Limits, fits and tolerances

Limits, fits and tolerances

1. Limits, Fits and Tolerances (Sub: Measurement and Metrology) Code: BMEC0003 Instructor Mr. Gaurav Bharadwaj Assistant Prof. Department of ME GLA University

2. INTRODUCTION No two parts can be produced with identical measurements by any manufacturing process. Every production process involves a combination of three elements viz , materials, machines and men. Variations in properties of the material being machined introduce errors. The production machines themselves may have some inherent inaccuracies. It is impossible for an operator to make perfect settings. While setting up the tools and work piece on the machine, some errors are likely to creep in.

3. Why study Limits & Fits? · Exact size is impossible to achieve. · Establish boundaries within which deviation from perfect form is allowed but still the design intent is fulfilled. · Enable interchangeability of components during assembly The designer has to suggest these tolerance limits, which are acceptable for each of the dimensions used to define shape and form, and ensure satisfactory operation in service.

4. TERMINOLOGY Basic Size: The size with reference to which the limits of size are fixed. Actual Size: Actual measured dimension of the part. Zero Line: It is a straight line corresponding to the basic size. The deviations are measured from this line. The positive and negative deviations are shown above and below the zero line respectively. Limits of Size: The two extreme permissible sizes of a part between which the actual size should lie. Maximum Limit of Size: The greater of the two limits of size. Minimum Limit of Size: The smaller of the two limits of size. Shaft: A term used by convention to designate all external features of a part, including those which are not cylindrical. Hole: A term used by convention to designate all internal features of a part, including those which are not cylindrical.

5. Allowance: It is the intentional difference between the hole dimensions and shaft dimension for any type of fit. Deviation: It is the algebraic difference between a limit of size and the corresponding basic size. Upper Deviation: It is the algebraic difference between the maximum limit of size and the corresponding basic size. It is denoted by letters ‘ES’ for a hole and ‘ es’ for a shaft. Lower Deviation: It is the algebraic difference between the minimum limit of size and the corresponding basic size. It is denoted by letters ‘EI’ for a hole and ‘ ei ’ for a shaft. Fundamental Deviation : It is the deviation, either upper or lower deviation, which is nearest to the zero line for either a hole or a shaft. It fixes the position of the tolerance zone in relation to the zero line.

6. CONVENTIONAL DIAGRAM OF LIMITS

7. SPECIFICATION OF TOLERANCES Unilateral Tolerance In this system, the dimension of a part is allowed to vary only on one side of the basic size, i.e. tolerance lies wholly on one side of the basic size either above or below it. e.g. Ø25 +0.18 +0.10 Basic Size = 25.00 mm Upper Limit = 25.18 mm Lower Limit = 25.10 mm Tolerance = 0.08 mm e.g. Ø25 -0.10 -0.20 Basic Size = 25.00 mm Upper Limit = 24.90 mm Lower Limit = 24.80 mm Tolerance = 0.10 mm

8. SPECIFICATION OF TOLERANCES Bilateral Tolerance In this system, the dimension of the part is allowed to vary on both the sides of the basic size, i.e. the limits of tolerance lie on either side of the basic size. e.g. Ø25 ±0.04 Basic Size = 25.00 mm Upper Limit = 25.04 mm Lower Limit = 24.96 mm Tolerance = 0.08 mm

9. Maximum and Minimum Metal Conditions : Let us consider a shaft having a dimension of 40 ± 0.05 mm. The maximum metal limit (MML) of the shaft will have a dimension of 40.05 mm because at this higher limit, the shaft will have the maximum possible amount of metal. The shaft will have the least possible amount of metal at a lower limit of 39.95 mm, and this limit of the shaft is known as minimum or least metal limit (LML). Similarly, consider a hole having a dimension of 45 ± 0.05 mm . The hole will have a maximum possible amount of metal at a lower limit of 44.95 mm and the lower limit of the hole is designated as MML. The higher limit of the hole will be the LML. At a high limit of 45.05 mm, the hole will have the least possible amount of metal.

10. Why unilateral system is preferred in interchangeable manufacturing? It is easy and simple to determine deviations. Another advantage of the system is that GO gauge ends can be standardized as the holes of different tolerance grades have the same lower limit and all the shafts have same upper limit. This form of tolerance greatly assists the operator, when machining of mating parts. The operator machine to the upper limit of shaft (lower limit for hole) knowing fully well that still has some margin left for machining before the parts are rejected. Tolerances are specified To obtain desired fits because it is not possible to manufacture a size exactly to obtain higher accuracy to have proper allowances

11. Relationship between Manufacturing Cost and Work Tolerance: If the tolerances are made closer and closer, the cost of production goes on increasing, because to manufacture the component with closer tolerances, we need: precision machines, tools, materials. tight inspection and more precise testing and inspection devices. it needs more concentration of the operators, frequent checking and more time which slows down the rate of production .

