Unit II System of Limits and Fits Limit Gauges

Unit IISystem of Limits and FitsLimit GaugesSystem of Limits and Fits : Introduction, Normal size, tolerance limits, deviations, allowance, fits and their types Unilateral and bilateral tolerance system, Hole and shaft basis systems interchangeability and selective assembly. Indian standard institution system British standard system, International standard system for plain and screwed work.Limit Gauges: Taylors Principle Design of go and No go gauges, plug, ring, snap, gap, taper, profile and Position gauges.Introduction 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., the parts must fit with each other in a desired way.

The standard reference temperature is 200 C for industrial measurements and, consequently, for dimensions defined by the system.

Due to the inevitable inaccuracy of manufacturing methods, a part cannot be made precisely to a given dimension, the difference between maximum and minimum limits of size is the tolerance.

When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit.

Terminology:NOMINAL (NORMAL) SIZE: It is the size of a part specified in the drawing. BASIC SIZE: It is the size of a part to which all limits of variation are determined.ACTUAL SIZE: It is the actual measured dimension of a part. Nominal and basic size are often the same.

Zero Line: It is the straight line corresponding to the basic size. The deviations are measured from this line.

Tolerance LimitsThere are two extreme possible sizes of a component.The largest permissible size for a component is called upper limit and smallest size is called lower limit.

DEVIATION Lower & UpperIt is the algebraic difference between any given size and actual size.ACTUAL DEVIATION: It is the algebraic difference between the actual size and the basic size.

Max Hole size Basic Size = Upper DeviationMin Hole size Basic Size = Lower DeviationHOLEMax shaft size Basic Size = Upper DeviationMin shaft size Basic Size = Lower DeviationSHAFT

ToleranceHow to decide tolerance?Functional requirements of mating partsCost of productionAvailable manufacturing process

Choose as coarse tolerance as possible without compromising functional requirementsProper balance between cost and quality of partsPOSITIONAL TOLERANCESTwo types of positional tolerances are used:Unilateral tolerancesBilateral tolerancesWhen tolerance is on one side of basic size, it is called unilateral and if it is both in plus and minus then it is known as bilateral tolerance.

FITSthe relation resulting from the difference between the sizes/dimensions of mating components before assembly is called a fit.

TYPES OF FITSCLEARANCE FIT: There is a clearance or looseness in this type of fits. These fits maybe slide fit, easy sliding fit, running fit etc.

Maximum shaft dimension < Minimum hole dimension

Figure Source: Technical Drawing, By David L. GoetschTYPES OF FITSINTERFERENCE FIT: There is an interference or tightness in these type of fits. E.g. shrink fit, heavy drive fit etc.

Maximum Hole size < Minimum Shaft size

TRANSITION FIT: In this type of fit, the limits for the mating parts are so selected that either a clearance or interference may occur depending upon the actual size of the mating parts.

Obtained by overlapping of tolerance zones of shaft and hole Does not guarantee neither clearance nor interference fit

BASIS OF LIMIT SYSTEMHOLE BASIS SYSTEM: In this system, the hole is kept as a constant member and different fits are obtained by varying the shaft size.SHAFT BASIS SYSTEM: In this system, the shaft is kept as constant member and different fits are obtained by varying the hole size.

To obtain different types of fits, it is general practice to vary tolerance zone of one of the mating parts

A fit is indicated by the basic size common to both components, followed by symbol corresponding to each component, the hole being quoted first.

E.g. 45 H8/g7

DIFFERENT LIMIT SYSTEMSThe Newall systemBritish Standard systemInternational Federation of National Standardization Association (ISA) systemISO systemBureau of Indian Standard (IS:1234 - yyyy) system

Source: Comprehensive Workshop Practice, R. K. RajputThe Newall systemISO System of Limits and Fits

IS 919 Indian Equivalent Standard for representing system of limits and fits

H : lower deviation of hole is zeroh : upper deviation of shaft is zeroE.S. upper deviationE.I. lower deviationRepresentation of Fit / Assembly Letter Symbol The selection of letter freezes one limit of hole / shaft (how much away from Basic size)45 E8/e7Basic SizeOne can have different possible combinations; eg. 45H6g7, 45H8r6, 45E5p7

FITS AND TOLERANCESThe components of the toleranced dimension shall be indicated in the following order:a) the basic size, andb) the tolerance symbol.If, in addition to the symbols it is necessary to express the values of thedeviations or the limits of size, the additional information shall be shownin brackets.

Permissible deviation

If a dimension needs to be limited in one direction only, this should be indicated by adding min or max to the dimension.

The upper deviation or the upper limit of sizeshall be written in the upper position and the lowerdeviation or the lower limit of size in the lowerposition, irrespective of whether a hole or a shaft istoleranced.

The tolerance symbol for the hole shall be placed before that for the shaft or above it, the symbols being preceded by the basic size indicated once only.

Indication of Tolerances on Angular Dimensions

FITS APPLICATIONS

Estimate kind of fit

30H7s6

30H8e6

30F8r6

30G7r6

STUFFING BOX