Iran Nezam Mechanical Design Exam key terms

A sketch of a body isolated from its surroundings showing every external force and moment acting on it, used to write equilibrium equations. Choosing the system boundary correctly is the main skill.

The condition in which the vector sum of forces and the sum of moments about any point both equal zero, so a structure at rest does not translate or rotate.

Forces and moments that a support such as a pin, roller, or fixed end exerts on a structure to prevent movement. Their number and direction depend on the support type and determine static determinacy.

A graph of internal bending moment along a beam's length, derived from shear by integration. Its maximum value locates the critical section for bending stress design.

A graph of the internal transverse force along a beam, equal to the integral of distributed load. Sudden jumps mark concentrated loads and reactions.

A geometric property of a cross-section equal to the second moment of area divided by the distance to the extreme fibre. Bending stress equals moment divided by section modulus.

A measure of how a cross-section's area is distributed about an axis, governing resistance to bending. It has units of length to the fourth power and depends on axis location.

The distance from an axis at which the entire area could be concentrated to give the same second moment. Used to compute slenderness and buckling capacity of columns.

The critical axial compression at which a slender ideal column becomes unstable, proportional to stiffness times the second moment of area divided by effective length squared.

Effective column length divided by radius of gyration. Larger values indicate buckling-dominated behaviour; small values indicate yielding or crushing governs instead.

The normal stress on a plane where shear stress is zero. Maximum and minimum principal stresses define the most severe tensile and compressive conditions in a stressed point.

An equivalent scalar stress combining normal and shear components, used to compare multiaxial stress states against a uniaxial yield strength for ductile metals.

The ratio of a material's strength or allowable capacity to the actual applied stress or load. Values above one indicate margin against failure, and codes prescribe minimum values.

A multiplier greater than one that accounts for local stress rise at holes, notches, fillets, or sudden section changes compared with the nominal stress in the member.

Progressive fracture of a part under repeated or fluctuating stress at levels below the static strength, usually starting at a surface defect or stress raiser.

A plot of applied cyclic stress amplitude against the number of cycles to failure for a material, used to estimate fatigue life and endurance limits.

An energy statement for steady, incompressible, frictionless flow along a streamline relating pressure, velocity, and elevation heads. It is the basis of many flow-metering calculations.

A mass-conservation statement requiring that the product of density, area, and velocity stays constant along a streamtube in steady flow, so velocity rises where area falls.

A dimensionless ratio of inertial to viscous forces that predicts whether pipe or duct flow is laminar or turbulent and sets the friction factor regime.

A formula expressing pipe friction head loss as a friction factor times length over diameter times velocity head. It applies to both laminar and turbulent flow.

Pressure energy lost to wall friction along straight pipe or duct length, proportional to length, friction factor, and velocity squared, and inversely proportional to diameter.

Pressure energy lost in fittings, valves, bends, and sudden changes of section, usually expressed as a loss coefficient times velocity head.

The energy per unit weight that a pump adds to a fluid, expressed in metres of the pumped liquid. It must overcome static lift, friction losses, and discharge pressure.

Net positive suction head, the absolute pressure margin above vapour pressure at the pump suction. Insufficient NPSH causes cavitation, noise, and impeller damage.

Formation and collapse of vapour bubbles in liquid where local pressure drops below vapour pressure, producing noise, erosion, and loss of pump or valve performance.

A dimensionless or dimensional index combining flow, head, and rotational speed that classifies impeller geometry as radial, mixed, or axial and guides pump selection.

Scaling relations for a geometrically similar pump or fan showing that flow varies with speed, head with speed squared, and power with speed cubed, at constant diameter.

Energy conservation for a system: heat added minus work done equals the change in internal energy, or for a control volume, the change in enthalpy plus kinetic and potential terms.

A statement that entropy of an isolated system never decreases, so heat flows spontaneously from hot to cold and no cycle can convert all heat input into work.

A combined property equal to internal energy plus pressure times volume, convenient for steady-flow devices where it tracks the energy carried by a moving stream.

The ratio of useful heating or cooling delivered to work input for a refrigerator or heat pump. It is the efficiency measure for reverse cycles.

The maximum possible thermal efficiency of a heat engine operating between two reservoir temperatures, equal to one minus the cold-to-hot absolute temperature ratio.

A single value combining convection on both sides and conduction through a wall, used to compute heat rate as the coefficient times area times temperature difference.

The opposition of a layer or surface to heat flow, equal to thickness over conductivity for a wall or the reciprocal of a convective coefficient for a surface.

The effective average temperature driving force in a heat exchanger, computed from the inlet and outlet temperature differences at the two ends of the unit.

A graphical tool relating dry-bulb temperature, humidity ratio, relative humidity, enthalpy, and dew point of moist air, used to analyse cooling and heating coils.

The fraction of total coil load that changes air temperature without changing moisture content. It determines the apparatus dew point and supply air condition.

A climatic index summing, over a season, the degrees by which daily mean outdoor temperature falls below a base temperature, used to estimate annual heating energy.