**1. Introduction**

54 Mechanical Engineering

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Although the simplest gear systems are those with just one gear engagement area between a pair of gears, alternatives are available for applications where it is necessary to transmit a very high torque in a very small space. One option to increase power density is to use the split torque systems that were mainly developed for the aviation industry. These gear systems are based on a very simple idea: division of the transmission of force between several contact areas, thereby increasing the contact ratio. This gives rise, however, to the problem of meshing four gears (Fig. 1).

Fig. 1. (a) Standard gearbox assembly; (b) Split torque gearbox assembly

Split torque gearboxes are configurations where a driving pinion (1) meshes with two intermediate idler pinions (2, 3), which simultaneously act on another gear (4). From now on, this assembly will be called four-gear meshing. In this case, the torque split is from gear (1) to gears (2) and (3) which engage gear (4). This gear assembly results in the reduction in gear speed causing an increase in available torque; hence, the split torque transmission means we can use smaller gears.

Split Torque Gearboxes: Requirements, Performance and Applications 57

 It allows transmission path redundancy. Thus, if one transmission path fails during flight, operation can always be assured through another path. In many cases, consequently, gear transmissions are sized so that a single path can handle 100% of

 It achieves final-stage transmission ratios of around 10:1 to 14:1 (Krantz, 1996; White, 1989). This improvement over the 5:1 to 7:1 ratios for planetary gearboxes (Krantz, 1996; White, 1989) is reflected in a corresponding reduction in the weight of the transmission

Several patents for transmission systems that apply split torque have been filed by Sikorky Aircraft Corporation and McDonnell Douglas Helicopters (Gmirya & Kish, 2003; Gmirya, 2005; Craig et al., 1998) that refer either to complete or improved power transmission systems from the rotorcraft or aircraft engine to the rotor or propeller. Other studies that describe various aspects of split torque transmission systems, particularly their use in helicopter gearboxes (White, 1974, 1983, 1989, 1993, 1998), conclude that such gears have a

Below we describe two helicopter transmission systems that use multiple path gearboxes. The first is a helicopter gearbox used for laboratory tests of torque divided into two stages, and the second is a commercial helicopter three-stage gearbox that combines bevel, spur and

The gear transmission described below was used to perform numerous tests on the operation of split-torque transmissions (Krantz et al., 1992; Krantz, 1994, 1996; Krantz & Delgado, 1996), which can be considered a standard for aeronautical applications. The full

engine power.

number of advantages over traditional gear systems.

**2.1 Helicopter gearbox for laboratory testing** 

Fig. 2. Helicopter transmission for laboratory testing

assembly is depicted in Fig. 2.

system.

helical gears.

The greater the number of gears that engage the same crown, the lower the torque exercised by each pinion. Gear assembles can have up to 14 gears engaging a single crown, as happens, for example, in tunnel boring machines.

This chapter explores four-gear meshing in a gear assembly that ensures a 50%torque split for each meshing area. Split torque gears are studied from two perspectives: first, the most common applications of split torque gearboxes in the aeronautical sector and second, the two most restrictive aspects of their application, namely:

