Presentation on theme: "8-6 Fits and allowances The FIT between two mating parts is the relationship between them wrt the amount of clearance or interference present when they."— Presentation transcript:
18-6 Fits and allowances The FIT between two mating parts is the relationship between them wrt the amount of clearance or interference present when they are assembled.Clearance fit: A fit between mating parts having limits of size so prescribed that a clearance always results in assembly.Interference fit: A fit between mating parts having limits of size so prescribed that an interference always results in assemblyTransition fit: A fit between mating parts having limits of size so prescribed as to partially or wholly overlap, so that either a clearance or an interference may result in assembly.
3Allowance: an intentional difference between the maximum material limits of mating parts. It is the minimum clearance (positive allowance) or maximum interference (Negative allowance) between parts.Basic size: The size to which limits or deviations are designed. The basic size if the same for both members of a fit.Deviation: The algebraic difference between a size and the corresponding basic size.Upper deviation: The algebraic difference between the maximum limit of the size and the corresponding basic size.Lower deviation: The algebraic difference between the minimum limit of size and the corresponding basic size.Tolerance: The difference between the maximum and minimum size limits on a part.Tolerance zone: A zone representing the tolerance and its position in relation to the basic size.Fundamental deviation: The deviation closest to the basic size.
4Description of fits Running and sliding fits: Running and sliding fits for which tolerance and clearances are given in the appendix represent a special type of clearance fit. These are intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes. Location fits: are intended to determine only the location of mating parts. Clearance fits: intended for parts that are normally stationary but can be freely assembled or disassembled. Transition fits: are a compromise between clearance and interference fits when accuracy of location is important but a small amount of clearance or interference is permissible. Interference fits: are used when accuracy of location is of prime importance.Drive and force fits: constitute a special type of interference fit, normally characterized by a maintenance of constant bore pressure throughout the ranges of sizes.
5Interchangeability of parts: the basis for mass production No part can be manufactured to exact dimensions as tool wear, machine variations, and human factors contribute to deviation from perfection.Three basic approaches to manufacturing:The completely interchangeable assembly. Parts are designed with tolerances.The fitted assembly. Mating features are fabricated either simultaneously or wrt one another. Individual members of mating features are not interchangeable.The select assembly. Parts are mass produced, but member of mating parts are individually selected to provide the required relationship to one another.
6Standard Inch fits: are designated for design purposes and not shown on shop drawings. RC Running and sliding fitLC locational clearance fitLT locational transition fitLN locational interference fitFN force or shrink fitThese letter symbols are used with numbers to represent classes of fit. For example FN4 represents class 4 force fit.Appendix tables in the textbook give the limits of size for the mating parts.
7Running and sliding fits: RC RC1 precision sliding fitRC2 Sliding fitRC3 Precision running fitRC4 Close running fitRC5 and RC6 Medium running fitsRC7 Free running fitRC8 and RC9 Loose running fits
8Locational clearance fits: Intended for parts that are normally stationary but that can be freely assembled or disassembledLC1 to LC4 minimum zero clearance (theoretical)LC5 and LC6 small minimum clearance(LC1-LC6 nonrunning parts)LC7 and LC11 loose clearance for assembly of bolts and similar parts
9Locational Transitional fits are a compromise between clearance and interference fits LT1 and LT2 slight clearance. Assembly by pressure or light hammer blowsLT3 and LT4 virtually no clearance also referred to as “easy keying fit” for shaft keys. Assembly is by pressure or hammer blows.LT5 and LT6 slight interference. These fits are useful for heavy keying, for ball race fits subject to heavy duty and vibration, and as light press fits for steel parts.
10Locational Interference Fits are used where accuracy of location is of prime importance. LN1 and LN2 Light press fitsLN3 heavy press fitLN4 to LN6 press fits primarily intended for press fits for more elastic or soft materials such as light alloys and more rigid plastics
11Force or shrink fits are normally characterized by maintenance of constant bore pressure throughout the range of sizesFN1 light drive fit requiring light assembly pressure and produces more or less permanent assembliesFN2 medium drive fitFN3 heavy drive fitFN4 and FN5 force fits suitable for parts that can be highly stressed and/or for shrink fits where the heavy pressing forces required are impractical.
12Basic Hole system is recommended for general use The basic size is the design size of the holeThe tolerance will be plusThe design size for the shaft will be the basic size minus the minimum clearance or plus the maximum interference and the tolerance will be minus as given in the tables in the AppendixExample: 1-in RC7 fit, values of , .0025, and given; hence the limits areholeФ-.0000Shaft Ф-.0012
13Basic Shaft system: the basic size is the design size for the shaft and the design size for the hole is found by adding the minimum clearance or subtracting the maximum interference from the basic sizeSymbol:the letter S following the fit symbol RC7SExample: 1-in RC7S fit, values of , .0025, (see previous example)Hole: Ф-.0000Shaft: Ф-.0012
14Preferred Metric Limits & fits International tolerance grade for internal & external dimensionsGrade 1: precise ……Grade 16 courseA fundamental deviation establishes the position of the tolerance symbol wrt basic sizeTolerance position letters give this fundamental deviation.Internal dimension: Capital letterExternal dimension: lower case letterExample: 40H8 international tolerance gradeBasic size Fundamental deviation (internal)
15Tolerance symbol: Metric Combination of IT grade number and tolerance position letterInternational tolerance grade table in the appendixHole basis fit: basic size is minimum size of holeExample: Ф25H8/f7 fit Use the table 48 in appendixHole limits ФShaft limits ФMin interference (fit)Max interference (fit)
16Tolerance symbol: metric Shaft basis fits system: Basic size is the maximum shaft sizeAppendix 49 in book gives informationExample: Ф16C11/h11 fitHole limits: ФShaft limits: ФMin clearance (fit)Max clearance (fit)