For North American industrial buyers, the safest choice between inch vs metric gears is usually the system already used by the equipment, mating gears, drawings, inspection standard, and spare-part program.
Inch gears are often the practical choice for legacy U.S. machinery, inch-based drawings, and MRO replacement work where only one gear is being replaced. Metric gears may be required for imported machines, global OEM platforms, and module-based gear sets. Neither system is automatically better. The right choice depends on compatibility, drawing control, inspection requirements, and how the gear will be sourced or replaced.
For replacement gears, buyers should avoid choosing by unit conversion alone. DP, module, pressure angle, tooth count, bore, keyway, face width, center distance, backlash, material, heat treatment, and quality documentation all affect whether a gear can work in the assembly.
Quick Answer: Match the Gear System to the Equipment
Use inch gears when the installed equipment, mating gear, drawing package, spare-part record, and inspection process are inch-based. Use metric gears when the machine, drawing, CAD model, and mating gear set are module-based. If only a sample or partial data is available, request a gear review before substituting one system for the other.
Keep inch specifications for legacy U.S. equipment, inch drawings, and single-gear MRO replacements.
Keep metric specifications for imported machines, metric CAD, module-based assemblies, and global OEM platforms.
Do not treat DP-to-module conversion as proof of interchangeability.
Confirm pressure angle, tooth count, bore, keyway, face width, center distance, backlash, material, heat treatment, and inspection scope before ordering.
Inch vs Metric Gears: Core Differences
Item | Inch gears | Metric gears |
|---|---|---|
Tooth-size term | Diametral pitch, often shortened to DP. | Module. |
Drawing units | Inches; often tied to legacy North American equipment and plant records. | Millimeters; common in imported equipment and global OEM drawings. |
Typical buyer context | Legacy U.S. machinery, inch plant standards, MRO replacement, and spare-part programs. | Imported machinery, module-based gear sets, metric CAD, and global OEM platforms. |
Replacement risk | Risk increases if a metric gear is substituted without full review. | Risk increases if an inch gear is substituted without full review. |
RFQ focus | DP, pressure angle, tooth count, bore/keyway, face width, mating gear data, material, and inspection scope. | Module, pressure angle, tooth count, bore/keyway, face width, mating gear data, material, and inspection scope. |
What Are Inch Gears?
Inch gears are gears specified with inch-based dimensions and tooth-size terms. The most common tooth-size term is diametral pitch, or DP. DP describes tooth size in relation to the pitch diameter and number of teeth.
North American buyers often encounter inch gears in legacy U.S. equipment, older plant machinery, inch-based drawings, spare-part programs, and MRO replacement projects. Preserving the inch specification can reduce mismatch risk with the installed mating gear, inspection tools, and maintenance records.
An inch gear RFQ should still include more than DP. Buyers should confirm pressure angle, tooth count, outside diameter, bore, keyway, hub details, face width, material, heat treatment, and any inspection requirements before ordering.
What Are Metric Gears?
Metric gears are gears specified with millimeter-based dimensions and tooth-size terms. The main tooth-size term is module. Module describes the size of each tooth in metric units and is commonly used in metric drawings, imported equipment, and global OEM platforms.
North American buyers often work with metric gears when maintaining imported machinery, sourcing replacement parts for module-based assemblies, or building new designs around metric CAD and global supplier standards.
Metric does not automatically mean newer, more accurate, or more suitable. If the machine, mating gears, drawing package, and inspection process are metric, module-based specifications should usually be preserved unless engineering controls a redesign.
Module vs Diametral Pitch: Why Conversion Alone Is Not Enough
Module and diametral pitch both describe gear tooth size, but they use different measurement systems. A common comparison formula is:
module = 25.4 / DP
This formula is useful for estimating tooth-size relationships. It does not prove that an inch gear and a metric gear can mesh correctly.
Term | What it helps identify | What it does not prove |
|---|---|---|
Diametral pitch / DP | Tooth-size category for inch gears. | Full compatibility with a metric gear. |
Module | Tooth-size category for metric gears. | Full compatibility with an inch gear. |
DP-module comparison | Approximate comparison between systems. | Pressure angle, tooth form, backlash, bore, material, heat treatment, or inspection fit. |
Gear compatibility also depends on pressure angle, tooth count, tooth profile, profile shift if applicable, bore, keyway, hub geometry, face width, center distance, backlash, mating gear condition, material, heat treatment, finishing process, and inspection requirements.
For a replacement project, close enough is not a reliable specification. If an inch gear and a metric gear appear similar, the buyer should have the gear data, drawing, or sample reviewed before placing an order.
When North American Buyers Should Keep Inch Gear Specifications
Keeping the inch specification is usually the safer path when the existing equipment and mating parts are already inch-based.
