Spur Gears, Bevel Gears & Gear Rack
Spur Gears, Bevel Gears & Gear Rack
Spur gears, bevel gears, and gear rack are open gearing components that transmit rotation between shafts or convert it into linear travel. Every replacement must match diametral pitch and pressure angle exactly. Texas Belting stocks 590+ steel gears and racks, including our BBMfg line, at our Houston, TX warehouse.
Shop gears & gear rack by type
On this page: Identify a gear · Diametral pitch chart · Pressure angle · Rack & pinion · Spur vs miter vs bevel · Backlash setup · Houston · Mistakes · FAQ
How do you identify a replacement gear?
Identify a replacement spur gear with two measurements: count the teeth (N), measure the outside diameter (OD) across the tooth tips, then calculate diametral pitch as DP = (N + 2) / OD and round to the nearest standard pitch. Confirm pressure angle, then measure bore, keyway, hub style, and face width to complete the spec.
- 1. Count the teeth. Mark one tooth and count around once. Wear does not change tooth count, so it is your most reliable number.
- 2. Measure the OD. With an even tooth count, caliper across two opposing tooth tips through the center. Odd tooth counts have no opposing tips and read low – measure bore centerline to tooth tip and double it, or ask us to confirm.
- 3. Calculate DP = (N + 2) / OD. This holds for standard full-depth inch spur gears. Common stock pitches run 4 through 48 DP, so round to the nearest one.
- 4. Confirm the pressure angle – 14.5° or 20° (see below). The two never interchange.
- 5. Spec the mount. Bore diameter, keyway and setscrews (finished bore) or plain bore for machining, hub style (flat/no hub vs. hub projecting on one side), and face width.
Worked example: a gear with 40 teeth and a measured OD of 4.200 in gives (40 + 2) / 4.2 = 10 DP. Cross-check against a published part: Boston Gear's H2412 lists 12 teeth, 24 DP, OD 0.583 in – (12 + 2) / 24 = 0.583 in, matching the catalog.
What is diametral pitch? DP chart with module conversions
Diametral pitch (DP) is the number of teeth per inch of pitch diameter – the tooth-size standard for inch gearing. Higher DP means smaller teeth: a 4 DP tooth is roughly 3/4 in from one tooth to the next, while 32 DP teeth sit less than 1/10 in apart. Two gears mesh only if their DP is identical.
| Diametral pitch | Circular pitch (in) | Tooth size | Metric module equivalent (mm) |
|---|---|---|---|
| 4 | 0.7854 | Coarse | 6.350 |
| 5 | 0.6283 | Coarse | 5.080 |
| 6 | 0.5236 | Coarse | 4.233 |
| 8 | 0.3927 | Medium | 3.175 |
| 10 | 0.3142 | Medium | 2.540 |
| 12 | 0.2618 | Medium | 2.117 |
| 16 | 0.1963 | Fine | 1.588 |
| 20 | 0.1571 | Fine | 1.270 |
| 24 | 0.1309 | Fine | 1.058 |
| 32 | 0.0982 | Extra fine | 0.794 |
| 48 | 0.0654 | Extra fine | 0.529 |
Circular pitch (tooth-to-tooth distance along the pitch line) equals pi / DP; module equals 25.4 / DP. Browse spur gears and gear rack by pitch.
14.5 vs 20 degree pressure angle: how to tell and why it matters
Stock inch gearing is cut with either a 14.5° or a 20° pressure angle, and the two never mesh with each other. 20° teeth have a thicker root and are generally recognized as carrying more load; 14.5° teeth have a higher contact ratio and run smoother and quieter provided the teeth are not undercut. Match the pressure angle of the existing gearing exactly, or replace the mating pair together.
| Property | 14.5° pressure angle | 20° pressure angle |
|---|---|---|
| Tooth form | Narrower root, more gradual flank | Thicker root, wider base |
| Load capacity | Lower | Higher (typical published guidance) |
| Running behavior | Higher contact ratio; smoother, quieter if not undercut | Slightly noisier at the same quality level |
| Backlash sensitivity | Less: 1 unit of backlash change needs 1.933 units of center distance change | More: 1 unit of backlash change needs only 1.374 units of center distance change |
| Minimum pinion teeth to avoid undercut | About 32 | About 17–18 |
| Where you find it | Legacy US-built machinery; still widely stocked | Current general-purpose standard |
To identify the pressure angle on an unmarked gear, use a pressure angle gauge or mesh it against a gear of known angle and check for full-flank contact. Older US-built equipment is often 14.5°, newer designs 20° – treat that as a starting assumption and verify. A healthy pinion under 17 teeth is almost certainly 20°, since a 14.5° pinion that small would be badly undercut.
