1. The Motor and the Gearbox Are One Discussion
The gearmotor, a motor and a speed reducer assembled as one unit, is the most common drive in industry, and its selection is where the mechanical and the electrical engineer meet over one shaft. The motor wants to spin fast and deliver modest torque; the load wants high torque and a human speed; the gearbox reconciles the two by trading speed for torque, and the gearmotor is the discipline of doing that trade once, honestly, on a selection sheet rather than on a hot motor.
This article is a reference for that selection. Section 2 fixes the vocabulary. Section 3 covers the load and the speed ratio. Section 4 covers the torque curve and the mechanical rating. Section 5 treats the thermal reality. Section 6 closes with the service factor and a sizing procedure with a worked example.
2. The Vocabulary of the Geared Drive
| Term | Definition | Where It Bites |
|---|---|---|
| Ratio | Input speed divided by output speed | The headline number of the gearbox |
| Rated torque | Torque the gearbox can carry continuously | The mechanical ceiling, from the catalog |
| Overhung load | Radial force at the output shaft from the pulley or sprocket | Limits the shaft and bearing, often above the torque |
| Service factor | Multiplier applied to the load for shock and duty | The safety margin against the catalogs |
| Thermal capacity | Heat the gearbox can dissipate without overheating | The limit that torque tables hide |
Three of the five terms matter because they are the ones the catalog table assumes away: the rated torque assumes a clean, steady load; the service factor restores the reality; and the thermal capacity is the number that makes a correctly sized gearbox run hot anyway.
3. The Load and the Ratio: Where the Selection Begins
The selection begins with two numbers that live on the machine, not in the catalog: the required output speed and the required output torque at the load shaft. The ratio is then the motor speed divided by the required output speed, and the motor power is the required torque times the output speed divided by the efficiency, converted by the usual constants.
The load has a personality, and the personality decides how big the margin must be.
- [ ] The steady load: a conveyor at constant speed, where the torque is flat and the shock is small, and the service factor stays modest.
- [ ] The inertial load: a rotating mass that must accelerate and stop, where the starting torque and the stopping stress exceed the steady value, and the sizing must cover the peaks.
- [ ] The shock load: a crusher, a press or a mixer, where the torque spikes irregularly, and the shock is absorbed by a service factor that the catalog warns about.
- [ ] The reversing load: a machine that cycles direction, where the gear teeth see the load on reversing flanks and the backlash and the bearing take the punishment.
The ratio and the load personality together fix the required torque at the gearbox output, and the honest first move is to write that number with the acceleration and the shock factors already inside it, so the catalog comparison, when it comes, is a comparison of like with like.
4. The Motor Torque Curve and the Mechanical Rating
The motor brings a torque curve, not a single number, and the skill of the integration is reading the curve against the load. The motor delivers a starting torque, a breakdown torque where the curve peaks, and a rated continuous torque where it can run forever, and the honest integration matches three things. The available torque must exceed the load torque at the operating speed; the starting and the peak torques must clear the acceleration and the shock peaks; and the continuous rating must survive the duty cycle, because a motor that peaks fine but runs hot is the classic mis-selection.
The gearbox side brings its own rating, the rated torque of the unit, and the rule of the integration is that neither side is allowed to be the weak link. The gearbox rated torque must exceed the motor’s maximum developed torque, not merely its rated torque, because the gear teeth are sized for the torque, and the motor that can deliver a peak above the gearbox rating will eventually fatigue the teeth even at a low average load. The overhung load, the radial force that rides on the output shaft through a pulley or a sprocket, is checked separately, and it is the check that surprises the most people, because a small gearmotor driving a heavy sprocket can fail the overhung load long before it fails the torque.
The classic sizing sentence: the gearbox is sized by the peak, the motor is sized by the RMS, and the thermal check decides whether the pair survives the shift.
5. The Thermal Reality: When the Sizing Table Lies
Every gearmotor catalog prints a table of rated torques, and every table hides a condition: the rating assumes adequate cooling, a continuous running duty, and an ambient temperature near twenty-five degrees Celsius. Run the unit at high ambient, at low speed with poor airflow, or with a heavy duty cycle, and the effective thermal capacity drops while the table number stays printed. The thermal check is the question “can this unit get rid of the heat this duty generates?” and it is the check that separates the shopper from the integrator.
The heat comes from two places. The motor dissipates I squared R losses plus iron losses, and the gearbox dissipates meshing and churning losses; both scale with load and speed, and both are routed into the same housing and out through the same surface. The practical consequence is a short list of thermal rules.
- The torque that can be sustained continuously at low speed is lower than the catalog rating, because the fan on the motor cools the motor, not the gearbox, and at low output speed the gearbox loses its own airflow.
- High ambient temperature cuts the allowable duty directly, so the unit sized for a clean factory must be rechecked for a furnace-adjacent machine.
- The cooling mode matters: a fan-cooled motor can sustain more than a naturally cooled one, and an externally cooled gearbox more than a ribbed, naturally ventilated case.
- The duty cycle is the honest test: a unit that runs half the time can carry a heavier peak than a unit that runs constantly, because the resting time is part of the thermal budget.