The gear is being replaced in legacy U.S. equipment.
The mating gear uses DP rather than module.
The plant drawing, spare-part record, or inspection plan is inch-based.
MRO teams need continuity with installed assets.
Only one gear is being replaced while the mating gear remains in service.
The equipment center distance, backlash, shaft layout, or housing cannot be redesigned.
Maintenance teams already stock or inspect related components using inch dimensions.
In these situations, converting to module without a controlled engineering review can create avoidable risk. Even if the tooth size appears close, pressure angle, profile, bore, keyway, and face width may not match.
When Metric Gears May Be the Better Fit
Metric gears may be the better fit when the machine, drawing package, or OEM platform is already metric.
Imported equipment uses module-based gears.
The replacement must mesh with an existing metric gear set.
The OEM drawing, CAD model, inspection plan, or spare-part list is metric.
A global design program uses metric dimensions and approved metric suppliers.
A redesign is being controlled through metric CAD, engineering review, and formal approval.
For North American buyers, the practical question is not whether inch or metric is preferred locally. The question is what the equipment and controlled documentation require. If a metric gear set is already installed, preserve the module-based specification unless a full redesign is approved.
Replacement Gears: What to Check Before Ordering
Replacement gear risk usually comes from missing information. A buyer may know the gear diameter and tooth count but not the pressure angle, material, heat treatment, or mating gear data. That is not enough for a controlled replacement.
Is one gear being replaced, or the full mating set?
Does the mating gear use DP or module?
Is the pressure angle known?
Are tooth count, outside diameter, bore, keyway, hub details, and face width confirmed?
Are helix angle and hand confirmed for helical gears?
Are center distance and backlash controlled by the housing or assembly?
Is the original 2D drawing or CAD file available?
Is a physical sample available for review?
Are material, heat treatment, hardness, and surface finish known or required?
Are load, speed, duty cycle, lubrication, temperature, and contamination conditions known?
Are inspection and documentation requirements defined before ordering?
If no drawing is available, a physical sample can help a supplier review the project. Photos and OEM numbers may support identification, but they should not be treated as the only controlled manufacturing input.
Custom Inch and Metric Gears: What Buyers Should Confirm Before RFQ
Custom gears can be useful when standard catalog gears do not match the equipment, drawing, or application requirements. For inch and metric projects, LILY can review both DP / inch-based and module / metric gear requirements when buyers provide enough project information.
LILY supports standard and non-standard gear manufacturing projects based on drawings, samples, gear data, operating conditions, quantity, and documentation requirements. Gear types that can be discussed, subject to drawing and order review, include spur gears, helical gears, bevel and miter gears, worm gears and worms, gear racks, internal gears, pinion shafts, compound gears, plastic gears, stainless steel gears, and custom gear assemblies.
LILY can manufacture gears according to customer drawings and applicable AGMA, DIN, ISO, JIS, or customer-specified requirements. Precision level depends on gear type, size, material, heat treatment, finishing process, and inspection scope. For suitable projects, LILY can discuss precision gear machining up to Grade 4 / high precision levels, subject to drawing review, gear type, process route, and order-specific inspection requirements.
Gear RFQ Checklist for Buyers
RFQ item | What to provide |
|---|---|
Controlled input | 2D drawing, CAD file, or physical sample. Photos can support review but should not be the only controlled input. |
Gear type | Spur, helical, bevel, miter, worm, rack, internal gear, pinion shaft, compound gear, plastic gear, stainless steel gear, or custom assembly requirement. |
Tooth data | Module or DP / diametral pitch, pressure angle, tooth count, tooth profile if known, helix angle and hand for helical gears. |
Main dimensions | Face width, OD, ID, bore, hub, shaft details, axial structure, and mounting method. |
Features | Keyway, spline, set screw, bearing journal, mounting holes, thread, positioning surfaces, or other fit-critical details. |
Mating parts | Mating gear information, assembly drawing, center distance, backlash, gear-pair constraints, and whether one gear or the full set is being replaced. |
Material and processing | Current or expected material, heat treatment, hardness, surface treatment, corrosion exposure, cleanliness, or packaging requirements if known. |
Operating conditions | Load, torque, speed, duty cycle, rotation direction, lubrication, temperature, contaminants, corrosion exposure, noise target, and space limits. |
Commercial scope | Quantity, prototype / pilot / repeat production status, annual demand if available, and requested timing. |
Documents | Inspection report, material certificate, gear inspection data, heat treatment or hardness report, FAI, PPAP, CoC, or traceability requirements. |
Material options, heat treatment, surface treatment, and process route should be confirmed according to gear type, size, load, speed, operating environment, precision target, finishing needs, quantity, and order-specific requirements.
Quality Documents Buyers May Request
Quality documentation should be defined before the order is placed. Buyers should not assume that every document is included by default.
