TOLERANCES AND ALLOWANCES

LIMIT SYSTEM

for interchangeable parts, to control the size of finished part, with due allowance for error

shaft for cylindrical part

• part which enters into the other

• enveloped surface

hole for cylindrical part

• the other in which one enters

• enveloping surface

Nominal size

size of a part specified in the drawing as a matter of convenience

Basic size

size of a part to which all limits of variation are applied to arrive at final dimensioning

Actual size

actual measured dimension of the part

Limits of sizes

• UPPER/HIGH/MAXIMUM LIMIT - largest permissible size for a dimension

• LOWER/MINIMUM LIMIT - smallest size of the part

ALLOWANCE

difference between the basic dimensions of the mating parts

TOLERANCE

• difference between the upper limit and lower limit of a dimension

• the maximum permissible variation in a dimension

• UNILATERAL SYSTEM - tolerance is allowed on one side of the nominal size
  BILATERAL SYSTEM - tolerance is allowed on both sides of the nominal size

Tolerance zone

zone between the maximum and minimum limit size

Zero line

a straight line corresponding to the basic size

Upper deviation

• the algebraic difference between the maximum size and the basic size.

• represented by a symbol ES (Ecart Superior)

Lower deviation

• algebraic difference between the minimum size and the basic size

• represented by a symbol EI (Ecart Inferior)

Actual deviation

the algebraic difference between an actual size and the corresponding basic size

Mean deviation

arithmetical mean between the upper and lower deviations

Fundamental deviation

one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to zero line

degree of tightness or looseness between the two mating parts

MAXIMUM CLEARANCE

MAX. CLEARANCE = MAX.HOLE - MIN.SHAFT

MINIMUM CLEARANCE

MIN.CLEARANCE = MIN.HOLE - MAX.SHAFT

CLEARANCE FIT

size limits for mating parts are so selected that clearance between them always occur

CLEARANCE

amount by which the actual size of the shaft is less than the actual size of the mating hole

INTERFERENCE

amount by which the actual size of a shaft is larger than the actual finished size of the mating hole

INTERFERENCE FIT

size limits for the mating parts are so selected that interference between them always occur

Transition fit

• the size limits for the mating parts are so selected that either a clearance or interference may occur depending upon the actual size

• the tolerance zones of hole and shaft overlap

Hole basis system

• The hole is kept as a constant member

• different fits are obtained by varying the shaft size

Shaft basis system

• The shaft is kept as a constant member

• different fits are obtained by varying the hole size