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Choose an industrial gearbox by the driven machine’s actual torque, speed, duty, environment, and maintenance constraints—not by ratio or catalog torque alone. The buying process is: document the application, select an architecture, apply a duty-specific service factor, verify mechanical and thermal ratings, match lubrication to the manufacturer’s instructions, and contract for commissioning and long-term support. That sequence prevents a gearbox that has adequate tooth strength on paper from overheating, leaking, or failing in service.
Start with an application specification
Before requesting quotations, create a data sheet for the driven machine. Suppliers can only size a gearbox correctly when the load profile and installation conditions are explicit. A ratio and nominal motor power are not enough.
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Record the driven load and speed
- Required output torque, including normal, peak, and starting torque where those values are known.
- Required output speed and allowable speed range.
- Input speed, motor type, and any variable-frequency-drive operating range.
- Reduction ratio, including whether the machine needs one stage or several.
- Duty cycle: operating hours per day, starts and stops per hour, reversals, inching, jogging, and periods at low speed.
- Shock, impact, cyclic, or high-inertia loads. A crusher, conveyor with a loaded belt, and a smooth fan do not impose the same duty even if their average power is similar.
Record the installation and environment
- Ambient-temperature range and expected bulk-oil temperature.
- Dust, water spray, washdown chemicals, humidity, corrosive atmosphere, and explosive-area requirements.
- Mounting orientation, shaft and flange dimensions, keyway or shrink-disc details, and the available footprint.
- Alignment capability, foundation stiffness, and coupling arrangement.
- Access for filling, draining, oil sampling, inspection, seal replacement, and lifting the unit during a rebuild.
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 maintenance accessibility. Treat each as a required line on the specification rather than an assumption.
Select the gearbox architecture
Choose the arrangement only after the load and space envelope are defined. The architecture affects efficiency, heat rejection, allowable ratios, backdriving behavior, noise, and the way the unit is serviced.
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Helical gearboxes
Helical arrangements are often considered where efficiency, continuous duty, and moderate noise are important. Confirm the available ratio range, shaft arrangement, bearing loads, and cooling capability for the actual speed and torque.
Bevel-helical gearboxes
Bevel-helical units redirect power through an angle while retaining the efficiency characteristics expected from helical gearing. They are useful when the motor and driven shaft must be arranged at 90 degrees, but the quotation must state the permitted overhung and thrust loads at the output shaft.
Worm gearboxes
Worm drives can provide compact right-angle transmission and, in some combinations, resistance to backdriving. That behavior is not automatic: it depends on the ratio, helix angle, load, lubrication, and operating condition. Their efficiency and heat generation must be checked at the intended duty rather than inferred from the catalog ratio.
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Planetary arrangements provide high torque density in a compact envelope. Compare their mechanical and thermal ratings, bearing-life basis, service access, and repair strategy with less compact alternatives.
Cycloidal and other arrangements
Cycloidal and specialty gearboxes may suit high shock loads, low-speed torque, or unusual mounting constraints. Ask the supplier to explain why the proposed arrangement fits the load spectrum and what parts are normally replaced during a rebuild.
For every architecture, the ratio must deliver the machine’s required speed and torque. Then check both mechanical and thermal ratings after applying the service factor. A unit that meets tooth-strength requirements but cannot reject heat during continuous operation is underspecified.
Apply a service factor that reflects real duty
Service factor is a multiplier that represents real loading relative to a steady catalog condition. It accounts for operating hours, starts, stops, reversals, shock, and driven-machine characteristics. It is not a universal quality score and should not be selected from a generic rule of thumb.
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Use the application table, not a default
The AutomationDirect IronHorse manual, dated 31 August 2022, provides AGMA application tables that vary the factor by machine and operating hours. Examples include:
| Application example | Service-factor example |
|---|---|
| Cement kilns and dryers | 1.50 |
| Reversing metal-mill drives | 2.00 |
| Heavy conveyors | 1.50–1.75, depending on operating hours |
These are examples from that manual, not defaults for every gearbox. Give the supplier the complete load profile and ask which table and duty category were used. The quotation should show the selected factor and the resulting design torque.
Calculate the design requirement
As a screening step, multiply the machine’s stated continuous torque by the selected service factor, then check whether starting, peak, shock, and inertia torques exceed that result. The supplier must still verify tooth strength, shaft strength, bearings, keys or shrink discs, and thermal capacity. If the machine reverses or starts under load, provide those events explicitly; an average torque calculation can hide the damaging load case.
Verify mechanical and thermal ratings
Request a rating sheet that identifies the standard and assumptions behind every number. “Rated torque” without a basis is not a useful comparison.
Mechanical rating
- Ask for the mechanical torque rating and the rating standard used.
- Confirm the bearing-life basis and the loads used for the calculation.
- Check allowable overhung and thrust loads at the actual shaft extension, speed, and direction of load.
- Verify shaft diameter, keyway or shrink-disc capacity, shaft tolerances, and coupling fit.
