Definitions Formulas


Torque is a unit of measure used to represent the twisting or turning effort around an axis. Torque is usually expressed in pound-feet, pound-inches, kilogram-meters or Newton-meters.

Horsepower is a unit of power equal to 33,000 foot-pounds per minute or the rate of work required to raise 33,000 pounds one foot in one minute.

RPM is an abbreviation for revolutions per minute.


Torque (in-lbs) = HP x 63025 / RPM
HP = Torque x RPM / 63,025
RPM = HP x 63,025 / Torque (in-lbs)

Service Factor

Before an enclosed gear drive can be selected for an application, an “equivalent power rating” must be determined. This can be done by multiplying the specified transmitted power by the service factor. Since service factors represent the normal relationship between gear unit design power rating and the maximum potential transmitted power, it is suggested that the service factor be applied to the nameplate rating of the driven machine or prime mover as applicable.

Manufacturer and user must agree upon which power, prime mover rating or driven machine requirements, should dictate the selection of the gear drive. It is necessary that the gear drive selected have a rated load capacity equal to or in excess of this “equivalent power rating”. Refer to Service Factor Table* for application service factor.

Extracted from ANSI/AGMA 6010-F97, Standard for Spur, Helical, Herringbone and Bevel Enclosed Drives, with the permission of the publisher, the American Gear manufacturers Association, 1500 King Street, Suite 201, Alexandria, Virginia 22314

We are anxious to assist you in determining an adequate service factor for your application. Tables are available to provide guidance in determining service factors, or they may be determined by an analytical method. Important factors to consider if using the analytical method include operational characteristics, system conditions and any special considerations.

Constant Horsepower versus Constant Torque

Different applications have different horsepower and torque characteristics. Two common types are constant torque and constant horsepower. When selecting an industrial gearbox, it is important to distinguish between the two in order to properly size a gearbox and it’s components.

Constant Horsepower – In this situation, the torque rating on the output shaft is the torque rating on the input shaft multiplied by the reduction ratio. This assumes full motor horsepower is required at the output shaft.

When multi-speed gearboxes are used for torque multiplication, the torque at the input shaft is multiplied by the reduction ratio. An extreme example would be the raising of a draw bridge. A surprisingly small motor can be used when coupled to sufficient speed reduction thus moving a tremendous load slowly.

Constant Torque – In this situation the torque rating required on the output shaft is the same as the input shaft.
When multi-speed gearboxes are used in constant torque applications, they are used only as speed reducers. An example would be a conveyor system which needs to move at one speed to fill small boxes and a slower speed to fill large boxes.