Choose an industrial gearbox by starting with the driven machine, not a catalog part number. Record the torque, speed, ratio, duty cycle, starts and stops, reversals, shock loads, environment, mounting and maintenance access; apply a service factor appropriate to that duty; then verify mechanical and thermal ratings, interfaces, lubrication and supplier support. A gearbox that meets tooth-strength calculations but overheats, cannot be aligned, or lacks the correct oil is still an underspecified drive.
This guide gives a practical selection method, explains service-factor and lubrication decisions, and provides a purchase-and-support checklist you can put into a specification or request for quotation.
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Start with an application data sheet
Send the gearbox supplier a complete application description before asking for a model recommendation. Malloy Electric identifies commonly missed items as the service factor for the specific duty, mounting integrity, environmental rating, lubrication matched to ambient conditions, alignment provisions and accessibility for maintenance. Missing any of these can produce a drive that looks adequate on paper but fails during operation.
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Record the driven-machine requirements
| Data to record | What the supplier needs it for |
|---|---|
| Required output torque | Sets the load that the gearbox must transmit at the output shaft. |
| Output speed and required ratio | Determines whether the reduction ratio can deliver the machine’s operating speed. |
| Input speed | Determines gear mesh speed, lubrication needs and thermal load. |
| Duty cycle and operating hours | Separates intermittent catalog conditions from continuous thermal duty. |
| Starts, stops and reversals | Captures transient loads that a steady-state torque value misses. |
| Shock, impact and inertia loads | Provides the basis for selecting a realistic service factor. |
| Ambient temperature | Affects oil viscosity, heat rejection and allowable thermal rating. |
| Dust, water, chemicals and washdown exposure | Determines enclosure, seals, corrosion protection and environmental rating. |
| Mounting orientation and available space | Controls oil level, breather position, shaft arrangement and installation feasibility. |
| Shaft, flange and key dimensions | Ensures the gearbox physically interfaces with the motor and driven equipment. |
| Maintenance access | Determines whether technicians can inspect, sample and change oil safely. |
Include a sketch or dimensional envelope showing the motor, coupling, driven shaft, base, reaction arm and access clearances. State whether the output is horizontal, vertical or at another angle. Orientation can change the required oil fill and the position of breathers, level plugs and drain plugs.
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Match the gearbox architecture to the duty
Architecture is a functional choice, not simply a brand preference. Compare candidate arrangements against the required ratio, available space, efficiency, backdriving or holding behavior, torque density, noise and duty cycle. The selected ratio must produce the driven machine’s required speed and torque; after that, check both mechanical and thermal ratings with the service factor applied.
Helical gearboxes
Helical stages are commonly considered when smooth transmission, moderate noise and good efficiency are important. Confirm the actual ratio range, continuous thermal capacity and allowable output loads for the proposed frame size rather than assuming that a familiar arrangement is adequate.
Bevel-helical gearboxes
Bevel-helical layouts are useful when the motor and driven shaft need an angled arrangement while retaining the characteristics of helical gearing. Verify the shaft angle, mounting faces, rotation direction and the thrust and overhung loads created by the machine.
Worm gearboxes
Worm arrangements can be attractive where a large ratio in a compact package or a particular backdriving or holding behavior is required. Their thermal rating and efficiency must be checked carefully for continuous duty; a ratio that works mechanically can still generate excessive heat.
Planetary gearboxes
Planetary designs are considered when high torque density, compact dimensions or a specific load-sharing arrangement is needed. Require the supplier’s mechanical and thermal ratings at your exact speed, ratio, duty cycle and mounting condition.
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Cycloidal and other arrangements
Cycloidal and specialized drives may suit high shock, low-speed or unusual packaging requirements. Treat them as application-specific selections: compare torque capacity, allowable loads, efficiency, maintenance provisions and supplier support on the same basis as conventional arrangements.
Calculate the service factor from the real duty
Service factor is a multiplier representing real loading relative to a steady catalog condition. A simple selection check is:
Design torque = required operating torque × service factor.
The factor must reflect starts, stops, reversals, shock, inertia and operating hours. Do not use a convenient default without documenting why it represents the machine.
Illustrative AGMA application values
The AutomationDirect IronHorse manual, dated 31 August 2022, provides AGMA tables organized by application and operating hours. Examples include:
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| Application example | Example service factor | Qualification |
|---|---|---|
| Cement kilns and dryers | 1.50 | Example from the manual’s AGMA application table, not a universal default. |
| Reversing metal-mill drives | 2.00 | Example from the manual’s AGMA application table, reflecting reversing duty. |
| Heavy conveyors | 1.50–1.75 | Range varies with operating hours in the manual. |
These figures illustrate the method; they do not replace the table for your exact machine. Ask the supplier to show the selected factor, the application category and the operating-hours assumption on the quotation.
Check transient and external loads separately
Torque multiplication does not automatically cover every shaft load. Specify the maximum overhung load from sheaves, sprockets or couplings and the maximum thrust load from the driven machine. Include acceleration torque, jam or stall events, reversing frequency and any brake torque. The supplier should state allowable overhung and thrust loads at the actual speed and shaft position.
