Why These Terms Matter
Mechanical engineering spans an unusually wide range of subject matter — thermodynamics, fluid mechanics, solid mechanics, materials science, machine design, controls, and manufacturing all fall under one discipline. Each of these sub-fields has its own precise vocabulary, and terms often carry meanings that differ subtly from their everyday use. "Stress" is not "strain," "power" is not "torque," and "yield strength" is not "ultimate strength" — mixing these up on a calculation, a drawing note, or a specification can lead to an undersized shaft, a mischaracterized failure, or a rejected part.
This glossary collects 55 of the terms mechanical engineers and engineering students encounter most often, organized alphabetically with plain-language definitions and, where applicable, references to the governing ASME, ISO, or ASHRAE standard. It is meant as a working reference for coursework, the FE/PE exam, and day-to-day design and manufacturing communication.
A
- Ambient Temperature
- The temperature of the surrounding environment in which equipment operates, distinct from the temperature of the equipment or fluid itself. Ambient temperature affects convective heat transfer rates, material property derating, and the sizing of cooling or insulation systems.
- Angular Velocity (ω)
- The rate of rotation of a body, expressed in radians per second (rad/s). Related to rotational speed in revolutions per minute (RPM) by ω = 2πN/60, where N is RPM. Angular velocity is used directly in power calculations for rotating shafts: Power = Torque × ω.
- ASME (American Society of Mechanical Engineers)
- The professional and standards-development organization that publishes many of the codes mechanical engineers work to daily, including ASME Y14.5 (Geometric Dimensioning and Tolerancing), the ASME Boiler and Pressure Vessel Code (BPVC), and ASME B31 piping codes. "ASME certified" or "ASME stamped" on a pressure vessel indicates the vessel was designed, fabricated, and inspected to BPVC requirements.
- Ambient Pressure
- The pressure of the surrounding atmosphere, typically taken as 14.7 psia (101.3 kPa) at sea level. Gauge pressure readings are relative to ambient pressure; absolute pressure is gauge pressure plus ambient pressure.
B
- Bearing
- A machine element that constrains relative motion to the desired type (typically rotation or linear translation) while reducing friction between moving parts. Rolling-element bearings (ball or roller) use rolling contact; journal (plain/sleeve) bearings rely on a hydrodynamic or hydrostatic oil film. Bearing selection depends on load type (radial vs. thrust), speed, and required life (L10 life, in rolling-element bearing terms).
- Bending Moment
- The internal moment (torque) within a structural or machine member caused by external loads that tend to bend it. Bending moment, combined with the section's moment of inertia, determines bending stress via the flexure formula σ = Mc/I, where c is the distance from the neutral axis to the outer fiber.
- BTU (British Thermal Unit)
- A unit of energy equal to the heat required to raise one pound of water by one degree Fahrenheit (≈1,055 joules). Widely used in HVAC and thermal system sizing in the U.S.; a "ton" of cooling equals 12,000 BTU/hr.
- Buckling
- A sudden lateral or torsional instability failure mode in a slender member under compressive load, occurring at a stress well below the material's yield strength. Governed by Euler's formula for long, slender columns: Pcr = π²EI/(KL)², where K is the effective length factor depending on end conditions.
C
- Cavitation
- The formation and violent collapse of vapor bubbles in a liquid when local pressure drops below the fluid's vapor pressure, typically at a pump impeller inlet or valve throat. Cavitation causes noise, vibration, reduced efficiency, and pitting erosion of metal surfaces. Prevented by ensuring Net Positive Suction Head Available (NPSHA) exceeds Net Positive Suction Head Required (NPSHR).
- Coefficient of Thermal Expansion (CTE)
- The fractional change in a material's dimension per degree of temperature change, typically expressed in units of per °C or per °F (e.g., µm/m·°C). CTE mismatch between joined materials — a bolted flange of one metal to a pipe of another, for example — causes thermal stress and must be accounted for in piping and pressure vessel design.
- Compressive Strength
- The maximum compressive stress a material can withstand before failure. Brittle materials like concrete and cast iron are characterized primarily by compressive strength since they fail well before reaching comparable tensile strength.
