You select the right gear in 6 steps: gear type → ratio → tooth count → module → material → standard or custom. Anyone who proceeds without this systematic approach risks incorrectly dimensioned components, unnecessarily high costs, or costly redesign work later.
This guide takes you through 6 steps from specification to decision — from choosing the gear type, through module and material, to the question of whether a catalog part is sufficient or a custom-made part is the better choice. It is intended for engineers and buyers who want to make an informed preliminary selection before entering detailed design discussions with the manufacturer.
Key takeaway: Step 1 determines the gear type (spur, bevel, worm, or rack and pinion). Steps 2–5 specify the ratio, tooth count, module, and material. Step 6 decides between a catalog part and a custom-made part. The selection table below gives quick orientation for typical applications.
Record requirements first
Before the first step in the decision tree can be taken, the constraints must be clear. The following parameters should be fully known:
- Torque and load: Rated load, start-up peaks, shock factor — directly affect the module and material.
- Speed: Input and output speed, whether constant or variable — together with torque, determines the transmitted power.
- Gear ratio: Desired i = n₁ / n₂ — determines tooth counts and, if applicable, the number of stages. For more information, see the article Basic Concepts of Gear Technology.
- Shaft orientation and installation space: Parallel, crossed, or offset shafts; available center distance and installation length.
- Noise requirement: Whether quiet operation (e.g., medical equipment, office environments) or industrial noise levels are acceptable.
- Environmental conditions: Corrosion resistance (moisture, aggressive media), temperature range, lubricant availability.
- Quantity: Single piece or series — determines the most economical manufacturing process and material choice.
- Accuracy requirement: Tolerances on transmission accuracy (e.g., for robot joints or positioning axes) determine the required gear quality.
Decision tree in 6 steps
Step 1 – Select the gear type
The geometry of the power transmission determines the basic design — for a quick recommendation, use the gear type selector:
- Spur gear (cylindrical gear): parallel shafts, simplest design, high efficiency. Straight-cut gears are easier to manufacture; helical gearing runs more quietly and transmits more torque, but generates axial forces. Comparison in the article Helical vs. Straight Tooth Profiles. More fundamentals in the article Spur Gearboxes: Fundamentals and Design.
- Bevel gear: shafts at an angle, typically 90°. Straight bevel gears for medium speeds; spiral bevel gears for higher speeds and lower noise. More on designs and selection criteria in the article Bevel Gearboxes: Design Types and Selection Criteria.
- Worm and worm wheel: high ratios in a single stage (i = 5 to over 100), compact design, high noise reduction, self-locking depending on the lead angle. Efficiency is significantly lower than for spur gears. Comparison in the article Worm Gear vs. Planetary Gear.
- Rack and pinion: conversion of rotary motion to linear motion — for feed axes, hoists, and transfer systems.
Step 2 – Determine the gear ratio and tooth count
The gear ratio i = z₂ / z₁ follows directly from the rotational speeds. Given the center distance and module, this determines the tooth counts. Important: if z₁ falls below the minimum tooth count (approximately 17 teeth at a pressure angle of 20°), undercut occurs — the tooth flanks are weakened at the root. The remedy is profile shift, which allows fewer teeth without undercut but changes the center distance.
Step 3 – Determine the module
The module m = d / z (pitch circle diameter divided by tooth count) is the measure of tooth size. A larger module means more tooth material and thus higher load capacity — at the expense of installation space and weight. The module is selected based on transmissible torque, material strength, and center distance. Standardized modules (1, 1.25, 1.5, 2, 2.5, 3 …) simplify procurement. More on the calculation in the article Calculate gear module.
Step 4 – Select the material
The material is selected based on four criteria simultaneously:
- Load and strength: Tempered steel (e.g., 42CrMo4) or case-hardened steel (16MnCr5) for high loads; grey cast iron for medium loads with good damping properties.
- Noise: Plastic gears (POM, PA6, PEEK) significantly dampen structure-borne noise and are suitable for quiet drives under light to medium loads.
- Corrosion and environment: Stainless steel or plastic for damp, corrosive, or hygienic environments; bronze for worm gear mating parts (wear compatibility). — Enquire about custom gears in stainless steel or PEEK
- Quantity and production: Plastic injection moulding is very economical for larger quantities; steel can be machined cost-effectively for one-offs and small batches. Overview in the article A Comparison of Gear Materials.
Step 5 – Specify the gear quality
Gear quality according to DIN 3961/ISO 1328 governs smooth running, noise, and transmission accuracy. Quality 8–10 covers simple industrial gearboxes. Quiet or precision-critical applications (servo, robotics, machine tools) require quality 5–7. Precision engineering and measuring-machine applications can require grades 3–4. Each quality grade higher means tighter manufacturing tolerances and disproportionately higher production costs — specify only as precisely as necessary.
Step 6 – Standard or custom-made?
Catalog part (standard): When module, tooth count, bore, and material match the standard range, the catalog part is the most economical choice — immediately available, proven, and with no tooling cost component.
Custom-made: When the center distance is precisely fixed and no standard combination fits, special materials or surface treatments are required, the quantity justifies an optimized geometry, or the gear must be integrated into an existing machine, manufacturing to drawing is the right choice. TEA manufactures custom gear teeth from single units to small production runs.
Selection table by application
The following table gives quick orientation for typical application scenarios. It is not a substitute for detailed design, but aids the initial selection.
| Application / Requirement | Recommendation |
|---|---|
| Quiet drive, high speed | Helical spur gear, hardened steel, high quality (5–7) |
| 90° deflection | Bevel gear set (spiral at higher speeds, straight at lower speeds) |
| High reduction ratio, possibly self-locking | Worm gear set (bronze/steel), pay attention to efficiency |
| Rotation to linear motion | Rack and pinion (straight or helical) |
| Quiet precision drive, light load | Plastic spur gear (POM or PA6), straight-cut |
| Corrosive or hygienic environment | Stainless steel version or plastic gear (POM, PEEK) — enquire about custom gears in stainless steel or PEEK |
These values are for preliminary selection only. Final dimensioning requires a load capacity analysis per DIN 3990 / ISO 6336 or manufacturer consultation.
Practical tip from TEA:
TEA manufactures everything from standard spur gears to bevel gears, gear racks, and worm gear sets through to complex custom gear teeth. Whether a single unit or a small batch — design consultation is part of our service. Contact us early in the design phase: request custom gears to drawing.
Enquire about a gear or custom gear teeth?
Our engineers support you on gear type, module, material, and quality — from catalog parts to custom-made products based on drawings.
To custom gear teeth →Related articles
Helical gearing vs. spur gearing
Smooth running, axial forces, manufacturing costs: when helical gearing is worthwhile and when it is not.
A Comparison of Gear Materials
Steel, cast iron, bronze, plastic: a direct comparison of strength, noise, corrosion resistance, and cost.
Gear Technology: Topic Overview
An overview of all guides, fundamentals, and tools related to gears and gear technology.