Fits and allowances Flra Hajdu B 406 hajdflsze

Fits and allowances Flóra Hajdu B 406 hajdfl@sze. hu

Content • Interchangeability of parts • Fits and allowances • Metric fits • Calculation of fits 2020. 10. 30. Hajdu Flóra

Interchangeability of Parts • Increased demand for manufactured products -> the development of new production techniques. • Interchangeability of parts became the basis for mass-production, lowcost manufacturing • it brought about the refinement of machinery, machine tools, and measuring devices. • design for 100% percent interchangeability. • No part can be manufactured to exact dimensions. • Tool wear, machine variations, and the human factor all contribute to some degree of deviation from perfection. • It is therefore necessary to determine the deviation and permissible clearance, or interference, to produce the desired fit between parts. 2020. 10. 30. Hajdu Flóra

Interchangeability of Parts • Modern industry has adopted 3 basis approaches to manufacturing • The completely interchangeable assembly • any and all mating parts of a design are toleranced to permit them to assemble and function properly without the need for machining or fitting at assembly. • The fitted assembly • mating features of a design are fabricated either simultaneously or with respect to one another. • Individual members of mating features are not interchangeable. • The selected assembly • all parts are mass-produced, but members of mating features are individually selected to provide the required relationship with one another. 2020. 10. 30. Hajdu Flóra

Fits and allowances • In order that assembled parts may function properly and to allow for interchangeable manufacturing, it is necessary to permit only a certain amount of tolerance on each of the mating parts and a certain amount of allowance between them. • The fit between two mating parts is the relationship between them with respect to the amount of clearance or interference present when they are assembled. • An allowance is an intentional difference between the maximum material limits of mating parts. It is the minimum clearance (positive allowance) or maximum interference (negative allowance) between such parts. 2020. 10. 30. Hajdu Flóra

Fits • Types: • 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 assembly. • Transition 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. 2020. 10. 30. Hajdu Flóra

Terminology • Basic Size: the size to which limits or deviations are assigned. • Deviation: the algebraic difference between a size and the corresponding basic size. • Upper Deviation: the algebraic difference between the maximum limit of 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. 2020. 10. 30. Hajdu Flóra

Terminology Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design 2020. 10. 30. Hajdu Flóra

Fits • Running and sliding fits • Drive and force fits • Locational fits • Clearance fits • Transition fits • Interference fits 2020. 10. 30. Hajdu Flóra

Running and sliding fits • A special type of clearance fit. • These are intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes. 2020. 10. 30. Hajdu Flóra

Drive and force fits • A special type of interference fit, normally characterized by maintenance of constant bore pressures throughout the range of sizes. • The interference varies almost directly with diameter • The difference between its minimum and maximum values is small to maintain the resulting pressures within reasonable limits. 2020. 10. 30. Hajdu Flóra

Locational Fits • Locational fits are intended to determine only the location of the mating parts • Locational clearance fits: • are intended for parts that are normally stationary but that can be freely assembled or disassembled. • They run from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as ball, race, and housing, to the looser fastener fits for which freedom of assembly is of prime importance. • Locational transition fits • compromise between clearance and interference fits when accuracy of location is important but a small amount of either clearance or interference is permissible. • Locational interference fits • used when accuracy of location is of prime importance and for parts requiring rigidity and alignment with no special requirements for bore pressure. • Such fits are not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit these conditions are covered by force fits. 2020. 10. 30. Hajdu Flóra

Standard Inch Fits • • • RC - Running and sliding fit LC - Locational clearance fit LT - Locational transition fit LN - Locational interference fit FN - Force or shrink fit • These letter symbols are used in conjunction with numbers representing the class of fit • Each of these symbols (two letters and a number) represents a complete fit, for which the minimum and maximum clearance or interference, and the limits of size for the mating parts 2020. 10. 30. Hajdu Flóra

Metric limits and fits • Establishes the designation symbols used to define specific dimensional limits on drawings • An "International Tolerance grade" (IT tolerance grade number) establishes the magnitude of the tolerance zone or the amount of part size variation allowed for internal and external dimensions alike • The smaller the grade number, the smaller the tolerance zone. • 1 -4: very precise grades intended primarily for gage making and similar precision work, although grade 4 can also be used for very precise production work. • 5 -16: represent a progressive series suitable for cutting operations, such as turning, boring, grinding, milling, and sawing. • 12 -16: intended for manufacturing operations such as cold heading, pressing, rolling, and other forming operations. • For work in other materials, such as plastics, it may be necessary to use coarser tolerance grades for the same process. 2020. 10. 30. Hajdu Flóra

International tolerance grade • Establishes the magnitude of the tolerance zone • Identified by prefix IT + grade number (e. g. IT 6) • The smaller the grade the smaller the tolerance zone • Grade 1 -4: very precise grades intented primarily for gage making and similar precision work • Grade 5 -16: a progressive series suitable for cutting operations, such as turning, boring, grinding, milling and sawing Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design scan 2020. 10. 30. Hajdu Flóra

Tolerance symbol Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design 2020. 10. 30. Hajdu Flóra

Fundamental deviation - shafts 2020. 10. 30. Hajdu Flóra

Fundamental deviation - holes 2020. 10. 30. Hajdu Flóra

Fits system Hole basis Fits System Shaft basis fits system the basic size will be the minimum size of the hole More recently used the basic size is the maximum shaft size. more than two fits are required on the same shaft- shaft basis fits system is recommended. • Clearance: H 6/g 5; H 8/d 9 • Transition: H 6/j 5, H 7/k 6 • Interference: H 7/p 6, H 8/s 7 2020. 10. 30. • Clearance: F 7/h 6; D 10/h 9 • Transition: J 6/h 6, K 7/h 6 • Interference: P 6/h 5, S 7/h 6 Hajdu Flóra