Introduction to Machine Element and Design

Foundational vocabulary and core concepts for Machine Element and Design I, covering design types, stress analysis, failure modes, and industrial standards.

Design

The transformation of concepts and ideas into useful machinery through the processes of conceiving, inventing, and contriving.

Engineering (Thomas Tredgold)

The art of directing the great sources of power in nature for the use and convenience of man.

Engineering Design

The process of applying various techniques and scientific principles for the purpose of defining a device, a process, or a system in sufficient detail to permit its realization.

Machine Design

The creation of new and better machines and improving the existing ones that work safely, reliably, and well.

Machine

An apparatus consisting of interrelated units (elements) that modifies force or motion, or a combination of mechanisms that transforms, transmits, or uses energy, load, or motion for a specific purpose.

Machine Element

An individual component or unit (such as a gear, shaft, or bearing) that comprises a mechanical system and functions as a load transmitter, torque transmitter, energy absorber, or seal.

Adaptive Design

A type of design where the designer's work is concerned with minor modifications or adaptations of existing designs, requiring ordinary technical training.

Development Design

A type of design that modifies existing designs into new ideas by adopting new materials or different methods of manufacture, requiring considerable scientific training.

New Design

A type of design that requires significant research, technical ability, and creative thinking to develop a product that differs markedly from existing items.

Stress ($\sigma$)

The internal force per unit area at any section of a body which resists external forces, expressed as $\sigma = \frac{P}{A}$.

Pascal ($Pa$)

The S.I. unit of stress, equivalent to $1\,N/m^{2}$.

Strain ($\epsilon$)

The deformation per unit original dimension of a body when a system of forces or loads is applied, expressed as $\epsilon = \frac{\delta l}{l}$.

Hooke's Law

A principle stating that when a material is loaded within its elastic limit, the stress is directly proportional to the strain.

Working Stress

Also known as safe, allowable, or design stress, this is the stress level kept lower than the maximum/ultimate stress at which failure of the material occurs.

Factor of Safety

The ratio of the maximum stress to the working stress ($\text{FS} = \frac{\text{Maximum Stress}}{\text{Working Stress}}$).

Yielding

A form of failure in a component where the applied stress exceeds the material's yield stress, causing plastic deformation without necessarily causing fracture.

Fracture

A form of failure where the material separates into pieces due to applied stress.

Cyclic Stress

A type of stress that occurs when a material is subjected to repeated loading and unloading cycles, where stresses vary from compressive to tensile or vice versa.

Fluctuating Stresses

Stresses that vary from a minimum value to a maximum value of the same nature (e.g., both tensile or both compressive).

Repeated Stresses

Stresses that vary from zero to a certain maximum value.

Alternating Stresses

Stresses that vary from a minimum value to a maximum value of the opposite nature.

Stress Concentration

The localization of high stresses significantly higher than the surrounding area due to irregularities or abrupt changes in cross-section.

Theoretical Stress-Concentration Factor ($K_{t}$)

The ratio of the maximum stress in a member (at a notch or fillet) to the nominal stress, depending only on the geometry of the part.

Notch Sensitivity Factor ($q$)

A property of a material measuring its sensitivity to stress concentration, ranging in value from $0$ to $1$.

Fatigue

The failure of a material subjected to repeated stresses at levels below the yield point, caused by progressive microscopic crack formation.

Endurance Limit ($\sigma_{e}$)

The maximum value of stress that a material can withstand for an infinite number of cycles without failure.

Standard

A set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency, and a specified quality.

Code

A usually legally binding document compiled by a governmental agency to achieve a specified degree of safety, efficiency, and performance or quality.