Four material groups, four application profiles
The material determines a gear’s load-carrying capacity, noise level, wear, corrosion resistance, temperature limit, and cost:
- Steel (quenched and tempered or case-hardened) – highest load-carrying capacity, the first choice for high-performance gearboxes; case-hardened steels (e.g., 16MnCr5) achieve maximum tooth root strength and pitting resistance.
- Plastic (POM/PA)—very quiet, often dry-running operation, corrosion-free, but with limited load and temperature capacity (approx. 80–120 °C).
- Sintered metal—manufactured using powder metallurgy to closely match the final contours, cost-effective for high-volume production, with a load-bearing capacity between that of plastic and steel; its porous structure can absorb oil (emergency operation).
- Bronze—standard material for worm gears in contact with hardened steel: low friction, good running-in properties, sufficient compressive strength.
| Material | Load capacity | Noise | Temperature | Corrosion |
|---|---|---|---|---|
| Steel (tempered/hardened) | very high | mittel | hoch | low (rusting) |
| Plastic (POM/PA) | low–medium | very quiet | limited (~80–120 °C) | Very good |
| Sintered metal | mittel | mittel | mittel | mittel |
| Bronze (e.g., CuSn12) | medium (friction pair) | leise | hoch | gut |
Gear quality according to DIN 3961/3962 (internationally ISO 1328-1; grades are only approximately comparable)
The manufacturing accuracy of a gear is classified into quality grades 1–12 (Grade 1 = highest precision):
| Level | Scope of Application |
|---|---|
| 5–6 | Robotics, medical technology, measuring instruments |
| 7–8 | Automotive, Mechanical Engineering |
| 9–10 | Conveyor Technology, General Machinery |
| 11–12 | Raw castings, rough machining |
The assignment of quality levels to industries is a guideline from the guide accompanying this learning unit; it is not a fixed standard requirement, and individual applications may differ from it.
Description of the figure
The scale ranges from quality grade 5 (highest precision) to grade 12 (lowest precision). Grades 5–6 are used for robotics and medical technology, grades 7–8 for the automotive industry and general mechanical engineering, grades 9–10 for material handling, and grades 11–12 for raw castings and rough machining. As a rough guideline: Grade 6 costs about 2 to 3 times as much as Grade 9—this is just an example value, not a consistent cost curve.
A higher grade means tighter manufacturing tolerances and a disproportionately higher cost (Grade 6 costs approximately 2–3 times more than Grade 9)—therefore, select only the level of precision that is actually necessary.
Backlash: necessary, but limited
Backlash j—the distance between the meshing and non-meshing tooth flanks—is necessary for lubrication and to compensate for thermal expansion. Excessive backlash causes backplay and noise, while insufficient backlash leads to jamming or seizing. The gear tooth tolerance class defines the permissible backlash based on tooth thickness and center distance tolerances.
Mnemonic
Briefly applied
A servo drive in a placement machine with high requirements for positioning accuracy and low noise calls for a hardened steel gear in quality grade 5–6. In contrast, a variable-speed gearbox in a household appliance with low load and cost constraints is a classic case for a standard-grade POM plastic gear. If the same application operates in a humid or hygienically sensitive environment, stainless steel or a suitable plastic (e.g., PEEK) may also be considered.
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