallest size of the part is known as lower

limit.

Fig.


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Tolerance: It is the difference between the upper and lower limit of the

dimension. The tolerence can be unilateral and bilateral.

When tolerance is allowed one side of the nominal size. Then it is said to

have unilateral system of tolerence.

When the tolerence is allowed on both sides of the of the nominal size,

then it is said to have bilateral system of tolerance.


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Basic size It is the size of a part to which all limits of variations (i.e.

tolerances) are applied to arrive at final dimensioning of parts. The

nominal size or basic size of a part is often same.

Actual size It is the actual measured dimension of the part.


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Allowance It is the difference between the basic dimensios of the

mating parts. The allowance may be positive or nagative .

When the shaft size is less than the hole size allowance is positive

When the shaft size is greater than the hole size allowance is

negative


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Tolerence Zone: It is the zone between the maximim limit and minimum

limit size as shown in Fig

Zero line: It is the straight line corresponding to the basic size. The

deviations are measured from this line.

Upper deviation: It is the difference of dimension between the maximum

possible size of the component and its nominal size.


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Fundamental deviation : It defines the location of the tolerance zone

with respect to the nominal size. For that matter, either of the deviations

may be considered.

Lower deviation : Similarly, it is the difference of dimension

between the minimum possible size of the component and its

nominal size.


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In this type of fit, the shaft of largest possible diameter can

also be fitted easily even in the hole of smallest possible

diameter

Clearance Fit

Fig. Schematic view of fit system

Fit: The degreeof tightness or looseness between two mating

parts is known as fit of the part.


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Transition Fit : In this case, there will be a clearance between

the minimum dimension of the shaft and the minimum

dimension of the hole.

Fig. Schematic view of fit systemInterference Fit

In this case, no matter whatever may be the tolerance level in

shaft and the hole, there is always a overlapping of the mating

parts. This is known as interference fit. Interference fit is a form

of a tight fit.


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Hole and Shaft Basis System

HOLE BASED SYSTEM

Size of hole is kept constant, shaft size is varied to get

different fits.

Fig. Hole basis system


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SHAFT BASED SYSTEM

Size of shaft is kept constant, hole size is varied to get

different fits

Fig. Shaft basis system

interchangeability.pptx

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