Short answer: Gearbox catalog ratings are published at a
service factor of 1.00, which AGMA calls Class I service. The service factor is the multiplier that accounts for your real duty. Multiply the motor's nameplate power by it, and the result must not exceed the gearbox's mechanical rating at the actual input speed. Pick the factor from the gearbox maker's table using three inputs: what the machine is, how many hours a day it runs, and how often it starts, stops or reverses.
A gearbox sized at a 1.00 service factor on a shock-loaded, 24-hour machine is carrying more than its catalog rating every hour it runs. The arithmetic is simple. The error to avoid is a factor picked by habit, such as "1.5 to be safe" or the number used on the last job, instead of one read from the table for this machine and this schedule.
This guide walks through how AGMA service factor selection works in a real manufacturer's catalog. It uses Winsmith's published worm gear tables as the worked reference.
What the service factor actually does
Winsmith states that it publishes its ratings in accordance with AGMA Class I service, a service factor of 1.00. When selecting from the catalog, the service factor for the application and prime mover is applied to the
input power or the prime mover's nameplate power. The result is the
design power, and it must not exceed the unit's mechanical rating at the proper input speed.
Design HP = motor nameplate HP × service factor
Two limits sit outside this calculation. Winsmith states that service factors are
not applied to thermal ratings, so a continuously running unit still needs a separate thermal check. Overhung loads on the shafts from sprockets, sheaves or gears are checked separately too, although Winsmith applies the same service factor to them.
Reading the application table
The application table lists machine types down the side and three daily-duty columns across the top: up to 3 hours a day, 3 to 10 hours, and over 10 hours. "Hours" means total operating time per day, not shift length. A few conveyor-related rows from Winsmith's worm gear table, for an electric or hydraulic motor:
- Conveyor, uniformly loaded or fed: up to 3 hr/day not listed · 3–10 hr/day 1.00 · over 10 hr/day 1.25
- Conveyor, not uniformly fed: up to 3 hr/day 1.00 · 3–10 hr/day 1.25 · over 10 hr/day 1.50
- Conveyor, reciprocating or shaker: up to 3 hr/day 1.25 · 3–10 hr/day 1.50 · over 10 hr/day 1.75
- Bucket elevator: up to 3 hr/day 1.00 · 3–10 hr/day 1.25 · over 10 hr/day 1.50
- Screw feeder: up to 3 hr/day 1.00 · 3–10 hr/day 1.25 · over 10 hr/day 1.50
The same machine moves up the table as its hours increase, and a rougher version of the same machine moves up again. A conveyor that is fed unevenly and runs around the clock is a 1.50 application. The same conveyor fed evenly for 8 hours a day is a 1.00 application. Some rows are marked
RF, "refer to factory". Those applications are not covered by a standard factor and go to the manufacturer.
When the machine is not on the list: load classification
If your machine is not listed, Winsmith also publishes an AGMA chart based on load classification. You choose the driven machine's load class (uniform, moderate shock, heavy shock or extreme shock) and the daily duration:
- Occasional ½ hour: uniform 1.00 · moderate shock 1.00 · heavy shock 1.00 · extreme shock 1.25
- Less than 3 hours: uniform 1.00 · moderate shock 1.00 · heavy shock 1.25 · extreme shock 1.50
- 3–10 hours: uniform 1.00 · moderate shock 1.25 · heavy shock 1.50 · extreme shock 1.75
- Over 10 hours: uniform 1.25 · moderate shock 1.50 · heavy shock 1.75 · extreme shock 2.00
Starts, stops, peaks and reversing
The application table assumes normal starting. Winsmith's explanatory notes set out where that assumption ends:
- Frequent starts and stops means more than 10–20 starts per hour. These applications use a separate, higher chart. For example, a uniform load over 10 hours a day goes from 1.25 to 1.50, and a heavy-shock load over 10 hours goes from 1.75 to 2.00.
