Limit fit and tolerance

Hole-

All the internal feature of a machine including those which are not cylindrical

It is denoted by capital letter (H,G ,E)

Shaft-

All the external feature of a machine including those which is not circular.

It is denoted by small letter (h,g,e)

Allowance

it is the difference between basic dimensions of holes and shafts

+ve - when Hole > Shaft

-ve  - when Hole < Shaft

Limit-

The maximum permissible value of a dimension are known as limits.

Fit-

The degree of tightness or looseness is known as the fit of the parts.

Type of fits

1. Clearance fit
2. Interference fit
3. Transition fit

Clearance fit -   

• In this case,there is a space between Hole and Shaft
• It is used at that place where loose joint is required.
• Clearance fit is Positive allowance (i.e. Hole > Shaft)

Interference fit-

• In this case there is negative difference between Hole and Shaft.(i.e. Hole < Shaft)
• Shaft is always larger than hole
• there is no relative movement between Hole and Shaft
• Power or motion is transfer through it

Transition fit-

• It is between clearance fit and interference fit
• it is used where accurate location is strongly required ( small clearance is permissible)
• Used in railway Wheels,Keys,Pins etc

Tolerance-

The difference between the maximum limit and minimum limit of parts is known as tolerance.

Type of Tolerance-

• Unilateral tolerance-In this type of tolerance the dimension is allowed to vary in only one direction of basic size either upper or lower.
• Bilateral tolerance- in this type of tolerance the variation of dimension is allowed  in both upper or lower direction. 

Requirements of Tolerance-

• It is not possible to create a part which is exact size and dimension.
• It affects the price value of product.(Large tolerance leads to minimum price value)
• Large variation in tolerance may affect the functioning of parrts.

• Tolerance Zone- It  is Zone between maximum and minimum limit size.
• Upper deviation- it is algebraic difference maximum limit and basic size of parts.
• Lower deviation- It is algebraic difference between minimum size and basic size of parts.