What mechanical engineers do
Mechanical engineering is about anything that moves, carries a load or uses heat and energy. Mechanical engineers:
- design parts and machines (often with CAD and 3D modelling),
- analyse them using physics: forces, motion, energy, heat and fluids,
- choose materials (steel, aluminium, plastics, composites),
- make them by machining, casting, welding and assembly,
- test, run and maintain them safely.
Big application areas include cars and motorbikes (automotive), ships (shipbuilding), aircraft (aerospace), power stations, air-conditioning, factory robots and medical devices.
Mechatronics joins mechanics, electronics and computer control: sensors measure, a controller decides, and motors or actuators move. A washing machine and a robot arm are mechatronic systems.
Mechanisms: gears, gear ratio and torque
A mechanism changes one kind of motion or force into another. Gears are toothed wheels that mesh together.
- The gear that is turned by the motor is the driver; the one it turns is the driven gear.
- Gear ratio = teeth on driven gear ÷ teeth on driver gear.
- Output speed = input speed ÷ gear ratio.
- Output torque ≈ input torque × gear ratio (ignoring friction).
Torque is turning force: force × distance from the centre (N m). A big driven gear turns slowly but strongly; a small driven gear turns fast but weakly. Meshing gears turn in opposite directions. Other mechanisms: levers, pulleys, belt and chain drives, cams, and the crank and slider, which changes rotation into back-and-forth (reciprocating) motion.
Energy: engines, motors and efficiency
Machines convert energy from one form to another:
- Engine: burning fuel heats gas, which pushes a piston; the crank turns this push into rotation (chemical → heat → kinetic).
- Electric motor: electrical energy → kinetic energy.
- Turbine: moving steam, water or wind turns blades (used in power stations).
No machine is perfect: some energy is always lost as heat and sound through friction. Efficiency = useful energy out ÷ energy in × 100%. Engineers raise efficiency with lubrication, bearings, light materials and good design.
Power = work done ÷ time (watts). For a rotating shaft, power = torque × angular speed.
Strength of materials: load, stress and safety
Every part carries forces. A load on a beam makes it bend; pulling a rod stretches it (tension); pressing a pillar squashes it (compression).
- Stress = force ÷ area (N/m², or pascal). The same force on a thinner part makes a larger stress.
- Each material has a maximum stress it can take before it breaks or bends for good.
- Engineers add a factor of safety: they design the part to take, say, 2 to 4 times the expected load.
Shape matters as much as material: an I-beam or a tube is much stiffer for its weight than a solid flat bar.
Making parts: machining, fitting and assembly
Machining
Machine tools remove metal to make exact shapes: lathe (turning round parts), milling machine (flat faces, slots), drilling machine (holes), grinding (very smooth surfaces). Today many are CNC (computer-controlled).
Fitting (bench work)
A fitter works at a bench with hand tools: marking out with a scriber and square, cutting with a hacksaw, shaping with files, drilling, tapping threads with a tap and die, and checking with a vernier caliper or micrometer.
Assembly and tolerances
Parts are joined with bolts, nuts, screws, rivets, pins, keys and welds. Shafts run in bearings. No part can be made to an exact size, so drawings give a tolerance, e.g. 20.00 ± 0.05 mm. A clearance fit lets a shaft slide or turn in a hole; an interference (press) fit makes it grip tightly.
Safety: wear eye protection, tie back hair, keep guards on machines and never measure a moving part.
Key formulas and definitions
- Gear ratio = teeth on driven gear ÷ teeth on driver gear
- Output speed = input speed ÷ gear ratio
- Output torque ≈ input torque × gear ratio (no friction)
- Torque = force × perpendicular distance from the axis (N m)
- Efficiency = useful energy (or power) out ÷ energy (or power) in × 100%
- Stress = force ÷ area (Pa = N/m²)
- Power = work ÷ time; for a shaft P = torque × angular speed
Worked examples
1. A driver gear has 12 teeth and the driven gear has 36. Find the gear ratio.
Gear ratio = 36 ÷ 12 = 3 (written 3 : 1).
2. The driver above turns at 300 rpm. How fast does the driven gear turn?
Output speed = 300 ÷ 3 = 100 rpm.
3. A motor gives 2 N m of torque to a 10-tooth gear meshing with a 40-tooth gear. What torque comes out (no friction)?
Ratio = 40 ÷ 10 = 4. Output torque = 2 × 4 = 8 N m.
4. A spanner 0.25 m long is pushed with 80 N at its end, at right angles. What is the torque on the nut?
Torque = force × distance = 80 × 0.25 = 20 N m.
5. A motor takes in 500 J of electrical energy and gives 400 J of useful kinetic energy. Find its efficiency.
Efficiency = 400 ÷ 500 × 100% = 80%. The other 100 J is wasted as heat and sound.
6. A steel rod of cross-section area 0.0002 m² holds a load of 10 000 N. Find the stress.
Stress = 10 000 ÷ 0.0002 = 50 000 000 Pa = 50 MPa.
Common mistakes
- Dividing the wrong way for gear ratio. It is driven teeth ÷ driver teeth.
- Thinking a big gear gives more speed. It gives more torque and less speed.
- Forgetting that meshing gears turn in opposite directions.
- Mixing units in stress: area must be in m² (1 mm² = 0.000001 m²) to get pascals.