LIMITS AND FITS

Two extreme permissible sizes of a part between which the actual size is contained are called limits The relationship existing between two parts which are

to be assembled with respect to the difference on their sizes before assembly is called a fit. Tolerance—Tolerance is defined as the total permissible

variation of a size. It is the difference between maximum limit and minimum limit of size

REASON OF HAVING TOLERANCE

• No manufacturing process is perfect.

• Nominal dimension (the "d" value) can not be achieved exactly.

• Without tolerance we lose the control and as a consequence cause functional or assembly failure.

Tolerance is "the total amount by which a specific

dimension is permitted to vary. The tolerance is the

difference between the maximum and minimum limits".

Variations occur in any manufacturing process, which

are manifested as variations in part size.

Tolerances are used to define the limits of the

allowed variation.

- Probably the most common Type.

- Variation is permitted in both positive and negative

directions from the nominal dimension.

- Two types are found; balanced (a) and unbalanced (b);

Bilateral Tolerance

Balance

d

Unbalanced

(b)

TOLERANCE

Dimensional tolerance - conventional

Geometric tolerance - modern

unilateral

bilateral

1.00 0.05+-

nominal dimension

tolerance

0.95 + 0.10- 0.00 1.05

+ 0.00- 0.10

1.00 0.05+-

0.95 - 1.05means a range

TOLERANCE STACKING

"TOLERANCE IS ALWAYS ADDITIVE" why?

What is the expected dimension and tolerances?

d = 0.80 +1.00 + 1.20 = 3.00

t = ± (0.01 + 0.01 + 0.01) = ± 0.03

0.80 ' ±0.01 1.20 ' ±0.01

1.00 ' ±0.01

?

1. Check that the tolerance & dimension specifications are reasonable - for assembly.

2. Check there is no over or under specification.

What is the expected dimension and tolerances?

d = 3.00 - 0.80 - 1.20 = 1.00

t = ± (0.01 + 0.01 + 0.01) = ± 0.03

0.80 ' ±0.01 1.20 ' ±0.01

3.00 ' ±0.01

?

Maximum x length = 3.01 - 0.79 - 1.19 = 1.03Minimum x length = 2.99 - 0.81 - 1.21 = 0.97

Therefore x = 1.00 ± 0.03

0.80 ' ±0.01 1.20 ' ±0.01

3.00 ' ±0.01

?

x

TERMINOLOGY

Basic size — It is the exact theoretical size arrived at by design. It is also called nominal size.Actual size—The size of a part as may be found by measurement.Maximum limit of size—The greater of the two limits of size.Minimum limit of size—The smaller of the two limits of size.Allowance—It is an intentional difference between maximum material limits of mating parts. It is a minimum clearance or maximum interference between mating parts.Deviation—The algebraic difference between a size (actual, maximum, etc.) andthe corresponding basic size.Actual deviation—The algebraic difference between the actual size and the corresponding basic size.Upper deviation—The algebraic difference between the maximum limit of size and the corresponding basic size.Upper deviation of hole = ESUpper deviation of shaft esLower deviation—The algebraic difference between the minimum limit of size and the corresponding basic size.Lower deviation of hole = ElLower deviation of shaft = ei

Zero line—It is the line of zero deviation and represents the basic size.Tolerance zone—It is the zone bounded by the two limits of size of the parts and defined by its magnitude, i.e. tolerance and by its position in relation to the zero line.Fundamental deviation—That one of the two deviations which is conveniently chosen to define the position of the tolerance zone in relation to zero line

Basic shaft— A shaft whose upper deviation is zero.Basic hole — A hole whose, lower deviation of zero

Clearance — It is the positive difference between the hole size and the shaft size.Maximum clearance—The positive difference between the maximum size of a hole and the minimum size of a shaft.Minimum clearance—The positive difference between the minimum size of a hole and the maximum size of a shaft.

Terms related to Metric Limits & Fits

Max Hole size – Basic Size = Upper Deviation

Min Hole size – Basic Size = Lower Deviation

HOLE

Max shaft size – Basic Size = Upper Deviation

Min shaft size – Basic Size = Lower Deviation

SHAFT

The ISO System of Limits and Fits is a coordinated system of hole and shaft tolerances for engineering and manufacturing

The hole basis fits have four preferred hole tolerances (H11, H9, H8, and H7); Upper case used for holes

the shaft basis fits have four preferred shaft tolerances (h11, h9, h7, and h6) lower case used for shafts

Types of Fits

-HOLE BASED SYSTEMSize of hole is kept constant,

shaft size is varied to get different fits.

To obtain different types of fits, it is general practice to vary tolerance zone of one of the mating parts

-SHAFT BASED SYSTEMSize of shaft is kept constant,

hole size is varied to get different fits.

Interference FitsInterference fits are those in which the inside member is larger than the outside member, requiring the application of

force during assembly.

There is some deformation of the parts after assembly, and a pressure exists at the mating surfaces.

A transition fit results in either a clearance or an interference condition between two assembled parts

For members that must move together is the running or sliding clearance fit (RC).

International Tolerance Grade (IT):

They are a set of tolerances that varies according to the basic size and

provides a uniform level of accuracy within the grade.

A fit is indicated by the basic size common to both components, followed by symbol corresponding to each component, the hole being quoted first.

H : lower deviation of hole is zero

h : upper deviation of shaft is zero

E.S. – upper deviation

E.I. – lower deviation

Representation of Tolerance

1) Letter Symbol

The selection of letter freezes one limit of hole / shaft

(how much away from Basic size)

45 E8/e7Basic Size

One can have different possible combinations; eg. 45H6g7,

45H8r6, 45E5p7

RANGE IN A GIVEN TOLERANCE GRADE

APPLICATIONSFITS