UNITS AND CONSTANT
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Definitions:
Nominal size:
The size designation used for general identification. The nominal size of a shaft and a hole are the same. This value is often expressed as a fraction.
Basic size:
The exact theoretical size of a part. This is the value from which limit dimensions are computed. Basic size is a four decimal place equivalent to the nominal size. The number of significant digits imply the accuracy of the dimension.
example: nominal size = 1 1/4
basic size = 1.2500
Design size:
The ideal size for each component (shaft and hole) based upon a selected fit. The difference between the design size of the shaft and the design size of the hole is equal to the allowance of the fit. The design size of a part corresponds to the Maximum Material Condition (MMC). That is, the largest shaft permitted by the limits and the smallest hole. Emphasis is placed upon the design size in the writing of the actual limit dimension, so the design size is placed in the top position of the pair.
Tolerance:
The total amount by which a dimension is allowed to vary. For fractional linear dimensions we have assumed a bilateral tolerance of 1/64 inch. For the fit of a shaft/hole combination, the tolerance is considered to be unilateral, that is, it is only applied in one direction from design size of the part.Standards for limits and fits state that tolerances are applied such that the hole size can only vary larger from design size and the shaft size smaller.
Basic hole system:
Most common system for limit dimensions. In this system the design size of the hole is taken to be equivalent to the basic size for the pair (see above). This means that the lower (in size) limit of the hole dimension is equal to design size. The basic hole system is more frequently used since most hole generating devices are of fixed size (for example, drills, reams, etc.) When designing using purchased components with fixed outer diameters (bearings, bushings, etc.) a basic shaft system may be used.
Allowance:
The allowance is the intended difference in the sizes of mating parts. This allowance may be: positive (indicated with a "+" symbol), which means there is intended clearance between parts; negative("-"), for intentional interference: or "zero allowance" if the two parts are intended to be the "same size".
Base and Supplementary Units
| Quantity | Unit | Symbol |
| Length | meter | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Thermodynamic temperature | Kelvin | K |
| Luminous intensity | candela | cd |
| Molecular substance | mole | mol |
| Plane angle | radian | rad |
| Solid angle | steradian | sr |
Derived Units
| Quantity | Unit | Symbol |
Space and Time
| ||
| Area | square meter | m² |
| Volume | cubic meter | m³ |
| Velocity | meter per second | m/s |
| Acceleration | meter per second per second | m/s² |
| Angular velocity | radian per second | rad/s |
| Angular acceleration | radian per second per second | rad/s² |
| Frequency | hertz | Hz (cycle/s) |
| Rotational speed | revolution per second revolution per minute | r/s r/m |
Mechanics
| ||
| Density | kilogram per cubic meter | kg/m³ |
| Momentum | kilogram meter per second | kg·m/s |
| Moment of inertia | kilogram meter squared | kg·m³ |
| Force | newton | N (kg·m/s²) |
| Torque, moment of force | newton meter | N·m |
| Energy, work, heat quantity | joule | J (N·m) |
| Power | watt | W (J/s) |
| Pressure, stress | pascal | Pa (N/m²) |
Heat
| ||
| Customary temperature | degree Celsius | °C |
| Thermal conductivity | watt per meter Kelvin | W/(m·K) |
| Entropy | joule per Kelvin | J/K |
| Specific heat | joule per kilogram Kelvin | J/(kg·K) |
Light
| ||
| Luminous flux | lumen | lm (cd·sr) |
| Illumination | lux | lx (lm/m²) |
| Luminance | candela per square meter | cd/m² |
Viscosity
| ||
| Kinematic viscosity | square meter per second | m²/s |
| Dynamic (absolute) viscosity | pascal second | Pa·s |
| Quantity | Equivalent | Dimensions | S.I. units |
| Mass | M | Kilogram (kg) | |
| Length | L | Metre (m) | |
| Time | T | Second (s) | |
| Frequency | cycles/unit time | T-1 | Hertz (Hz) |
| Area | length x width | L2 | m2 |
| Volume | length x height x width | L3 | m3 |
| Density | Mass/unit volume | ML-3 | kg/m3 |
| Velocity | Distance/unit time | LT-1 | m/s |
| Acceleration | Velocity/unit time | LT-2 | m/s2 |
| Force | mass x acceleration | MLT-2 | Newton |
| Weight | mass x gravitational acceleration | MLT-2 | Kilogram |
| Pressure or Stress | force/unit area | ML-1T-2 | Pascal (Pa) |
| Moment of Inertia | mass x length2 | ML2 | kg m2 |
| Work | force x distance | ML2T-2 | Joule (J) |
| Energy | Work capacity | ML2T-2 | Joule (J) |
| Potential Energy | mass x gravitational acceleration x height raised | ML2T-2 | Joule (J) |
| Kinetic Energy | 1/2 mass x velocity2 | ML2T-2 | Joule (J) |
| Power | Work/unit time | ML2T-3 | Watt (W) |
| Momentum | Mass x velocity | MLT-1 |
CONVERSIONS
Millibar (mb): 1 mb = 100 Pa; 1 Pa = 0.01 mb
Celsius: oC = K – 273.15; K = oC + 273.15
Fahrenheit: oF = 9/5(oC) + 32; oC = 5/9(oF-32)
USEFUL NUMERICAL CONSTANTS
Universal Gas Constant (R) 8.3143 J K-1 mol-1Stefan-Boltzmann constant (s) 56.696 x 10-9 W m-2 K-4
Planck constant (h) 0.66262 x 10-33 J s
Velocity of light (c) 299.8 x 106 m s-1
Wien’s constant 2897 mm
Acceleration due to gravity 9.80665 m s-2
Molecular weight of dry air 28.97 g mol-1
Density of dry air 1.209 kg m-3
Specific heat of air at constant pressure (Cp) 1004 J K-1 kg-1
Gas constant for dry air (Rd) 287 J kg-1 K-1
Standard atmospheric pressure 101.3 kPa
Gas constant for water vapor (Rv) 461 J kg-1 K-1
Specific heat of water vapor at constant pressure 1952 J K-1 kg-1
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