Fits in Mechanical design

FITS

A fit is defined as the condition of clearance or interference between two parts.

Terminology associated with fits

Allowance - The amount of clearance or interference that exists between two parts

Maximum Allowance - The greatest amount of clearance or interference that can exist between mating parts. ( i.e) Largest female size ( upper limit ) minus the smallest male size ( lower limit )

Minimum Allowance - The least amount of clearance or interference that can exist between mating parts. ( i.e) Smallest female size ( lower limit ) minus the largest male size ( upper limit )

Types of fits

There are three types. They are

• Clearance fit
• Interference fit
• Transition fit

Clearance Fit

• There will always be some amount of space between mating parts
• No force required to put these parts together.
• When calculation is performed the allowance values will be positive.

Clearance Fit

Interference Fit

• There will always be some amount of contact between mating parts.
• Usually force will be required to put these parts together.
• When calculation is performed the allowance values will be negative.

Interference Fit

Transition Fit

• There could exist between mating parts sometimes a condition of clearance or interference.
• Force depends on how the parts were toleranced.
• When calculation is performed the allowance values will be positive & negative.

Transition Fit

Tolerance in Mechanical design

History of Tolerance

• In the 1800's manufacturing used the " cut & try, file & fit " approach.
• The plus-minus or co-ordinate system of tolerance was next developed.
• In the 1900's the first GD&T standards came out to improve quality & utility of engineering drawings.
• In 1966, the united GD&T standards was published " ANSI - Y14.5M".

Definition of Tolerance

A tolerance is the acceptable difference between the maximum and minimum size of a mechanical part as a basis for determining the accuracy of its fit with another part.

Tolerance

For Example: A dimension gives as Ø 28±0.2 means that the manufactured part may be Ø 28.2 or Ø 27.8, or anywhere between these limit dimensions.

Terminology

Nominal Dimension - The dimension that the tolerance are applied to.

Upper Limit - The maximum allowable size of the part based on the tolerance given

Lower Limit - The minimum allowable size of the part based on the tolerance given.

Interval of Tolerance - The upper limit minus lower limit, also known as range of  tolerance.

WHY Tolerance

No one cannot manufacture a part to its accurate dimension. If you leave a dimension without a tolerance, no one else will know the importance or the unimportant of that dimension. Not only a lack of tolerance lead to improper fits, it will also add to delay and higher costs.

Unit price vs Tolerance

How Effective tolerance helps

• The part functions correctly.
• Fabrication cost is minimum.

Types of Tolerance

• Special tolerance
• General workshop tolerance

Special Tolerance

• Limit Allowance
• Unilateral Tolerance
• Bilateral Tolerance

Examples

Examples of Special Tolerance

General workshop tolerance

General workshop tolerance are usually found in the tittle block of a detail drawing. These tolerance set the acceptable limits when the fabricator or machinist has no other tolerances given on the drawings.