- Peak loads: normal starting, or occasional momentary peaks two or three times a day of up to 300% of catalog rating at 1,800 rpm, are permissible. If either is exceeded, use a service factor of 1.5.
- Heavy starting loads: where the output shaft is coupled directly to large gears or heavy masses, use a service factor of 2.
- Reversing, repeated severe shock, stalling, overrunning or "wind-up" drives are not covered by the tables at all. Winsmith recommends a service factor of 3 and asks that each case be referred to its engineering department.
Engine-driven equipment
The tables assume an electric or hydraulic motor. For an internal combustion engine, Winsmith converts the factor upward:
- Motor factor 1.00 becomes 1.50 for a single-cylinder engine, or 1.25 for a multi-cylinder engine.
- Motor factor 1.25 becomes 1.75 for a single-cylinder engine, or 1.50 for a multi-cylinder engine.
- Motor factor 1.50 becomes 2.00 for a single-cylinder engine, or 1.75 for a multi-cylinder engine.
- Motor factor 1.75 becomes 2.25 for a single-cylinder engine, or 2.00 for a multi-cylinder engine.
- Motor factor 2.00 becomes 2.50 for a single-cylinder engine, or 2.25 for a multi-cylinder engine.
Worked example
Winsmith's catalog sizes a reducer for a non-uniformly loaded belt conveyor running 24 hours a day. A 3/4 HP (about 0.56 kW) motor at 1,750 rpm drives the headshaft at 30 rpm.
- Service factor from the table: conveyor, not uniformly fed, over 10 hours =
1.50.
- Ratio: 1,750 ÷ 30 = 58.33, so a standard 60:1 is used. The
gearbox ratio calculator handles this step.
- Design power: 0.75 HP × 1.50 =
1.125 HP (about 0.84 kW).
- Select the unit size whose mechanical rating at 60:1 and 1,750 rpm input meets 1.125 HP in the 1.00 service-factor column. In the catalog that is a size 930, and its thermal capacity is not exceeded.
Rerun step 1 with the conveyor fed evenly for 8 hours a day and the factor drops to 1.00. The design power becomes 0.75 HP. That is why the hours and the loading pattern need to be stated, not assumed.
One table per gearbox
The figures above are Winsmith's worm gear tables. Other manufacturers, and other gear types such as helical and bevel units, publish their own tables and rating bases. When comparing gearboxes from different makers, apply each maker's factor to that maker's rating. Do not carry one company's number into another company's catalog. As an independent distributor working across manufacturers, we run that comparison from each manufacturer's own catalog.
A service factor also does not predict how long a gearbox will last, and it does not fix a wrong torque, speed or ratio. It only scales the duty so it can be compared fairly with a 1.00 catalog rating.
What to send us for a gearbox selection or quote
- Gearbox make and full model or part number, plus a nameplate photo
- Motor HP and rpm, and whether it is electric, hydraulic or an engine
- What the gearbox drives, and whether the load is even, uneven or shock
- Hours of operation per day
- Starts per hour, and whether the drive reverses
- Required output speed
Frequently asked questions
What does a service factor of 1.00 mean?
It is the basis the catalog ratings are published on: AGMA Class I service. Any application rougher or longer-running than that basis uses a factor above 1.00.
Is a service factor the same as a safety factor?
No. A service factor adjusts the rating for a known duty: hours, load type, starts and prime mover. It is read from a table, not chosen as a margin.
Can I use one manufacturer's service factor chart for another brand?
No. Factors are tied to each manufacturer's own rating basis. Use the table for the gearbox you are actually selecting.
Does the service factor apply to the thermal rating?
Not in Winsmith's catalog. Thermal capacity is checked separately for continuous duty.
Summary
Catalog ratings assume a service factor of 1.00. Read the real factor from the gearbox maker's table using the machine type, hours per day, starts and prime mover. Multiply the motor power by it, and choose the size that carries the result at the actual input speed. Then check thermal capacity and overhung load separately.
Sources & References
Send us your motor, machine and duty details and we will match a gearbox from our
gearing products to the right service factor.
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