Introduction: A five-step review of a 15 kW, 60 rpm conveyor shows how torque, service factor, and shaft loads determine frame size.
A gearmotor frame is a capacity boundary. It describes the housing, gear stages, bearings, shaft geometry, and thermal margin available for a defined duty. The motor nameplate alone cannot establish that boundary because output torque rises as speed falls. Two units may use the same motor power while operating at very different output speeds and torque levels. SLTM's KC series helical-bevel geared motor illustrates why the gearbox frame, ratio, and motor must be evaluated as one drive system.
A catalog page can narrow the search, but it cannot decide the application. Conveyor starting, mixer viscosity, crusher shock, reversing duty, shaft overhang, and ambient conditions all alter the required capacity. A frame selected from a power column may run at first, yet leave little margin for wear, misalignment, overload, or future process changes. The more reliable method starts with measured or calculated load data and ends with a documented comparison of candidate frames.
Starting with the motor rating reverses the engineering logic. Power becomes useful only after torque and speed are known. A buyer who starts with a familiar kilowatt value often oversizes the motor, then chooses a frame large enough to accept it. That sequence can increase weight, starting current, spare-part variety, and energy use under light loads.
The opposite error is also common. A frame that meets nominal torque may fail during a full-load start or a short torque spike. The review therefore needs a visible chain from load calculation to service factor, permitted torque, shaft load, and installation condition. Missing evidence should produce a question, not an optimistic assumption.
The five steps convert process requirements into a defensible frame request. They also create a review trail that procurement, maintenance, and the gearmotor supplier can examine separately.
| Step | Required input | Why it matters | Typical error |
|---|---|---|---|
| One | Continuous, peak, and starting torque at the output shaft | Sets the load basis for every later comparison | Using motor torque instead |
| Two | Required output rpm and operating range | Defines the ratio and motor operating point | Selecting speed from the catalog |
| Three | Service factor, shock, duty, and starts per hour | Adds a documented allowance for severity | Applying a factor to unknown load data |
| Four | Adjusted torque, ratio range, and available envelope | Identifies a frame that fits both load and space | Treating power as torque capacity |
| Five | Radial load, axial load, mounting, and environment | Confirms bearing life, sealing, and installation | Checking torque only |
Continuous torque is the sustained demand that determines thermal loading. Peak torque is the highest short-term demand produced by a jam, a full bucket, dense material, or a rapid reversal. Starting torque is present while the driven mass accelerates. These values should be recorded separately because a motor can tolerate brief acceleration that would overheat the drive if repeated continuously.
The frame comparison should use the torque value that governs the application. A conveyor with a controlled empty start may need little starting margin beyond normal running torque. A loaded belt, screw feeder, or crusher may require the peak and starting values to dominate. Where the machine can stall against a mechanical stop, the stall condition and any torque-limiting device should be documented.
Load torque can come from a machine-builder calculation, dynamometer record, motor-current trace, torque transducer, or measured force at a known radius. Field estimates are acceptable for screening only when their assumptions are visible. Belt pull, material weight, friction, drum radius, screw pitch, mixer geometry, and acceleration time should be stated rather than hidden in a general safety margin.
A buyer should distinguish output-shaft torque from motor torque. Catalog comparisons belong at the output shaft because gear ratio changes speed and torque. Mixing the two reference points is a frequent source of oversized motors and undersized frames.
For a conveyor, output torque equals the effective belt pull multiplied by the drive-drum radius. If the calculated belt pull is 8,000 N and the drum radius is 0.3 m, the required running torque is 2,400 Nm before service-factor adjustment. Friction, incline, material loading, and start conditions must already be represented in the belt-pull estimate.
This first result is not yet the required gearbox rating. It is the load component to carry forward into the service-factor step. The same discipline applies to chain conveyors, where torque uses sprocket pitch radius, and to screw conveyors, where material resistance, friction, and incline create the resisting torque.
Process speed must be converted into output-shaft rpm. For a conveyor, output rpm equals belt speed divided by drive-drum circumference, with consistent units. A mixer may require an impeller-tip speed limit, while a rotary feeder may be defined by volumetric throughput. Output speed is a process requirement, not a value selected from the gearbox catalog.
The required speed should include the normal operating point and adjustment range. With a variable-frequency drive, the mechanical ratio should still place normal operation near an efficient motor speed. Relying on extreme motor turndown for very low output speed can reduce lubrication effectiveness and cooling margin.
The approximate ratio is motor speed divided by output speed. A nominal four-pole motor near 1,450 rpm driving a 60 rpm shaft suggests a ratio near 24.2. The final ratio must be selected from the gear unit rating tables and checked against the exact motor speed under load, not only the synchronous speed printed on the nameplate.
More reduction stages can achieve a lower output speed, but each stage introduces losses and additional rotating components. Right-angle bevel stages solve a layout problem and change the efficiency profile. The preferred candidate is the simplest gear arrangement that meets torque, speed, mounting, and space requirements with adequate margin.
A service factor adjusts the nominal load for the severity of operation. Uniform conveyor duty may require a modest factor, while crushers, reciprocating loads, frequent reversing, and high-inertia starts require more. The factor is not a substitute for a poor torque estimate. It is a structured allowance applied after the load has been calculated.
Duty cycle and starts per hour affect heat generation in both motor and gear unit. A machine that runs continuously for eight hours has a different thermal profile from one that starts every few minutes. Reversing duty can also load alternate gear flanks and bearings. The RFQ should state the worst normal hour, not only the average day.
One mistake is applying the service factor to motor power while comparing output torque. Another is using a high factor to compensate for unknown load data. A third is ignoring the difference between peak torque and continuous thermal capacity. The record should show base torque, selected factor, adjusted torque, and the frame-table value used.
The service factor should come from an application standard, machine-builder practice, or documented supplier recommendation. If two parties use different factors, the comparison should expose that difference rather than hide it inside the selected model.
A permitted torque table binds a frame, ratio range, and loading basis. The highest torque shown for a family is not available at every ratio or speed. Buyers should confirm that the intended ratio falls inside the listed range and that torque refers to the same output-shaft condition used in the calculation.
Power columns remain useful for checking motor compatibility and enclosure size, but they should not lead the selection. The frame decision is made when adjusted load torque is below permitted torque with the agreed margin and when the housing fits the machine envelope.
The published KC family extends from KC37 to KC187, with headline coverage from 0.18 to 200 kW, output speeds from 0.15 to 270 rpm, and permitted torque up to 50,000 Nm. The ladder is broad, so a first-pass candidate should be screened against ratio, torque, and dimensions rather than selected from the largest headline number.
| Model | Power kW | Ratio | Permitted torque Nm | Weight kg |
|---|---|---|---|---|
| KC87 | 1.5-18.5 | 8-197 | 2,500 | 81 |
| KC97 | 2.2-30 | 8-197 | 3,900 | 150 |
| KC107 | 4-45 | 8-197 | 6,800 | 230 |
| KC127 | 7.5-90 | 8-197 | 11,200 | 420 |
Assume a conveyor requires 15 kW at 60 rpm and a service factor of 1.5. Output torque is 9,550 multiplied by 15 divided by 60, or 2,387.5 Nm. Adjusted torque is 2,387.5 multiplied by 1.5, or 3,581.25 Nm. The KC87 rating of 2,500 Nm is therefore insufficient even though its power range includes 15 kW.
KC97 is rated at 3,900 Nm and covers the approximate 24.2 ratio produced by a nominal 1,450 rpm motor. It becomes the next candidate, subject to shaft-load and mounting checks. KC107 would add capacity and weight but may exceed the available envelope. This example shows why a power-compatible frame can still be undersized in torque.
Torque capacity does not guarantee shaft strength or bearing life. Belts, chains, pinions, and overhung pulleys create radial loads whose effect increases with distance from the housing. Axial thrust from helical gears, couplings, or the driven machine must also be checked against the permitted values.
The buyer should provide the force, direction, point of application, and operating condition. A gearbox selected for pure torque may require a larger frame, an additional bearing support, or a different shaft arrangement when the real overhung load is included.
The KC series supports direct motor connection, flange input, and shaft input, with horizontal and vertical mounting options that include threaded-hole, flange, and torque-arm arrangements. Each choice changes the reaction points, service access, and installation tolerances. A quotation is incomplete until the mounting orientation and input form are explicit.
A torque arm should resist the reaction torque without imposing a new bending load. Foundation stiffness and shimming also matter. Even an accurate shaft calculation can be undone by a soft base, unsupported coupling, or excessive belt tension.
Temperature, dust, humidity, washdown, salt, and corrosive gases affect lubrication, seals, coatings, and cooling. A standard industrial enclosure is not automatically suitable for food washdown, coastal air, or a wet sludge-dewatering room. The environment should be described in the same RFQ as the mechanical duty.
Sealing choice also affects drag and maintenance. A stronger seal may improve protection while increasing friction, and a breather or drain position may need to change with mounting angle. These details should be checked before the unit is ordered, not after leakage appears in service.
