14 - Gears

Abstract

This chapter focuses on gearing system used in gas turbines. There are basically two types of gearing systems, single- and double-helical gearing. Double-helical speed gear is the first choice for calculating the accuracy of loading and smoothness of operation. Predictable performance renders unnecessary any deviations from easily defined and measured geometry. These gear sets are more efficient and have unmatched reliability if properly applied. In single-helical gearing, all forces externally generated have to be added to the thrust produced by the gear itself, and the total is used on each shaft to select the high-speed shaft thrust beating. An error in the thrust or beating capacity estimation may result in frequent failures of the thrust beating, or the associated shafting. The major factors affecting gear performance are pressure angle, helix angle, tooth hardness, scoring, gear accuracy, beating types, service factor, gear housing, and lubrication. The decision regarding the pressure angle is made by the designer early in the design stage. Helix angles vary from 5º to 45º, and are selected to obtain a minimum overlap ratio and to provide good load sharing. An increase in helix angle increases the thrust, and this increase is the main reason for the lower helix angles in single-helical gearing. The advantages of single-helical gearing are greater accuracy, less sensitivity to coupling thrust, no measurable apex runout, and it is less expensive to cut teeth. The disadvantages of single-helical gears are that they require expensive thrust bearing and thrust faces, and are less efficient due to heat load on the thrust bearing.