Verify mechanical, thermal and interface ratings
Require a rating statement rather than accepting a nominal catalog torque. A complete quotation should identify:
- the rating standard used for gear and bearing calculations;
- mechanical torque rating and the service factor applied;
- thermal rating for the stated ambient temperature, speed, mounting orientation and duty cycle;
- bearing-life basis and the assumed load spectrum;
- efficiency and expected heat rejection;
- allowable overhung and thrust loads;
- shaft diameters, keyways, tolerances and permissible coupling fits;
- mounting arrangement, reaction-arm requirements and alignment limits;
- enclosure, seal and environmental protection details; and
- oil capacity, breather location and level, drain and sampling provisions.
Mechanical and thermal ratings answer different questions. Tooth and shaft calculations may show enough torque capacity while continuous losses raise the oil temperature beyond the gearbox’s thermal limit. Review both ratings at the same duty point.
Check installation geometry before ordering
Confirm motor adapter dimensions, output shaft extension, flange or foot pattern, rotation direction, center height and service clearances. Check the manufacturer’s permitted mounting orientations; an orientation that is physically possible may not provide the correct oil level for bearings or gears. Establish alignment limits for the motor-to-gearbox coupling and gearbox-to-machine coupling, then plan how technicians will measure alignment after installation.
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Specify lubrication from the OEM instruction first
Use the gearbox manufacturer’s oil type, viscosity grade, additive package, fill level and change procedure whenever those instructions are available. AGMA 955-A22, approved by the AGMA Technical Division Executive Committee in August 2022, describes generalized lubrication guidance for open and enclosed metallic gearing, including spur, helical, worm, bevel and face gears. It explicitly is not a substitute for the gear manufacturer’s instructions.
Why ISO VG alone is not enough
ISO viscosity grade identifies a kinematic-viscosity range, not a complete lubricant specification. Speed, bulk-oil temperature, gear type, load, seals, metallurgy and additive compatibility also affect the correct product. The Horsburgh & Scott maintenance manual uses mm²/s (commonly called centistokes, or cSt) as the preferred kinematic-viscosity unit and reproduces tables from ANSI/AGMA 9005-F16.
Those enclosed-drive tables include ISO VG 150, ISO VG 220, ISO VG 320 and ISO VG 460. The correct grade depends on pitch-line speed and operating temperature, so a grade number by itself cannot identify the right oil.
Is ISO VG 220 suitable?
ISO VG 220 may be suitable for some enclosed industrial gearboxes, but it is not a universal fill. Treat “ISO VG 220 industrial gear oil” as a search description only, then verify all of the following against the gearbox documentation:
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- whether mineral, PAO or another synthetic base oil is permitted;
- the required extreme-pressure or other additive system;
- seal, paint and metallurgy compatibility;
- the approved operating-temperature range; and
- the specified fill level and whether the gearbox is splash-, bath- or forced-lubricated.
Horsburgh & Scott states: “EP or synthetic gear lubricants in the corresponding viscosity grades may be substituted where deemed acceptable by the gear drive manufacturer.” That manufacturer approval is the condition; matching the viscosity number alone is insufficient.
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Never mix oils casually
Before changing product or base-oil type, obtain written compatibility guidance from the gearbox maker or lubricant supplier. Mixing incompatible additive systems can reduce film strength, cause foaming or attack seals. If a change is approved, follow the specified drain, flush and refill procedure instead of topping up indefinitely with a different product.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Build a maintenance and oil-change plan
Oil-change timing is model-specific. First-week, first-month and recurring intervals should come from the OEM manual because they depend on load, speed, temperature, contamination and lubricant type. Do not turn one gearbox’s interval into a universal calendar rule.
Typical tasks in the documented maintenance workflow
The Horsburgh & Scott enclosed gear-drive schedule includes these actions:
- Check mounting bolts, base hardware and reaction-arm fasteners for correct tightness.
- Operate the unit briefly and observe for abnormal noise, vibration, leakage or temperature rise.
- Stop safely and check the oil level using the specified level point and orientation.
- Drain and discard used oil in accordance with site and local disposal requirements.
- Flush with SAE 10 flushing oil when the manual calls for flushing; do not substitute an unapproved solvent.
- Replace the filter element where the gearbox is fitted with one.
- Inspect accessible gear teeth for undue wear, scoring, pitting or other damage.
- Refill with the approved oil to the specified level, reinstall the breather and plugs, and check for leaks during a controlled restart.
Record oil brand and grade, batch or container information, fill quantity, operating hours, temperature, vibration observations and any particles or discoloration found. Provide a clean sampling point if oil analysis will be part of the maintenance program; drawing samples through a dirty drain plug can make trend data unreliable.
Use operating symptoms as inspection triggers
- Rising temperature: verify oil level and viscosity, mounting orientation, ambient temperature, load, speed and cooling path before increasing the service interval.
- New noise or vibration: check alignment, loose mounting, coupling condition, bearing condition and gear-tooth damage.