Thermal rating
- Obtain the thermal rating for the stated ambient temperature, input speed, duty cycle, and mounting orientation.
- Ask how heat is rejected: housing surface area, fan, cooling coil, or another arrangement.
- Check whether the thermal rating changes with low-speed operation, high ambient temperature, washdown guards, or restricted airflow.
- Require an oil-temperature limit and the alarm or shutdown point recommended for continuous service.
Compare the service-factor-adjusted load with both ratings. Mechanical margin does not compensate for insufficient thermal capacity, and a large thermal rating does not make an overloaded gearset safe.
Confirm interfaces, mounting, and alignment
Many gearbox problems begin at the installation interface rather than inside the gearset. Put the following dimensions and tolerances on the purchase order:
- Input and output shaft dimensions, key or spline details, and allowable fits.
- Flange, foot, torque-arm, or hollow-shaft mounting dimensions.
- Permitted shaft misalignment in angular, parallel, and axial directions.
- Foundation flatness, bolt grade and torque requirements, and reaction-arm details.
- Coupling type, guard clearance, and access for removing seals or bearings.
- Orientation limits for oil level, breathers, drain plugs, and inspection covers.
Require the supplier to identify the enclosure or environmental protection rating and the seal material. A rating suitable for dry indoor service may not survive abrasive dust, washdown, or chemically contaminated water.
Specify lubrication correctly
Lubricant selection is a system decision. ISO viscosity grade is only one element; speed, bulk-oil temperature, gear type, load, seals, metallurgy, and additive compatibility also matter.
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Follow the gearbox manufacturer first
Use the original equipment manufacturer’s oil type, viscosity, additive package, and fill-level instruction whenever they are available. Record the exact product specification on the maintenance bill of materials so a replacement is not chosen by viscosity alone.
AGMA 955-A22 provides generalized lubricant guidance for open and enclosed metallic gearing, including spur, helical, worm, bevel, and face gears. It was approved by AGMA’s Technical Division Executive Committee in August 2022. The publication itself says its guidance is not intended to replace the gear manufacturer’s instructions.
Use ISO VG grades as a qualified starting point
If the manufacturer’s instruction is unavailable, consult the AGMA-derived tables in the Horsburgh & Scott enclosed gear-drive maintenance manual. Those tables map ISO viscosity grades to pitch-line speed and operating temperature for enclosed drives and include grades such as:
| ISO viscosity grade | Where it appears in the cited guidance |
|---|---|
| ISO VG 150 | Enclosed-drive viscosity tables related to pitch-line speed and operating temperature |
| ISO VG 220 | Enclosed-drive viscosity tables related to pitch-line speed and operating temperature |
| ISO VG 320 | Enclosed-drive viscosity tables related to pitch-line speed and operating temperature |
| ISO VG 460 | Enclosed-drive viscosity tables related to pitch-line speed and operating temperature |
The manual identifies the preferred kinematic-viscosity unit as mm²/s, commonly called centistokes (cSt). Select a grade from the applicable speed and temperature row, then verify the choice against seals, metallurgy, additives, and the gearbox maker’s approval.
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ISO VG 220 can be suitable for some enclosed industrial gearboxes, but the grade is not a universal fill. A gearbox running at a different pitch-line speed or oil temperature may require another grade, and the correct additive package may differ. Treat “ISO VG 220 industrial gear oil” as a search description only; confirm the exact gearbox requirement before filling.
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 qualification matters: an extreme-pressure or synthetic product is not interchangeable merely because its ISO grade matches.
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Check compatibility before changing oil
- Do not mix mineral, synthetic, and extreme-pressure products unless the gearbox maker and lubricant supplier confirm compatibility.
- Verify seal compatibility, especially when changing base oil chemistry or additive package.
- Check whether the unit uses splash, bath, forced, or circulating lubrication and fill it to the specified level for its orientation.
- Label the installed oil and keep a retained sample when contamination or wear trending is important.
Build a maintenance and oil-change plan
Oil-change timing is model-specific. First-week, first-month, and recurring intervals must follow the gearbox OEM manual; the cited maintenance document does not create one interval that applies to every model.
Use a controlled first-service workflow
The Horsburgh & Scott enclosed gear-drive maintenance schedule includes the following actions:
- Check housing, foundation, coupling, and mounting-arm bolting for correct tightness.
- Operate the unit briefly while observing noise, vibration, leakage, and temperature behavior.
- Stop the unit safely and check the oil level using the specified sight glass, dipstick, or level plug.
- Drain and discard the oil according to the site’s waste-handling procedure.
- Flush with SAE 10 flushing oil where the manual calls for it, then drain the flushing oil completely.
- Replace the filter element if the gearbox has a filter.
- Inspect accessible gear teeth for undue wear, scoring, pitting, or abnormal contact.
- Refill with the approved lubricant to the specified level and record the product, batch, date, and quantity.
Do not infer that every gearbox needs SAE 10 flushing oil or a filter change; those steps are included in the cited schedule and must be reconciled with the model’s own instructions.