12. S.NO. TOLERANCES ALLOWANCE 1. It is the permissible variation in dimension of a part (either a hole or shaft) It is the prescribed difference between the dimensions of two mating parts (hole and shaft) 2. It is the difference between higher and lower limits of a dimension of a part. It is the intentional difference between the lower limits of hole and higher limit of shaft. 3. The tolerance is provided on the basic size of a part. Allowance is to be provided on the dimension of mating parts to determine desired type of fit. 4. It has absolute value without sign. Allowance may be positive or negative. Difference between Tolerance and Allowance

13. FIT A fit may be defined as the degree of tightness and looseness between two mating parts. An ideal fit is required for proper functioning of the mating parts. Three basic types of fits can be identified, depending on the actual limits of the hole or shaft.

14. Clearance Fit The largest permissible diameter of the shaft is smaller than the diameter of the smallest hole. A clearance fit has positive allowance. Allows rotation or sliding between the mating parts. Used in tailstock spindle of lathe machine, feed movement of the spindle quill in a dill machine, cylinder and piston, shaft pulley, etc. The clearance fits may be slide fit, easy sliding fit, running fit, slack running fit and loose running fit.

15. Interference fit The minimum permissible diameter of the shaft exceeds the maximum allowable diameter of the hole. Interference fit has negative allowance. When two mating parts are assembled with an interference fit, it will be an almost permanent assembly, that is, the parts will not come apart or move during use. To assemble the parts with interference, heating or cooling may be required. Used in keyed pulley and shaft, car wheels, etc. The interference fits may be shrink fit, heavy drive fit and light drive fit.

16. Transition Fit It may result in either clearance fit or interference fit depending on the actual value of the individual tolerances of the mating components. Transition fits are a compromise between clearance and interference fits. They are used for applications where accurate location is important but either a small amount of clearance or interference is permissible. Used in railway wheels, fixing keys, changing gears, etc. The transition fits may be force fit, tight fit and push fit.

17. Systems of obtaining different types of fits: Hole Basis: In this system, the basic diameter of the hole is constant while the shaft size is varied according to the type of fit. Shaft Basis system: In this system, the basic diameter of the shaft is constant while the hole size is varied according to the type of fit.

18. Significance of Hole basis system: The bureau of Indian Standards (BIS) recommends both hole basis and shaft basis systems, but their selection depends on the production methods. Generally, holes are produced by drilling, boring, reaming, broaching, etc. whereas shafts are either turned or ground. If the shaft basis system is used to specify the limit dimensions to obtain various types of fits, number of holes of different sizes are required, which in turn requires tools of different sizes. If the hole basis system is used, there will be reduction in production costs as only one tool is required to produce the ole and the shaft can be easily machined to any desired size. Hence hole basis system is preferred over shaft basis system.

19. S.NO. Hole basis system Shaft basis system 1. Size of the hole whose lower deviation is zero is assumed as the basic size. Size of the shaft whose upper deviation is zero is assumed as the basic size. 2. Limits on the hole are kept constant and those of shaft are varied to obtain desired type of fit. Limits on the shaft are kept constant and those of hole are varied to obtain desired type of fit. 3. Hole basis system is preferred in mass production. Not suitable for mass production. 4. It requires less amount of capital and storage space for tools needed to produce shafts of different sizes Needs large amount of capital and storage space for large number of tools required to produce holes of different sizes. Difference between hole basis and shaft basis system

20. Holes and Shaft Designations According to BIS 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. Tolerance grades 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 IT15 to IT16 – For processes like casting, general cutting wo rk Standard Tolerance: Various grades of tolerances are defined using the ‘standard tolerance unit’, ( i ) in µ m. where, D (mm) is the geometric mean of the lower and upper diameters of a particular diameter step within which the chosen the diameter D lies.

21. Diameter steps in I.S-919 are: 1-3, 3-6, 6-10, 10-18, 18-30, 30-50, 50-80, 80-120, 120-180, 180-250, 250- 315, 315-400 and 400-500 mm It is understood that the tolerances have parabolic relationship with the size of the products. As the size increases, the tolerance within which a part can be manufactured also increases. IT01 – 0.3 + 0.008D IT0 – 0.5 + 0.012 D IT1 – 0.8 + 0.020D Grades of Tolerance IT2 to IT4 – the values of tolerance grades are placed geometrically between the tolerance grades of IT1 and IT5. IT6 – 10 i ; IT7 – 16i; IT8 – 25i; IT9 – 40i; IT10 – 64i; IT11 – 100i; IT12 – 160i; IT13 – 250i; IT14 – 400i; IT15 – 640i; IT16 – 1000i.