The thermal reality is the reason the same gearmotor that sizes correctly on paper can trip its thermal overload in a hot corner of the plant, and the integrator either catches it in the calculation or the maintenance log catches it later.
6. The Service Factor: The Margin and Its Honest Size
The service factor is the multiplier that restores the real world onto the catalog rating, and it is the most argued single number in the design office because its value is a judgment, not a lookup. The manufacturers publish tables that tie the factor to the load class and the duty cycle, and the integrator’s job is to read those tables against the actual machine, not against the warranty of the brochure.
| Load Class | Typical Duty | Service Factor Range |
|---|---|---|
| Uniform, light | Conveyor, fans, light mixers | 1.0 – 1.25 |
| Moderate shock | Machine tools, agitators, pumps | 1.25 – 1.5 |
| Heavy shock | Crushers, presses, heavy mixers | 1.5 – 2.0 |
The rule of the factor is that it applies to the required output torque before the selection, so the chosen unit is the one whose rated torque clears the multiplied load. Three cautions complete the picture. First, the factor is not a license to ignore the peak: the peak overhung load and the starting torque are still checked against the unit’s maximums. Second, the factor and the thermal check interact: the same unit chosen for the same load can be comfortable under a uniform duty and marginal under a reversing one, because the shock adds heat as well as stress. Third, the factor should be written on the selection sheet, because the reader of the sheet, the maintainer or the next engineer, needs to know what margin the design committed to, and an undocumented factor is a silent bet against the future.
7. The Sizing Procedure: An Eight-Step Method
Bring the discipline into a repeatable method that any project engineer can run.
- State the output requirement: write the required output speed, the required output torque at the load shaft, and the direction, and separate the steady part from the peak part.
- Identify the load class: judge the duty as uniform, moderate shock or heavy shock, and note whether it reverses or starts frequently, because the class sets the service factor.
- Select the ratio: divide the motor nominal speed by the required output speed, and round to the nearest standard ratio, watching the effect the rounding has on the output speed.
- Size the motor: choose the motor whose torque curve clears both the steady torque and the peak, and check the RMS torque against the continuous rating over the duty cycle.
- Size the gearbox by the peak: choose the gearbox whose rated torque clears the service-factored load and the motor peak, so the teeth survive the worst torque the motor can deliver.
- Check the overhung load: calculate the radial force from the pulley or sprocket at the output shaft and compare it with the catalog allowance, using an effective load factor when the force is offset.
- Check the thermal capacity: estimate the heat from the duty cycle, the ambient and the cooling mode, and confirm the unit can reject it continuously; if not, upsize or add cooling.
- Reconcile and document: confirm the chosen unit satisfies every check, write the service factor, the ratios and the margins on the selection sheet, and record the assumptions, because the next engineer inherits the sheet.
8. A Worked Example: The Conveyor Drive
Size a gearmotor for a conveyor that must move a load at 0.5 metres per second, requiring 60 N.m at the output shaft, running continuously for ten hours a day in a clean factory at light shock.
- Requirement: 60 N.m at the output, at the belt speed converted to about 30 revolutions per minute at the driven pulley, running constantly.
- Load class: uniform with light shock, so a service factor near 1.25 covers the start and the occasional jam; the duty is continuous, so the thermal check will be the demanding one.
- Ratio: a four-pole motor runs near 1450 revolutions per minute; dividing by 30 gives a ratio near 48, rounded to the standard 50, with the resulting output speed accepted by the process.
- Motor: the required power is the torque times the speed converted to about 0.2 kilowatts; a 0.37 kilowatt motor is chosen, and its torque curve is checked to clear the start of the conveyor with its load.
- Gearbox: the required torque of 60 N.m is multiplied by the service factor 1.25, giving 75 N.m, and the gearbox with a rated torque comfortably above 75 N.m at the ratio 50 is selected, so the teeth keep their margin against the motor peak.
- Overhung load: the pulley produces a radial force at the output shaft, checked against the catalog allowance, and since the drive is close-coupled and tensioned properly, the force clears with margin; if it had not, a heavier output bearing or a supported shaft would have been specified.
- Thermal: at ten hours continuous running in a clean ambient, the unit’s thermal capacity is checked against the heat of the constant duty; a fan-cooled motor and a ribbed gearbox reject the heat comfortably, and the unit is passed on temperature.
- Documentation: the selection sheet records the required 60 N.m, the service factor 1.25, the ratio 50, the motor 0.37 kilowatts and the gearbox rating, together with the assumption of light shock and the clean ambient, so a future revision can be judged against a stated baseline.
The worked example is the article compressed: the load named with its personality, the ratio fixed, the motor sized by the curve, the gearbox sized by the peak, the overhung load checked, the thermal reality confronted, and the margin written down. The gearmotor integration succeeds not because any single calculation is dramatic but because every check is honest, and the honest checks are what keep the pulley turning, the housing cool and the selection sheet safe to inherit. The motor and the gearbox, sized and documented as one machine, become the quiet, repeatable drive that the plant forgets, in the best sense, is there at all.