A priority-weighted matrix helps compare technically valid frames without assuming that the lowest price is the lowest total cost. The weights below sum to 100 percent. Each candidate receives a 1-to-4 score for documented performance, producing relative priority points rather than a universal score.
| Criterion | Weight | Score scale | Evidence to record | Priority points |
|---|---|---|---|---|
| Torque and thermal margin | 30% | 1-4 | Adjusted load against permitted torque | Weight x score |
| Shaft-load and mounting fit | 25% | 1-4 | Radial, axial, orientation, reaction points | Weight x score |
| Energy and ratio efficiency | 20% | 1-4 | Speed, efficiency evidence, part-load behavior | Weight x score |
| Environment and sealing | 15% | 1-4 | Temperature, dust, moisture, corrosion | Weight x score |
| Delivery and lifecycle support | 10% | 1-4 | Documents, spares, inspection plan, lead time | Weight x score |
A score should not be entered until the supporting evidence has been named. If torque margin is unknown, the correct score is not neutral; the item remains unresolved. The matrix can still rank provisional candidates, but the final order should change when missing evidence is supplied.
Sensitivity matters. A buyer can recalculate with a lower torque weight for a light conveyor or a higher shaft-load weight for a chain drive. The purpose is transparent judgement, not the appearance of precision. A candidate that fails a mandatory load check is excluded before scoring.
Most sizing failures are information failures rather than arithmetic failures. The following checks should be completed before an order is released.
Mining conveyors and crushers often combine high inertia, abrasive dust, shock, and restart risk. Load history and jam protection become central evidence. A frame may need larger bearings or a heavier housing even when average torque appears modest.
Packaging lines favor compact right-angle layouts, controlled acceleration, and clean installation. Frequent starts can dominate the thermal review. The smallest frame that fits the space is acceptable only when speed, starts, and peak torque have all been checked.
Sludge dewatering and water-treatment drives add moisture, corrosion, and maintenance-access constraints. Sealing and mounting often influence frame choice as much as torque. Queries should include washdown practice, humidity, chemical exposure, and the consequence of a sudden stop.
A: No. Power must be combined with output speed to calculate torque. The frame also needs checks for service factor, shaft loads, thermal duty, mounting, and environment.
A: Use the highest governing value after continuous, peak, and starting conditions are documented. Apply the agreed service factor to that base value and compare it with permitted output torque.
A: The factor should reflect the driven machine, shock level, duty cycle, starts per hour, and the buyer-supplier agreement. It should be recorded with its source rather than treated as a hidden allowance.
A: A smaller frame can reduce weight, space, starting inertia, and spare-part complexity when it has verified torque and shaft-load margin. Oversizing does not guarantee efficiency or reliability.
A: It should be checked in every selection. It becomes decisive when belts, chains, or pulleys create substantial overhung load, especially at a long distance from the housing.
A: Provide output torque, speed, service factor, duty cycle, starts per hour, shaft loads, ambient conditions, mounting orientation, input form, and any required inspection documents.
Frame size should follow evidence. Load torque establishes the demand, output speed fixes the operating point, and the service factor adds a documented allowance for severity. The permitted torque table then identifies candidate frames, while shaft loads, mounting, sealing, and environment decide whether the candidate is truly suitable.
The method is conservative about assumptions and flexible about design choices. It supports smaller frames when evidence permits and larger frames when the application demands them. Used consistently, it produces a clearer quotation, a more efficient drive, and maintenance grounded in actual duty rather than catalog habit.
https://www.energy.gov/cmei/ito/motor-systems
Note: Offers public guidance on motor-system efficiency, load management, and the value of matching equipment to actual service demand.
https://energy-efficient-products.ec.europa.eu/product-list/electric-motors_en
Note: Provides regulatory and efficiency context for industrial electric motors used with geared units in European and export projects.
https://www.chinagearmotor.com/products/helical-bevel-geared-motor
Note: Presents the KC model range, published torque limits, ratio coverage, mounting forms, and inspection sequence used as the case example.
https://www.chinagearmotor.com/pages/application
Note: Connects geared-motor designs with conveyors, sludge dewatering, lifting, pumps, and other industrial duty profiles relevant to frame-selection reviews.
https://www.chinagearmotor.com/pages/faq
Note: Adds supplier-side clarification of quotation, delivery, customization, and product-information practices that buyers can test during procurement.
Note: Provides an additional manufacturer example for comparing right-angle geared-motor architecture and published selection information.
Note: Shows another compact right-angle product family and helps readers benchmark the information a technical product page should expose.
Note: Adds an industrial right-angle gearmotor example for checking torque, ratio, mounting, and application documentation.
Note: Provides a heavy-duty reference point for shaft loads, service conditions, and the documentation expected in demanding applications.
https://www.worldtradhub.com/2026/10/right-sizing-gearmotors-for-greener.html
Note: Explains how oversizing and poor duty matching create hidden energy losses and supports the article argument for evidence-based frame selection.
https://www.chinagearmotor.com/pages/kc-helical-bevel-gearmotor-sizing-hub
Note: Organizes the eight practical inputs, inline versus right-angle layout decision, KC37 to KC187 range, and mounting choices used in the selection review.
This post was reproduced from: https://www.dietershandel.com/2026/10/helical-bevel-gearmotor-sizing-five.html
