Choose an industrial gearbox from the driven machine backward, not from the motor nameplate or a convenient catalog size. Define the machine’s continuous, starting and peak torque; required output speed; input speed; starts, stops and reversals; operating hours; load variation; ambient conditions; contamination; mounting and shaft interfaces. Calculate the ratio, apply a service factor based on the real duty cycle, then verify tooth and shaft torque, thermal capacity, external loads, lubrication and service support. A gearbox is correctly selected only when all of those checks pass at the same time.
The practical sequence is: load profile → ratio → gearbox architecture → service factor → thermal check → interfaces and external loads → maintenance and support specification. The sections below explain each decision and show what to put in a supplier’s written selection.
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Start with the driven machine
The gearbox transmits the load that the machine actually imposes. Motor horsepower alone does not describe that load: two machines with the same motor can have radically different startup torque, shock, reversing frequency and thermal demand. STOBER’s sizing guidance states, “To achieve the best gearbox solution, customers should size from the load.” Record the following before looking at a model number.
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- Continuous output torque: the torque required during normal steady operation.
- Startup or breakaway torque: the torque needed to accelerate the mechanism from rest, including static friction and product load.
- Peak or jam torque: the highest torque that can occur during a transient, jam, collision or process upset. State whether a torque limiter, clutch or drive current limit prevents the event.
- Emergency or fault torque: a short-duration value that may govern shaft, key, coupling and bearing strength even when it does not govern the gear tooth rating.
- Inertia: reflected load inertia affects acceleration torque and the number of starts the gearbox sees.
If torque is supplied as power, convert it using the units and speed used by your supplier. Do not silently substitute motor-rated torque for output torque; account for the ratio and the efficiencies of the motor, drive and gearbox.
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Describe speed and motion
- Input speed, including the normal range and any overspeed condition.
- Required output speed, including allowable tolerance and whether the speed is fixed or variable.
- Acceleration and deceleration times.
- Number of starts, stops and reversals per hour.
- Whether the output rotates continuously, indexes, oscillates or holds position.
- Any back-driving, overhauling or braking condition.
For a simple fixed-speed application, the basic reduction ratio is i = n1 / n2, where n1 is input speed and n2 is required output speed. The ratio may need to be adjusted for the actual motor speed, drive frequency, slip and the gearbox’s available nominal ratios.
Capture operating environment
- Hours per day, days per week and expected service life.
- Load pattern: constant, intermittent, cyclic, pulsating or impact-loaded.
- Ambient temperature range, enclosure temperature and exposure to radiant heat.
- Dust, washdown, moisture, chemicals, abrasive particles or explosive atmospheres.
- Mounting orientation and available cooling airflow.
- Noise, backlash, positional accuracy and allowable vibration.
These details affect service factor, lubricant selection, sealing, thermal capacity and the permissible mounting position. A gearbox selected without them is not fully sized, even if its catalog torque appears adequate.
Calculate the ratio and design torque
Derive the reduction ratio
Use the required input and output speeds rather than a rounded catalog label. For example, an illustrative 1,750 rpm input and 35 rpm output require i = 1,750 / 35 = 50, or a nominal 50:1 reduction. If the motor is on a variable-frequency drive, calculate at the highest and lowest operating frequencies and check both the resulting output speeds and the gearbox’s thermal behavior.
Multi-stage gearboxes may offer a range of ratios, but the exact ratio can change efficiency, noise, backlash, envelope and thermal loss. Ask the supplier to show the selected ratio and the resulting output speed at the stated input speed and frequency.
Apply the required service factor to torque
KG International’s catalog selection method expresses design torque as Mc2 = Mr2 × fs', where Mr2 is required output torque and fs' is the required service factor. The gearbox’s rated output torque must satisfy Mn2 ≥ Mc2. Use the same torque units throughout and state whether torque is continuous, intermittent or peak.
Illustrative calculation: if a driven machine needs 1,200 N·m continuously and the application analysis establishes a service factor of 1.4, the calculated design torque is 1,680 N·m. A candidate whose published rated output torque is below 1,680 N·m does not meet that criterion, regardless of its motor horsepower. The example demonstrates the method; the correct factor comes from the application and the selected manufacturer’s tables.
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Check torque at every operating point
Compare continuous and peak torque separately. A gearbox can have enough continuous tooth capacity but fail a shaft, key, coupling or bearing check during a jam. Conversely, a brief peak may be acceptable when a manufacturer specifies a permissible peak or emergency torque and the control system limits its duration. Obtain written limits for:
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- acceleration and startup torque;
- reversing torque;
- maximum intermittent or peak torque;
- emergency or stall torque;
- allowable torque at reduced speed or elevated ambient temperature.
