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Mechanical Drawing (MDP 115) FirstYear, Mechanical Engineering Dept., Faculty of Engineering, Fayoum University Dr. Ahmed Salah Abou Taleb FITS and TOLERANCES 2 Fits 3 Fits Between Mating Parts 4 ILLUSTRATION OF DEFINITIONS 5 Fits Between Mating Parts Fit: degree of tightness between two parts. Fit types: – Clearance Fit – tolerance of mating parts always leaves a space – Interference Fit – tolerance of mating parts always results in interference – Transition Fit – sometimes interferes, sometimes clears 6 Clearance Fit The mating parts have such upper and lower limits that a clearance always results when the mating parts are assembled. 7 Clearance Fit (e.g.: H7/f6) 8 Clearance Fit (pl. H7/f6) 9 Clearance Fit (pl. H7/f6) 10 Interference Fit The mating parts have such limits that the lowest shaft diameter is larger than the largest hole diameter.. 11 Interference Fit (e.g. H7/p6) 12 Interference Fit (e.g. H7/p6) 13 Interference Fit (e.g. H7/p6) 14 Transition Fit Either a clearance or an interference may result depending on the exact value of the dimensions of the machined shaft and hole within the specified tolerance zones. 15 Transition Fit (e.g.: H7/j6) 16 Transition Fit (e.g.: H7/j6) 17 Transition Fit (e.g.: H7/j6) 18 Transition Fit (e.g.: H7/j6) 19 Together (Letter & Grade) on both mating components decide quality of fit Representation of Fit 0.021 INTERFERENCE FIT 0.022 0.013 Φ30.035 Φ30.022 Φ30.021 Φ30.000 H7 : Tol Grade 7 mean 21μ variation (H means upper deviation zero) p6 : Tol Grade 6 means 13μ variation 20 (p means upper deviation is 22 μ) 21 Φ30G7r6 Φ30F8r6 Φ30H8e6 Φ30H7s6 Estimate kind of fit 22 FITS APPLICATIONS 23 24 Basic Systems for Fit Specification In order to standardize dimensioning of fits, two basic systems are used: 1) Basic Hole System: Minimum hole diameter is taken as the basis. Lower deviation for the hole is equal to zero. Dmax is prescribed according to the specified tolerance. 2) Basic Shaft System: Maximum shaft diameter is taken as the basis. Upper deviation for the Shaft is equal to zero. dmin is prescribed according to the specified tolerance. 25 Basic Hole System • Toleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size. 26 Fits in Basic Hole System 27 Basic Shaft System • In this system, the maximum shaft is taken as the basic size and is used only in specific circumstances. 28 Fits in Basic Shaft System 29 Indicating Tolerances 30 Two ways of indicating tolerances on technical drawings Limits of a dimension or the tolerance values are specified directly with the dimension. Questions ? 31 Indicating tolerances The dimension is given by: • a shape symbol, • nominal size, • a letter indicating the position of the tolerance zone in relation to zero line, • a number indicating the width of the tolerance zone. (quality of production?) 32 Specifying Fits in Technical Drawings 33 DIMENSIONING OF TOLERANCES -RULES • The upper deviation should be written above the lower deviation value irrespective of whether it is a shaft or a hole. • Both deviations are expressed to the same number of decimal places, except in the cases where the deviation in one direction is nil. 34 Example For a nominal diameter of 25 mm and for a fit specification of H7/j5 determine the following: • Type of the tolerancing system • Tolerance on the hole • Tolerance on the shaft • Upper and lower limits of the hole (Dmax, Dmin) • Upper and lower limits of the shaft (dmax, dmin) • Type of the fit 35 Key a) H7/j5 Basic Hole System b) D = 25 mm, from the given table: H7 21 m 0 m + - j5 5 m - 4 m nominal size H7 j5 Th 21 m c) Ts 9 m 36