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Basic terminology for Limits and Fits

Short notes on Limits and Fits for GATE Mechanical Engineering and Production Engineering.

  • Shaft: The term shaft refers not only to the diameter of a circular shaft but also to any external dimension on a component.
  • Hole: The term hole refers not only to the diameter of a circular hole but also to any internal dimension on a component.
  • Basic Size: Basic or Nominal 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 for strength.
  • Actual Size: Actual size is the dimension as measured on a manufactured part. As already noted 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.
  • Maximum limit: Maximum limit or High limit is the maximum size permitted for the part.
  • Minimum limit: Minimum limit or low limit is the minimum size permitted for the part.
  • Tolerance: It is the difference between maximum limit of size and minimum limit of size.
  • Allowance: It is an international difference between the maximum material limits of mating parts. For shaft, maximum material limit will be its high limit and for hole, it will be its low limit. If the shaft is smaller than hole, the allowance is positive, but if the shaft is larger than the hole, it is negative.
  • Deviation: It is the algebraic difference between a size (actual, maximum etc.) and the corresponding basic size.
  • Actual deviation: It is the algebraic difference between an actual 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 a positive quantity when the maximum limit of size is greater than the basic size and is a negative quantity when the maximum limit of size is less than the basic size. 
  • Lower deviation: It is the algebraic difference between the minimum limit of size the corresponding basic size. It is a positive quantity when the minimum limit of size is greater than the basic size and a negative quantity when the minimum limit of size is less than the basic size.
  • Mean deviation: It is the arithmetic mean deviation between the upper deviation and lower deviation.
  • Zero line: It is a straight line to which the deviations are referred to in a graphical presentation of limits and fits. 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 the negative deviations below this line. 
  • Fundamental deviation: This is the deviation, either the 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. 
  • Unilateral limits: In the method of presenting the limits, both the limits of size are on the same side of zero line. That is, the permitted tolerance is stated or indicated as wholly positive or wholly negative, e.g.  or . One of the limits of the size may be the basic size.
  • Bilateral limits: Here, one of the limits of size is one one side of the zero line and the other limit of size is on the other side of the zero line, i.e. permitted tolerance is indicated partly positive and partly negative, e.g. .

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