Assam ASSEB HS Skill Mechanical Engineering Technology flashcards

34 cards. Read the term and the definition here. The deck at the top of this page is optional.

First-angle projection

A projection method where the object lies between the observer and the plane of projection. The top view appears below the front view and the left-side view appears on the right.

Representative fraction

The ratio of drawing length to actual length, written as 1/n. An RF of 1:50 means 20 mm on paper represents 1000 mm in reality.

Isometric drawing

A pictorial drawing with three axes at 120 degrees where true lengths are marked along the axes. It is larger than an isometric projection, which foreshortens by 0.816.

Hole-basis fit

A limit system in which the hole size is fixed and the shaft is varied to obtain clearance or interference. The hole's lower deviation is zero.

Lathe cutting speed

The surface speed of the workpiece past the tool, calculated as V = pi times diameter times rpm divided by 1000. HSS tools cut mild steel near 25 to 30 m/min.

Up-milling versus down-milling

Up-milling feeds the work against cutter rotation, so chip thickness grows from zero. Down-milling feeds with rotation, starting at maximum chip and giving better finish on rigid machines.

Riser in casting

A reservoir of molten metal that feeds the casting as it solidifies, preventing shrinkage cavities. Chills are metal inserts that speed local solidification instead.

Bend allowance

The length of the neutral axis in a sheet-metal bend, added to flat-pattern dimensions. It depends on bend radius, angle, and sheet thickness.

Brinell hardness test

A hardness test pressing a hardened steel ball into the surface under a fixed load. The load divided by ball diameter squared is kept constant for a given material class.

Proof stress

The stress producing a specified permanent strain, usually 0.2 percent, used for materials such as aluminium alloys that show no distinct yield point.

Eutectoid steel

Steel containing about 0.8 percent carbon, whose microstructure at room temperature is fully pearlite. Lower carbon gives ferrite plus pearlite; higher carbon gives pearlite plus cementite.

Annealing

A heat treatment of slow heating and very slow cooling that softens the metal, refines grain structure, removes internal stresses, and improves machinability.

Lami's theorem

For three coplanar concurrent forces in equilibrium, each force is proportional to the sine of the angle between the other two forces, so P/sin alpha = Q/sin beta = R/sin gamma.

Angle of friction

The angle whose tangent equals the coefficient of friction, so tan phi = mu. At this angle a body on an incline is on the point of sliding.

Parallel-axis theorem

The moment of inertia about any axis equals that about a parallel centroidal axis plus the area times the square of the distance between the axes.

Thermal stress

Stress produced when a bar is prevented from expanding or contracting. For full constraint it equals alpha times E times temperature change, compressive on heating.

Bending equation

M/I = sigma/y = E/R relates bending moment, second moment of area, extreme fibre stress, distance from neutral axis, and radius of curvature for a beam.

Mechanical advantage

The ratio of load lifted to effort applied in a machine. Velocity ratio is effort distance over load distance, and efficiency is mechanical advantage divided by velocity ratio.

Screw jack velocity ratio

For a screw jack with handle length L and pitch p, the velocity ratio equals two pi L divided by p, since one turn moves the load by one pitch.

First law of thermodynamics

Energy is conserved: heat supplied minus work done equals the change in internal energy for a closed system. For steady flow, enthalpy and kinetic changes replace internal energy.

Otto cycle efficiency

The air-standard efficiency of a constant-volume cycle equals one minus one over compression ratio raised to gamma minus one. Higher compression ratio raises efficiency.

Equivalent evaporation

The mass of water evaporated from 100 degrees Celsius to dry saturated steam at 100 degrees Celsius per unit mass of fuel. It compares boilers using a standard latent heat of 2257 kJ/kg.

Boiler accessories

Efficiency-improving auxiliary equipment such as economisers, superheaters, and air preheaters. Mountings, by contrast, are safety and working fittings like safety valves and water gauges.

Bernoulli's equation

For steady, frictionless, incompressible flow along a streamline, pressure head plus velocity head plus elevation head is constant. It follows from energy conservation.

Reynolds number

A dimensionless ratio of inertia to viscous forces, equal to velocity times diameter divided by kinematic viscosity. Pipe flow is laminar below about 2000 and turbulent above 4000.

Coefficient of discharge

The ratio of actual to theoretical discharge through an orifice, equal to the product of contraction and velocity coefficients. For a sharp-edged orifice it is about 0.6.

Specific speed of a pump

A dimensionless-type number equal to speed times square root of discharge divided by head to the power three-quarters. It determines whether a centrifugal, mixed, or axial impeller suits the duty.

Synchronous speed

The speed of the rotating magnetic field in an AC machine, equal to 120 times supply frequency divided by number of poles. An induction motor runs slightly slower, with slip.

Star-delta starter

A starter that runs a motor briefly in star, reducing phase voltage by a factor of root three, then switches to delta. Starting current and torque are reduced to one third.

Thermocouple

A temperature sensor of two dissimilar metals joined at a hot junction, producing an EMF proportional to the temperature difference by the Seebeck effect.

Fire classes

Classification of fires by fuel: A solids, B flammable liquids, C gases, D metals, and electrical equipment. Water is unsuitable for electrical and liquid fires; CO2 or dry powder is used.

Lockout and tagout

A procedure isolating all energy sources, applying personal locks and tags, releasing stored energy, and verifying isolation before maintenance begins on machinery.

Break-even quantity

The output at which total revenue equals total cost, found by dividing fixed cost by the difference between selling price and variable cost per unit.

Prime cost

The sum of direct material, direct labour, and direct expenses for a product. Factory cost adds factory overheads, and total cost further adds administration and selling overheads.