HOW THE MAIN GEARBOX (MGB) WORKS IN A HELICOPTER?
Power transfer function from the engine to Main Rotor, Tail Rotor and other necessary accessories is performed by the Main Gear Box in helicopters. The main purpose of a helicopter main gearbox or transmission is to reduce output rpm of the engine to most suitable level for driving the main rotors and tail rotors of a helicopter. For example, a helicopter having an engine output rpm of 6600 but a rotor speed of 300 rpm. This reduction function would be provided by the main gearbox or commonly called Transmission of the helicopter.
The function of a Clutch or Freewheeling Unit is disengaged the Main Transmission Drive from the engine and allow it to drive tail rotor system in case of an Autorotation. The freewheeling unit may be located in the accessory gearbox in some helicopters as it is a part of helicopter transmission or it may be a part of engine accessory gearbox in some cases.
MGB Oil System
Transmissions of helicopters normally have their own lubrication system and functions of which include lubrication, cooling and internal cleaning of it. Level of oil is checked through a sight gauge which may be two or more at different locations. Many main gearboxes have chip detectors located at sump for detection and early warning of the wear and tear stuff. Chips detectors are electrically connected to warning lights in the pilot’s instrument panel that lights up to warn pilots in the event of an internal failure/ wear and tear happening inside the gearbox/ transmission assembly. In modern helicopters, chip detectors may have a burn-off capability which may attempt to correct the situation without a pilot action or remedial measure. Otherwise, pilot has to refer to the specific emergency procedure of that helicopter in order to decide further action.
Normally, located directly forward of the engine, a transmission is joined to isolation mounts on structural support assemblies of the airframe structure. It is further connected to the cold end of the engine through a drive shaft.
Functions. Main purposes which it serves are given as under:-
a. Provides angle change to the drive and speed reduction by a train of spiral bevel gears and one/ two-stage planetary gears in order to drive the rotor mast.
b. A freewheel clutch in the input drive quill coupling disengages to allow the main rotor and gear train to turn freely when the engine is stopped or is idling below rotor driving speed as in case of an auto-rotational.
c. A secondary gear train drives the tail rotor through a shaft supported by hanger bearings or support bearings.
d. Power is transmitted through the 42 and 90-degree gearboxes to the tail rotor.
e. 42-degree gearbox change angle of the drive only and provides a gear ratio of 1:1.
f. 90-degree gearbox changes the angle of drive and provides a gear reduction.
g. The transmission also drives the tail rotor drive quill, the rotor tachometer/ hydraulic pump quill and the cockpit blower quill as accessories on the transmission.
Figure 1: Main Gearbox and Drive System
Mounting. A lift – link and five pylon isolation mounts attach the transmission to the fuselage structure. The forged steel lift – link is connected to the forward underside of the transmission support case and a fuselage beam, to give direct single point suspension from transmission to the main fuselage. Four main mounts support the pylon through the transmission support case with the aft pair restrained by friction dampers. The fifth isolation mount is centred on a support fitting, bridged across the rear side of the pylon support structure, with its eyebolt attached to a welded beam, extending between the aft legs of the support case.
Operation. Statically, the transmission supports the weight of the mast, the rotating controls, and the main rotor. In flight, the transmission along with the mast is the main and the only link between the main rotor and the airframe. Output reduction ratios vary from transmission to transmission. After a further reduction in the 90-degree gearbox, the tail rotor turns at much-reduced rpm as compared to the engine. Engine rpm is reduced to main rotor rpm through internal transmission gear reduction. This reduction is accomplished by a system of bevel gears and a two-stage planetary, for an increased overall reduction for the transmission.
The operation is accomplished by the engine being connected to the transmission by the main drive shaft. The main drive shaft, commonly called as a short shaft, connects the engine to the freewheeling clutch portion of the transmission input quill. The input quill pinion drives the horizontal spiral bevel gear, providing the first stage of reduction. The horizontal spiral bevel gear, in turn, drives the cockpit air blower and vertical shaft. Through the vertical shaft, torque is transmitted up to the two-stage planetary gears and to the mast.
Main Sections. Generally, the transmission is divided into five sections which are discussed in succeeding paragraphs.