How to match a gear rack to a pinion
A rack and pinion must share the same diametral pitch and the same pressure angle – both, exactly. Then match the rack face width to at least the pinion face width, and check pitch-line backing so your mounting height works out. Rack travel per pinion revolution = N x (pi / DP), so a 20-tooth, 20 DP pinion moves the rack 3.14 in per turn.
| Pinion (DP, teeth) | Circular pitch (in) | Rack travel per revolution (in) | Shop |
|---|---|---|---|
| 20 DP, 20 teeth | 0.1571 | 3.14 | Gear rack · Pinions |
| 12 DP, 24 teeth | 0.2618 | 6.28 | Gear rack · Pinions |
| 10 DP, 20 teeth | 0.3142 | 6.28 | Gear rack · Pinions |
| 6 DP, 18 teeth | 0.5236 | 9.42 | Gear rack · Pinions |
Reading a rack spec: a published listing such as Martin R206X6 breaks down as 6 DP, 20° pressure angle, 2 in face width, 1.5 in overall thickness, 1.333 in pitch-line backing, 6 ft long, low-carbon steel. Pitch-line backing – the distance from the pitch line to the back of the rack – is the number that sets your pinion mounting height, and it equals overall thickness minus the tooth addendum (1 / DP). Match five fields when reordering: DP, pressure angle, face width, pitch-line backing, and length.
Keep pinions at or above roughly 18 teeth at 20° (about 32 at 14.5°) to avoid undercut. For travel longer than one stick, rack sections butt end-to-end – ask whether the ends come machined square for butting before ordering a continuous run.
Spur vs miter vs bevel gears: which one do you need?
Spur gears connect parallel shafts. Miter and bevel gears connect intersecting shafts, usually at 90°: miter gears are a 1:1 pair with equal tooth counts, while bevel gears change speed with unequal tooth counts. Gear rack converts pinion rotation into straight-line travel. Miter and bevel gears are cut to run as sets, so replace them in pairs.
| Type | Shaft arrangement | Ratio | Typical use | Shop |
|---|---|---|---|---|
| Spur gear | Parallel shafts | Any (set by tooth counts) | Open drives, change gears, rack pinions | Spur gears |
| Gear rack | Rotary to linear | N x pi/DP travel per rev | Slides, positioners, gate and door drives | Gear rack |
| Miter gear | Intersecting, 90° | 1:1 only | Right-angle transfer at the same speed | Miter & bevel gears |
| Bevel gear | Intersecting, 90° | Speed change (2:1, 3:1...) | Right-angle drives with reduction | Miter & bevel gears |
| Change & drive gears | Varies | Varies | Machine tools, legacy equipment | Industrial gears |
Backlash and center distance setup for open gearing
Backlash is the small running clearance between meshing teeth, and open gearing needs some. Mount spur gears at the standard center distance C = (N1 + N2) / (2 x DP): a 20-tooth pinion driving a 40-tooth gear at 10 DP mounts at (20 + 40) / (2 x 10) = 3.000 in. At that spacing, stock gears deliver their designed backlash; pushing centers tighter to "take out the slack" causes binding, heat, and rapid tooth wear.
Typical published average backlash for stock spur gears at standard centers runs from about 0.003 in at fine pitches (14–32 DP) to roughly 0.010–0.013 in at coarse pitches (3–4 DP); small increases over nominal centers are acceptable, tighter is not. For rack installs, set pinion-to-rack spacing at the tightest point of rack runout – a strip of printer paper between pinion and rack there is a common shop starting point for clearance – then roll the pinion through full travel and verify it never binds.