- Creep
- The slow, time-dependent plastic deformation of a material under sustained load, typically significant only at elevated temperatures (generally above 30-40% of the material's melting point on the absolute scale). Creep governs the design life of turbine blades, boiler tubes, and other high-temperature components, and is distinct from fatigue, which is caused by cyclic rather than sustained loading.
D
- Datum
- A theoretically exact point, axis, or plane derived from a physical feature of a part, used as the origin from which the location or geometric characteristics of other features are established. Datums are the foundation of GD&T (Geometric Dimensioning and Tolerancing per ASME Y14.5) and are identified on drawings with datum feature symbols.
- Ductility
- A material's ability to undergo significant plastic deformation before fracture, typically measured as percent elongation at break in a tensile test. Steel and aluminum are ductile (they yield and stretch visibly before breaking); cast iron and ceramics are brittle (they fracture with little or no plastic deformation).
- Dynamic Viscosity
- A fluid's resistance to shear flow, expressed in units of Pa·s (or centipoise, cP). Distinct from kinematic viscosity, which is dynamic viscosity divided by fluid density. Viscosity governs pipe friction losses, pump power requirements, and lubrication film thickness in bearings.
E
- Efficiency (Thermal/Mechanical)
- The ratio of useful output (work or heat) to total input energy, always less than 100% for real systems due to irreversibilities (friction, heat loss, turbulence). Carnot efficiency represents the theoretical maximum for a heat engine operating between two temperatures and is never achieved in practice.
- Elastic Modulus (Young's Modulus, E)
- The ratio of stress to strain within a material's linear elastic region, representing its stiffness. Steel has E ≈ 200 GPa (29,000 ksi); aluminum has E ≈ 69 GPa (10,000 ksi). Young's modulus is a fixed material property that does not change with heat treatment or alloy strengthening — only yield and ultimate strength change.
- Enthalpy (h)
- A thermodynamic property equal to internal energy plus the product of pressure and volume (h = u + Pv), representing the total heat content of a system per unit mass. Enthalpy is the key property used in steam tables, refrigeration cycle analysis, and psychrometric calculations because it directly gives the heat added or removed in a constant-pressure process.
- Entropy (S)
- A thermodynamic property representing the degree of disorder or unavailability of energy to do useful work. The second law of thermodynamics states that total entropy of an isolated system never decreases — this is why no real process is 100% efficient and why heat flows spontaneously from hot to cold, never the reverse.
F
- Factor of Safety (FOS)
- The ratio of a material's failure strength (yield or ultimate, depending on the failure mode being guarded against) to the maximum stress expected in service. A factor of safety of 2 on yield strength means the part is designed to operate at half the stress that would cause permanent deformation. FOS values are set higher for uncertain loading, brittle materials, or safety-critical applications, and lower for weight-critical, well-characterized applications like aerospace.
- Fatigue
- Progressive, localized structural damage that occurs when a material is subjected to cyclic (repeated) loading, even at stress levels well below the material's yield strength. Fatigue failure proceeds through crack initiation, crack propagation, and final fracture, and is responsible for the majority of in-service mechanical failures. Characterized by S-N curves (stress amplitude vs. number of cycles to failure) and, for steels, an endurance limit below which fatigue failure theoretically never occurs.
- Flexure Formula
- The equation σ = Mc/I relating bending stress (σ) to bending moment (M), distance from the neutral axis (c), and the section's second moment of area (I). Fundamental to beam and shaft bending stress calculations.
- Fluid Dynamics
- The study of fluids (liquids and gases) in motion, governed by the Navier-Stokes equations, conservation of mass (continuity), and conservation of energy (Bernoulli's equation for simplified, inviscid, incompressible flow). Used to analyze pipe flow, pump and turbine performance, aerodynamic drag, and HVAC duct design.
G
- Gear Ratio
- The ratio of the number of teeth (or diameters) between a driven gear and a driving gear, determining the speed reduction (or increase) and corresponding torque multiplication between them. A 4:1 gear ratio reduces output speed to 1/4 of input speed while multiplying torque by approximately 4 (minus mechanical losses).
