RRB JE Mechanical Engineering — Complete CBT-2 Study Guide

• First law: dQ = dU + dW (energy conservation) — heat added equals rise in internal energy plus work done by the system. Second law: heat cannot flow spontaneously from cold to hot without external work (Clausius) and no engine converts heat to work with 100% efficiency (Kelvin-Planck). • Carnot cycle efficiency η = 1 − T2/T1 (temperatures in Kelvin) — the theoretical maximum for any heat engine operating between two temperatures; no real cycle can exceed it. • Otto cycle (petrol engines): constant-volume heat addition; efficiency depends only on compression ratio r: η = 1 − 1/r^(γ−1). Diesel cycle: constant-pressure heat addition, generally lower efficiency than Otto at the SAME compression ratio, but diesel engines use much higher compression ratios in practice, giving them better real-world efficiency. • Entropy: a measure of disorder/unavailable energy; total entropy of an isolated system never decreases (increases for irreversible processes, constant for reversible ones). • PV diagrams: area enclosed = net work done per cycle; steeper adiabatic curves vs shallower isothermal curves on a P-V plot at the same points (adiabatic has no heat exchange, isothermal has constant temperature).

• Factor of Safety (FoS) = Ultimate/Yield Stress ÷ Allowable (design) Stress — chosen based on load type (static/dynamic/impact), material reliability, and consequence of failure; ductile materials typically use a lower FoS than brittle ones. • Stress concentration occurs at geometric discontinuities (holes, fillets, keyways, sharp corners) — stress concentration factor Kt = max local stress / nominal stress; fillets/rounded transitions reduce Kt compared to sharp corners. • Fatigue failure occurs under repeated/cyclic loading at stress levels below the material's ultimate strength; the S-N curve plots stress amplitude against number of cycles to failure — steels typically show an endurance limit (stress below which infinite cycles are survived), non-ferrous metals often do not. • Types of gears: Spur (parallel shafts, straight teeth, simplest, noisy at high speed), Helical (parallel/crossed shafts, angled teeth, smoother/quieter but generates axial thrust), Bevel (intersecting shafts, e.g. differential), Worm (perpendicular non-intersecting shafts, high reduction ratio, self-locking). • Bearings: Ball/roller (anti-friction/rolling contact, lower friction, used for moderate loads) vs Journal/plain (sliding contact, needs lubrication film, used for heavy loads and high-speed shafts with proper oil supply).

• Pascal's law: pressure applied to a confined, incompressible fluid is transmitted equally and undiminished in all directions — basis of hydraulic jacks/lifts/presses. • Continuity equation: A1V1 = A2V2 (for incompressible flow) — velocity increases where cross-sectional area decreases, and vice versa. • Bernoulli's theorem: total energy head (pressure + kinetic + potential) is constant along a streamline for ideal, steady, incompressible flow — explains venturi effect and pitot tube operation. • Viscosity: Newton's law of viscosity τ = μ(du/dy) — shear stress is proportional to velocity gradient; Newtonian fluids (water, air) obey this linearly, non-Newtonian fluids (paint, blood) do not. • Types of flow measurement: Venturimeter (low head loss, higher cost), Orifice meter (simple, cheap, higher head loss), Pitot tube (measures point velocity via stagnation pressure difference, used in flow velocity profiling and aircraft airspeed indicators).

• Casting processes: Sand casting (cheap, suited for large/complex low-volume parts, rough surface finish); Die casting (metal mold, high precision, high volume, suited for non-ferrous metals like aluminium/zinc); Investment/lost-wax casting (very high precision/complex shapes, higher cost per part). • Welding: Arc welding uses an electric arc to melt base+filler metal (SMAW, common manual process); Gas welding (oxy-acetylene) gives more control for thin sheets but slower/lower penetration than arc welding; TIG gives the cleanest, highest-quality welds for critical/thin-section work; MIG is faster and more suited to production-line welding. • Machining operations: Turning (rotating workpiece, stationary single-point tool, on a lathe — produces cylindrical parts); Milling (rotating multi-point cutter, workpiece typically stationary/fed — produces flat surfaces, slots, complex profiles); Drilling (rotating tool creates round holes); Grinding (abrasive wheel, used for fine finishing and hard materials). • Metal forming: Forging (compressive force shapes metal, improves grain structure/strength — used for high-strength parts like crankshafts); Rolling (passing metal between rollers to reduce thickness, produces sheets/plates/rails); Extrusion (metal pushed through a die to create a constant cross-section, e.g. aluminium window frames, pipes). • Quality control: Go/No-Go gauges quickly check if a dimension is within tolerance without measuring the exact value; Statistical Process Control (SPC) uses control charts to monitor whether a production process remains within statistically expected variation over time.