📖 The lesson
Bearings
- A bearing supports a moving part, reduces friction and wear, and carries radial loads (perpendicular to the shaft), axial / thrust loads (along the shaft) or both.
- Plain (journal, sleeve) bearings — a shaft turning in a bush. They carry high and shock loads in little space but have higher starting friction and need lubrication (oil film) or self-lubricating liners (PTFE, nylon). Typical materials are bronze, babbitt (white metal), lead-bronze, copper-lead, silver-lead and sintered (oil-impregnated) bronze.
- Rolling-element bearings: ball (radial, some thrust; angular-contact for combined loads), cylindrical roller (heavy radial), needle (heavy radial in small diameters), tapered roller (combined radial and thrust, e.g. wheel bearings), and spherical / self-aligning types, which tolerate misalignment (rod ends, control linkages). Races and rolling elements are hardened through-hardened or case-hardened steel (e.g. AISI 52100) or stainless steel; cages are steel, brass, bronze or polymer.
- Sealed or shielded bearings are lubricated for life; open bearings are greased or oil-lubricated.
Bearing defects
| Defect | Appearance and cause |
|---|---|
| Brinelling | Indentations in the races at ball/roller spacing from impact or static overload (hard landing, careless installation). |
| False brinelling | Polished/etched marks from vibration while static — fretting with no rolling (e.g. during transport). |
| Spalling / flaking | Pieces breaking out of the surface — fatigue at end of life or after overload. |
| Galling, smearing | Metal transfer from sliding with poor lubrication. |
| Overheating | Blue or straw discoloration — lack of lubricant, over-tightening, overload. |
| Corrosion, pitting, etching | Moisture or contaminated grease; electrical arcing gives pitting/fluting. |
| Cage damage | Cracked or worn separators from lubrication failure. |
- Cleaning: solvent wash. Never spin a bearing with compressed air, which can damage it or injure you. Dry, inspect, then lubricate at once with the specified grease to prevent corrosion.
- Install by pressing on the ring that carries the interference fit, never through the rolling elements. Protect bearings from dirt; handle with clean gloves.
Transmissions
- Gear ratio = teeth on driven gear ÷ teeth on driver (= driver speed ÷ driven speed). A reduction gear lowers speed and multiplies torque by the same ratio, less losses. An idler gear changes direction without altering the ratio.
- Spur (parallel shafts, straight teeth), helical (angled teeth, smoother and quieter but produce end thrust), double-helical/herringbone (cancels thrust), bevel and spiral bevel (shafts at an angle — helicopter tail-rotor drives), worm and wheel (high ratio, right-angle, often irreversible), planetary/epicyclic (propeller reduction gearboxes, helicopter main gearboxes) and rack and pinion (rotary to linear).
- Backlash — the free play between meshing teeth. Too little causes binding and heat; too much causes noise and shock loads. It is measured with a dial indicator. The tooth contact pattern (checked with marking compound/engineer's blue) should be central on the flank.
- Gear inspection: pitting, scoring, spalling, broken or cracked teeth, wear steps, discoloration; check oil and magnetic chip detectors for debris.
- Belts and pulleys: flat, V- and toothed (timing) belts. Speed ratio = driven pulley diameter ÷ driver pulley diameter. Check tension, alignment, cracks, fraying and glazing.
- Chains and sprockets: roller chains give a positive drive (some flap/trim actuators, older control systems). Inspect for wear, stiff links, stretch (elongation) and lubrication; check sprocket teeth for hooking.
- Screw jacks (Acme or ball-screw jacks) convert rotation into large linear force — e.g. trimmable horizontal stabiliser and flap actuators. Ball screws recirculate balls for low friction and are reversible; Acme screws are often self-locking.
- Lever devices: levers, bellcranks and quadrants change direction and mechanical advantage (MA = effort arm ÷ load arm).
- Push-pull rod systems: rigid rods with adjustable rod ends (often self-aligning spherical bearings). Check thread engagement — the rod-end thread must be visible beyond (or block) the inspection (witness) hole — and that lock nuts are tight. Look for bends, corrosion and play in bearings.