Limits fits


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Limits fits

  2. 2. Fig. 1.1 Graphical Illustration of Fits and Tolerances ILLUSTRATION OF BASIC SIZE DEVIATIONS, LIMITS AND TOLERANCES
  3. 3. LIMITS Actual machine size - between the maximum and minimum limits MAXIMUM LIMIT Maximum permissible machine size corresponding to basic size. Fig. 1.1, Ø 30 mm is = Ø 30 + 0.035 = Ø 30. 035 mm. Basic size Maximum limit MINIMUM LIMIT Minimum allowable machine size corresponding to basic size. In Fig. 1.1, Ø 30 mm is = Ø 30 - 0.215 = Ø 29. 785 mm. Basic size Minimum limit
  4. 4. TOLERANCE The difference between the maximum and minimum limits of a size is tolerance. In Fig. 1.1 the tolerance is = Ø 30.035 - Ø 29.785 = 0.25m. Max. limit Min. limit Tolerance DEVIATION (Actual, or maximum, or minimum) – (Basic size). UPPER DEVIATION - (Maximum limit) – (Basic size) In Fig.1.1, the upper deviation is = Ø 30.035 - Ø 30 = 0.035 mm.
  5. 5. LOWER DEVIATION - (Minimum limit) – (Basic size) In Fig. 1.1, the lower deviation is = Ø 29.785 - Ø 30 = - 0.215 mm. ACTUAL DEVIATION - (Actual measured size) – (Basic size) In Fig. 1.1, the actual deviation = Ø 29.925 - Ø 30 = - 0.075 mm. ZERO LINE • A straight line drawn for the basic size is called zero line. • Upper and lower deviation are referred to zero line. • Zero line is drawn horizontally. • Above zero line - positive deviations. • Below zero line - negative deviations.
  6. 6. TOLERANCE ZONE The zone bounded by the upper and lower limits of the basic size. In Fig. 1.1, tolerance zone is shown by hatched line. FITS • The relationship between the mating surfaces of the parts,because of the difference in dimensions is called fit. • Thus fit has a characteristic effect on the performance of mating parts. • Example: width of the slot & the thickness of the key.
  7. 7. CLEARANCE  (Hole dimension) – (Dimension of shaft that fit to it). Maximum - hole size is maximum and the shaft size is minimum. Minimum - shaft size is maximum and the hole size is minimum. CLASSIFICATION OF FITS Clearance fit Interference fit Transition fit
  8. 8. A B Fig. 1.2 Clearance Fit Min hole – Max Shaft = + ve – clearance fit. Fit with positive clearance between the hole and the shaft. CLEARANCE FIT
  9. 9. INTERFERENCE FIT Fit with negative clearance between the hole and the shaft. Fig. 1.3 Interference Fit Max hole – Min Shaft = - ve – interferance fit
  10. 10. TRANSITION FIT Fit established when the dimensions of the hole are such that there exists either a positive clearance or a negative clearance when the shaft is fitted into the hole. B
  11. 11. Fig 1.4 Transition Fit
  12. 12. TRANSITION FITS AND INTERFERENCE FITS Combination of Hole and Shaft Quality of Fit Typical uses H6 j5 fine Clearance transition Very small clearance is obtained - used for fits where a slight interference is permissible - coupling spigots and recesses, gear rings clamped to steel hubs. H7 j6 normal H8 j7 coarse H6 k5 fine True transition Fits averaging no clearance where slight interference can be tolerated , with the object of eliminating vibration - ball bearings races of light duty. H7 k6 normal H8 k7 coarse H6 m5 fine Interference transition Fits averaging slight interference - used for ball bearing races of medium duty. H7 m6 normal H8 m7 coarse H6 p5 fine Press fit Light press fit for nonferrous parts which can be dismantled when required-bearing bushes-press fit for steel, cast iron or brass to steel assemblies- bush in a gear.H7 p6 normal H6 r5 fine Drive fit Medium drive fit for ferrous parts and light drive fit for non-ferrous parts that can be dismantled.H7 r6 normal H6 s5 fine Drive fit Permanent or semi permanent assemblies of steel and cast iron with considerable gripping force- collars pressed on to shafts, valve seatings etc. For light alloys this gives a press fit. H7 s6 normal H6 u5 fine Force or shrink fit High interference fit a thorough investigation into the degree of grip and the stresses in the part must be made.H7 u6 normal
  13. 13. SYSTEMS OF FITS HOLE BASIS SYSTEM: the hole is constant in diameter and various types of fits are obtained by suitably varying the limits of the shaft. SHAFT BASIS SYSTEM: the shaft is constant in diameter and various types of fits are obtained by suitably varying the limits of the hole.
