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Engineering Fits And Types Of Fits in Mechanical Engineering

Engineering 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 a relationship between two meeting parts, viz., shaft and hole. This relationship is nothing but the algebraic differences  between their sizes

Types of Engineering Fits

  • There are Three types of fits between parts:
    • Clearance Fit
    • Interference Fit
    • Transition Fit

Engineering Fits Video Animation

Fits and Types of Fits (Youtube)

Clearance Fit

A clearance fit, in which the shaft is always smaller than the hole into which it fits. 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.

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Clearance Fit

Minimum air space is 0.002”. This is the allowance and is always positive  in a clearance fit

Interference Fit

An interference fit , in which the shaft is always bigger than the hole into which it fits. 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.

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Interference Fit Diagram

Transition Fit

A transition fit, in which the shaft may be either bigger or smaller than the hole into which it fits — it will therefore be possible to get interference or clearance fits in one group of assemblies.

Transition Fit Diagram

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”.

Classification Of Fits

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Classification Of Fits

Basis of Fits

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  • Hole basis System: A 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.
  • Shaft basis system: The 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.

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