Gear Torque Ratings – AGMA and ISO Gear Standards

Gear torque ratings always seem to be a source of misunderstanding. While all engineers know what a "foot-pound" or "Newton-meter" of torque is, many have difficulty in understanding the torque rating of a gearbox.

For example, commercial gearbox "S" has a torque rating of 3,000 foot-pounds (4.068 Nm) and industrial gearbox "D" has the same torque rating. Are these gearboxes equivalent? The answer is: "No."

Projecting Life Spans

Commercial gearboxes are rated with a L10 bearing life of 5,000 hours, or seven months. Industrial gearboxes are rated with a L1 0 life of 100,000 hours or 11 years. Even when the commercial gearbox service factors are applied for "continuous duty," the life will not approach that of the industrial rated gearbox.

Note that in the case above, both gearboxes have American Gear Manufacturers Association (AGMA) torque ratings. The difference is that AGMA and International Standards Organization (ISO) have specific standards for "Enclosed Commercial Gearboxes." These standards spell out a minimum bearing life of 5,000 hours. The gearing may have sufficient power or torque capacity but when the bearings fail, so do the gears. Industrial gearboxes will use gearing designed per AGMA standards and support the gears on large heavy-duty bearings that will provide a life of 100,000 hours.

Wastewater equipment operators should require industrial 100,000-hour ratings for all equipment that is intended for continuous duty service.

AGMA and ISO Gear Standards

AGMA and ISO publish the two most common standards for rating gearing. These two gear rating systems are similar but not identical. That is, a gear rated per AGMA standards would not have the same torque and power rating as the same gear rated per the ISO standards. Typically, the ISO standards provide a higher torque and power rating than do the AGMA standards.

For typical wastewater and industrial gear drives, spur or helical gearing is normally used. The AGMA standard for spur and helical gears is AGMA standard 2001-D04, and the ISO standard is 6336. These two standards are under review by the international committees that oversee these standards, in an effort to form one integrated international standard for rating gears. Hopefully, in the future we will have a unified gear standard. Until then, DBS can provide gear ratings that meet AGMA or ISO standards — whichever our customers require.