Introduction to Design of Machine Elements
- 1. 08/07/2025 1 DESIGN OF MACHINE ELEMENTS
- 2. 08/07/2025 2 What is “Design of Machine Elements” Design of Machine Elements (DoME) is defined as the selection of material and the dimensions for each geometrical parameter so that the element satisfies its function and undesirable effects are kept within the allowable limit.
- 3. 08/07/2025 3 •BASICS: •What is Stress and What is strain? •Stress vs Strain Curve •Types of Stresses
- 4. 08/07/2025 Before Proceeding – These pictures share a connection 4 Titanic He co-founded a company in late 2000’s He is a first-of all a Businessman and then an engineer Guess him!!!
- 5. 08/07/2025 Ignoring the “Basics” in design isn’t just a risk— it’s a recipe for disaster. Imploded Titan Submersible at 3800 m below sea level Boeing 737 Max failure at 2500 m above sea level
- 6. 08/07/2025 6 Stress The force of resistance per unit area, offered by a body against deformation is known as Stress. σ = ( ) Unit is N/mm2 If its External resistance/Area, then its called as Pressure Strain It is defined as the change in dimension to original dimension is known as strain. e= Young’s modulus (or) Modulus of Elasticity The ratio of tensile or compressive stress to the corresponding strain is a constant. E = ( )
- 7. 08/07/2025 7 Stress vs Strain Curve
- 8. 08/07/2025 8 Curve Region Titan Analogy Proportional (σ ε) ∝ Normal ocean pressure — hull compresses but returns to shape. Elastic Limit Minor thermal changes or loading cycles — no damage yet. Yield Point Microcracks, early delamination under thermal mismatch. Strain Hardening Hull becomes tougher for a short time (composite binding tighter). UTS Maximum stress the sub could take (perhaps due to hull shape flaws). Necking Internal failure — rapid decrease in load-bearing area. Fracture Final implosion — catastrophic material failure. The Final Dive: A Stress Strain Adventure Under Pressure ‑
- 9. 08/07/2025 9 1 ️ 1️ ⃣ Compressive Stress & Strain: The Ocean’s Embrace Definition: Compressive stress is the resistance per unit area when a body is subjected to two equal and opposite push forces. Formula: σ = P/A Compressive strain: Decrease in length/original length. As pressure pushes inward on Titan’s hull, its wall thickness and length slightly decrease—just like hugging a soda can until it buckles.
- 10. 08/07/2025 10 2 ️ 2️ ⃣ Tensile Stress & Strain: The Stretch Within Definition: Tensile stress is induced by two equal and opposite pull forces. Formula: σ = P/A Tensile strain: Elongation/original length. Inside Titan, bolts and seals resist being pulled apart by any internal pressure or leak—imagine your bolts are in a constant long-distance relationship.
- 11. 08/07/2025 11 3 ️ 3️ ⃣ Shear Stress & Strain: Layers at War Definition: The stress induced in a body, when subjected to two equal and opposite forces which are acting tangentially to the body Formula: τ = F/A Shear strain: Angular deformation. At carbon-fiber/titanium interfaces, layers can slip under load—like sliding two cake layers with frosting in between. Delicious… but dangerous for composites. Another example: Opening a tight bottle cap involves applying torque to twist the cap. That torque causes shear stress in the plastic threads as they try to resist rotation.
- 12. 08/07/2025 12 Shear Modulus (or) Rigidity Modulus (or) Modulus of Rigidity The ratio of shear stress to the corresponding shear stain within elastic limit is called shear modulus C/G/N = Factor of Safety It is defined as the ultimate tensile stress (or) ultimate load to the permissible stress (or) permissible load. F.O.S =
- 13. 08/07/2025 13 Lateral strain The strain induced in a body at right angles to the direction of applied load is known as lateral strain Lateral strain = Longitudinal strain The strain induced in a body along the axial direction of applied load. The ratio of axial deformation to the original length of the body is known as longitudinal strain Longitudinal strain =
- 14. 08/07/2025 14 4 ️ 4️ ⃣ Under Pressure — Longitudinal & Lateral Strain & Poisson’s Ratio As external pressure squeezes Titan lengthwise, its diameter changes too: Longitudinal strain: Decrease in length. Lateral strain: Decrease in diameter (cross-section). Poisson’s Ratio (μ) = Lateral strain / Longitudinal strain Most materials have a positive Poisson’s ratio: Compress them (↓length) → they bulge outward (↑diameter). Stretch them (↑length) → they narrow (↓diameter). Negative Poisson’s ratio (auxetic foam materials) behave oppositely—shrinking in all directions when compressed. Helmet design uses this concept: Traditional foams (μ > 0) spread and absorb impact by expanding laterally. Advanced auxetic foams (μ < 0) become denser under compression, offering superior shock absorption in sports and aerospace helmets. Visualization: Squeezing a sponge makes it fatter—positive Poisson’s. A magical sponge that shrinks uniformly—that’s auxetic. Poisson's ratio It is a material property that describes how much a material deforms in a direction perpendicular to the applied force, relative to the deformation in the direction of the force.
- 15. 08/07/2025 15 5 ️ 5️ ⃣ Thermal Stress: Cold Outside, Hot Inside Definition: Stress induced when a body is restrained from expanding/contracting due to ΔT. Formula: σ = α·ΔT·E Titan’s hull sees near-freezing seawater outside and warm electronics inside. Carbon fiber and titanium expand differently, so thermal stress builds—like gluing a metal and plastic ruler and heating one end.
- 16. 08/07/2025 16 Principal plane: The planes which have no shear stress are known as the principal plane. Principal stress: The magnitudes of normal stress which are acting on a principal plane are known as principal stress. At any point, three orthogonal principal stresses exist. The greatest one governs where yielding or fracture will initiate—like the strongest argument winning in court. 6 ️ 6️ ⃣
- 17. 08/07/2025 17 7 ️ 7️ ⃣ Volumetric Strain: Crushed in an Instant Definition: Change in volume/original volume, ε_v = ΔV/V. When Titan imploded, its volume collapsed nearly instantly—like a soda can under a hydraulic press, shriveling to a fraction of its size. 8️ ⃣ Bending Stress: Uneven Forces, Dangerous Curves Definition: Stress from a bending moment, causing one side in tension and the other in compression. Formula: σ = M·y/I If Titan’s shell weren’t perfectly uniform, bending stresses arise under pressure—one side caves in while the opposite side stretches. 9 ️ 9️ ⃣ Circumferential (Hoop) & Longitudinal Stresses: The Cylindrical Reality Bulk Modulus It is defined as the ratio of direct stress to the corresponding volumetric strain is a constant and it is called as bulk modulus. K =
- 18. 08/07/2025 18 while the opposite side stretches. 9 ️ 9️ ⃣ Circumferential (Hoop) & Longitudinal Stresses: The Cylindrical Reality Hoop stress (σ₁): Acts around the circumference, σ₁ = p·d/(2·t). Longitudinal stress (σ₂): Acts along length, σ₂ = p·d/(4·t). Because hoop stress is twice longitudinal stress, failures tend to run around the cylinder—like a balloon bursting in a ring rather than lengthwise.
- 19. 08/07/2025 19 Titan’s tragedy wasn’t a single oversight—it was the cumulative effect of multiple stress types interacting under extreme conditions. As engineers, you must anticipate all these stresses in your designs to ensure safety and reliability
- 20. 08/07/2025 20 So class, whether it's material life or married life… you only succeed if you know how to handle stress!!!
- 21. Introduction to the Machine Design Machine design is a systematic, iterative procedure crucial for developing and refining mechanical components and systems that meet stringent performance, safety, and economic demands. This methodical approach ensures a balance of functionality, cost-effectiveness, and reliability in engineering solutions. Problem Definition & Requirements Gathering Clearly defining the engineering challenge and compiling all necessary performance, safety, and economic criteria. Conceptual Design Generating diverse initial ideas and potential solutions that address the defined problem, exploring various mechanical principles. Detailed Design & Analysis Performing precise calculations, creating CAD models, and simulating behaviour to refine the chosen concept into a workable design. Prototyping & Testing Building physical prototypes and rigorously testing them against the initial requirements to validate performance and identify flaws. Optimization & Iteration Refining the design based on test results, addressing deficiencies, and continuously improving the solution through repeated cycles.
- 22. 1. Problem Definition & Requirements Gathering Design a low-cost portable fan: • What airflow is needed? • How long should it run? • Safety and cost constraints? Every successful design starts by clearly defining the problem and listing all constraints (cost, performance, safety, etc). 2. Conceptual Design Creative thinking is necessary: • Should the fan be hand-powered? Solar-powered? Foldable? Battery Powered? 3. Detailed Design & Analysis Technical Details: like what kind of motor (BLDC – Brushless DC motor) Calculate the motor torque, draw 3D CAD models, and run simulations for blade performance. The initial idea will become a technically viable design. 4. Prototyping & Testing Building it and seeing whether it actually works: Building a working prototype of the fan and testing battery life, airflow, and noise. •To find out if the design meets expectations & here it often reveals flaws in the design. Optimization & Iteration Fix the issues and improve the design: Is the fan too loud? Change the blade material, or if the Battery drains fast, then optimise the circuit or have a bigger battery capacity. In the iterative phase, going back to tweak and improve the design is done until it’s ready for production. EXAMPLE OLD WAY NEW WAY
- 23. Factors Influencing Machine Design The criteria and constraints that dictate a machine element's performance and manufacturability under specific operating conditions are paramount. These factors ensure the final design is robust, efficient, and safe. Load Characteristics Considering static, dynamic, impact, and cyclic loads to ensure the component withstands all anticipated forces. Motion Type Designing for rotary, linear, or oscillatory motions to optimise component interaction and longevity. Strength & Stiffness Requirements Ensuring the material can resist deformation and fracture under applied stresses and maintain dimensional stability. Weight & Size Constraints Adhering to physical limitations and efficiency requirements, especially in applications where space or mass is critical. Wear & Surface Interaction Addressing potential material loss due to friction and contact between surfaces, crucial for extending component life. Manufacturability & Material Availability Selecting designs and materials that are practical to produce and readily available, impacting cost and production timelines. Safety Standards & Environmental Conditions Complying with regulations and designing for extreme temperatures, humidity, or corrosive atmospheres.
- 24. Material Selection for Static Strength Choosing materials capable of withstanding constant, non-fluctuating loads without permanent deformation or failure is fundamental for static strength. The design stress must always be less than the allowable stress. The allowable stress is typically derived by dividing the material's yield strength by a predetermined factor of safety, which accounts for uncertainties in loading, material properties, and manufacturing processes. This ensures the component operates well within its elastic limits, preventing permanent deformation under maximum expected static loads. For example, in the design of a crane boom, engineers must account for significant bending loads. The material selected must possess sufficient static strength to support the maximum intended load with a safe margin, ensuring structural integrity and preventing catastrophic failure. Common Materials for Static Strength: Carbon & Alloy Steels Versatile and widely used, offering a good balance of strength and cost. Examples include AISI 4140, known for its toughness. High-Strength Aluminum Alloys Lighter alternatives, such as 7075-T6, provide excellent strength-to-weight ratios for aerospace and automotive applications. Titanium Alloys Premium materials like Ti-6Al-4V offer exceptional strength and corrosion resistance for demanding environments.
