Limits Fits Tolerances Brief Notes GATE 2021

Define Limits

It is the maximum and minimum permissible sizes within which the actual size of a component lies.

Define Fits

  • It is basically an assembly condition between a shaft and a hole.
  • Or condition of looseness or tightness between two mating parts being assembled together

Hole- The component which engulfs another component, a shaft is called a hole.

Shaft- The component which is engulfed by another component.

Types of Fits

  1. Clearance Fits
  2. Transition Fits
  3. Interference Fits

Clearance Fit

  • The type of fit where easy sliding, running fit, or loose running fit is desirable.
  • The tolerance zones never met.
  • The maximum shaft dimension is smaller than the minimum hole dimension.

Maximum and Minimum Clearances Formula Clearance Fit

Maximum Clearance = UL of Hole – LL of Shaft

Minimum Clearance = LL of Hole – UL of Shaft

Transition Fits

  • This type of fit may be tight and push-fit, wringing fit. Ex- Gear, the pulley on a shaft, etc.
  • Tolerance zones always overlap.

Maximum and Minimum Clearances Formula for Transition Fits

Maximum Clearance = UL of Hole – LL of Shaft

Minimum Clearance = LL of Hole – UL of Shaft

Interference Fits

  • This type is a shrink-fit, heavy drive fit and light drive.
  • Tolerances zones never meet but cross each other.
  • Maximum Hole size < Minimum Shaft size.

Maximum and Minimum Clearances Formula for Interference Fit

Maximum Interference = LL of Hole – UL of Shaft

Minimum Interference = UL of Hole – LL of Shaft

Allowance Definition

  • It is the minimum clearance or maximum interference.
  • The intentional clearance between the basic dimensions of mating parts.
  • It may be positive or negative.

Ok, let us try to solve these questions-

Q- Which of the fits are based on the interference system?

  • I. C. Engine Cylinder and Piston
  • Ball-bearing outer race and housing
  • Ball-bearing inner race and shaft

Q- Allowance in limits and fits means?

  • Maximum Clearance between shaft and hole
  • Minimum Clearance between shaft and hole
  • Difference between Maximum and Minimum size of the hole
  • Difference between Maximum and Minimum size of the shaft

Define Tolerances

We know that it is impossible to make anything to an exact size, therefore it is essential to allow a definite tolerance or permissible variation on every specified dimension.

  • Tolerances on a dimension may either be unilateral or bilateral.

When the two limit dimensions are only on one side of the nominal size, (either above or below) the tolerances are said to be unilateral.

When the two limit dimensions are above and below nominal size, (i.e. on either side of the nominal size) the tolerances are said to be bilateral.

  • Unilateral tolerance is preferred over bilateral tolerance. It is because the operator can machine to the upper limit of the shaft (or lower limit of a hole) still having the whole tolerance left for machining to avoid rejection of parts.

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