Four designs, four principles
Spur, planetary, bevel, and worm gearboxes solve the same problem—adjusting speed and torque—in fundamentally different ways. This determines what each type is suitable for:
- Helical gearboxes—typically with parallel shafts (coaxial configurations are also possible, e.g., two-stage with input and output on a single shaft), involute gearing; at 95–99%, the most efficient standard design.
- Planetary gearboxes—coaxial design, load distributed across multiple planetary gears, 95–98% efficiency per stage, very compact.
- Bevel gearbox—axis deflection (usually 90°), spiral bevel gears 94–97%, hypoid gears with axis offset 90–96%.
- Worm gearbox – shafts cross at right angles, high sliding ratio, 35–90% (highly dependent on the gear ratio), self-locking possible (rule of thumb: i ≈ 30 or greater, depending on the lead angle and coefficient of friction; use only as specified by the manufacturer).
Comparison Table
| Criterion | Spur gear | Planetary | Bevel gear | Worm gear |
|---|---|---|---|---|
| Shaft position | parallel (coaxial also possible) | koaxial | right-angled (hypoid: + shaft offset) | rechtwinklig |
| Efficiency per stage | 95–99 % | 95–98 % | Spiral 94–97% / Hypoid 90–96% | 35–90% (highly dependent on i) |
| Translation by level | Depends on the application (example from the guide: i = 4) | 3:1–10:1 | Helical-bevel gear 1:1–8:1 | 5:1–100:1 |
| Torsional backlash | Manufacturing-dependent (DIN 3961/3962) | Low; reduced backlash of 1–3 arcmin possible | Not quantified in sources | medium to high |
| Noise | low–medium (diagonal is quieter than straight) | moderate to quiet | niedrig | leise |
| Self-locking | nein | nein | nein | Possible (rule of thumb: for i ≈ 30 or greater, depending on the lead angle and coefficient of friction; use only if specified by the manufacturer) |
| Costs | low–medium | medium–high | medium (hypoid high) | affordable |
Values from the guides on worm/planetary gearboxes, bevel gearboxes, and helical gearboxes. Cells without a number are not assigned a guideline value in these sources; “no” for bevel gears is derived from the relationship shown in Unit 2 (η significantly above 50% ⇒ not self-locking).
Mnemonic
Briefly applied
A conveyor belt drive with parallel shafts and a focus on efficiency calls for a helical gearbox. If the same application requires a 90° deflection without the need for self-locking, a bevel gearbox is a viable option. If a flap or an actuator is to maintain its position without a continuous current, a self-locking worm gearbox is an option—with the efficiency trade-off shown in Unit 2 as a compromise. In hoisting systems, it does not replace a holding brake (see next unit).
Knowledge check
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