Technical Articles
Custom Bore Coupling RFQ: 8 Key Dimensions to Confirm
A custom bore or keyway is justified when a catalogue coupling cannot match the existing shaft connection, flange, brake wheel, spacer, DBSE or installation envelope. The supplier should not receive only an outside diameter and a photograph. A reliable RFQ separates measured dimensions, design requirements and values that still need confirmation before a drawing is released.
Quick answer: Confirm eight areas before quotation: shaft bores, keyways, hub lengths, DBSE and envelope, load and speed, materials and heat treatment, tolerances and balance, and inspection and delivery documents. This prevents a part that looks similar but cannot be installed or safely transmit the required load.
1. Lock the drawing revision and units
Send one controlled drawing or measurement sketch with drawing number, revision, date and units. DWG, DXF, STEP or IGS files can support review, but the approved drawing should identify functional interfaces and acceptance dimensions. If two files conflict, record the query and obtain a written decision before machining starts.
For an obsolete machine, use the old coupling measurement guide. Mark each value as measured, inferred from a mating part or still to be confirmed. Do not allow a worn bore, damaged keyway or distorted flange to become the new design baseline without review.
2. Confirm both shaft bores and keyways
Provide shaft diameters d1 and d2, bore lengths, shaft extensions, keyway width and depth, key length, fit, shoulder fillets and the shaft material if it affects the connection. State whether the interface is a straight bore, tapered bore, spline, shrink fit or locking assembly. The coupling hub must have enough wall thickness and length for the peak torque and the actual shaft condition.
Photograph the shaft shoulders and keyway contact surfaces. Reddish fretting debris, crushed key flanks, scoring or a loose fit can indicate a shaft-line problem rather than a simple dimensional requirement. If the old component has been re-bored, provide the original drawing and the current measured condition separately.
3. Define hub length, DBSE and the installation envelope
Confirm hub length, overall coupling length, distance between shaft ends (DBSE), maximum outside diameter, guard clearance, removal direction and the available axial movement. Add flange pilots, bolt-circle diameter, bolt size, brake-wheel or brake-disc dimensions, spacer length and any intermediate shaft details.
A custom bore is not interchangeable if the installer must move the motor, change the guard or remove a bearing to fit it. The custom replacement coupling product page can be linked from the RFQ, but the drawing must still control all interface dimensions. For replacement work on a steel rolling mill, compare the relevant steel rolling mill coupling application before locking the interface.
4. Provide load, speed and service conditions
The connection dimensions do not determine the coupling size by themselves. Provide motor power P, the operating speed n at the coupling shaft, reducer ratio, direction of rotation, normal torque, startup method, braking, reversing, shock or jam loads, duty cycle and operating temperature. Base torque can be estimated with T = 9550P / n in N·m, then verified against the actual peak-load basis.
For a Gear Coupling or Drum Gear Coupling, state the lubrication and sealing environment. For a high-speed or balanced assembly, state the complete-machine balance requirement and the maximum operating speed. A custom bore should preserve the interface, not hide an unverified load assumption.
5. State materials and heat-treatment requirements
Avoid descriptions such as “high-strength steel” or “hard material.” Identify the material designation system, the component that requires heat treatment, the controlled hardness or mechanical-property requirement and any surface treatment. If an equivalent material is proposed, compare the original requirement, the proposed grade, process condition and service risk in the drawing review.
Material certificates, heat-treatment records and nondestructive testing are delivery documents only when they are included in the agreed scope. Do not present a planned document as evidence that a particular batch or customer project has already been inspected.
6. Define tolerances, fits and balancing
List critical bore and keyway tolerances, flange runout, pilot fit, bolt-circle position, concentricity, face runout, overall length and any datum references needed for inspection. If a brake wheel or spacer is integrated, define the functional face and its relationship to the shaft axis.
For high-speed equipment, state the balancing grade, correction plane and whether balancing is required on the complete assembly or individual parts. If the existing drawing does not provide a balancing basis, flag it as an engineering decision rather than assuming a generic value.
7. Define inspection, export and delivery documents
Specify which dimensions are checked, the sampling or 100% inspection basis, report format, material documents, heat-treatment records, balancing report, nondestructive testing and first-article approval if required. The inspection plan should use the same revision and datum system as the manufacturing drawing.
For first-time or high-risk replacements, agree whether a static trial assembly, staged dimensional approval or witness inspection is required. Inspection should confirm the interfaces and acceptance criteria; it should not be invented after production because a marketing statement sounded reassuring.
Export and delivery information
An international RFQ should include quantity, destination country, packaging, rust prevention, shipping marks, document language, Incoterms, required delivery date and the consignee information. Link the international buyer coupling RFQ checklist when the project needs export documents in addition to the mechanical drawing.
Also identify the equipment shutdown date, spare-part urgency, installation support requirement and whether the old coupling must be returned for inspection. These details help the supplier plan a safe replacement sequence without treating an urgent delivery date as permission to skip drawing approval.
How should a custom coupling drawing be approved?
Use a written revision process. The supplier should return a confirmation drawing that shows the shaft bores, keyways, hubs, DBSE, envelope, materials, heat treatment, critical tolerances, lubrication or sealing details and inspection scope. The buyer should approve the revision or list changes before production. Any later change should trigger a new revision and impact review.
Common custom bore coupling RFQ mistakes
- Sending only a worn outside diameter and assuming the original bore is still correct.
- Omitting shaft speed, peak torque, braking or reversing from the drawing request.
- Mixing dimensions from different revisions or using different units without a controlled baseline.
- Describing material, hardness or inspection with non-verifiable marketing language.
- Forgetting DBSE, guard clearance, removal direction, brake interfaces or balancing requirements.
FAQ | Custom Bore RFQ
What dimensions are essential for a custom bore coupling?
At minimum, provide both shaft bores, keyways, bore lengths, hub lengths, shaft extensions, DBSE, overall length, outside diameter, flange or brake interfaces and the removal clearance. Load and speed data are also required to verify the design.
Can a supplier manufacture from photographs only?
Photographs are useful for identifying interfaces and failure conditions, but they do not replace measured dimensions or a controlled drawing. Use photographs with a measurement sketch and clearly mark values that still need confirmation.
What should be included in the final custom coupling inspection package?
The agreed package may include the approved drawing, dimensional report, material and heat-treatment documents, balancing report, nondestructive-testing record, packing list and installation instructions. The exact scope should be agreed before quotation.
Need an engineering selection or replacement review?
Send power, speed, shaft diameters, keyway, installation dimensions, drawings or old-part photos. We will review the working condition before confirming the model and quotation.