Make condition checks part of normal operation
- Trend oil temperature, vibration, noise, and leakage against a known baseline.
- Inspect breathers, seals, drain plugs, and sight glasses during routine rounds.
- Take oil samples through a clean, repeatable sampling point when the duty or criticality justifies analysis.
- Investigate a sudden change rather than waiting for the next calendar service.
Require meaningful supplier service support
Support belongs in the purchase specification, not in an informal promise after delivery. AGMA’s enclosed-drive publication scope includes specifying, selecting, testing, procuring, operating, and maintaining gear drives, which reflects how much of the lifecycle the supplier should help document.
Documents to obtain before acceptance
- Installation, alignment, lifting, and commissioning instructions.
- Lubricant type, viscosity, additive, fill volume, fill level, and seal specifications.
- Inspection points, sampling method, temperature limits, and recommended change intervals.
- General arrangement drawings, shaft and flange dimensions, and mounting tolerances.
- Parts lists with identification for gears, bearings, seals, breathers, filters, and couplings.
- Warranty terms, exclusions, required commissioning records, and claim procedure.
- Troubleshooting guidance for overheating, leakage, vibration, noise, and abnormal wear.
Put response and repair terms in writing
Ask for named contacts, response times for production-stopping failures, remote diagnostic support, field-service coverage, and the location and capability of repair or rebuild facilities. Confirm whether replacement parts are stocked, made to order, or subject to long lead times. For a critical drive, require a recommended spare-parts list and a plan for preserving a removed gearbox while a failure is investigated.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Compare gearbox quotations on lifecycle value
Once multiple units meet the basic torque and speed requirement, compare them on the factors that affect operating cost and availability:
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| Comparison axis | Questions to ask |
|---|---|
| Ratio and output speed | Does the selected ratio deliver the required speed across the operating range? |
| Service-factor margin | What factor was used, and how much margin remains for starts, reversals, and shock? |
| Mechanical versus thermal rating | Which rating governs at the stated ambient temperature and duty? |
| Efficiency and heat rejection | What losses and oil temperatures should be expected at the actual speed? |
| Overhung and thrust loads | Are the allowable loads adequate at the real pulley, sprocket, or coupling location? |
| Mounting and alignment tolerance | What installation accuracy and foundation conditions are required? |
| Enclosure and environment | Will seals, breathers, and housing protection withstand dust, water, chemicals, or washdown? |
| Lubricant and change interval | Which oil is approved, how is it sampled, and what interval does the OEM specify? |
| Inspection access | Can technicians reach the level point, drain, breather, filter, and sampling port safely? |
| Spare-parts lead time | Which failure-critical parts are stocked and how quickly can they ship? |
| Commissioning and rebuild support | Who performs alignment, startup checks, diagnostics, and repair? |
| Documentation quality | Are drawings, tolerances, oil instructions, and parts identification complete? |
| Total cost of ownership | How do purchase price, oil, energy loss, inspections, downtime exposure, and rebuildability compare? |
A lower purchase price can be outweighed by higher heat rejection requirements, difficult oil changes, poor access, or long parts lead times. Conversely, a more expensive gearbox is not automatically better if its rating, support, or documentation does not match the application.
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Common buying and commissioning failures
Choosing by ratio alone
A ratio can be correct while torque capacity, thermal capacity, or shaft loading is wrong. Require the complete rating calculation for the actual duty.
Using a generic service factor
Applying one default factor to a reversing, shock-loaded, or long-hour drive hides the real load spectrum. Provide operating hours and transient events and document the table category used.
Filling by viscosity grade only
ISO VG 220, 320, or another grade does not identify the complete lubricant. Confirm additive chemistry, base oil, seals, temperature range, and OEM approval.
Ignoring mounting orientation
Changing orientation can move the oil level away from bearings or gears and can place a breather or seal in the wrong location. Get an orientation-specific fill instruction before installation.
Leaving support undefined
Without response times, parts identification, and warranty conditions, a failure can become a procurement exercise during an outage. Make those terms deliverables before acceptance.
A purchase-order checklist
- Application torque, speed, ratio, input speed, duty cycle, starts, stops, reversals, and inertia loads attached.
- Service factor, source table, and design torque stated.
- Mechanical and thermal ratings stated with standards and assumptions.
- Overhung and thrust limits, shaft tolerances, mounting dimensions, and alignment limits approved.
- Environmental protection, seals, breathers, and washdown or contamination requirements documented.
- Approved lubricant, ISO grade, additive type, fill level, and compatibility notes listed.
- Commissioning, oil sampling, inspection, and change instructions supplied.
- Parts list, spare recommendations, warranty terms, troubleshooting route, and response times included.
- Repair, rebuild, and field-service capability identified, with lead-time commitments.
Use the completed checklist to evaluate both the gearbox and the supplier. The best choice is the unit that satisfies the load and temperature calculations while remaining installable, inspectable, lubricatable, and supportable for the life of the machine.
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