- Foaming or milky oil: investigate contamination, water ingress, overfilling and lubricant compatibility.
- Repeated seal leakage: inspect shaft finish, breather operation, pressure, seal material compatibility and alignment.
- Metal particles in oil: stop relying on a routine top-up; identify the source through inspection or analysis and involve the supplier.
Specify the supplier’s service support before purchase
Support is part of the gearbox specification. Require the supplier to include the following with the quotation and order acknowledgement:
- installation, alignment and commissioning instructions;
- approved oil, grease and seal specifications, including alternatives and compatibility limits;
- oil capacity, fill-level diagram and breather, drain and sampling locations;
- inspection tasks and model-specific oil-change intervals;
- spare-parts identification with exploded views or part numbers;
- warranty terms, exclusions and the records needed to preserve coverage;
- troubleshooting guidance for temperature, noise, vibration and leakage;
- repair, rebuild and inspection capability, including what work can be performed in the field;
- expected response times for technical questions and breakdown support; and
- document revision, gearbox serial-number traceability and a named technical contact.
AGMA’s enclosed-drive publication scope covers specifying, selecting, testing, procuring, operating and maintaining gear drives. Use that full life-cycle scope when evaluating a supplier instead of comparing only purchase price.
Compare compliant gearbox options on equal terms
First eliminate any option that fails the required ratio, design torque, thermal duty, shaft loads, mounting or environmental conditions. Then compare the survivors using the same duty assumptions and documentation requirements.
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|---|---|
| Ratio and output speed | Does the actual ratio produce the machine’s required speed across its operating range? |
| Service-factor margin | What factor was used, which application category supports it, and what margin remains? |
| Mechanical versus thermal rating | Are both ratings adequate for continuous duty at the stated ambient temperature and speed? |
| Efficiency and heat rejection | How much power becomes heat, and how is that heat removed in the proposed mounting? |
| Overhung and thrust capacity | Do allowable loads cover the real pulley, sprocket, coupling and machine loads? |
| Mounting and alignment tolerance | Can the installation meet the stated limits, and are measurement points accessible? |
| Enclosure and environmental protection | Will seals, coating and breathers withstand dust, water, chemicals and washdown? |
| Lubricant and change interval | Is the approved oil available at the site, and can it be changed without contamination? |
| Inspection and sampling access | Can technicians check level, temperature, vibration and oil condition safely? |
| Spare-parts lead time | Which bearings, seals, couplings and gear elements are stocked, and how quickly can they ship? |
| Commissioning, repair and rebuild | Who performs startup assistance, failure analysis and factory or field rebuilds? |
| Documentation quality | Are drawings, manuals, lubrication data, tolerances and revision information complete? |
| Total cost of ownership | What do energy losses, oil, inspections, downtime, spares and rebuilds add over the planned life? |
Procurement and commissioning checklist
Put these items on the purchase order
- Gearbox model, serial-number format and exact ratio.
- Input and output speeds, design torque and service factor with its application basis.
- Mechanical and thermal ratings for the stated ambient, duty and orientation.
- Output shaft, flange, key, mounting and alignment dimensions.
- Allowable overhung and thrust loads.
- Approved lubricant, viscosity grade, additive or base-oil restrictions, fill quantity and level method.
- Breather, drain, level and oil-sampling accessories.
- Required documentation, training, commissioning assistance and response-time commitments.
- Warranty conditions and the maintenance records required for coverage.
Verify before handing over to production
- Inspect the delivered gearbox for shipping damage, correct model and correct orientation accessories.
- Confirm that the mounting surface, shaft fits, keys, fasteners and coupling match the approved drawing.
- Align the motor, gearbox and driven machine within the supplier’s stated limits.
- Install the specified breather and verify that drain and level plugs are in the correct positions for the installed orientation.
- Fill with the approved lubricant to the specified level; document product and quantity.
- Run unloaded or at the supplier’s prescribed commissioning load, watching temperature, noise, vibration and leakage.
- Recheck fasteners, oil level and alignment after the initial operating period required by the OEM.
- Enter baseline temperature, vibration, speed, load and oil data in the maintenance record.
Common buying mistakes to avoid
- Choosing by ratio alone: the correct speed does not prove adequate torque, thermal capacity or shaft-load capacity.
- Using a generic service factor: cement-kiln, reversing-mill and heavy-conveyor examples differ; apply the category and hours that match the machine.
- Treating ISO VG 220 as universal: viscosity, temperature, additives, seals and OEM approval all matter.
- Ignoring mounting orientation: incorrect oil level or breather placement can starve bearings or force leakage.
- Checking only tooth strength: continuous heat can exceed the thermal rating even when gears pass a mechanical calculation.
- Leaving maintenance access to the installer: an inaccessible level plug or sampling point turns routine care into avoidable downtime.
- Accepting undocumented support: without parts identification, repair capability and response commitments, a low purchase price can create a long outage.
No independent market-size, failure-rate or service-life statistic is established by the authoritative material used for this guide. Base the decision on your documented duty, verified ratings, lubricant compatibility and the supplier’s measurable support commitments.
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