Choose the gearbox architecture for the application
Once the load, ratio and design torque are known, compare architectures that can meet them. The following are engineering tendencies, not substitutes for a manufacturer’s rating tables.
| Architecture | Typical strengths | Important trade-offs and checks |
|---|---|---|
| Helical | Efficient power transmission, compact inline packages and broad industrial availability. | Confirm ratio range, thermal rating, radial and axial bearing loads, and whether the required output torque fits the available frame size. |
| Bevel-helical | Efficient right-angle transmission with a relatively compact envelope when the motor and output must be perpendicular. | Check the right-angle mounting arrangement, overhung load, output direction, lubrication position and space for the motor and coupling. |
| Worm | Simple right-angle arrangement, high reduction in a compact package and often a lower initial purchase cost. | Sliding contact can produce more heat and lower efficiency than rolling or helical arrangements. Verify thermal capacity, duty cycle, lubricant temperature and whether the unit is self-locking under the exact load and lubrication conditions; never assume self-locking without a written specification. |
| Planetary | High torque density, compact coaxial packaging and the ability to handle substantial torque in a small envelope. | Compare backlash, bearing and carrier loads, ratio availability, lubrication, cooling and the supplier’s service procedures. The catalog method still requires calculated torque to be no greater than rated torque. |
| Cycloidal | High reduction ratios, robust shock tolerance and low backlash in designs intended for demanding cyclic or positioning service. | Confirm allowable input speed, output bearing and pin load ratings, lubrication interval, noise, envelope and the manufacturer’s peak-torque definition. |
When several architectures pass the basic rating, compare them on the factors that affect the machine rather than on a single efficiency number: ratio and speed range, continuous and peak torque margin, heat rejection, shock and reversing capability, backlash, radial and axial load capacity, mounting envelope, noise, lubrication access, expected maintenance and local parts or repair support.
Set the service factor from duty cycle
Service factor is a capacity margin for conditions that make an application harder than a smooth, continuous reference load. It accounts for non-uniform loading, hours of service, elevated ambient temperature, shock, cyclic operation and reversals. It is not a universal safety factor that can be selected independently of the gearbox family.
Interpret the named reference values correctly
STOBER explains that a service factor of 1.0 means the unit has only just enough capacity for the application. The same guidance identifies 1.4 as adequate for most industrial applications, subject to the manufacturer’s specifications and tables. Those values are reference points, not permission to use 1.4 for every machine. A high-inertia indexer with frequent reversals, a conveyor with severe impact, or a unit operating in a hot enclosure may require a different value.
Build the factor from the real duty
- Operating time: continuous 24-hour operation generally demands more capacity than occasional use.
- Load uniformity: smooth fan or pump torque is less severe than a crusher, mixer or indexing mechanism with pulsation.
- Starts and stops: each acceleration adds torque and heat; frequent cycling can govern both mechanical and thermal sizing.
- Reversals: direction changes impose transient tooth, shaft and coupling loads and can expose backlash or looseness.
- Shock and jams: impacts, product blockages and tool engagement may require a higher factor or a torque-limiting device.
- Ambient temperature: hot surroundings reduce the ability to reject heat and may require derating.
- Control behavior: a drive’s current limit, ramp time and braking method can reduce or increase transient torque.
Ask the supplier to state the service-factor basis in writing: load class, hours per day, starts and reversals per hour, ambient temperature, peak torque and any assumed coupling or torque limiter. If the supplier supplies only a bare number, the selection is difficult to audit later.
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Verify thermal capacity, not just tooth strength
Continuous low-speed or high-power operation can overheat a gearbox even when the gears have adequate mechanical torque capacity. Thermal rating depends on transmitted power, efficiency losses, input speed, output speed, duty cycle, ambient temperature, lubricant viscosity, mounting position and cooling method.
What to check on the thermal chart
- Actual ambient temperature and any heat from nearby motors, ovens or process equipment.
- Continuous input power and output torque at the lowest operating speed.
- Variable-speed operation: identify the worst combination of high torque and low airflow.
- Lubricant type, viscosity, fill level and allowable oil temperature.
- Natural convection, fan cooling, cooling coil or external heat exchanger.
- Mounting orientation, because it can change oil distribution and the effective cooling surface.
Use the selected model’s thermal chart or calculation, not a generic efficiency assumption. If the chart does not cover the actual ambient or duty, request a derating or a cooled version. A temperature switch, oil-level device or condition-monitoring point may be appropriate where overheating would stop production or damage the load.
Check shafts, bearings and external loads
The gearbox sees more than torque. The output mechanism can apply radial, axial and overhung loads that govern the shaft or bearing selection. STOBER warns that output mechanisms alter radial and axial loading and that the overhung-load rating must exceed the application load.
Calculate the external loads
- For a chain, belt or gear mounted on the output shaft, calculate the tangential force from transmitted torque and the pitch radius.