Document or record | When buyers may request it |
|---|---|
Material Certificate / MTR | Material-controlled industrial projects where material grade, certificate format, and traceability scope must be confirmed. |
Dimensional Inspection Report | Replacement gears, fit-critical parts, and controlled drawings where bore, keyway, hub, face width, or mounting features matter. |
Gear Inspection Report | Projects where tooth profile, lead, pitch error, runout, span, or measurement over pins affects mating performance. |
Heat Treatment Report / Hardness Report | Hardened gears, wear-critical applications, or orders with specified hardness target, case depth, or treatment process. |
Surface Treatment Report | Coating, passivation, plating, black oxide, corrosion protection, or cleanliness-controlled projects. |
FAI / First Article Inspection | New parts, new tooling, or controlled production approval before repeat production. |
PPAP | OEM or repeat-production programs where the customer requires a production approval package. |
CoC / Certificate of Conformance | Orders requiring formal compliance confirmation against a drawing, revision, standard, or purchase order scope. |
Traceability records | Projects that require batch, material, process, or inspection traceability. |
LILY Bearing operates under company-level quality systems including ISO 9001, AS9100, and IATF 16949 where applicable. For a specific gear order, buyers should confirm applicable documentation, inspection scope, and project-specific quality requirements during RFQ.
Practical Decision Checklist
Is this a replacement part or a new design?
What system do the mating gears use: DP or module?
Is one gear being replaced, or the complete gear set?
Is there a controlled drawing, or only a physical sample?
Are pressure angle, tooth count, bore, keyway, face width, and center distance confirmed?
Are material, heat treatment, hardness, and finishing requirements known?
Are inspection and quality document requirements defined?
Does the equipment use inch or metric inspection standards?
Is any DP/module conversion reviewed by engineering?
Are drawings, CAD, samples, operating conditions, quantities, and documentation needs ready for RFQ?
If the answer to several of these questions is unknown, the next step should be a gear review, not a direct unit conversion.
Request a Gear Review
Send drawings, samples, gear specifications, mating-part details, operating conditions, quantities, and required inspection documents. LILY can review the project scope and discuss inch, metric, or custom gear manufacturing options based on drawing and order requirements.
FAQ
What is the difference between inch and metric gears?
Inch gears are typically specified with diametral pitch, an inch-based measure of tooth size. Metric gears are typically specified with module, a millimeter-based measure of tooth size. Buyers should also confirm pressure angle, tooth count, bore, face width, and mating gear data before assuming compatibility.
Are diametral pitch and module the same thing?
No. Diametral pitch and module both describe tooth size, but they use different systems. DP is inch-based, while module is metric. They can be mathematically compared, but conversion alone does not prove that two gears are interchangeable.
Can inch and metric gears mesh together?
They should not be assumed to mesh together. Some converted values may look close, but meshing depends on pressure angle, tooth profile, tooth count, center distance, backlash, face width, and mating gear condition. Engineering review is needed before substitution.
Can metric gears replace inch gears?
Metric gears can replace inch gears only when the full gear geometry, mating parts, shaft features, material, heat treatment, and assembly requirements are confirmed. For a single replacement gear in legacy inch equipment, keeping the original inch specification is often safer.
How do I identify whether a gear is inch or metric?
Start with the drawing, equipment documentation, or OEM spare-part record. If those are not available, check tooth count, outside diameter, pitch diameter if known, pressure angle, bore, keyway, and mating gear data.
Should U.S. buyers choose inch or metric gears?
U.S. and North American buyers should usually choose the system that matches the installed equipment, mating gears, drawings, and inspection standard. Inch gears are often used in legacy U.S. equipment, while metric gears may be required for imported machines or global OEM programs.
What information is needed for a custom gear RFQ?
A custom gear RFQ should include a drawing, CAD file, or sample; gear type; DP or module; pressure angle; tooth count; bore; keyway; face width; material; heat treatment; quantity; application conditions; mating gear data; and required inspection documents.
What causes replacement gear mismatches?
Common causes include using DP/module conversion alone, missing pressure angle data, incorrect tooth count, unconfirmed bore or keyway dimensions, face width differences, uncontrolled center distance, unknown material or heat treatment, and missing mating gear information.
What quality documents should buyers request for custom gears?
Depending on project risk, buyers may request material certificates, dimensional inspection reports, gear inspection reports, heat treatment reports, hardness reports, surface treatment reports, FAI, PPAP, CoC, or traceability records.
Can a supplier quote a gear from a sample if no drawing is available?
A supplier may be able to review a physical sample, but the final quote and manufacturing scope depend on measurable gear data, material and heat treatment requirements, mating-part information, operating conditions, quantity, and inspection requirements.
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