A shared vocabulary is the foundation of engineering communication — this reference defines 100 terms that appear regularly in mechanical design work, organized by subject area for quick lookup.
Engineering terminology is precise because precision matters: “clearance fit” and “interference fit” describe opposite conditions, and confusing them produces a part that either falls out or cannot be assembled. This glossary covers terms from materials science, design analysis, GD&T, manufacturing, and fastening — the domains a working mechanical designer encounters daily. Terms are defined with practical context, not just textbook definitions.
Materials and Properties
Tensile Strength (UTS): Ultimate tensile strength — the maximum stress a material can sustain in uniaxial tension before fracture. Measured in MPa or ksi. Used to size members against static failure.
Yield Strength (σ_y): The stress at which permanent (plastic) deformation begins. For ductile metals, the 0.2% offset proof stress is the practical engineering yield point. Structural and machine members should be designed so working stress stays below yield.
Elastic Modulus (Young’s Modulus, E): Ratio of stress to strain in the elastic region. Controls stiffness, not strength. Steel: 200 GPa; aluminum: 69 GPa; PEEK: 3.6 GPa. Deflection calculations require E.
Hardness: Resistance to surface indentation. Common scales: Brinell (HB) for bulk comparison, Rockwell (HRC for hard materials, HRB for softer metals), Vickers (HV) for precision and thin materials. HRC is the standard for hardened steels (above 20 HRC).
Fatigue Strength (Endurance Limit): The stress amplitude below which a material can withstand an essentially unlimited number of load cycles without fracture. Steel has a true endurance limit at approximately 10⁶–10⁷ cycles; aluminum does not (fatigue strength continues to decrease with more cycles — specify life-based fatigue strength).
Fracture Toughness (K_IC): Resistance to crack propagation. Materials with high K_IC (titanium, ductile steels) tolerate larger cracks before fracture. Critical in fatigue-loaded aerospace and pressure vessel design.
Ductility: Ability of a material to deform plastically before fracture, measured as elongation (%) or reduction in area (%). Ductile materials (steel: 15–30% elongation) give warning before failure; brittle materials (cast iron, hardened steel, ceramics: <2%) fracture suddenly.
Toughness: Energy absorbed per unit volume before fracture — the area under the stress-strain curve. Combines strength and ductility. Measured by Charpy or Izod impact test (J or ft·lbf).
Creep: Time-dependent plastic deformation under sustained stress. Significant for metals above approximately 0.4 Tm (homologous temperature) and for polymers at room temperature. Relevant for bolted joints at elevated temperature (loss of preload) and plastic components under sustained load.
Hardenability: The ability of a steel to be through-hardened by quenching — a measure of the depth to which martensite forms. Quantified by the Jominy end-quench test. High hardenability = alloy steel (SCM440, 4140); low hardenability = plain carbon steel (S45C).
Design and Analysis
Safety Factor (Factor of Safety, FS): Ratio of failure load (or stress) to design load (or stress). FS = strength / stress. Typical values: 2–4 for static ductile materials; 4–8 for brittle materials; higher for fatigue or unknown loading. Provides margin for material variability, load uncertainty, and simplified analysis assumptions.
Stress Concentration Factor (K_t): Ratio of peak local stress to nominal stress at a geometric discontinuity (hole, notch, fillet radius). K_t for a circular hole in a wide plate = 3. Sharp inside corners produce higher K_t than generous fillets — this is why minimum inside fillet radius is a critical design rule in fatigue-loaded components.
Moment of Inertia (Second Moment of Area, I): Geometric property of a cross-section that quantifies resistance to bending. I = ∫y²dA. Units: mm⁴. Higher I = stiffer/stronger beam in bending. I-sections and box sections maximize I per unit weight by placing material away from the neutral axis.
Section Modulus (S = I/c): I divided by the distance from the neutral axis to the extreme fiber (c). Used to calculate bending stress: σ = M/S. Units: mm³.
Buckling: Sudden lateral collapse of a slender column or plate under compressive load, occurring below the material yield strength. Euler’s critical load: P_cr = π²EI / (KL)², where K is the effective length factor (1.0 for both ends pinned; 0.5 for both ends fixed). Slenderness ratio (KL/r, where r = √(I/A)) determines whether buckling or crushing governs.
Deflection: Elastic displacement of a beam or structure under load. For a simply supported beam with central point load: δ = PL³/(48EI). Deflection limits are usually more restrictive than stress limits in precision machinery (L/500 to L/1000 is typical).
Hertz Contact Stress: Stress distribution under elastic contact between curved bodies (ball-race, cam-follower, gear tooth contact). Maximum contact pressure for a sphere on a flat: p_max = (6FE*²/π³R²)^(1/3). Relevant for bearing selection, gear design, and any rolling contact application.
Thermal Stress: Stress induced by constrained thermal expansion: σ = E × α × ΔT, where α is the coefficient of thermal expansion. Critical in dissimilar material joints (aluminum bracket on steel frame) at elevated temperatures.
