Chapter 13: Mechanical Technology

Mechanisms and power transmission

Belt drives transmit power between pulleys; the speeds are inversely proportional to the diameters: (ignoring slip). Example: a 1440 rpm motor with a 100 mm pulley driving a 300 mm pulley gives rpm. Chain drives (bicycles, motorcycles) do not slip. Gears transmit torque positively: spur, helical, bevel, worm, and rack-and-pinion. The speed ratio is (teeth numbers). Other mechanisms: cams and followers, cranks and connecting rods, linkages, clutches, brakes, bearings (plain, ball, roller), couplings, and flywheels.

Power from rotation: (watts, with in rpm and torque in N m). A motor delivering a torque of 50 N m at 1440 rpm has a power of kW.

Thermodynamics and heat engines

Energy cannot be created or destroyed (first law); heat cannot flow spontaneously from cold to hot, and no engine converts all heat to work (second law) (Çengel & Boles, 2019). A heat engine takes heat from a hot source, converts some to work, and rejects the rest; its thermal efficiency is .

The four-stroke internal combustion (IC) engine has four strokes: intake (air or air–fuel mixture drawn in), compression, power (combustion pushes the piston down), and exhaust. The spark-ignition (petrol) engine is modelled by the Otto cycle, and the ideal efficiency depends on the compression ratio ; for and , (real engines achieve much less). The compression-ignition (diesel) engine compresses air to a higher ratio and injects the fuel, so it is more efficient. Engines also have cooling, lubrication, ignition, fuel, intake and exhaust systems; two-stroke engines complete the cycle in one revolution.

Fluid power: hydraulics and pneumatics

Hydraulic systems use liquid (oil) to transmit force: . A cylinder of bore 50 mm at a pressure of 6 bar ( Pa) produces a force kN. Components: pump, reservoir, valves, cylinders, motors, filters, pipes. Pneumatic systems use compressed air: they are clean, fast, and safe but less rigid. They are used in automation, tools, brakes on heavy vehicles, and packaging. Fluid flow: .

Refrigeration and air conditioning

The vapour-compression cycle uses a compressor, condenser, expansion valve, and evaporator. Its performance is measured by the coefficient of performance ; an air conditioner removing 3 kW of heat with 1 kW of electrical input has COP = 3. Refrigerants must be chosen with ozone depletion and global warming in mind.

Automotive technology

A road vehicle has an engine (or electric motor), transmission (clutch, gearbox, differential), suspension, steering, brakes (hydraulic with discs or drums; ABS), electrical systems (battery, alternator, starter), and body and chassis. Hybrid and electric vehicles use batteries and electric motors, with regenerative braking recovering energy; they shift maintenance toward electrical systems and battery management. Regular servicing of oil, filters, brakes, tyres, and cooling fluid extends life and safety.

Maintenance

Types of maintenance: breakdown (run to failure), preventive (scheduled), predictive (condition monitoring with vibration, temperature, and oil analysis), and corrective. Lubrication reduces friction and wear; fault finding follows a logical sequence from symptom to cause. Keep maintenance records, because they reveal patterns, support planning, and satisfy audits.

Common mistakes

Treating slip-free belt equations as exact for real belts.

Confusing hydraulic pressure with force.

Skipping lock-out and tag-out before maintenance.

Practice questions

A gear of 24 teeth drives a gear of 72 teeth at 900 rpm. Find the driven speed. [300 rpm]

Find the ideal Otto efficiency for and . [≈58.5%]

Describe the four strokes of a petrol engine.