Spur Gear Calculator

When to use this CAD/mechanical design tool

• Use for quick gear-pair layout checks.
• Use before choosing center distance in CAD.
• Use when comparing a gear ratio against pulley or chain alternatives.

How to use

1. Enter one mechanical-design case using typed dimensions, counts, RPM values, tolerances, or drafting selections.
2. Click the exact action button for the selected tool; default values, typing, URL-prefilled values, and worked examples do not generate results automatically.
3. Review the output cards, warnings, assumptions, visual summary, copy text, and downloads before using the result in CAD, drawing, supplier, or inspection workflow.
4. Verify final values against applicable standards, supplier catalogs, validated CAD documentation, manufacturing limits, inspection practice, and qualified engineering review.

Input guide

Driver and driven teeth

Positive integer tooth counts for the two external spur gears.

Module or diametral pitch

Defines pitch diameter in metric or inch convention.

Driver RPM

Used to estimate driven RPM and pitch-line velocity.

Notation

Output interpretation

• Center distance is based on pitch diameters.
• Driven speed is ideal and does not include losses.
• Pitch-line velocity supports preliminary speed review only.

Supported CAD/mechanical design use cases

• Two-gear external spur meshes
• Module or DP geometry
• Speed ratio planning

Unsupported patterns

• Planetary gears
• worm gears
• bevel gears
• strength rating
• profile shift optimization

How to interpret the output

• A ratio above one reduces driven speed.
• Center distance follows pitch diameter, not outside diameter.
• Velocity warnings should trigger supplier and lubrication review.

CAD and mechanical-design guidance

• Keep module or DP identical between mating gears.
• Check pressure angle compatibility.
• Confirm shaft spacing and housing constraints before release.

Worked examples

Assumptions

• Inputs are typed planning values and are not extracted from CAD files, supplier catalogs, drawings, PLM, CAM, ERP, or inspection systems.
• Metric calculations use millimetres unless the output explicitly states another unit; inch calculations use inches unless the output explicitly states another unit.
• The result is a preliminary calculator or drafting helper and not a certified standards database, CAD kernel, inspection report, or supplier approval.
• External gears are assumed to mesh on standard pitch circles.
• No profile shift, backlash, strength, wear, or efficiency calculation is included.

Practical guidance

• Use the Gear Dimensions Calculator for a single gear envelope.
• Use Shaft Key Calculator to review torque transfer at the gear hub.

Common mistakes

• Using driven/driver ratio backwards.
• Computing center distance from outside diameters.
• Ignoring pressure-angle and module compatibility.

Limitations

• No gear strength, tooth contact, backlash, interference, noise, lubrication, material, or heat-treatment analysis is included.
• ScholarTool CAD and mechanical-design calculators provide general engineering calculation and planning support. They do not replace supplier requirements, certified standards, CAD software documentation, manufacturing review, inspection standards, or professional engineering judgement.
• The tool does not open, upload, parse, repair, or validate CAD files, drawings, STEP, STL, DXF, DWG, CAM, or CNC data.
• The tool does not perform strength certification, fatigue validation, finite-element analysis, supplier catalog selection, standards compliance certification, or inspection programming.

Related workflow links