4.1.6.1 Vehicle final drive gearsFo

4.1.6.1 Vehicle final drive gearsFor vehicle final drive gears, which are relatively low speed, coarse pitch teeth, are chosen for adequate strength. As a consequence, pinions have small numbers of teeth (z1 of about 14), whereas a value z1 of about 28 would be chosen for a comparatively high speed gear of similar size. Thus, the tooth bending strength of the former would be about twice that of the latter.The computed reliability of vehicle gears can be as low as 80 % to 90 % whereas that of high speed industrial gears should be at least 99 %.In general, the material used in high volume vehicle gear production may be of more uniform quality than that used for gears produced in small numbers.Comparison of applied gear designs has indicated that for about 10 000 cycles, the load transmitted by truck final drive gears is about four times greater than that transmitted by aircraft or space vehicle gears, where the material, quality, size and design are the same.For low speed vehicle gears which are intended to have short lives (less than 100 000 cycles), small amounts of plastic deformation, pitting and abrasive wear can usually be tolerated. Consequently, the levels of surface stress which are permissible are substantially higher than would be permissible for long life, high speed gears.4.1.6.2 Main drive for aircraft and space vehiclesFor main drives of aircraft and space vehicles, which are found in helicopter rotor drives and the main pump drives of space vehicle boosters, gears of the highest quality material and accuracy are used. Such gears are extensively tested. For example, 10 to 20 transmissions of the same production series may be tested under operational conditions for the full design life. The tolerable wear rate is established on the basis of test results. Lubricant spray rate, position of injection points and direction of spray is optimized.For these reasons, higher loading is permissible for a design life up to 100 times longer (in cycles of tooth loading), and speeds about 10 times greater than those of a typical vehicle transmission. The probability of damage in such cases shall not exceed 0,1 % to 1 %. Overall loading cannot be as high as for vehicle gears, since neither surface wear nor minor damage can be tolerated.4.1.6.3 Industrial high speed gearsFor industrial high speed gears, where the pitch line velocities exceed 50 m/s, the pinions are often designed with 30 or more teeth with the objective of minimizing the risk of scuffing and wear. A typical gear pair would consist of a pinion with 45 teeth and a wheel with 248.Industrial high speed gears should be better than 99 % reliable for a normal life of more than 1010 cycles. Extensive prototype testing is normally excluded because of the cost. As a consequence, the load capacity ratings of high speed gears tend to be conservative with relatively high safety factors.