Limits and fits

In mechanical engineering, limits and fits are a set of rules how many thousandths of an inch or Millimetre a part's measurement is to be under or over the theoretical size to achieve various sorts of fit (sliding fit, rotating fit, non-sliding fit, loose fit, etc.)

LIMITS & FITS. LIMITS‐‐‐ High limit. Low limit. FITS‐‐‐ Clearance fit, Interference fit, Transition fit, LIMITS & FITS. LIMITS:‐ These are two extreme permissible sizes of dimension between which actual size of dimension is contained.The greater of these two is called high limit and the smaller low limit LIMITS & FITS FITS:‐ • It is the relationship existing between two mating parts amount of play or interference which is present when they are assembled together. • It is the degree of tightness or looseness between two mating parts to perform a definite function

Terms used in limits & fits:‐ • Shaft • Hole • Basic size • Actual size • Limits of size– HL & LL • Tolerance = HL‐LL • Allowance = Difference between MMC of mating parts

Deviation‐‐• Upper deviation • Lower deviation • Mean deviation Zero Line

Fundamental Deviation FITS

TYPES OF FITS‐ • Clearance fit– Min & Max clearance • Interference,Press or Force fit– Min & Max interference • Transition fit

HOLE/SHAFT BASIS SYSTEM

Various allowances for different fits maybe obtained in two ways:‐ • Hole base system • Shaft base system HOLE BASIS SYSTEM • HOLE SIZE IS KEPT CONSTANT AND SHAFT IS VARIED TO GIVE VARIOUS TYPES OF FIT. • BASIC SIZE TAKEN IS LOW LIMIT OF HOLE • HIGH LIMIT OF HOLE & TWO LIMITS (HL&LL) GIVE THE DESIRED FIT HOLE BASIS SYSTEM 3

1 SHAFT 1 ‐‐‐ Clearance fit 2—Transition fit 3 Interference fit SHAFT BASIS SYSTEM • SHAFT SIZE IS KEPT CONSTANT & HOLE SIZE IS VARIED TO GIVE VARIOUS FITS. • BASIC SIZE IS TAKEN AS MAX LIMIT SIZE OF SHAFT. • LL OF SHAFT AND TWO LIMITS (HL & LL) OF HOLE GIVE THE DESIRED FIT. • METHOD NOT PREFERRED IN LARGE PRODUCTION SHAFT BASIS SYSTEM 3

1 HOLE 1 ‐‐‐ Interference fit 2—Transition fit 3 ‐‐Clearance fit TOLERANCE • Permissible variation in size or dimension is called tolerance. • Amount by which a job is allowed to go away from accuracy without any functional trouble 25 ± 0.005 cm

ISO SYSTEM OF Limits & fits • Covers holes and shafts up to 3150 mm • For any basic size there are 28 different holes progressively oversize and undersize. • 28 holes are ‐‐ A,B,C,CD,D,E,EF,F,FG,G,H,J,JS,K,M,N,P,R,S,T,U,V, X,Y,Z,ZA,ZB,& ZC • Each hole has a choice of 18 grades of tolerance i.e. IT01,ITO,IT1 to IT16 (Similarly for shafts) • Tolerance grades decide the accuracy of manufacture VALUES OF TOLERANCES

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Wikipedia