Fits and tolerances in engineering
- 1. P R E S E N T E D B Y : A N K I T K U M A R ( 1 4 5 7 0 7 8 ) N A Y A N K U M A R ( 1 4 5 7 0 8 0 ) P R A N T I K C H O W D H U R Y ( 1 4 5 7 0 8 2 ) Fits and Tolerances in Engineering Heritage Institute of Technology Department of Mechanical Engineering SEMINAR-II MECH-3221
- 2. Contents History Introduction Interchangeability System Terminologies Tolerances Fits Hole and Shaft-basis System Limit Systems Designation of holes and shafts Conclusion
- 3. History Replacement of hand made tools with machine made tools. This change in production, is now known as the Industrial Revolution. Began in England in 18th century. Instead of utilizing artisans to produce hand-made items, machines started to help and eventually take the place of the artisans. Inventions aided in speeding up the production of manufactured items.
- 4. Introduction In the manufacturing of a machine, quality is a primary consideration. Manufacturing precision taken into the product determines its quality, its cost and selling price. Precision is the degree of accuracy necessary to ensure the functioning of a part as intended. There are two types of parts: o Non-mating parts o Mating parts
- 5. Interchangeability The concept of mass production originated with the automobile industry. MODEL-T of Ford Motors was the first machine to be mass-produced. Till the 1940’s, every component was manufactured in-house. When one component assembles properly (and which satisfies the functionality aspect of the assembly/product) with any mating component, both chosen at random, then it is known as interchangeability.
- 6. Ford Motor Co./AP The 1914 Ford Model T touring car is shown in this Ford Motor Co. handout. The 1914 model was the first version built on Henry Ford's moving assembly line.
- 7. System Terminologies Shaft: The term ‘shaft’ used in this standard has a wide meaning and serves for specification of all outer elements of the part, including those elements, which do not have cylindrical shapes. Hole: The term ‘hole’ can be used for specification of all inner elements regardless of their shape. Basic Size: The basic size or normal size is the standard size for the part and is the same both for the hole and its shaft. This is the size which is obtained by calculation of strength.
- 8. System Terminologies (Contd.) Actual Size: Actual size is the dimension as measured on a manufactured part. As already mentioned, the actual size will never be equal to the basic size and it is sufficient if it is within predetermined limits. Limits of Size: These are the maximum and minimum permissible sizes of the part (extreme permissible sizes of the feature of the part). Maximum Limit: The maximum limit or high limit is the maximum size permitted for the part Minimum Limit: The minimum limit or low limit is the minimum size permitted for the part. Zero Line: In a graphical representation of limits and fits, a zero line is a straight line to which the deviations are referred to. It is a line of zero deviation and represents the basic size. When the zero line is drawn horizontally, positive deviations are shown above and negative deviations are shown below this line. Deviation: It is the algebraic difference between a size (actual, limit of a size, etc.) and the corresponding basic size.
- 9. System Terminologies (Contd.) Upper Deviation: It is designated as ES (for hole) and es (for shaft). It is the algebraic difference between the maximum limit of the size and the corresponding basic size. When the maximum limit of size is greater than the basic size, it is a positive quantity and when the maximum limit of size is less than the basic size then it is a negative quantity. Lower Deviation: It is designated as EI (for hole) and ei (for shaft). It is the algebraic difference between the minimum limits of size and the corresponding basic size. When the minimum limit of size is greater than the basic size, it is a positive quantity and when the minimum limit of size is less than the basic size then it is a negative quantity. Fundamental Deviations (FD): This is the deviation, either upper or the lower deviation, which is the nearest one 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. Actual Deviation: This is the algebraic difference between an actual size and the corresponding basic size. Mean Deviation: It is the arithmetical mean between the upper limit and the lower limit.
- 10. System Terminologies (Contd.) Tolerance: It is the difference between the upper limit and the lower limit of a dimension. It is also the maximum permissible variation in a dimension. Tolerance Zone: It is a function of basic size. It is defined by its magnitude and its position in relation to the zero line. It is the zone bounded by the two limits of size of a part in the graphical presentation of tolerance. Tolerance Guide: It is the degree of manufacturing. It is designated by the letters IT (stands for International Tolerance). Numbers, i.e., IT0, IT01, IT1, follow these letters up to IT16; the larger the number, the larger the tolerance. Tolerance Class: The term is used for a combination of fundamental deviation and tolerance grade. Allowance: It is an intentional difference between the maximum material limits of mating parts. For a shaft, the maximum material limit will be its high limit and for a hole, it will be its low limit. Fits: The relationship existing between two parts, shaft and hole, which are to be assembled, with respect to the difference in their sizes is called fit.
