PE Mechanical Machine Design key terms

A technique of carrying units through calculations and converting them with ratios equal to one, so unwanted units cancel and the result carries the correct unit; it catches errors in stress, torque, and power computations.

The digits in a measured or computed value that reflect its precision; rules govern rounding and the number of digits retained so reported results do not imply more accuracy than the input data support.

The accumulated variation in a gap, clearance, or dimension caused by combining the tolerances of several parts in an assembly, calculated by worst-case arithmetic or statistically by root-sum-square.

The ratio of a strength or allowable stress to the stress expected in service, giving margin for uncertainty in loads, material properties, analysis, and the consequences of failure.

A standardized symbolic language on engineering drawings that uses feature control frames to specify allowable form, orientation, location, and profile variation of features, replacing ambiguous plus/minus tolerancing.

The professional duties of an engineer, including holding public safety paramount, working only within one's competence, avoiding conflicts of interest, and reporting unsafe conditions; state licensing boards enforce these obligations.

Consensus technical documents such as the ASME Boiler and Pressure Vessel Code or ANSI standards that prescribe design rules, material requirements, and testing; they define the accepted minimum practice for a component.

The probability that a component or system performs its required function without failure for a specified time under stated operating conditions; used in statistical design and life testing.

Classifications of mating part dimensions: a clearance fit always leaves a gap, an interference fit always requires forcing together, and a transition fit may produce either depending on the actual sizes.

The ratio of the maximum local elastic stress at a geometric discontinuity, such as a hole, fillet, or groove, to the nominal stress computed on the net or gross section.

The alternating stress amplitude below which a ferrous material survives an essentially unlimited number of fatigue cycles without failure; a key basis for infinite-life shaft and spring design.

A plot of cyclic stress amplitude versus number of cycles to failure for a material at a specified mean stress and surface condition; used to estimate finite fatigue life.

A straight-line failure criterion plotted on mean stress versus alternating stress axes, running from the endurance limit at zero mean stress to the ultimate strength at zero alternating stress, defining a safe design region.

A linear cumulative damage rule in which the fractions of fatigue life consumed at each stress level are summed; failure is predicted when the total reaches one.

The tensile clamping force created in a threaded fastener during tightening; it determines joint separation resistance, bolt fatigue stress, and how external load splits between bolt and clamped members.

The shortest distance from the root to the face of a fillet weld, equal to 0.707 times the leg size; multiplied by weld length it gives the effective area resisting shear.

The number of revolutions or hours at which 90 percent of a population of identical rolling-element bearings will survive without fatigue failure under a specified load and speed.

A rotational speed at which the shaft's running frequency coincides with a natural bending frequency, causing resonant vibration and large deflection; operation must be kept away from these speeds.

The ratio of a helical spring's mean coil diameter to its wire diameter; it controls the Wahl stress correction, the spring rate, and how easily the spring can be manufactured.

A threaded device that converts torque and rotation into linear force and motion, characterized by lead, thread friction, efficiency, and whether it is self-locking; used in jacks and presses.

Elastic instability of a slender column under axial compression, occurring at a critical load proportional to flexural rigidity EI and inversely proportional to the square of the effective length.

An equivalent uniaxial stress derived from the distortion-energy failure theory that combines multiaxial stress components so results can be compared with a material's tensile yield strength.

The circumferential tensile stress in the wall of a thin-walled cylindrical pressure vessel, equal to internal pressure times radius divided by wall thickness, and roughly twice the longitudinal stress.

The frequency at which a structure oscillates freely after a disturbance; when an excitation frequency matches it, resonance amplifies response, so designs shift natural frequencies away from operating speeds.

A heat treatment that heats steel above its critical temperature then cools it slowly, usually in the furnace, producing softness, relief of internal stress, and improved machinability.

Rapid cooling of austenitized steel in oil, water, or polymer to form hard martensite; the cooling rate must exceed the critical rate for the alloy to achieve full hardness.

A post-quench heat treatment at a temperature below the critical range that reduces brittleness and relieves residual stress while sacrificing some hardness and strength to gain toughness.

A surface treatment such as carburizing or induction hardening that raises the carbon content or locally austenitizes the outer layer, producing a hard wear-resistant case over a tough core.

Resistance of a material to localized plastic deformation, measured by indentation tests such as Rockwell or Brinell; hardness correlates roughly with tensile strength for steels.

The stress at which a material exhibits a specified permanent strain, commonly 0.2 percent offset for metals; it marks the onset of unacceptable plastic deformation in design.

The maximum engineering stress a material sustains before necking and fracture in a tension test, calculated as peak load divided by the original cross-sectional area.

Time-dependent plastic deformation of a material held under constant stress at elevated temperature; design against creep limits stress and temperature for components such as turbine and boiler parts.

Accelerated corrosion that occurs when two dissimilar metals are electrically connected in an electrolyte, the more active metal becoming the anode and dissolving preferentially.

Brittle crack growth caused by the combined action of a tensile stress and a specific corrosive environment, occurring at stresses well below the material's normal strength.

A formula combining carbon and alloying element percentages to express a steel's weldability and hardenability; high values require preheat or post-weld heat treatment to avoid cracking.

A suffix such as T6 or O following an aluminum alloy number that identifies its heat treatment and strain-hardening condition, and therefore its strength and ductility.