Most gearmotor orders start with horsepower and end with a shaft. The two numbers that actually decide whether the drive lives or dies get filled in somewhere in the middle, often by guesswork. Here is the order that keeps a plant out of trouble.
Step One: Ratio
Ratio is input speed divided by output speed. If a 1,750 rpm motor has to turn a conveyor head pulley at 70 rpm, the ratio is 25:1. That is the whole calculation — but note what it depends on. The output speed requirement has to come from the machine, not from whatever ratio was on the shelf last time.
Step Two: Output Torque
Output torque is roughly motor torque multiplied by ratio, less the losses in the gear train. A motor rated 10 ft-lb at its running speed through a 25:1 reducer produces something in the neighborhood of 250 ft-lb at the output before efficiency losses, not 250 ft-lb at the motor. Getting that multiplication right is what keeps you from buying a drive that is nominally the correct ratio but structurally too light for the load.
Step Three: Service Factor
This is the step that gets skipped, and it is the step that determines life. Catalog ratings are published for a defined, comparatively gentle duty. Real machines add shock loading, frequent starts, reversing, high ambient temperature, and hours per day that the catalog never saw.
The service factor is a multiplier applied to the required output torque to arrive at the rating the unit must actually carry. The classic application table runs from smooth, uniform loads at the low end to heavy shock at the high end, with separate columns for hours per day and starts per hour. A uniform-load conveyor running eight hours a day and a crusher running around the clock with heavy shock are not the same application, even at identical torque.
Practical rule: size the gearbox on factored torque, not on the motor nameplate. The motor is usually the cheap part; the reducer is the one that grinds itself apart.
Step Four: The Loads the Ratio Does Not Cover
Two more checks catch most field failures:
- Overhung load — a sprocket or sheave mounted on the output shaft applies a bending load the catalog rating may not cover. Long centers and heavy chain make it worse.
- Thermal capacity — a reducer in an enclosure, at high ambient, has a lower real capacity than the mechanical rating suggests. Heat, not torque, is often what actually kills it.
Step Five: Mounting and Accessory Match
The drive is only as good as what bolts to it. Sheaves, sprockets, bushings, couplings and chain have to be matched to the output shaft and to the duty. That is where an integrated supplier saves a plant real time, because sizing the reducer in isolation from the driven components is how you end up with a correct gearbox and a wrong installation.
Where RBC Fits
RBC Industrial supplies motors and gearboxes alongside the power-transmission components that bolt to them — sheaves, sprockets, gears, bushings, couplings and chain from the Martin, Morse and Browning lines. If you give us motor speed, required output speed, duty cycle and the driven load, we will size the reducer on factored torque and hand you the complete drive.
RBC Industrial · (915) 845-8188 · www.rbc-industrial.com