- GD&T (Geometric Dimensioning and Tolerancing)
- A symbolic drafting language, standardized in ASME Y14.5, used to define and communicate engineering tolerances for form, orientation, location, and runout of part features relative to datums. GD&T replaces ambiguous coordinate (plus/minus) tolerancing with a mathematically defined tolerance zone for each feature.
- GPM (Gallons Per Minute)
- A volumetric flow rate unit common in pump, chiller, and piping system specifications in the U.S. Converting to SI: 1 GPM ≈ 0.0631 L/s.
H
- Hardness
- A material's resistance to localized plastic deformation, typically measured by an indentation test (Rockwell, Brinell, or Vickers). Hardness correlates roughly with tensile strength for a given material family and is often used as a quick, non-destructive quality-control check after heat treatment.
- Heat Exchanger
- A device used to transfer heat between two or more fluids without mixing them, typically via a solid wall separating the streams. Common configurations include shell-and-tube, plate, and finned-tube (used in air-cooled applications). Sized using the Log Mean Temperature Difference (LMTD) method or the effectiveness-NTU method.
- Hydraulic Diameter (Dh)
- An equivalent diameter used to apply circular-pipe flow correlations (friction factor, Reynolds number) to non-circular ducts and channels, defined as Dh = 4A/P, where A is cross-sectional flow area and P is the wetted perimeter.
I
- Ideal Gas Law
- The equation of state PV = nRT (or Pv = RT on a per-unit-mass basis) relating pressure, volume, temperature, and quantity of an ideal gas. A reasonable approximation for many gases at low pressure and high temperature relative to their critical point, but inaccurate for real gases (like steam) near saturation conditions.
- Interference Fit
- An assembly method in which a shaft's diameter is slightly larger than the bore it is pressed into, creating a mechanical joint held together entirely by friction from the resulting elastic (or, in shrink fits, thermally induced) clamping pressure — no fasteners or adhesive required. Common for gear-to-shaft and bearing-to-housing assemblies.
- ISO (International Organization for Standardization)
- The body that publishes internationally recognized standards relevant to mechanical engineering, including ISO 9001 (quality management systems), ISO 286 (limits and fits), and ISO 1101 (geometrical tolerancing — the international counterpart to ASME Y14.5).
K
- Kinematics
- The branch of mechanics describing motion (position, velocity, acceleration) without reference to the forces that cause it. Contrasted with kinetics (or dynamics), which relates motion to the forces and moments producing it.
- Kinematic Viscosity (ν)
- Dynamic viscosity divided by fluid density (ν = µ/ρ), expressed in units of m²/s (or centistokes, cSt). Used directly in the Reynolds number calculation, Re = VD/ν, which determines whether pipe flow is laminar or turbulent.
L
- Laminar Flow
- Smooth, orderly fluid flow in parallel layers with no cross-mixing, occurring at Reynolds numbers below approximately 2,300 in pipe flow. Contrasted with turbulent flow (Re above approximately 4,000), which is chaotic and mixes rapidly. Laminar flow has a parabolic velocity profile and lower friction losses than turbulent flow at the same average velocity, but is uncommon in most industrial piping due to the higher flow rates involved.
- Load (Static vs. Dynamic)
- A static load is applied gradually and remains constant (or nearly so) over time; a dynamic load varies with time and may include impact, shock, or cyclic components. Dynamic loads are typically analyzed with a dynamic load factor or through fatigue analysis rather than simple static stress calculations, since the effective stress under impact can be many times the equivalent static stress.
M
- Mach Number
- The ratio of an object's (or flow's) velocity to the local speed of sound. Flow is subsonic below Mach 1, transonic near Mach 1, and supersonic above Mach 1. Compressibility effects become significant above roughly Mach 0.3, at which point incompressible flow assumptions (like standard Bernoulli's equation) begin to break down.
- Machinability
- A qualitative measure of how easily a material can be cut, drilled, or otherwise machined to a desired shape and finish, considering tool wear, achievable surface finish, and cutting speed. Free-machining steels (with added sulfur or lead) have high machinability; austenitic stainless steels and titanium alloys are comparatively difficult to machine.
- Modulus of Elasticity
- See Elastic Modulus (Young's Modulus).
- Moment of Inertia (Area)
- A geometric property of a cross-section (denoted I) describing its resistance to bending, distinct from mass moment of inertia, which describes resistance to rotational acceleration. A larger I for a given cross-sectional area (achieved by placing material farther from the neutral axis, as in an I-beam) increases bending stiffness and reduces bending stress for a given moment.
N
- Newton's Laws of Motion
- The three foundational laws of classical mechanics: (1) a body at rest or in uniform motion stays that way unless acted on by a net force; (2) F = ma, force equals mass times acceleration; (3) for every action there is an equal and opposite reaction. These underpin virtually all static and dynamic analysis in mechanical engineering.
- NPSH (Net Positive Suction Head)
- The difference between the absolute pressure at a pump's suction and the fluid's vapor pressure, expressed as a head (in feet or meters of fluid). NPSH Available (a system characteristic) must exceed NPSH Required (a pump characteristic, from the manufacturer's curve) to avoid cavitation.
P
- Poisson's Ratio (ν)
- The ratio of transverse (lateral) strain to axial strain when a material is stretched or compressed, typically around 0.3 for structural metals and 0.5 for incompressible materials like rubber. Used in multiaxial stress-strain relationships and finite element analysis.
- Power
- The rate of doing work, expressed in watts (W) or horsepower (hp); 1 hp = 745.7 W. For rotating machinery, Power = Torque × Angular Velocity — this is why a low-torque, high-speed motor and a high-torque, low-speed motor can deliver identical power output.
- Preload
- The tensile force induced in a bolt (or clamping force between two mating faces) during initial tightening, before any external service load is applied. Adequate preload keeps a bolted joint from separating under cyclic external loads, which is essential to avoiding bolt fatigue failure — most of a properly preloaded bolt's fatigue life is protected because the external load only slightly increases total bolt tension rather than fully cycling it from zero.
- Pressure Vessel
- A closed container designed to hold gases or liquids at a pressure substantially different from ambient pressure. Design, fabrication, inspection, and testing of pressure vessels in the U.S. are governed by the ASME Boiler and Pressure Vessel Code (BPVC), particularly Section VIII.
R
- Reynolds Number (Re)
- A dimensionless ratio of inertial forces to viscous forces in a fluid flow, Re = ρVD/µ (or VD/ν), used to predict whether flow will be laminar or turbulent. Low Re (viscous forces dominate) indicates laminar flow; high Re (inertial forces dominate) indicates turbulent flow.
- Rockwell Hardness (HRC/HRB)
- A hardness scale measured by the depth of indentation from a standardized indenter under a specified load. HRC (the "C" scale, using a diamond cone indenter) is used for hardened steels; HRB (the "B" scale, using a hardened steel ball) is used for softer materials like unhardened steel, aluminum, and brass.
S
- Shear Stress
- Stress acting parallel (tangential) to a material cross-section, as opposed to normal stress, which acts perpendicular to it. Shear stress governs bolt and pin failure in single or double shear, torsional stress in shafts (τ = Tc/J), and beam shear near supports.
- Specific Heat (Cp, Cv)
- The amount of heat energy required to raise the temperature of a unit mass of a substance by one degree, measured at constant pressure (Cp) or constant volume (Cv). For an ideal gas, Cp − Cv = R (the specific gas constant), and the ratio k = Cp/Cv (specific heat ratio) governs isentropic (adiabatic, reversible) compression and expansion processes.
- Stress vs. Strain
- Stress (σ) is internal force per unit area within a material (units of Pa or psi); strain (ε) is the dimensionless ratio of deformation to original length. The two are related through the material's elastic modulus in the linear region (σ = Eε, Hooke's Law) but diverge beyond the yield point, where strain increases much faster than stress as the material deforms plastically.
- Surface Finish (Ra)
- A quantitative measure of surface roughness, most commonly expressed as Ra (arithmetic average roughness), in microinches (µin) or micrometers (µm). Tighter surface finish requirements increase machining cost and are specified only where functionally required — for sealing surfaces, bearing journals, or fatigue-critical fillets, for example.
T
- Thermal Conductivity (k)
- A material property describing its ability to conduct heat, expressed in W/(m·K). High-k materials (copper, aluminum) are used for heat sinks and heat exchangers; low-k materials (insulation foams, air) are used to resist heat flow. Governs Fourier's Law of conduction: q = -kA(dT/dx).
