The size and therefore the price of a gearbox depend on the gear-rating method specified. This comparison of eight commonly used standards published by the American Petroleum Institute (API), American Gear Manufacturers Association (AGMA) and the International Organization for Standardization (ISO) highlights the differences between them. Let us begin with a brief description of these standards:
API 613 5th edition (2003): Special Purpose Gear Units for Petroleum, Chemical and Gas Industry Services. Since its initial publication in 1968, most of the main gearboxes in refineries have had to conform to this specification. If you specify this standard, you will probably pay substantially more for the gearbox than if another standard is used. API 613 covers not only gear rating, but also related lubricating systems, controls, and instrumentation. The conservative rating stems mainly from basing the material allowable stresses on the lowest grade materials (grade 1) from the American Gear Manufacturers Assn. (AGMA) standard that was in effect in 1977, even though use of the better “grade 2” materials is now required. Although AGMA material-allowable stresses have increased over the years to reflect increasingly stricter metallurgical requirements, improved metallurgy and extensive field experience, the API ratings have remained unchanged.
The sixth edition of API 613 is currently in development, and should be published sometime this year. It will not change the basic rating method or material allowables. However, it does incorporate language to allow the use of alternate rating methods if the API method would result in excessive pitch line velocity or excessive face width.