A production line’s conveyor drive loses a tooth from a spur gear. The original part is no longer available, so a local shop makes a copy by counting the teeth and measuring the outside diameter. The new gear fits on the shaft and meshes with its partner. Within days it is howling, and after a month the teeth on both gears are pitted. The copy had the right tooth count but the wrong module and pressure angle. It never truly meshed. Custom spur gear machining starts with identifying the gear, not just measuring it.
The Numbers That Define a Spur Gear
| Parameter | What it means | Why it must match |
|---|---|---|
| Module (m) or diametral pitch | Tooth size; pitch diameter divided by number of teeth | Gears of different modules cannot mesh properly |
| Number of teeth (z) | Sets the ratio and pitch diameter | Changes speed ratio and center distance |
| Pressure angle (usually 20°, sometimes 14.5°) | Angle of the tooth flank at the pitch point | Mismatched angles contact only at tips or roots |
| Profile shift | Tooth form moved outward or inward | Common in older machines; ignoring it changes backlash and strength |
| Face width | Tooth length along the axis | Affects load capacity |
| Bore, keyway, hub | How the gear mounts to the shaft | Concentricity and fit decide runout |
Identifying a Worn Gear
Outside diameter alone can mislead. On a standard gear, outside diameter equals module times (teeth + 2), but profile shift, wear, and older inch-based standards all change that. A reliable identification uses several measurements together:
- Count the teeth on both mating gears
- Measure the center distance between the two shafts in the machine
- Measure the outside diameter, and the span over a set number of teeth (base tangent length)
- Check the tooth form with a gear gauge if available
The center distance is the most valuable clue. It is fixed by the machine and does not wear, so it pins down the module and any profile shift far more reliably than a worn tooth can.
Backlash: Not Too Tight, Not Too Loose
Backlash is the small clearance between meshing teeth. Too little, and the gears bind as they heat up, causing noise and rapid wear. Too much, and the drive becomes loose, knocks during reversals, and loses positioning accuracy. Backlash is controlled by tooth thickness and center distance, which is one more reason the copy must be measured against its partner and its housing, not in isolation.
Material and Hardening
Gear material should match the load and the partner gear. A soft gear running against a hardened one will wear quickly. Many industrial spur gears use medium-carbon or alloy steels, with surface hardening on the teeth where load and wear are high. When only one gear of a pair is being replaced, the new one should be compatible with the old one’s hardness so wear does not concentrate on either side.
Replace One Gear or Both?
If the mating gear shows pitting, scoring, or a polished, hooked tooth profile, replacing only the broken gear can shorten the new gear’s life. A new gear running against a worn partner inherits its wear pattern. When in doubt, inspect both, and plan to replace as a pair if the surviving gear is far along.
How Merdeka Precision Makes Custom Gears
Gears are part of the component range in our fabrication service. Each job begins by receiving or creating a part drawing. When only a broken gear is available, we measure it together with any available information on the mating gear and center distance before producing the drawing for approval.
Blanks are turned on our lathes, hubs and features are machined on our vertical milling machines and the Mori Seiki MV 50 E machining center, and internal keyways are cut on our Mao Jung slotter machine. Every gear passes QC inspection before delivery. Since gears are usually needed one or a few at a time, they are typically produced as job orders.
Our ISO 9001:2015 quality system and decades of experience supplying manufacturers are part of why customers choose Merdeka Precision for parts that cannot simply be bought off the shelf.
Before You Send a Broken Gear
Send the broken gear, its mating gear if possible, the shaft or bore size, and the machine’s center distance. If any of the gears have markings, photograph them. These details turn a guess into a gear that runs quietly from the first day.
Is there a gear in your plant that has already been copied once and still runs noisy? Tell us about it and we will help identify what the original really was.