Karnataka SSLC Elements of Mechanical Engineering IV - JTS (KSEAB) key terms

The ability of a metal to be stretched into thin wire under tensile force without breaking. Mild steel is ductile; cast iron is not. Measured roughly by percentage elongation before fracture.

The property that lets a metal be hammered or rolled into thin sheets without cracking. It differs from ductility because the metal is compressed rather than pulled. Aluminium and copper are highly malleable.

A material's resistance to scratching, indentation or wear. Tested by Brinell or Rockwell methods. Hard materials usually machine poorly and are brittle, so hardness is often traded against toughness.

The amount of energy a material absorbs before it fractures. A tough material resists shock loading without breaking, unlike a brittle one. It combines strength with some deformation capacity.

The tendency of a material to fracture with little or no plastic deformation. Cast iron and glass are brittle; they fail suddenly under impact or bending instead of bending first.

The maximum pulling stress a material withstands before it necks and breaks, expressed in newtons per square millimetre. It is found from a tensile test on a standard specimen.

A measuring instrument with a main scale and a sliding vernier scale that reads to 0.02 mm. It measures outside, inside and depth dimensions using its jaws and depth bar.

The smallest measurement an instrument can read reliably. For a vernier, it is one main-scale division divided by the number of vernier divisions. A 0.5 mm main division with 50 vernier divisions gives 0.01 mm.

A precision instrument using a calibrated screw and thimble to measure small thicknesses and diameters, typically to 0.01 mm. It works on the principle that one rotation advances the spindle by one pitch.

A holding device bolted to a workbench, with two jaws closed by a screw, used to grip work during filing, sawing or chipping. Soft jaw caps protect finished surfaces from the hardened jaws.

A hardened steel hand tool with cut teeth used to remove small amounts of metal and smooth surfaces. Single-cut files finish, double-cut files cut faster, and the grade names the tooth spacing.

A hand saw with a thin, fine-toothed blade held under tension in a frame, used to cut metal bar and pipe. Blade teeth point forward, and pressure is applied on the forward stroke.

A matched pair of threading tools: the tap cuts an internal thread in a drilled hole, and the die cuts an external thread on a rod. Tapping requires a correctly sized tapping drill.

The axial distance between corresponding points on adjacent thread turns, or between successive teeth on a gear or chain. It fixes how far a nut travels per revolution and how gears mesh.

A threaded fastener with a head, passed through clearance holes in the parts and tightened by a nut. It can be removed without damaging the parts, unlike a rivet or weld.

A threaded female fastener that screws onto a bolt or stud to clamp parts together. Hexagonal nuts are standard because they can be turned from six positions with a spanner.

A thin disc placed under a nut or bolt head to spread clamping load over a wider area and protect the surface. Spring washers also resist loosening from vibration.

A rolling-element bearing in which hardened steel balls run between inner and outer rings, replacing sliding friction with rolling friction. It carries radial loads well and needs little lubrication.

A plain bearing in which a shaft journal rotates inside a lubricated sleeve or bush. It suits heavy loads and high speeds, provided an oil film separates the surfaces.

The small gap between a shaft and its bearing or bush. Too little clearance causes seizure from heat; too much causes vibration and noise. It is checked with a feeler gauge or plastigauge.

A power transmission using an endless flexible belt running on two or more pulleys. It is quiet and absorbs shock but can slip, so it suits moderate torque over longer centre distances.

A grooved or flat-rimmed wheel mounted on a shaft that carries a belt or rope. A pair of pulleys of different diameters changes speed and torque in inverse proportion to their diameters.

The ratio of the distance moved by the effort to the distance moved by the load in a machine. For a belt drive it equals the ratio of the driven pulley diameter to the driving pulley diameter.

The ratio of load lifted to effort applied in a machine. It equals velocity ratio multiplied by efficiency, so it can never exceed the velocity ratio for a real machine.

