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# TOLERANCES AND FITS OF SHAFTS AND HOLES

The dimension tolerance is the difference between the upper and lower limit of dimension. On the technical drawings there are usually shown the nominal dimensions, such as the designer sets up. Due to the difficulty of detail manufacturing to achieve the nominal dimension requires a large precision and time, and hence the cost of the final product. Therefore, the constructor assumes some forks depart from the nominal dimension. In many cases, the constructor forces of high precision specifying the nominal size or designation permissible tolerance. An example would be fit the bearing in the sleeve or on the shaft, intentional ensuring adequate clearance between the cooperating elements such as gears or hydraulic cylinders, or interference in order to ensure inseparable elements. For holes, we have to deal with a tolerance dimension of the hole, in the case of cylindrical elements tolerance dimension of the shaft .

Tolerance:
T = B - A
Upper deviation of the hole:
Es = B - N
Upper deviation of the shaft:
es = B - N
Lower deviation of the hole:
Ei = A - N
Lower deviation of the shaft:
ei = A - N

where:
T - tolerance
A - lower dimension limit
B - upper dimension limit
Es - upper deviation of the hole
es - upper deviation of the shaft
Ei - lower deviation of the hole
ei - lower deviation of the shaft
N - nominal dimension

Deviations can take positive or negative values, depending on the assumptions of constructor. Deviation tolerance are normalized values, and Polish Standard specifies the location of the tolerance in relation to the nominal dimensions by the letter of the Latin alphabet, capilal letter for holes and small in the case of shafts, and provides 19 classes of accuracy numbered in Arabic numerals.

The tolerances zone of holes:
A to G, where both dimension limits are higher than the nominal dimension
H, when the upper dimension limit is higher than the nominal dimension, and the lower equals
J, when the nominal dimension is between the upper and lower dimension limit
K, when the lower dimension limit is lower than the nominal dimension, and the upper equals
L to Z, where both dimension limits are lower than the nominal dimension

The tolerances zone of shaft:
a to g, where both dimension limits are lower than the nominal dimension
h, when the lower dimension limit is lower than the nominal dimension, and the upper equals
j, when the nominal dimension is between the upper and lower dimension limit
k, when the upper dimension limit is higher than the nominal dimension, and the lower equals
l to z, where both dimension limits are higher than the nominal dimension

Class of accuracy:
1 to 4 - are used for the most accurate precision equipment
5 to 12 - are used for typical applications of machine
12 to 17 - are used for less accurate devices.

Similarly to the tolerance of cylindrical shafts and holes are defined the cone tolerances. In case of cone tolerances after the coding letter of the tolerance zone adds a small letter s.

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