Design study of a split-torque helicopter
transmission
G White
Transmission Research Inc., Cleveland, Ohio, USA
Abstract: A helicopter transmission with split-torque drive trains to the main rotor and tail rotor drive
shaft is described. A design based on two 1200 kW engines supplying torque to a 350 r/min main shaft
demonstrates a weight reduction of 40 per cent, extreme low height and reduced losses.
Four double-helical pinions drive a combining gear and provide a 10:1 reduction ratio to the main shaft;
flexible torque tubes, set by fine-difference splines, control the division of torque between pairs of pinions.
Rotor loads bypass the transmission housings and pass to the fuselage through a large-diameter mast that
encloses an integral sump and cooler.
Keywords: helicopter transmission, split-torque gearbox, lightweight gears, high-ratio gears
1 INTRODUCTION
During the past five decades a variety of speed reduction
gears have been developed for turning the main rotor of a
helicopter. These different designs, employing from three
to six stages of gearing, evolved from attempts to improve
reliability and operational life while reducing the fraction
of transmission to aircraft weight.
One outcome of these efforts is the realization that a
major improvement in the established ratio of output torque
to weight is not feasible by refining the details of an
existing design, for the development cycle of materials and
components is mature. There remains, however, the pos-
sibility of weight reduction by a change in drive train
configuration, obtained from an alternative combination of
gear sets in the engine to main rotor driveline. This aspect
of helicopter transmission design is addressed.
2 GEAR TRAIN ARRANGEMENT
A gear arrangement with the potential to deliver a weight
benefit relative to planetary designs is the split-torque type,
so named on account of torque being divided between
parallel sets of fixed-axis gears. Applied to helicopters in
recent years [ 1±4], with a history spanning early watches
[4] and presses to marine propeller drives [5], such
transmissions terminate in a number of equally loaded
pinions that drive a combining gear and output shaft. The
higher the number of pinions that can be positioned around
the gear, the greater are the benefits realized in respect of
the speed reduction ratio, output gear torque capacity and
weight reduction. A number of split-torque gears with from
two to five torque-balanced pinions downstream of each
engine were developed and reported in reference [6]; the
present design with two pinions per engine corresponds to
the simplest of these arrangements.
Planetary reduction gears, commonly employed at the
final reduction stage of a helicopter transmission, also
divide torque between planet pinions, but the reduction
ratio from a five-planet assembly is only 4.6:1, and higher
ratios obtained with either fewer planet pinions or stepped
pinions bring a major weight gain. Conversely, two low-
ratio units in series reduce weight but introduce higher
losses and component totals.
To bypass theselimitations the design described adopts a
double-helical gear at the output stage. This gear brings the
ability to fit pinions of greater length than diameter which,
in combination with reduced tooth loads, allows a speed
ratio about twice that of a simple planetary unit and
concurrent reductions in gear weight, bearing weight and
losses.
Figures 1 and 2 show in outline the double-helical output
stage incorporated in a twin-engine transmission that
includes the following:
(a) at each engine input, an overrun clutch and single bevel
train;
(b) at each intermediate reduction stage, dual offset spur
gearsthat are driven by a single pinion;
117
The MS was received on 6 January 1998 and was accepted for publication
on 6 April 1998.
G00198 # IMechE 1998 Proc Instn Mech Engrs Vol 212 Part G