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Types of Fit and Tolerance. Hole basis sstem and shaft basis system

- 1. Presented By Mr.S.Saravanakumar, M.E., Assistant Professor Easwari Engineering College
- 2. TERMINOLOGY
- 3. BASIC SIZE: It is the standard size of a part , with reference to which all the limits of variations of size are determined. ZERO LINE: the line corresponding to basic size is called as zero line. SHAFT AND HOLE: These terms are used to designate all the external and internal features of any shape and not necessarily cylindrical. HOLE DESIGNATION: By upper case letters from A, B, ... Z, Za, Zb, Zc (excluding I, L, O, Q, W and adding Js, Za, Zb, Zc) - 25 nos. Indian Stds. SHAFT DESIGNATION: By lower case letters from a, b, ... z, za, zb, zc (excluding i, l, o, q, w and adding js, za, zb, zc) - 25 nos.
- 4. DEVIATION UPPER DEVIATION:The algebraic difference between the maximum limit of size (of either hole or shaft) and the corresponding basic size LOWER DEVIATION:The algebraic difference between the minimum limit of size (of either hole or shaft) and the corresponding basic size FUNDAMENTAL DEVIATION:It is one of the two deviations which is chosen to define the position of the tolerance zone
- 5. The algebraic difference between upper and lower deviations. It is an absolute value.
- 6. Unilateral tolerance : if the tolerance is allowed on one side of the basic size, the system of tolerance is said to be unilateral.
- 7. Bilateral tolerance : if the tolerance is allowed on both side of the basic size, the system of tolerance is said to be unilateral.
- 8. The limits are two extreme permissible sizes of a part between which, the actual size of that part is contained. They are fixed with reference to the basic size of that dimension.
- 9. Basic shaft: it is the shaft, whose upper deviation is zero or whose max. limit of size is equal to basic size . Basic hole: it is the hole, whose lower deviation is zero or whose min. limit of size is equal to basic size .
- 10. Allowance is the prescribed difference between the hole dimension and shaft dimension for any type of fit. It is the intentional difference between the lower limit of the hole and higher limit of the shaft.
- 11. The relation between the two parts, where one is inserted into the other with a certain degree of tightness or looseness is known as fit. Fit is the degree of tightness or looseness between two mating parts to perform a definite function . Fit is the relation between dimensions of two mating parts before their assembly. Classification of fits
- 12. In this type of fit, the largest permitted shaft diameter is smaller than the Diameter of the smallest hole, so that, the shaft can rotate or slide through, with Different degrees of freedom according to the purpose of the mating members
- 13. In this type of fit, diameter of minimum allowable shaft is greater than that of Maximum allowable hole. In this type of fit, the sizes of the mating parts are so selected that, interference Or negative allowance will always occur.
- 14. In a fit of this type, the diameter of the largest allowable hole is greater than that of the smallest shaft, but the smallest hole is smaller than the largest shaft, so that, small positive or negative allowance between the shaft and hole members are employable. In this type of fit, the size limits of mating (shaft and hole) parts are so selected that, either clearance or interference may occur depending upon the actual size of the parts.
- 15. In this system, the design size of hole, whose lower deviation (fundamental deviation) is zero, is assumed as basic size and different class of fits are obtained by varying the limits of the shaft only. In other words, the limits of the hole are kept constant and those of the shaft are varied so as to obtain the necessary fit.
- 16. In this system, the design size of a shaft, whose upper deviation (fundamental deviation) is zero, is assumed as basic size and different class of fits obtained by varying the limits of the hole only. In other words, the limits of shaft are kept constant and limits of holes are varied to obtain the necessary type of fit.
- 17. To describe completely a hole or a shaft, its basic size followed by appropriate letter and the number of tolerance grade is given . Holes are designated by capita letter Shafts are designated by small letter. Example, 20 mm hole ‘H' with tolerance grade IT7 is designated as 20H7. 20. mm 'f' shaft with tolerance grade IT8 is designated as 20f8. for shafts a to h -clearance fit, j to n - transition it, p to z -interference fit.
- 18. Grades of Tolerance Grade of Tolerance: It is an indication of the level of accuracy. There are 18 grades of tolerances – IT01, IT0, IT1 to IT16 IT01 to IT4 - For production of gauges, plug gauges, measuring instruments IT5 to IT 7 - For fits in precision engineering applications IT8 to IT11 – For General Engineering IT12 to IT14 -For Sheet metal working or press working 14 IT12 to IT14 –For Sheet metal working or press working IT15 to IT16 – For processes like casting, general cutting work
- 20. example: 50 H7g7 : is a fit indicated by its basic size 50mm, followed by symbols representing the limits of hole (H7 i.e hole having basic size 50mm and tolerance grade IT7) and shaft (g7 i.e shaft having basic size 50mm and tolerance grade IT7) the type of fit system is hole basis and obtained type of fit is clearance fit. 35 H8j7 :
- 28. Endurance Limit • Whenever a cyclic (repetitive) load is applied on the material, If the material shows no evidence of fracture then this property of the material is called, “Endurance Limit” • While if the material shows any evidence of fracture during the loading this property is called, “Fatigue Limit” • The conventional fatigue testing has been concerned primarily with the testing of specimens with smooth surfaces under the conditions of rotating-bending or uni axial tension compression cycling. • The results of these tests are presented in the form of plots of stress verses the number N of stress cycles required to cause the fracture.
- 30. • These plots are called δ-N diagrams, it was suggested by “WhOler”. These diagrams are also called, “S-N diagrams” in some literatures. It is shown in the figure. Usually in mild steel or certain other steels, an endurance limit is observed. • Actually when the cyclic load is applied on the material stress decreases and once a point is arrived where the stress becomes constant, means there is no further • decrease in stress with the increase of N. However many materials do not exhibit a clear cut endurance limit, but δ- N curves continues downward as N increases.
- 31. • Most of mechanical components has irregularities in structure and abrupt change in cross section, which increase stress concentration at such changes. Some materials are more sensitive than others to stress raising notches under fluctuating loads , to account for this effect a parameter called notch sensitivity factor is found for each material.... • Notch sensitivity of a material is measure of how sensitive material is for notches or geometric irregularities... • Notch Sensitivity: Degree to which the theoretical effect of stress concentration has actually reached. q=(kf-1)/(kt-1) q - Notch Sensitivity Factor kf - Theoritical Stress Concentration Factor for Axial or Bending Loading. kt - Theoritical Stress Concentration Factor for Torsional or Shear Loading. Notch Sensitivity Factor