system of limits, fits and tolerancing.pptx

8/22/2019 System of Limits, Fits and tolerancing.pptx

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System of Limits, Fits and

tolerancing


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Tolerance

Tolerance is defined as the amount by whichthe job is allowed to go away from accuracy

and perfectness without causing any

functionable trouble, when assembled with its

mating parts and put it into actual service.

Primary purpose of tolerances is to permit

variation in dimensions without degradation of

the performance beyond the limits establishedby the specification of the design.

Tolerance may be unilateral or bilateral.


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Why Tolerance is specified

The tolerance is specified because it is

impossible to have actual dimensions due to:

Variations in the properties of the materialbeing machined, introduce errors.

The production machines have some

inherence problems and limitations.

Human effect, operator may do imperfect

settings.


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Limits: These are the maximum and minimum

permissible size of the part.

Go Limit: It refers to upper limit of the shaft

and upper limit of a hole. Corresponds tominimum material condition.

NoGo Limit: It refers to the lower limit of the

shaft and upper limit of the hole. Corresponds

to min. material condition.


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Upper deviation: Maximum limit of size basic size. It is positive when maximum limit of

size > basic size and vice versa.

Lower deviation: Minimum limit size basic

size positive when minimum limit of Size >

basic size and vice versa

Fundamental deviation: this is the deviation

either the upper or the lower deviation, which

the nearest one to the zero line


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Fits Fits: When two parts are to assemble, the

relation resulting from the difference betweenthe size before assembling is called fit.

Basic size of a fit: It is that basic size which is

common to the two parts of a fit. Clearance: This is the difference between the

size of the hole and shaft, before assembly,when the difference is positive (i.e. shaft smaller

than the hole). Interference: This is the arithmetic difference

between the sizes of the hole and the shaftbefore assembly, when the difference is negative.


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Type of fit:

Depending upon the actual limits of the hole orshaft, fits may be classified into the following 3

categories

Clearance fit

Interference fit

Transition fit


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Clearance fit: In this type of fit, the largest permitted

shaft diameter is smaller than the diameter of the

smallest hole, so that the shaft can rotate or slide

through the difference degrees according to purpose ofmating members Ex.Bearing and shaft.

Interference fit: In this type of fit, the minimum

permitted diameter of the shaft is larger than the

maximum allowable diameter of the hole. In this case theshaft and the hole members are intended to be attached

permanent and used as a solid component but according

to the application of this combination, this type of fit can

be varied. Ex. Bearing bushes, which are in interferencefit in their housing.

Transition fit: In this type of fit, the diameter of the

largest allowable hole is greater than that of the

smallest shaft, but the smallest hole is smaller than the

largest shaft, so that small positive or negative clearance


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Allowance

It is the prescribed difference between thedimensions of two mating parts for any type of fit.

It is intentional difference between the lower limit

of hole and higher limit of shaft.

Allowance may ne negative or positive.

Positive allowance is called clearance and the

negative allowance is called interference.

system of limits, fits and tolerancing.pptx

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