- 25. Material Selection for Fatigue Strength Fatigue strength refers to a material's ability to endure repetitive or cyclic loading without crack initiation or propagation over its intended life. This is critical for components subjected to fluctuating stresses, such as those in engines, rotating machinery, or vehicle suspensions. Key Concepts in Fatigue: S–N Curve (Stress amplitude vs. Number of Cycles) A graphical representation showing the relationship between the applied stress amplitude and the number of cycles to failure for a material. It is a graph that shows how many cycles a material can survive under repeated (cyclic) loading at a certain stress level. Endurance Limit (Fatigue Threshold) The maximum stress level below which a material can withstand an infinite number of load cycles without fatigue failure. Not all materials possess a true endurance limit. Improvement Techniques: Shot Peening A cold working process that introduces compressive residual stresses on the material surface, inhibiting crack initiation. Smooth Fillet Radii & Surface Finish Minimising stress concentrations at corners and improving surface quality to prevent microscopic flaws from becoming fatigue crack origins. Common Materials for Fatigue Strength: Spring Steels Such as 1095 and 4340, are designed for high elasticity and fatigue resistance, often used in springs and flexible components. Aluminum Alloys 6061 and 7075 are chosen for their light weight and reasonable fatigue performance in aerospace and automotive structures. Titanium Alloys Offer excellent fatigue properties, particularly in high-temperature and corrosive environments, making them ideal for critical aerospace components. Consider a bicycle crank arm, which experiences millions of pedal cycles throughout its life. Selecting a material with high fatigue strength, coupled with surface treatment techniques, is essential to prevent premature failure due to cyclic loading.
- 26. S-N Curve
- 27. Material Selection for Wear Resistance Wear resistance involves choosing materials and surface treatments that resist material loss due to friction, abrasion, adhesion, or surface fatigue. This is crucial for components in relative motion, where direct contact can lead to degradation and reduced service life. Common Wear Modes: Abrasive Wear Occurs when hard particles or rough surfaces slide against a softer surface, causing material removal. Adhesive Wear Results from strong bonds forming between contacting surfaces, followed by material transfer or tearing when these bonds break. Surface Fatigue Caused by repeated stresses on a surface, leading to cracks and pitting, often seen in rolling contact applications. The role of hardness is pivotal: harder materials generally offer better resistance to wear. However, a balance must be struck with toughness to prevent brittle fracture. Materials & Treatments for Wear Resistance: Case-Carburized Steels Such as 8620, are heat-treated to create a hard surface layer while maintaining a tough core, ideal for gears. Tool Steels & Nitriding High-carbon tool steels combined with nitriding (nitrogen diffusion) create extremely hard and wear-resistant surfaces. Bronze & Copper Alloys Used in bushings and bearings due to their excellent tribological properties and ability to form a lubricated surface. Consider gears in gearbox assemblies, which require high abrasion resistance due to constant meshing contact. Materials and treatments are chosen to extend their operational life and maintain efficiency.
- 28. Material Selection for Corrosion Resistance Corrosion resistance is about selecting materials and protective measures to prevent chemical or electrochemical deterioration in specific environments. This is particularly important for components exposed to harsh chemicals, moisture, or saline conditions. Common Corrosion Types: Uniform Corrosion Occurs evenly over the entire surface of the material, leading to a general thinning. Pitting Corrosion Localised attack forming small holes or "pits" on the surface, often difficult to detect. Galvanic Corrosion Happens when two dissimilar metals are in electrical contact in the presence of an electrolyte. Crevice Corrosion Localised corrosion occurring in confined spaces, like gaps or under gaskets, where stagnant solutions can exist. Resistant Alloys: Austenitic Stainless Steels Such as 304 and 316 (which contains molybdenum for enhanced resistance), widely used in general corrosive environments. Duplex & Superalloys Materials like Inconel and Hastelloy offer superior resistance to extreme temperatures and highly aggressive chemical media. Aluminum & Titanium Alloys Known for forming protective oxide layers, providing good resistance in many atmospheric and marine environments. Protection Methods: Anodizing, Plating, Coatings Applying protective layers to the surface to create a barrier against corrosive agents. Sacrificial Anodes More reactive metals are connected to the component to corrode preferentially, protecting the primary material. Consider components on offshore platforms exposed to harsh seawater. Selecting appropriate alloys and applying robust protection methods are essential to prevent rapid degradation and ensure long-term operational integrity.