Vibration Natural Frequency (f_n): The frequency at which a system oscillates freely after disturbance. For a mass on a spring: f_n = (1/2π)√(k/m). Resonance occurs when excitation frequency approaches f_n — avoid by designing f_n to be 2× or more away from excitation frequencies, or adding damping.
Modal Analysis: FEA technique that calculates a structure’s natural frequencies (modes) and mode shapes. Used to identify resonance risks and guide structural modification to shift natural frequencies away from operating excitation frequencies.
GD&T and Drawing Standards
Datum: A theoretically exact point, axis, or plane from which measurements are made. On a drawing, datums are identified by a datum feature symbol (triangle with identification letter). The datum reference frame defines the coordinate system for location and orientation tolerances.
Flatness: A form tolerance that controls how flat a surface is — the distance between two parallel planes within which all points of the surface must lie. Applies independently to each feature; does not control orientation or location.
Cylindricity: A form tolerance controlling the shape of a cylindrical feature in both cross-section (roundness) and along the axis (straightness). The tightest form tolerance for cylindrical features.
Concentricity / Coaxiality: A location tolerance requiring the median points or axis of a feature to lie within a cylindrical tolerance zone coaxial with a datum axis. Use runout or position with cylindrical tolerance zone for most shaft applications — concentricity is rarely specified in practice.
Total Runout: Composite tolerance controlling all variations (form, location, orientation) of a cylindrical or planar surface relative to a datum axis, measured by rotating the part 360°. Captures all geometric errors in one measurement. Total runout is more restrictive than circular runout.
True Position: Location tolerance specifying the allowable deviation of a feature’s axis or center plane from its theoretically exact (basic) location. Specified with a tolerance zone diameter (∅ symbol) for cylindrical features. The most important GD&T control for hole pattern and mating part alignment.
Maximum Material Condition (MMC): The condition where a feature contains the maximum amount of material — for a hole, MMC is the smallest allowed diameter; for a shaft, MMC is the largest allowed diameter. When MMC is applied to a position tolerance, the tolerance zone increases (bonus tolerance) as the feature departs from MMC toward LMC.
Basic Dimension: A theoretically exact dimension used to define the true profile, true position, or basic size of a feature. Basic dimensions are enclosed in a rectangle on drawings and have no tolerance — the tolerance is defined by the associated GD&T tolerance.
Surface Roughness (Ra): The arithmetic mean of the absolute deviations of the surface profile from the mean line. Ra is the most commonly specified parameter. Ra 0.8 µm: fine turned or ground; Ra 1.6 µm: standard machined; Ra 3.2 µm: rough machined; Ra 6.3 µm: saw cut or EDM. Specified using the ISO 1302 check-mark symbol with Ra value.
Tolerance of Form (Envelope Requirement): ISO requirement (circle E modifier) that a feature’s actual surface must lie within the envelope of perfect form at MMC. Ensures that the feature both meets the size tolerance and has the correct geometric form — critical for shafts in sliding fits.
Manufacturing Terms
Tolerance: The total permissible variation in a dimension — the difference between the upper and lower limit of size. Tolerance = Upper Limit − Lower Limit.
Clearance Fit: A fit in which the hole is always larger than the shaft — guaranteed clearance (space) between mating parts. Used for rotating/sliding interfaces. Example: H8/f7.
Interference Fit (Press Fit): A fit in which the shaft is always larger than the hole — requires force or thermal differential to assemble. Generates clamping pressure that transmits torque or axial force without fasteners. Example: H7/p6.
Transition Fit: A fit that may result in either a small clearance or a small interference depending on where within tolerance each part falls. Used for locating without guaranteed slack or excessive assembly force. Example: H7/k6.
Tolerance Grade (IT Grade): ISO 286 defines 18 International Tolerance grades (IT01 to IT18). IT5–IT7 are precision machined fits; IT8–IT11 are general machined; IT12–IT16 are for stamping and casting. The IT number defines the tolerance magnitude for a given nominal size.
Deep Drawing: Sheet metal forming process that stretches and draws a flat blank into a cup or box shape using a punch and die. Drawability depends on the r-value (plastic strain ratio) of the sheet material — high r-value materials (deep drawing steel, aluminum 3003) resist thinning and form deeper draws.
Minimum Bend Radius: The minimum inside radius of a bend that avoids cracking the outer surface during sheet metal bending. Typically specified as a multiple of material thickness (e.g., 1t for low-carbon steel, 2t for 6061-T6 aluminum). Harder materials require larger minimum radii.
Draft Angle: The taper angle applied to vertical surfaces of a casting or injection-molded part to allow ejection from the mold or die. Typically 1–3° for plastic injection molding; 1–2° for die casting. Insufficient draft causes sticking, scuffing, and ejection damage.
EDM (Electrical Discharge Machining): Material removal by controlled electrical discharges (sparks) between the workpiece and electrode. Can machine any electrically conductive material regardless of hardness — used for hardened tool steel, cemented carbide, and complex profiles impossible to mill. Wire EDM cuts profiles to ±0.005 mm accuracy; sinker EDM creates cavities (mold cores).