Figure 2: Main Gearbox Sections
a. Top case. It is also called as upper case in most of the transmissions. It serves as the support for the main rotor mast when installed or for the lifting adapter to transport the transmission when necessary. Also, the top case houses a vent that relieves internal pressure within the transmission.
b. Planetary Gear Case. The planetary gear case or commonly called as ring gear case is the only transmission case assembly which is made of steel. It houses the first and second stage planetary gear sections which give the transmission required gear reduction to the main rotor mast.
c. Main Case. The main case of the transmission has the provisions for the main input quill and output blower quill or any other outputs necessary to drive accessories.
d. Accessory Drive Case and Support case. The accessory drive/ sump case house the tail rotor output quill, transmission hydraulic pump, and tachometer drive quill and transmission oil pump. Sump case may have oil level sight gauge and a chip detector. Along with internal oil filter and the oil pump inlet screen oil lubrication system. The support case of the transmission allows the transmission to be attached to the airframe at six different positions.
e. Sump Case. It is the lowest part of a transmission and houses the oil pump and drain valve of the oil system.
Transmission Quills. The transmission contains a varied variety of quills. The quills may be replaced individually without shimming.
a. Main Input Quill. The main input quill (3, figure 9.2 and figure 9.7) is located on the aft side of the transmission. The engine transmits power to the transmission through the main driveshaft and the main input quill. A freewheel (one way) clutch located in the main input quill operates automatically, engaging to allow the engine to drive rotor or disengaging the idling engine during autorotational descent.
b. Tail Rotor Drive Quill. The tail rotor drive quill (figure 9.8) is located on the aft side of the transmission sump case. The forward tail rotor driveshaft is attached to a splined coupling which is part of the tail rotor drive quill.
c. Hydraulic Pump and Tachometer Drive Quill. The hydraulic pump and tachometer drive quill (9, figure 9.2) is located on the right side of the transmission sump case. The quill has pads for two hydraulic pumps and the rotor tachometer generator. It is shown in figure 9.9.
d. Fan Drive Quill. The fan drive quill (10, figure 9.2 and Figure 9.10) is located on the forward side of the transmission. This quill transmits power from the transmission input bevel gear to drive the air distribution blower (fan).
e. Alternator Drive Quill. The alternator drive quill (5, figure 9.2 and figure 9.11) is located on the left side of the transmission. The purpose of this quilt is to take power from the input bevel gear of the transmission to drive the alternator.
Main Rotor Mast Assembly. The main rotor mast assembly (Figure 9.12) is a tubular steel shaft fitted with two bearings, which support it vertically in the transmission. Mast driving splines are engaged with transmission upper stage planetary gear providing counterclockwise rotation as viewed from above. Splines on the upper portion of mast provide mounting for main rotor and control assemblies.
Main Drive Shaft. The main drive shaft with crowned tooth couplings is installed between an adapter on engine output shaft and the freewheel coupling of the transmission input drive quill. Two clamp sets, of split V-band type, hold the mating curved splined faces of couplings in secure contact. The flexibility of couplings is provided by sliding an inner coupling in splines of an outer coupling to accommodate movement of transmission on pylon mountings. A spring in each coupling assists centring of the shaft during operation and tends to hold shaft assembly in place if clamps are removed during maintenance.
Tail Rotor Drive Shaft. Five driveshaft sections (Figure 9.17) transmit power from the transmission to the tail rotor through two gearboxes. The shaft sections are identical and are supported by three hanger assemblies on the tail boom and engine deck.
Tail Rotor Driveshaft Hanger Assembly. Three hanger assemblies connect and support tail rotor driveshaft along the top of the tail boom and above the engine deck. Each assembly consists of couplings on a short, splined shaft, mounted through a single-row sealed ball bearing in a ring-shaped hanger equipped with two mounting lugs for attachment on a support fitting (figure 9.18).
Intermediate Gearbox. The intermediate gearbox 15, (figure 9.19) is located on the tail boom at the base of the vertical fin. The gearbox provides a forty-two-degree change in direction of tail rotor driveshaft. It consists of a case with a gear quill in each end. The case is fitted with a breather-type oil filler cap, an oil level sight gage and a drain plug equipped with a chip detector which activates warning lights on the pilot and gunner caution panels and the miscellaneous controls panel when excessive metal particles contamination occurs. The input and output quills have flexible couplings for attachment of drive-shafts. Access is provided by a cover with quick-release fasteners.
Intermediate Gearbox Quills. The intermediate gearbox input and output quills consist of a pinion gear bearing mounted in a sleeve. Each quill has a flexible coupling attachment of drive shafts. The output quill has an oil collector cone installed on the inboard end of the pinion gear.
Tail Rotor Drive Gearbox. A gearbox (Figure 9.25) at top of tail boom vertical fin provides ninety-degree or any desired change in direction of drive and speed reduction between the input drive shaft and the output shaft on which the tail rotor is mounted. The gearbox consists of mating input and output gear quill assemblies set into gear case provided with a breather-type oil filler cape oil level sight gage and a drain plug with a chip detector. The input quill has a flexible coupling for attachment of driveshaft. Control linkage is attached on the left side with a control rod extending through the rotor shaft.