**2. Applications**

Gear transmission requirements for aircraft are very demanding, with a standard gear ratio between engine and rotor of 60:1 (Krantz, 1996). Moreover, the gear transmission system should be safe, reliable, lightweight and vibration-free. One of the most limiting factors is weight and there are three fundamental transmission parameters that greatly affect this factor:


In helicopters, planetary gear systems are typically used for the final transmission stage, with planets consisting of between 3 and 18 gears and with planetary gearing transmission ratios between 5:1 and 7:1 (Krantz, 1996; White, 1989).

Using split-path arrangements with fixed shafts in the final transmission stage is a relatively recent development that offers a number of advantages over conventional systems, being several of them based on weight reduction for the overall transmission:

 It allows torque to be transmitted through various paths. This is a major advantage because when torque is split, the contact force between teeth is less and, hence, smaller, lighter gears can be used, therefore reducing the overall weight. Split torque however has the disadvantage that the torque must be shared equally between the paths. The problems associated with split torque are discussed in Section 4.

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

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

 The geometric limitation of the four-gear assembly that requires simultaneous engagement for all four gears. Note that the four gears do not mesh correctly in just any position, although they may seem to do so initially. We will describe the conditions for

 Torque split between the two gearbox paths must be as balanced as possible to ensure that neither of the paths is overloaded. The technology available to ensure proper

Gear transmission requirements for aircraft are very demanding, with a standard gear ratio between engine and rotor of 60:1 (Krantz, 1996). Moreover, the gear transmission system should be safe, reliable, lightweight and vibration-free. One of the most limiting factors is weight and there are three fundamental transmission parameters that greatly affect this

1. The number of transmission stages. The greater the number of stages used to achieve the final gear ratio, the heavier the transmission, given that more common elements

2. The number of transmission paths, the basis for split torque gearboxes. Torque is divided between several transmission paths, resulting in a contact force in the smaller

3. The final stage transmission ratio. Using a greater transmission ratio in the final stage enables weight to be reduced. This is because torque in previous steps is lower, making

In helicopters, planetary gear systems are typically used for the final transmission stage, with planets consisting of between 3 and 18 gears and with planetary gearing transmission

Using split-path arrangements with fixed shafts in the final transmission stage is a relatively recent development that offers a number of advantages over conventional systems, being

 It allows torque to be transmitted through various paths. This is a major advantage because when torque is split, the contact force between teeth is less and, hence, smaller, lighter gears can be used, therefore reducing the overall weight. Split torque however has the disadvantage that the torque must be shared equally between the paths. The

gear that means that smaller, and consequently lighter, gears can be used.

happens, for example, in tunnel boring machines.

**2. Applications** 

factor:

two most restrictive aspects of their application, namely:

simultaneous meshing of the four gears in general terms below.

torque split between two paths will be discussed below.

such as shafts and bearings are necessary.

ratios between 5:1 and 7:1 (Krantz, 1996; White, 1989).

several of them based on weight reduction for the overall transmission:

problems associated with split torque are discussed in Section 4.

it possible to use smaller gears.


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 number of advantages over traditional gear systems.

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 helical gears.
