SECTION 2 1 LIMIT AND TOLERANCE Basic Terminology

SECTION 2. 1 LIMIT AND TOLERANCE

Basic Terminology Nominal size(D, d): Also called basic size, refers to the size of a feature of perfect form as defined by the drawing specification. Actual size(Da, da): refers to size of the associated integral feature. It is obtained by measurement.

Basic Terminology Deviation(Ea, ea): refers to value of actual size minus its nominal size. for hole: Ea=Da-D for shaft: ea=da-d Limit deviation(ES/EI, es/ei): refers to algebraic different value between limits of size and nominal size. for hole: ES=Dmax-D for shaft: es=dmax-d EI=Dmin-D ei=dmin-d

Basic Terminology Size tolerance(Th, Ts): refers to the difference between the upper limit of size and the lower limit of size. for hole: Th=│Dmax-Dmin│ for shaft: Ts =│dmax-dmin│ =│ES-EI│ =│es-ei│ Diagram of limits and fits

Illustration of Limits and Fits The concepts of limits and fits can be illustrated in the figure bellow. Zero line: the nominal size, to which the deviations and tolerances are referred. Size tolerance interval : contained between two lines representing the upper and lower deviations，defined by the magnitude and position of the size tolerance interval. Illustration of limits and fits

Example 1 ：Calculate the Limit Deviation and Size Tolerance Illustration of size tolerance interval

Standard Tolerance Standard tolerance(IT): refers to any tolerance belonging to the ISO code system for tolerances on linear sizes. Standard tolerance grade: It is designated by the letters IT followed by the grade number (e. g. IT 7). There are 20 standard tolerance grades designated IT 01 , IT 0, and IT 1~18. Nominal size range: The standard tolerances are not defined individually for each specific nominal size, but for nominal size range. Standard tolerance grades Nominal size range

Calculation of Standard Tolerance Value IF nominal size range is from 0 to 500 mm, IT grade range is from IT 5 to IT 18, the value of standard tolerance is calculated as: Standard tolerance grade Nominal size(mm) IT 5 above - IT 6 Up to and including 500 IT 7 IT 8 IT 9 IT 10 … IT 18 Standard tolerance value IT 17 …

Example 2 ：Calculate the Standard Tolerance Value Question: Calculate the value of standard tolerance IT 7 of diameter 100 mm of the shaft.

Fundamental Deviation The fundamental deviation is that limit deviation, which defines that limit of size, which is the nearest to the nominal size. The fundamental deviations are identified and controlled by: • upper case letter(s) for holes (A. . . ZC) • lower case letter(s) for shafts (a. . . zc)

Fundamental Deviation Value Generally, the fundamental deviation value can be identified in tables below.

Example 3 ：Calculate the Fundamental Deviations Question: Calculate the fundamental deviations for the hole sizes 90 F 7.