A rigid bar turning about a fixed pivot or fulcrum, used to multiply force or distance. It is classed by the order of fulcrum, load and effort along the bar.

The fixed pivot point about which a lever rotates. Its position relative to load and effort decides whether the lever multiplies force, speed or merely changes direction.

A toothed wheel that meshes with another to transmit motion and force positively, without slip. The ratio of teeth sets the speed change, and the module must match for correct meshing.

The standard measure of gear tooth size, equal to pitch circle diameter divided by number of teeth, in millimetres. Two gears mesh only if their modules and pressure angles match.

Power transmission by an endless roller chain running on toothed sprockets. Unlike a belt it cannot slip, so it suits high torque and short centre distances, though it needs regular lubrication.

A device joining two shafts end to end so they rotate together. A rigid coupling demands accurate alignment; a flexible coupling tolerates small misalignment and absorbs shock.

Energy transferred between bodies because of a temperature difference. It flows spontaneously from hotter to colder bodies and is measured in joules, unlike temperature.

A measure of the average kinetic energy of particles in a body, indicating how hot or cold it is. It is measured in kelvin or degrees Celsius and decides the direction of heat flow.

The heat needed to raise the temperature of one kilogram of a substance by one kelvin, in joules per kilogram kelvin. Water's high value makes it a good cooling medium.

The heat absorbed or released during a change of state at constant temperature, such as boiling or melting. It does not raise the temperature, only change the phase.

A closed pressure vessel in which water is heated to produce steam at a pressure above atmospheric. It supplies steam to engines, turbines or process plant and needs safety valves and water-level controls.

A rotary machine in which high-pressure steam expands through fixed and moving blades, turning a shaft. It converts steam enthalpy into shaft work more efficiently than a reciprocating steam engine.

A reciprocating machine in which steam pushes a piston inside a cylinder, and the piston rod drives a crank through a connecting rod. It produces reciprocating motion converted to rotation.

The sequence of intake, compression, power and exhaust strokes completing one cycle in two crankshaft revolutions. Used by most petrol and diesel engines in vehicles and generators.

The ratio of cylinder volume before compression to volume after compression. Petrol engines run about 8:1 to 12:1; diesel engines run higher, around 16:1 to 22:1, for ignition by heat.

An internal combustion engine that draws in air only, compresses it to a high temperature, then injects fuel that ignites spontaneously. It needs no spark plug and is more thermally efficient.

A rotodynamic pump in which a spinning impeller throws liquid outward, raising its pressure and velocity. It suits large flows at low head and must be primed before starting.

Filling a pump casing and suction pipe with liquid before starting, so the impeller can create suction. A centrifugal pump running dry loses suction and may damage its seals.

The formation and collapse of vapour bubbles in a liquid where local pressure falls below vapour pressure. It causes noise, vibration and pitting damage to pump impellers and pipe walls.

A machine that raises the pressure of a gas by reducing its volume, usually air. Reciprocating and rotary types supply workshops, spray painting and pneumatic tools.

A pressure-relief device that opens automatically to release steam or gas when pressure exceeds a set limit. It protects boilers, receivers and compressors from bursting.

A maintenance safety procedure in which an energy source is isolated, locked and tagged before work begins. It prevents accidental start-up or release of stored energy and injury to workers.

Scheduled inspection, lubrication and replacement carried out before failure occurs, based on time or running hours. It reduces breakdowns and extends machine life compared with breakdown maintenance.

A drawing method showing a three-dimensional object as separate front, top and side views projected onto planes at right angles. Hidden edges are drawn as dashed lines and dimensions are added.

A pictorial drawing in which the three axes are 120 degrees apart and lengths are drawn true to scale along them. It shows shape in one view but is not used for dimensioned production drawings.

The ratio between a drawn length and the real length it represents. Reducing scales such as 1:10 are used for large objects, enlarging scales for small parts, and full scale is 1:1.