22. For Shaft ei = es – IT es = ei + IT For Hole EI = ES – IT ES = EI + IT es = upper deviation of shaft ei = lower deviation of shaft ES = upper deviation of hole EI= lower deviation of hole Calculation for Upper and Lower Deviation

23. Interchangeability: Interchangeability occurs when one part in an assembly can be substituted for a similar part which has been made to the same drawing. Interchangeability is possible only when certain standards are strictly followed. Universal interchangeability means the parts to be assembled are from two different manufacturing sources. Local interchangeability means all the parts to be assembled are made in the same manufacturing unit. Advantages: the operator is not required to waste his skill in fitting the mating components by trial and error and thus the assembly time is reduced. there is increased output with reduced production cost. it facilitates production of mating components at different places by different operators. the replacement of worn out or defective parts and repair becomes very easy. the cost of maintenance and shutdown period is also reduced to minimum.

24. Selective assembly: In the selective assembly the components produced are classified into groups according to their sizes by automatic gauging. This is done for both mating parts, holes and shaft, and only matched groups of mating parts are assembled. It results in complete protection against defective assemblies and reduces matching costs since the parts since the parts may be produced with wider tolerances. Examples: A practical example is the assembly of pistons with cylinder bores. Let the bore size be 50 mm and the clearance required for the assembly is 0.12mm on the diameter. Let the tolerance on bore and the piston each = 0.04 mm. Then, Dimension of bore diameter is 50+0.02 mm and 50-0.02 mm Dimension of piston shaft is 49.88+0.02 mm and 49.88-0.02 mm By grading and marking the bores and pistons they may be selectively assembled to give the clearance of 0.12 mm as given below: Cylinder bore 49.98 50.00 50.02 Piston 49.86 49.88 49.90

25. Expressing a dimension as 25.3 ±0.05 mm is the case of (a) Unilateral tolerance (b) Bilateral tolerance (c) Limiting dimensions (d) All of the above A shaft has a dimension, The respective values of fundamental deviation and tolerance are

26. Two shafts A and B have their diameters specified as 100 ± 0.1 mm and 0.1 ± 0.0001 mm respectively. Which of the following statements is/are true? (a) Tolerance in the dimension is greater in shaft A (b) The relative error in the dimension is greater in shaft A (c) Tolerance in the dimension is greater in shaft B (d) The relative error in the dimension is same for shaft A and shaft B

27. In an interchangeable assembly, shafts of size mm mate with holes of size mm. The maximum possible clearance in the assembly will be (a) 10 microns (b) 20 microns (c) 30 microns (d) 60 microns

28. A hole is specified as mm. The mating shaft has a clearance fit with minimum clearance of 0.01 mm. The tolerance on the shaft is 0.04 mm. The maximum clearance in mm between the hole and the shaft is (a) 0.04 (b) 0.05 (c) 0.10 (d) 0.11

29. Interference fit joints are provided for: (a) Assembling bush bearing in housing (b) Mounting heavy duty gears on shafts (c) Mounting pulley on shafts (d) Assembly of flywheels on shafts In order to have interference fit, it is essential that the lower limit of the shaft should be (a) Greater than the upper limit of the hole (b) Lesser than the upper limit of the hole (c) Greater than the lower limit of the hole (d) Lesser than the lower limit of the hole

30. A shaft and hole pair is designated as 50H7d8. This assembly constitutes (a) Interference fit (b) Transition fit (c) Clearance fit (d) None of the above Which of the following is an interference fit? (a) Push fit (b) Running fit (c) Sliding fit (d) Shrink fit

31. Consider the following fits: I.C. engine cylinder and piston Ball bearing outer race and housing Ball bearing inner race and shaft Which of the above fits are based on the interference system? (a) 1 and 2 (b) 2 and 3 (c) 1 and 3 (d) 1, 2 and 3 Allowance in limits and fits refers to (a) Maximum clearance between shaft and hole (b) Minimum clearance between shaft and hole (c) Difference between maximum and minimum size of hole (d) Difference between maximum and minimum size of shaft

32. Clearance in a fit is the difference between (a) Maximum hole size and minimum shaft size (b) Minimum hole size and maximum shaft size (c) Maximum hole size and maximum shaft size (d) Minimum hole size and minimum shaft size Basic shaft and basic hole are those whose upper deviations and lower deviations respectively are (a) + ve , - ve (b) - ve , + ve (c) Zero, Zero (d) None of the above

33. A fit is specified as 25H8/e8. The tolerance value for a nominal diameter of 25 mm in IT8 is 33 microns and fundamental deviation for the shaft is - 40 microns. The maximum clearance of the fit in microns is (a) -7 (b) 7 (c) 73 (d) 106 The dimensional limits on a shaft of 25h7 are (a) 25.000, 25.021 mm (b) 25.000, 24.979 mm (c) 25.000, 25.007 mm (d) 25.000, 24.993 mm