- Tolerance
- The total permissible variation in a dimension, defined as the difference between the maximum and minimum allowable limits. Tighter tolerances generally increase manufacturing cost, so tolerances are specified only as tight as functionally necessary — a principle central to both GD&T and general dimensioning practice.
- Torque
- A rotational (twisting) force, equal to the applied force times the perpendicular distance (moment arm) from the axis of rotation, expressed in N·m or lb-ft. Torque, not force alone, determines whether a bolt is properly tightened, a shaft can transmit a given power at a given speed, or a motor can start a given load.
- Turbulent Flow
- Chaotic, mixing fluid flow characterized by eddies and rapid velocity fluctuations, occurring at Reynolds numbers above approximately 4,000 in pipe flow. Most practical piping, HVAC duct, and pump applications operate in the turbulent regime, where friction losses scale roughly with velocity squared rather than linearly with velocity as in laminar flow.
U
- Ultimate Tensile Strength (UTS)
- The maximum stress a material can withstand while being stretched before necking and fracture, as measured on a tensile test's stress-strain curve. UTS is always higher than yield strength for ductile materials and represents the true upper bound of load-carrying capacity, though most designs are limited by yield strength (or a fatigue limit) well before reaching UTS.
V
- Vibration (Natural Frequency, Resonance)
- Every mechanical system has one or more natural frequencies at which it will oscillate freely if disturbed. Resonance occurs when an external forcing frequency (from an unbalanced rotating shaft, for example) matches a natural frequency, causing amplitude to grow dramatically and potentially leading to rapid fatigue failure. Machine design deliberately separates operating speed from natural frequencies by a comfortable margin (commonly at least 20-25%).
- Von Mises Stress
- A scalar "equivalent stress" derived from a full multiaxial stress state, used to predict yielding in ductile materials under combined loading via the distortion energy (von Mises) failure theory. A part yields when its von Mises stress reaches the material's uniaxial yield strength — this is the stress value most commonly reported by finite element analysis software for ductile-material static checks.
W — Y
- Weld Symbol
- A standardized drafting symbol (per AWS A2.4) specifying weld type, size, length, and process on an engineering drawing, attached to a reference line with an arrow pointing to the joint. Distinct from GD&T symbols, though the two frequently appear together on fabrication drawings.
- Work
- Energy transferred when a force acts on an object over a distance (Work = Force × Distance, for a constant force in the direction of motion), expressed in joules or foot-pounds. Distinguished from power, which is the rate at which work is done.
- Yield Strength
- The stress at which a material begins to deform plastically (permanently) rather than elastically. Below the yield strength, a part returns to its original shape when the load is removed; above it, permanent deformation remains even after unloading. Most structural and machine design calculations size parts against yield strength (with an appropriate factor of safety) rather than ultimate tensile strength, since permanent deformation is typically considered a functional failure even though the part has not fractured.
Quick Reference Table
| Term | Category | Key Point |
|---|---|---|
| Factor of Safety | Design | Ratio of failure strength to design stress |
| Yield Strength | Materials | Onset of permanent deformation |
| Ultimate Tensile Strength | Materials | Maximum stress before fracture |
| Fatigue | Failure Modes | Cyclic loading failure below yield |
| Creep | Failure Modes | Time-dependent deformation at high temperature |
| Buckling | Failure Modes | Compressive instability of slender members |
| Von Mises Stress | Failure Theory | Equivalent stress for ductile yield prediction |
| Reynolds Number | Fluids | Predicts laminar vs turbulent flow |
| Cavitation | Fluids | Vapor bubble collapse from low local pressure |
| NPSH | Fluids | Available vs required suction head for pumps |
| Enthalpy | Thermodynamics | Total heat content per unit mass |
| Entropy | Thermodynamics | Measure of energy unavailability / disorder |
| Thermal Conductivity | Heat Transfer | Material's ability to conduct heat |
| Torque | Mechanics | Rotational force × moment arm |
| GD&T | Drafting | ASME Y14.5 tolerancing language |
| Preload | Fasteners | Initial bolt tension before service load |