Spur Gears Are Simple — Until You Order One
A spur gear looks like a disk with teeth, but the part number carries real engineering information: diametral pitch, pressure angle, tooth count, style, and bore. Read it wrong and the gear will not mesh, will not fit the shaft, or will fail under load. This guide explains the five stock styles and the specification fields that matter, based on the Martin Sprocket & Gear stock gear catalog RBC Industrial references.
The Five Stock Styles
- Type A — Plain without hubs. All steel. A flat gear blank with teeth cut on the OD; used where a hub would interfere with the application or where the gear mounts on a special carrier.
- Type B — Plain with hubs. Steel or cast. The workhorse style: a hub on one side provides a longer bearing surface and a larger maximum bore.
- Type B1 — Web. Steel or cast. Solid web construction between hub and rim for a good strength-to-weight ratio at moderate sizes.
- Type B2 — Web with lightening holes. Steel or cast. Holes cut in the web reduce weight and rotational inertia without sacrificing rim support.
- Type B3 — Spoke. Cast only. Spokes connect hub to rim; used on the largest stock gears where weight savings matter and the casting process makes spokes economical.
Cast gears are machined on all operating surfaces, and Martin cast gears are cast with a larger hub to provide extra strength and allow larger bores than the equivalent steel part.
Reading the Specification
The three numbers that matter most are diametral pitch (DP), pressure angle, and tooth count. Diametral pitch is the ratio of teeth to pitch diameter — a 3 DP gear with 30 teeth has a 10-inch pitch diameter. Pressure angle must match between mating gears: a 14½° pressure angle gear will not operate with a 20° pressure angle gear, and the stock line is predominantly 14½°. Tooth count determines the speed ratio between the two gears in mesh.
In the stock tables, a part like S315 reads as: steel spur gear, 3 DP, 15 teeth, 5.000 in. pitch diameter, 5.666 in. outside diameter, Type A, with a 1-5/16 in. stock bore and a 2-3/4 in. hub. Larger sizes step up through B, B1, B2, and B3 styles with bigger stock bores — from 1-5/16 in. on the small sizes to nearly 2 in. on the 120-tooth gear.
Ordering Tips
Know your DP, pressure angle, tooth count, and required bore before you call. If the stock bore is close to but smaller than your shaft, the gear can often be rebored — but the catalog's recommended maximum bore with keyway and setscrew is the safe limit; exceeding it in both gears of a mesh invites failure. Keep a mating gear sample or a tooth profile gauge handy to confirm pressure angle.
RBC Industrial stocks steel and cast spur gears across the size range and can rebore to spec. Call (915) 845-8188 or visit www.rbc-industrial.com.