LIMITS , FITS , TOLERANCES&

SURFACE ROUGHNESS

SUBMITTED TO DEEPAK SHARMASUBMITTED BY

DEVANSH CHATURVEDIME-I(III YEAR)1214340056

OBJECTIVE INTRODUCTION AND STUDY OF- 1.STANDARDIZATION 2.INTERCHANGEABILITY 3.LIMITS 4.FITS 5.TOLERANCE 6.IS STANDARDS 7.LIMIT SYSTEM & SURFACE ROUGHNESS

STANDARDIZATION Standardization or standardization is

the process of developing and implementing technical standards. Standardization can help to maximize compatibility, interoperability, safety, repeatability or quality. It can also facilitate commoditization of formerly custom processes.

There are at least four levels of standardization: 

Compatibility InterchangeabilityCommonalityReference These standardization processes create compatibility, similarity, measurement and symbol standards.

There are typically four different techniques for standardization

Simplification or variety control Codification Value engineering Statistical process control

Effects On firms-shifting of competition from integrated systems

to individual components within the system.When the shift toward competition based on individual components takes place, firms selling tightly integrated systems must quickly shift to a modular approach, supplying other companies with subsystems or components.

On customers-Standards increase compatibility and interoperability between products & reduced uncertainty

Greatest downside of standardization for consumers is lack of variety. It is also possible that a consumer will choose a product based upon a standard that fails to become dominant. In this case, the consumer will have spent resources on a product that is ultimately less useful to him or her as the result of the standardization process.

Contd. On technology-Increased adoption of a new

technology as a result of standardization is important because rival and incompatible approaches competing in the marketplace can slow or even kill the growth of the technology.

The negative effects of standardization on technology have to do with its tendency to restrict new technology and innovation. Standardization in an area also rules out alternative technologies as options while encouraging others.

Interchangeability. The term refers to the mass production of

identical items within limits of sizes. Significance-In modern days production,

mass production technique has broken up into several smaller activities and tis led to specialization. Any one component selected at random should be assemble correctly with any other mating component, that too selected at random.

Interchangeability ensures increased output with reduced production cost.

Examples

LIMITS Definition-The two extreme permissible

size for any object is known as limit. The minimum size is called Lower limit. The maximum size is called Upper limit.

Terminologies

Nominal size-It is the close approximation to some basic size. Nominal size is referred to as a matter of convenience.

Basic size-it is described for the sake of convenience & is the size of parts in relation to which all limits of variation are determined. When used for fits between shaft and hole, some basic size is specified for both shaft and hole.

Zero line-represents the basic size such that deviation from the basic size is zero.

ALLOWANCES Allowances-difference between hole

dimension and shaft dimension for any type of fit is called allowance.

Maximum allowances=[ largest hole size-minimum shaft size.]

Minimum allowance=[ largest shaft size-smallest hole size.]

For clearance fit, allowance is positive For interference fit, allowance is negative

DEVIATION Deviation-defined as algebraic difference

between maximum limit or size(of either hole or shaft) and corresponding basic size.

Types- 1-Upper deviation-Algebraic difference

between maximum limit of size and corresponding basic size.for hole it is represented as ‘es’ & ‘ES’ for shaft.

Upper deviation is positive when maximum limit of size is less than basic size and vice versa.

2-lower deviation-algebraic difference between minimum limit of size(of either hole or shaft) and corresponding basic size.

3-Fundamental deviation-it is closest to zero line.it may be upper or lower deviation,

This fixes the position of zero line.

FIGURE

FITS Definition-during assembly of two

mating parts, there maybe either tightens or looseness between them. This degree of tightness or looseness between them is termed as fits.

Types- 1.Clearance fit 2.Interfernce fit.

3.Transition fit.

1-Clearance fit-In this,the maximum permissible shaft diameter is smaller than the minimum permissible limit of hole.So that the shaft can rotate or slide in hole.

Example-shaft rotating in a bearing.

2.Interfernce fit-the minimum permissible shaft diameter is larger than the maximum allowable diameter of hole.

Example-bearing bushes in their housing and smaller end of connecting rod in engine.

3.Transition fit-diameter of maximum allowable hole is larger than that of the smaller limit of shaft but the smaller limit of hole is smaller than the largest limit of shaft.

Example-coupling rings and recesses.

FIGURE

TOLERANCE Definition-Defined as difference

between the high and low limits of size. Types- 1.Unilateral tolerance-It is the one in

which total allowable variation from the basic size is in one direction only(may be positive or negative)

2.Bilateral tolerance-it is the one in which allowable variation from the basic size is in both positive and negative direction.

FIGURES.

IS STANDARDS Indian standard (IS : 1919-1963) the

system of limits and fits comprises 16 grades of fundamental tolerance ie. Grades for manufacture and 25 fundamental deviation types indicated by letter symbols for both holes and shafts in diameter range from 1mm to 400mm.

A to Z for holes A to z for shafts. There are 16 tolerance grades designated as IT

1 to IT 16. These are called standard tolerance.

Bureau of Indian Standards Logo

LIMIT SYSTEM BASES OF LIMIT SYSTEM- 1.Hole basis-when the hole is kept as a

constant member and different fits are obtained by varying the shaft size, then the limit is said to be on a hole basis.

2.Shaft basis-when the shaft is kept as a constant member and different fits are obtained by varying hole size, then limit system is said to be on a shaft basis.

SURFACE ROUGHNESS Referred as primary texture. Caused due to the irregularities in the

surface roughness which results from inherent action of production process.

Deemed to include transverse feed marks and irregularities within them.

Methods to measure surface roughness

1.Centre line average method(CLA METHOD) defined as average value of ordinates between surface and mean line, measured on both sides of it.

CLA value or Ra(in microns)= [(y1+y2…+yn)/n]

Y1,y2…yn are ordinates measured on both sides of mean line

N=number of ordinates

2.ROOT MEAN SQUARE METHOD- Defined as square root of arithmetic mean of square

of ordinates. R.M.S VALUE

(in microns)=[(Y12+Y2

2…+YN2)/N)]1/2