- 11. Tolerances Tolerance is the total amount that a specific dimension is permitted to vary; It is the difference between the maximum and the minimum limits for the dimension. For Example a dimension given as 1.625 ± .002 means that the manufactured part may be 1.627” or 1.623”, or anywhere between these limit dimensions. How to decide tolerance? Functional requirements of mating parts Cost of production Available manufacturing process Choose as coarse tolerance as possible without compromising functional requirements Proper balance between cost and quality of parts
- 12. Tolerances The Tolerance is 0.001” for the Hole as well as for the Shaft
- 14. Specifications of Tolerances Limit Dimensioning The high limit is placed above the low limit. In single-line note form, the low limit precedes the high limit separated by a dash
- 15. Specifications of Tolerances (Contd.) Plus-or-minus Dimensioning • Unilateral Tolerance • Bilateral Tolerance
- 16. Fits Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts. A fit is the relationship between two meeting parts, viz., shaft and hole. This relationship is nothing but the algebraic differences between their sizes There are four types of fits between parts: Clearance Fit Interference Fit Transition Fit Line Fit
- 17. Clearance Fit When an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the parts. Minimum air space is 0.002”. This is the allowance and is always positive in a clearance fit
- 18. Interference Fit The internal member is larger than the external member such that there is always an actual interference of material. The smallest shaft is 1.2513” and the largest hole is 1.2506”, so that there is an actual interference of metal amounting to at least 0.0007”. Under maximum material conditions the interference would be 0.0019”. This interference is the allowance, and in an interference fit it is always negative.
- 19. Transition Fit This type of fit may result in either a clearance or interference condition. In the figure below, the smallest shaft 1.2503” will fit in the largest hole 1.2506”, with 0.003” to spare. But the largest shaft, 1.2509” will have to be forced into the smallest hole, 1.2500” with an interference of metal of 0.009”.
- 20. Line Fit The limits of size are so specified that a clearance or surface contact may result when mating parts are assembled.
- 21. Hole-basis System Minimum hole is taken as the basic size, an allowance is assigned, and tolerances are applied on both sides of and away from this allowance. 1. The minimum size of the hole 0.500” is taken as the basic size. 2. An allowance of 0.002” is decided on and subtracted from the basic hole size, making the maximum shaft as 0.498”. 3. Tolerances of 0.002” and 0.003” respectively are applied to the hole and shaft to obtain the maximum hole of 0.502” and the minimum shaft of 0.495”. Minimum clearance: 0.500”- 0.498” = 0.002” Maximum clearance: 0.502” – 0.495” = 0.007”
- 22. Shaft-basis System Maximum shaft is taken as the basic size, an allowance is assigned, and tolerances are applied on both sides of and away from this allowance. 1. The maximum size of the shaft 0.500” is taken as the basic size. 2. An allowance of 0.002” is decided on and added to the basic shaft size, making the minimum hole as 0.502”. 3. Tolerances of 0.003” and 0.001” respectively are applied to the hole and shaft to obtain the maximum hole of 0.505” and the minimum shaft of 0.499”. Minimum clearance: 0.502”- 0.500” = 0.002” Maximum clearance: 0.505” – 0.499” = 0.006”
- 23. International Tolerance Grade (IT) They are a set of tolerances that varies according to the basic size and provides a uniform level of accuracy within the grade.
- 24. Indian Standards Specifications and Application In India we have IS: 919 recommendation for limits and fits for engineering. This standard is mostly based on British Standards BS: 1916-1953. Sizes up to 500 min are covered in IS: 919 and sizes above 500 mm, up to 3150 mm, are covered in IS: 2101 However, it is yet to adopt several recommendations of ISO: 286.
- 25. Conclusion In manufacturing process after manufacturing the product we need to check the dimensions of the product. No manufacturing process is perfect to get perfect dimensions for a required product. After manufacturing process even if you are getting the errors in the dimensions of the product, those errors can be acceptable up to certain limit. These limits are decided during the designing process.
- 26. Bibliography Kulkarni, V.A. , Bewoor, A.K: Metrology and Measurement I.S. 1871: Commentary on Indian Standard wrought steels for general engineering purposes. ISO 286: ISO System of limits and fits (in two parts)