What is machine design?
A machine is a set of parts that work together to do a job. Machine design means choosing the right parts, their size and their material so the machine is safe, strong enough and not too costly. Designers reuse small standard parts called machine elements, so a part can be bought or replaced anywhere.
Fastening machine elements
Fasteners join parts.
- Bolt and nut: a bolt goes through holes; the nut screws on the thread and clamps the parts. Can be opened again.
- Screw and stud: screws go into a threaded hole; a stud has thread at both ends.
- Washer: a flat ring under the nut that spreads the load.
- Pin and cotter: a pin stops parts from sliding; a split pin stops a nut from turning loose.
- Rivet: a metal pin hammered flat at the end. It cannot be opened without breaking it (permanent).
Thread = a spiral groove. One turn of the nut moves it forward by the pitch.
Shaft-related machine elements
A shaft is a spinning rod that carries power. Parts on it:
- Key: a small block in a slot that locks a pulley or gear to the shaft so it cannot slip.
- Coupling: joins the ends of two shafts so that they turn together.
- Bearing: holds the shaft in place and lets it spin with little friction (ball bearing: small steel balls roll between two rings).
- Clutch and brake: a clutch joins or separates two shafts; a brake slows or stops a shaft.
Gear drives, belts and chains
A gear is a wheel with teeth. Two gears with teeth fitting together pass motion without slipping. The gear that is turned by the motor is the driver; the other is the driven gear. They turn in opposite directions.
Gear ratio = teeth on driven ÷ teeth on driver. Speed of driven = speed of driver ÷ ratio. A bigger ratio means slower speed but more turning force (torque).
Types: spur (straight teeth, simple), helical (slanted teeth, quiet), bevel (turns the direction by 90°), worm (big speed cut). A belt (flat or V) uses friction between belt and pulley; a chain uses links and toothed wheels, so it does not slip. Others: springs store energy, cams make a part move up and down, flywheels keep the speed even.
Key formulas and definitions
- Gear ratio = N driven / N driver
- Driven speed = driver speed × N driver / N driven
- Torque rises by the same ratio when speed falls (ignoring losses)
- Belt drive: N1 × D1 = N2 × D2 (N = rpm, D = pulley diameter)
- Thread: one turn moves a nut forward by its pitch
Worked examples
1. A driver gear has 20 teeth and the driven gear has 60 teeth. Find the gear ratio.
Ratio = 60 / 20 = 3. The driven gear turns 3 times slower.
2. The driver gear turns at 900 rpm. It has 15 teeth; the driven gear has 45 teeth. Find the speed of the driven gear.
Driven speed = 900 × 15 / 45 = 300 rpm.
3. A pulley of diameter 10 cm on a motor turns at 1200 rpm. It drives a pulley of diameter 30 cm by a belt. Find the speed of the big pulley.
N1 D1 = N2 D2, so N2 = 1200 × 10 / 30 = 400 rpm.
4. A bolt has pitch 1.5 mm. How far does the nut move along it in 8 full turns?
Distance = 8 × 1.5 = 12 mm.
Common mistakes
- Mixing up driver and driven: the ratio is driven teeth ÷ driver teeth, not the other way round.
- Thinking a bigger gear always means more speed. A bigger driven gear means less speed.
- Saying a rivet can be opened and reused. It is a permanent joint.
- Forgetting that meshing gears turn in opposite directions.