- Resolve belt, chain or gear forces into radial and axial components at the actual shaft position.
- Include the weight of the mounted element and any tension from pretensioning.
- Account for thrust from helical gears, screw conveyors, pumps or actuators.
- Check transient loads during acceleration, stopping, reversing and jams.
Compare those forces with the manufacturer’s radial, axial and overhung-load ratings at the actual speed, direction and distance from the bearing. A catalog rating quoted at the shaft shoulder may not apply at a pulley mounted farther outboard. If the load exceeds the integrated bearing capacity, use an external bearing arrangement or a different gearbox.
Confirm the mechanical interfaces
- Output shaft diameter, keyway, spline, shrink disc or keyless connection.
- Hollow-bore dimensions and the required torque arm or reaction support.
- Input flange, motor frame, coupling, adapter and allowable motor weight.
- Foot, flange, shaft-mounted or torque-arm mounting.
- Alignment tolerance, coupling engagement and shaft runout.
- Seals and enclosure suitable for dust, washdown, moisture or chemicals.
- Brake, backstop, encoder or external sensor provisions.
Check that the mounting arrangement can react gearbox torque without distorting the housing. Follow the supplier’s instructions for shaft fits, key length, tightening torque, alignment and reaction-arm clearance.
Use this step-by-step selection workflow
- Write the load profile. List continuous, startup, peak, emergency and reversing torque; inertia; speed range; starts and stops per hour; operating hours; and any torque limiter or drive limit.
- Define the speed requirement. State input speed or motor frequency, required output speed, allowable variation and acceleration/deceleration times.
- Calculate the ratio. Apply
i = n1 / n2at the relevant operating points and identify the nearest available nominal ratio. - Calculate design torque. Apply
Mc2 = Mr2 × fs'using the service factor justified by the duty. RequireMn2 ≥ Mc2for the selected rating. - Shortlist architectures. Eliminate designs that cannot provide the ratio, output direction, torque density, backlash, shock tolerance or envelope required by the machine.
- Check continuous thermal capacity. Use the model’s thermal chart at actual ambient temperature, speed, lubricant and mounting orientation. Include variable-speed worst cases.
- Check external loads. Submit radial, axial and overhung loads, distances from bearings, thrust direction and transient conditions for confirmation.
- Check interfaces and installation. Confirm shaft or bore, keyless connection, flange or foot pattern, motor adapter, coupling, alignment and reaction support.
- Specify lubrication and maintenance. Obtain lubricant grade, fill quantity, fill and drain positions, change interval, inspection access and oil-analysis requirements.
- Get a written selection. Require the supplier to list model, ratio, ratings, assumptions, deratings, thermal result, external-load checks, expected life, spare parts, commissioning and service response.
Compare candidates with an engineering scorecard
When two or more gearboxes meet the minimum rating, score the complete machine fit. A low purchase price is not a useful advantage if the unit needs an unplanned cooler, cannot accept the output pulley load or has no practical repair support.
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| Decision area | Questions to answer | Evidence to request |
|---|---|---|
| Torque margin | Does rated torque exceed calculated continuous and peak requirements? | Rating table, service-factor calculation and peak/emergency limits. |
| Speed and ratio | Does the ratio produce the required output speed across the full operating range? | Exact ratio, input-speed range and output-speed calculation. |
| Thermal behavior | Can it reject heat at low speed, high torque and the stated ambient? | Thermal chart, derating, cooling method and oil-temperature limit. |
| External loads | Can shafts and bearings carry pulley, chain, gear or thrust loads? | Radial, axial and overhung-load ratings at the actual load position. |
| Motion quality | Are backlash, torsional stiffness, noise and reversing behavior suitable? | Backlash or torsional data and application notes. |
| Installation | Will the mounting, coupling and envelope fit without forcing misalignment? | Dimensional drawing, mounting limits and shaft-interface details. |
| Maintenance | Can technicians inspect, drain, refill and repair it safely? | Lubricant specification, fill procedure, intervals and parts list. |
| Support | Can you obtain commissioning, spares and repair help where the machine operates? | Service contacts, response terms, warranty and spare-parts lead times. |
Specify lubrication, contamination protection and maintenance
Lubrication is part of the gearbox selection, not an afterthought. Use the lubricant grade and fill procedure specified for the exact model, ratio, mounting position and ambient range. A change in orientation can alter the required fill level. Mixing oils, overfilling or using an unapproved viscosity can increase churning losses and temperature or leave bearings inadequately lubricated.
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- Accessible fill, drain and level points in the installed orientation.
- Breather placement that does not ingest washdown water or process dust.
- Inspection windows, temperature measurement and vibration or oil-analysis access where justified by criticality.