Gear and gear rack supplier in Houston, TX
Texas Belting supplies spur gears, miter and bevel gears, change gears, and gear rack from Houston. Most stocked sizes typically ship same day from our Houston, TX warehouse. Call (888) 203-2358 to confirm availability. We serve maintenance and OEM buyers across the Texas Gulf Coast and ship nationwide, with fast quote turnaround on gear identification requests.
Common gear selection mistakes
- Buying by OD alone. Similar ODs can be different pitches. Compute DP from tooth count and OD together.
- Mixing pressure angles. A 14.5° gear meshed with a 20° gear makes edge contact and wears out both parts. Match the angle or replace the pair.
- Forcing inch gearing onto a metric machine. Module 2 is not 12 DP. If the machine is imported, source module gearing.
- Ignoring pitch-line backing on rack. Different backing changes the pinion center height and throws the mount out of mesh.
- Setting gears with zero backlash. Tight mesh binds and overheats. Mount at standard center distance and keep measurable clearance.
- Replacing half of a bevel or miter set. These are lapped or matched as pairs; a new gear against a worn mate runs rough and fails early.
When to call instead of self-serve
Some jobs are faster over the phone:
- Odd tooth counts or heavily worn gears where the DP math will not land on a standard pitch
- Unmarked gears where the pressure angle cannot be confirmed in the field
- Module gearing for imported machinery
- Hardened, wide-face, or non-stock bore requirements that need machining
- Long rack runs needing machined ends or cut-to-length sections
Call (888) 203-2358 or send us what you have – photos, tooth count, OD, and bore are enough to start.
Frequently asked questions
How do I measure a gear to find a replacement?
Count the teeth, then measure the outside diameter across the tooth tips with calipers. Diametral pitch = (teeth + 2) / OD, rounded to the nearest standard pitch. Then confirm the pressure angle (14.5 or 20 degrees), and measure the bore, keyway, hub style, and face width. Tooth count is the most reliable number on a worn gear because wear does not change it.
What is diametral pitch on a gear?
Diametral pitch (DP) is the number of teeth per inch of pitch diameter, and it defines tooth size for inch gearing. Higher DP means smaller teeth: 4 DP teeth are roughly 3/4 in apart, while 32 DP teeth sit under 1/10 in apart. Two gears mesh only if their DP matches exactly. Common stock pitches run from 4 to 48 DP.
Can I mix 14.5 and 20 degree pressure angle gears?
No. Gears with different pressure angles have different tooth flank geometry and never mesh correctly, even at identical diametral pitch. Running them together produces edge contact, noise, and rapid wear. Match the pressure angle of the existing gearing exactly; if you cannot determine the mating gear's angle, replace both gears as a pair.
How do I match a gear rack to a pinion gear?
Rack and pinion must share the same diametral pitch and the same pressure angle. Then match the rack face width to at least the pinion face width, and check pitch-line backing so the pinion center height works with your mount. Rack travel per pinion revolution equals tooth count x (pi / DP): a 20-tooth, 20 DP pinion moves the rack 3.14 in per turn.
What is the difference between a miter gear and a bevel gear?
Both connect intersecting shafts, usually at 90 degrees. Miter gears are a special bevel pair with equal tooth counts, so the ratio is always 1:1 - direction changes, speed does not. Bevel gears have unequal tooth counts and change speed as well as direction. Both types are cut to run as matched sets, so replace them in pairs, not individually.
How do I calculate gear ratio?
Divide the driven gear's tooth count by the driver's tooth count. A 20-tooth pinion driving a 40-tooth gear gives 40 / 20 = 2:1, so the output shaft turns at half the input speed with roughly twice the torque, minus friction losses. Because ratio depends only on tooth counts, you can change ratio by swapping gears as long as DP and pressure angle stay the same.
What causes gear teeth to wear out prematurely?
The usual causes are mounting error and mismatch: centers set too tight (binding and heat) or too loose (shock loading), shaft misalignment concentrating load on one tooth edge, mixed pressure angles, abrasive contamination on open gearing, inadequate lubrication, and overload beyond the tooth rating. A new gear meshed against a badly worn mate also wears fast - replace the pair when wear is heavy.
Related products and guides
Need help matching a gear or rack?
Send a photo, tooth count, OD, and bore. Our Houston team will identify the pitch and pressure angle and quote a stock replacement.
Request a Quote Call (888) 203-2358Last updated: July 2026