Annealing: Heat treatment process that softens hardened or work-hardened metal, restores ductility, and relieves residual stress. For steel: heat to above Ac1, hold, then slow cool (furnace cool). For copper and aluminum: solution anneal then quench (aluminum) or simple heat-and-cool (copper).
Case Hardening: Heat treatment process that hardens only the surface layer of a low-carbon steel part, leaving a tough core. Methods: carburizing, carbonitriding, nitriding. Produces a hard (58–62 HRC), wear-resistant case with a tough, impact-resistant core.
Induction Hardening: Surface hardening using induced electrical currents to rapidly heat the surface, followed by quenching. Used on medium-carbon steels (S45C, SCM440) for localized hardening of gear tooth flanks, shaft journals, and cam lobes without affecting the entire part.
Shot Peening: Controlled impact of the surface with small spherical shot particles. Induces compressive residual stress in the surface layer, significantly improving fatigue strength. Standard treatment for springs, gears, and fatigue-critical structural components. Specified per SAE J442/J443 (shot peening coverage and intensity — Almen arc height).
Bearings and Tribology
Dynamic Load Rating (C): The load under which 90% of a group of identical bearings will complete 1 million revolutions without pitting fatigue. Used with the bearing life equation: L₁₀ = (C/P)^p × 10⁶ revolutions, where P is equivalent dynamic bearing load and p = 3 for ball bearings, 10/3 for roller bearings.
B10 Life (L10 Life): The life in operating hours or revolutions that 90% of a population of bearings will reach or exceed without fatigue failure (the first 10% fail at L10). Standard for specifying bearing design life in SKF, NSK, NTN, and FAG catalogues.
Stribeck Curve: Graph showing the coefficient of friction between lubricated surfaces as a function of the Hersey number (η·N/P, where η = viscosity, N = speed, P = pressure). Identifies three lubrication regimes: boundary (high friction, metal-to-metal contact), mixed, and hydrodynamic (full fluid film, low friction). Good bearing design operates in the hydrodynamic regime.
Specific Film Thickness (Lambda Ratio, Λ): Ratio of minimum oil film thickness to composite surface roughness (Λ = h_min / √(Rq1² + Rq2²)). Λ > 3: full film (EHD) lubrication; 1 < Λ < 3: mixed lubrication; Λ < 1: boundary lubrication. Rolling contact bearings operate in EHD lubrication when properly selected and lubricated.
Preload (Bearing): Controlled interference applied to a bearing to eliminate internal clearance and improve stiffness, running accuracy, and fatigue life. Applied by spring, spacer shimming, or nut torquing against the inner race. Required for precision spindles and angular contact bearing pairs. Excessive preload causes overheating and premature failure.
Seals and Fluid Power
Compression Set: Permanent deformation of an elastomeric seal after sustained compression. Expressed as a percentage of original deflection. Low compression set is critical for long-life static and dynamic seals. NBR has moderate compression set; FKM (Viton) has low compression set; silicone has moderate compression set but good temperature range.
Squeeze (O-ring): The percentage radial compression of an O-ring in its groove. For static seals: 15–25% squeeze. For dynamic seals: 10–20% squeeze. Too little squeeze = leak; too much = excessive friction and wear.
Hydraulic System Pressure Classes: ISO 4413 defines standard working pressures. Medium pressure: 100–250 bar. High pressure: 250–700 bar. Very high pressure (offshore, aerospace): 700 bar+. Pipe and fitting wall thickness, valve ratings, and hose specifications all step up with pressure class.
Cavitation: Formation and violent collapse of vapor bubbles in a liquid when local pressure drops below vapor pressure. Causes pitting damage to pump internals, valve seats, and hydraulic actuator surfaces. Prevent by ensuring adequate inlet pressure to pumps (positive suction head), proper line sizing, and avoiding sharp restrictions that cause local velocity increase.
Fasteners and Joints
Clamp Load (Preload): The tensile force in a bolt (and compressive force on the joint interface) produced by tightening. Effective joint function depends on maintaining adequate clamp load throughout service life against all loosening forces. Target: 70–75% of proof load for most bolted joints.
Proof Load: The maximum load a fastener can sustain without permanent deformation. Approximately equal to 0.2% proof stress × tensile stress area. The proof load test (ISO 898-1) applies the proof load and verifies the fastener returns to original length.
Thread Engagement Length: The length of threaded engagement between a bolt and tapped hole necessary to develop full bolt tensile strength without stripping the internal thread. Rule of thumb: 1D (one bolt diameter) in steel of similar strength; 1.5D in cast iron; 2D+ in aluminum alloy.
Galling: Severe adhesive wear between contacting metal surfaces, characterized by tearing and transfer of material. Austenitic stainless steel fasteners are particularly susceptible (high adhesive tendency). Prevent by using anti-seize compound (Molykote, Never-Seez), nickel-plated fasteners, or specifying galling-resistant material pairs.
Conclusion
Mastery of engineering vocabulary is a career-long accumulation. This glossary covers the terms that appear most frequently in day-to-day mechanical design work — use it as a quick reference and as a starting point for deeper study in any area where the definition raises more questions than it answers. The cross-references embedded in each definition point to the adjacent concepts that form the full picture of each technical domain.
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