- Defined inspection and oil-change intervals based on hours and contamination, not only calendar time.
- Replacement seals, bearings and couplings identified by part number.
- Safe procedures for isolating the motor, brake and stored mechanical energy before service.
For a critical production line, include expected life, spare-unit strategy, commissioning assistance and repair response in the purchase specification. Those requirements can determine the practical choice between otherwise acceptable architectures.
Common selection mistakes and how to correct them
Choosing by motor horsepower alone
Symptom: the catalog selection appears adequate by motor size, but the gearbox trips on acceleration, overheats or develops bearing damage. Correction: provide the driven-machine torque profile, inertia, starts and reversals, then recalculate design torque and thermal capacity.
Using a service factor without stating its basis
Symptom: a nominal factor is quoted, but no one can explain whether it includes shock, hours, temperature or reversing. Correction: document each duty assumption and obtain the manufacturer’s application-table result. Treat 1.0 as only just enough capacity and 1.4 as a STOBER reference for many industrial applications, not a universal rule.
Checking gears but ignoring overhung load
Symptom: a belt or chain drive causes premature bearing or shaft failure even though torque is below the gear rating. Correction: calculate radial, axial and overhung forces at the actual mounting distance and compare them with the published ratings.
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Ignoring low-speed thermal limits
Symptom: the gearbox runs acceptably at rated speed but overheats during slow, high-torque operation. Correction: evaluate the worst speed-torque-ambient combination on the model’s thermal chart and add cooling or choose a larger unit if required.
Assuming a worm gearbox is automatically self-locking
Symptom: the load back-drives when power is removed, or the unit becomes excessively hot. Correction: obtain a written back-driving or holding specification for the exact ratio, load, lubrication and wear condition; use a brake or backstop when the safety function requires positive holding.
Forcing an unsuitable mounting or coupling
Symptom: seal leakage, noise, coupling wear or abnormal vibration appears after installation. Correction: follow the dimensional drawing, shaft-fit, alignment and reaction-support limits, and verify that the chosen mounting position is approved for the lubricant fill.
Worked selection example
Consider a hypothetical conveyor that needs 1,200 N·m continuous output torque at 35 rpm from a 1,750 rpm motor. It starts ten times per hour, reverses twice per hour, runs 16 hours per day and operates in a 40 °C ambient. The output sprocket applies a calculated radial load at a known distance from the gearbox bearing. These values are an illustration of the selection process, not a rating for a particular product.
- Ratio:
i = 1,750 / 35 = 50, so request a nominal 50:1 ratio and confirm the actual output speed. - Duty: starts, reversals and the conveyor’s load variation must be supplied to the manufacturer’s service-factor table rather than assumed to be smooth continuous duty.
- Design torque: if the documented application factor were 1.4,
Mc2 = 1,200 × 1.4 = 1,680 N·m. The selected rated output torque must be at least 1,680 N·m under the stated conditions. - Peak event: determine jam and emergency torque separately and verify shaft, key, coupling and bearing limits, plus any torque-limiter setting.
- Thermal check: evaluate 35 rpm output, 40 °C ambient, 16-hour operation and the actual input power on the selected model’s thermal chart.
- External-load check: compare the sprocket’s radial and any axial load at its actual overhung distance with the gearbox rating.
- Architecture: compare helical, bevel-helical, worm, planetary and cycloidal candidates that can provide the 50:1 ratio and pass all torque, thermal and load checks; then select on envelope, efficiency, backlash, maintenance and support.
The example shows why a model cannot be selected from ratio alone. Every candidate still needs a written confirmation of thermal capacity, external loads, interfaces and service assumptions.
What to request in the purchase specification
Send the supplier a complete data sheet and require the final selection to show each item below:
- Gearbox model, architecture, nominal ratio and exact ratio.
- Input speed range, motor interface, drive frequency and coupling details.
- Continuous, startup, peak, emergency and reversing torque.
- Service-factor value and the duty assumptions behind it.
- Hours per day, starts and reversals per hour, load variation and inertia.
- Thermal rating at the actual ambient, speed, lubricant, mounting and cooling method.
- Output shaft or hollow-bore dimensions, key or keyless connection and mounting pattern.
- Radial, axial and overhung loads, load position and bearing-life calculation.
- Lubricant type, fill quantity, approved orientation, inspection and change procedure.
- Sealing and enclosure requirements for dust, washdown, chemicals or moisture.
- Expected life, allowable backlash or torsional stiffness, noise limits and vibration criteria.
- Commissioning instructions, spare-parts availability, warranty and repair or service response.
A written selection that exposes its assumptions is easier to audit, install and replace than a catalog number chosen without a load record. It also makes later changes—such as a faster cycle, hotter ambient or heavier product—visible before they become gearbox failures.
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