This International Standard specifies the minimum requirements for enclosed, precision, single and double helical, one- and two-stage speed increasers and reducers of parallel shaft design with pinion speeds of 3000 min?1 or greater, or pitch line velocities of 25 m/s or greater, for special purpose applications. Such applications will typically be required to operate continuously for extended periods, without installed spare equipment and are critical to the continued operation of the installation. By agreement this International Standard may be used for other services. This International Standard also specifies a method of rating gears which meet the following criteria: a) gear accuracy ? teeth accuracy: accuracy grade 4 or better as given in ISO 1328-1:1995, for both single pitch deviation, fpt, and total cumulative pitch deviation, Fp, ? total helix deviation F? between the helices of the pinion and wheel: accuracy grade 4 or better as given in ISO 1328-1:1995; b) range of the transverse contact ratios: 1,2 < ?? < 2,0; c) overlap ratio ?? W 1,0; d) helix angle: 5 u ? u 35°; e) working flanks of the pinion or gear: provided with profile modifications to obtain a good conjugate tooth load distribution along the path of contact; f) working flanks of pinion or gear: modified as necessary to compensate for both torsional and bending deflections and, when necessary for gears with pitch line velocities in excess of 100 m/s, also for thermal distortions; g) gear lubrication: straight mineral oil, viscosity grade VG-32 or VG-46 (see ISO 3448); h) material of the gear teeth: quality MQ or better, in accordance with ISO 6336-5:1996.