LIMITS,FITS & TOLERANCES

In the production of engineering components also this fact is applicable. They may found to be closely similar. Every process is a combination of three elements namely MAN,MACHINE and METHOD. A slight change in any of the three will constitute a change in the PRODUCT. Thus when the process is in control most of the item will be on or about the mean value and other values, will be described in a more or less symmetrical way about the mean value and remaining with in expressible limits. It is therefore obvious that it is impossible to produce a part to exact size and some allowances known as TOLERANCE has to be allowed.

“NO TWO THINGS IN THE WORLD CAN BE IDENTICAL”

Such a system of giving tolerance to a dimension for the manufacturing is known as LIMIT SYSTEM.

TERMINOLOGY FOR LIMITS AND FITS

A pair of mating components are generally called SHAFT and HOLE.SHAFT A term used by convention to designate all the external features of a part including those which are not cylindrical. A shaft is commonly referred as male component.Eg : plugs, axles, spindles, locating spigots.

HOLE A term used by convention to designate all the internal features of a part including those which are not cylindrical. It is dented by capital letters. A hole is referred to the female component.Eg: bore, recess.

SIZE A number expressing the numerical value of a dimension in appropriate units is called size of a component.BASIC SIZE or NOMINAL SIZE It is the decided size of a component from which the limits are divided by the application of tolerance. Basic size is same for the hole and the shaft.ACTUAL SIZE The size of a finished component which is actually measurable with an instrument is known as actual size.DEVIATION The algebraic difference between the actual size of a component and its basic size is known as deviation. This can be of two types.UPPER DEVIATION The algebraic difference between the maximum limit size and corresponding basic size. It is denoted by ‘Es’ for hole and ‘es’ for a shaft.Es – Ecart superior.

LOWER DEVIATION The algebraic difference between the minimum limit size and corresponding basic size is called lower deviation. It is denoted by ‘Ei’ for hole and ‘ei’ for the shafts.ZERO LINE The line which represents the basic size of components in an assembly or a drawing is known as zero line. In limits and fits all the deviations are shown with respect to the zero line. LIMITS Limits are the extreme permissible dimension of any part. There are two extreme permissible sizes for a dimension. The largest permissible size is called upper or higher limits (H) where as the smallest size is known as the lower limit (L).ALLOWANCE The diff. b/w the maximum shaft and size and minimum hole size. It can be either positive or negative.

TOLERANCETolerance is the permissible variation in dimension of a component.Tolerance = Upper limit – Lower limitThe difference b/w the upper limit dimension and the lower limit dimension on the basic size of the component is termed as tolerance.

Eg: 11 ± 0.1 mm.Here basic size = 11mm11 + 0.1 = 11.1mm is the max. limit size.(upper limit size)11 – 0.1 = 10.9 mm is the min. limit size.(lower limit size)→ 11.1 – 10.9 = 0.2mm is the tolerance.

DIFFERENT WAYS OF EXPRESSING TOLERANCE.Tolerance is basically specified in to two forms.i) Unilateral tolerance.ii) Bilateral tolerance.

UNILATERAL TOLERANCE.If all the tolerance is allowed to vary on one side

of the basic size, the system is called as unilateral.

Ie. Tolerance lies wholly on one side of the basic size either above or below it.

Unilateral system is preferred in interchangeable manufacturing , especially when precision fits are required.

BILATERAL SYSTEMIf all the tolerance is allowed on both the side of basic size, then the system is called as bilateral system. The system is mainly used in mass production where machine setting is done for the basic size.

WHY TOLERANCE IS SPECIFIED?1)Variation in the properties of the materials being

machined introduces error.2)The production machines themselves have some in

accuracies to produce perfect parts.3)It is impossible for an operator to make perfect

settings. (i.e in adjusting tools and work pieces on the machines some errors are likely to creep in.)

USES OF TOLERANCE.1)It reduces machine time.2)It is used in mass production 3)Avoids rejection of components.4)Reduces the cost of product.

Relation b/w tolerance and cost. If the tolerance are made closer the

cost of production goes on increasing because to manufacture close tolerance parts we need,

5)Precision machines, tools, and materials.6)Trained and highly skilled operators.7)Tight inspection and more precise inspection

device.8)Close supervision and control is essential.

Difference b/w TOLERANCE and ALLOWANCE.

TOLERANCE

• It is the permissible variation in dimension of a part either a hole or a shaft.

• It is the difference b/w higher and lower limits of a dimension or a part.

• Tolerance is given on the dimension of a part as it is not possible to make a part to the exact specified dimension

ALLOWANCE

• It is the prescribed dimension of two mating parts.

• It is the intentional difference b/w the lower limit of the hole and higher limit of the shaft.

• Allowance is to be provided on the dimension of mating parts to obtain desired type of fit.

FITFit may be defined as a degree of tightness or looseness between the two mating parts to perform a definite function when they are assembled together. There are three basic types of fits are possible to achieve by varying the amount of allowance between the size of two mating parts.

FITS

CLEARNCE TRANSITION INTERFERANCE

CLEARANCE FIT In clearance fit shaft has smaller size than the hole. i.e. the largest permissible shaft diameter is smaller than the smallest hole diameter. The resulting allowance is always positive in value. INTERFERANCE FIT Here the shaft has a larger size than the hole size and an external force is required to press the shaft to hole for fitting. This fit is a permanent fit. The minimum permissible shaft diameter is larger than the maximum limit of hole. The resulting allowance has always a negative value. A hindrance will always be present , at the time of fitting.

TRANSITION FIT In this type of fit, the clearance fit is available for mating parts with one set of dimension when gives in limit system and interference fit is obtained with other set of dimension. The allowance varies from positive to negative corresponding to two given set of dimensions. Usually this fit provides either zero interference or a clearance. These are used where parts can be replaced without difficulty during minor repairs.

Basic Hole System or Hole BasisDefinition of the "Basic Hole System":

The "minimum size" of the hole is equal to the "basic size" of the fit.

Clearance = Hole – ShaftCmax = Hmax – SminCmin = Hmin – Smax

Both Cmax and Cmin >0 – Clearance fit

Both Cmax and Cmin <0 – Interference fit

Cmax > 0, Cmin < 0 – Transition fitAllowance = Hmin - Smax (i.e., Cmin)