  14. 14. Fig. 1.5 Hole Basis System Clearance Fit B Interference Fit C HOLE BASIS SYSTEM Single hole, whose lower deviation is zero. Minimum limit of the hole will be equal to its basic size.
  15. 15. Grades of Holes MANUFACTURING PROCESS USED H5 Fine Boring, Fine Grinding, Honing H6 Fine Boring, Honing, Hand Reaming H7 Int. Grinding, Broaching, Reaming H8 Boring, Machine Reaming H9 Boring & Reaming H10 Not for diameter fits; used for milled widths, drilled holes H11 Not used in fits; punching, coarse drilling drawn recesses
  16. 16. HOLE BASIS SYSTEM - Design Example Requirement Hole basic size - 20 mm diameter. Clearance of - 0.100 mm. Hole tolerance - 0.025 mm. Shaft tolerance - 0.050 mm. Minimum limit of the hole is Ø 20 mm. Maximum limit of the shaft = Lower limit of the hole – Minimum clearance = Ø 19.900 mm Design
  17. 17. Minimum limit of the shaft = Maximum limit of the shaft – Tolerance on the shaft. = Ø 19. 850 mm. Shaft Max. limit = Ø 19.900 mm. Min. limit = Ø 19. 850 mm. Maximum limit of the hole = Maximum limit of the hole + Tolerance on the hole. = Ø 20. 025 mm. Hole Max. limit = Ø 20.025 m. Min. limit = Ø 20. 000 mm.
  18. 18. Why the Hole Basis System is Preferred? Holes are produced by drilling, boring, reaming, broaching, etc., Shafts are either turned or ground. Shaft basis system - Holes of different sizes are required, (requires tools of different types and sizes). Hole basis system - Only one tool is required, to produce the hole and the shaft can be machined to any desired size.
  19. 19. SYSTEMS OF TOLERANCING 1. UNILATERAL SYSTEM • Deviations are allowed in only one direction from the basic size. • The deviation in the other direction is zero. • Hence, either the maximum limit or the minimum limit will be equal to the basic size. • Deviations are allowed in both the directions from the basic size. • The deviations are allowed in both the directions. • One of the limits - above the basic size. • Other limits - below the basic size. 2. BILATERAL SYSTEM
  23. 23. ATTAINABLE SIZE TOLERANCE Sl. No. Machining Process IT Value Tolerance value (for a size of 20 mm) 1 Drilling IT12 0.21 2 Milling IT10 0.084 3 Turning/ Boring IT8 0.013 4 Grinding IT6 0.006 5 Lapping IT4 0.006 6 Broaching IT7 0.021 7 Shaping IT12 0.210 8 Reaming IT8 0.033 9 Hobbing IT7 0.021 10 Surface grinding IT4 0.006 11 Honing IT7 0.021
  25. 25. What machining process should be adopted to each step of the following components. Also give step drawing details ? (a)
  26. 26. (b) (c)
  27. 27. REFERENCE 1. Gopalakrishna K R, “Machine Drawing”, Tenth Edition, Bangalore, 1996. 2. Faculty of Mechanical Engineering, PSG College of Technology, “Design Data Book”, Coimbatore, 1993. 3. ASME Y 14.5M- 1994, “Dimensioning and Tolerancing”, ASME, New York, 1995.
  28. 28. THANK YOU