ACCESSORY DRIVE GEARBOXES .1 INTRODUCTION

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8.3 ACCESSORY DRIVE GEARBOXES 8.3.1 INTRODUCTION

Gearboxes provide the power for aircraft hydraulic, pneumatic and electrical systems in addition to providing various pumps and control systems for efficient engine operation. The high level of dependence upon these units requires an extremely reliable drive system.

The drive for the gearbox is typically taken from a rotating engine shaft usually the HP shaft, via an internal gearbox, to an external gearbox that provides a mount for the accessories and distributes the appropriate geared drive to each accessory. A starter may also be fitted to provide an input torque to the engine. An accessory drive system on a high by-pass engine takes between 400 and 500 horsepower from the engine.

8.3.2 INTERNAL GEARBOX

The location of the internal gearbox within the core of an engine is dictated by the difficulties of bringing a driveshaft radially outwards and the space available within the engine core.

Mechanical Arrangements of Accessory Drive Gearboxes.

Figure 8.8.

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Thermal fatigue and a reduction in engine performance, due to the radial driveshaft disturbing the gasfiow, create greater problems within the turbine area than the compressor area. For any given engine, which incorporates an axial-flow compressor, the turbine area is smaller than that containing the compressor and therefore makes it physically easier to mount the gearbox within the compressor section. Centrifugal compressor engines can have limited available space, so the internal gearbox may be located within a static nose cone or, in the case of a turbo-propeller engine, behind the turbo-propeller reduction gear as shown in fig.8.8.

On multi-shaft engines, the choice of which compressor shaft is used to drive the internal gearbox is primarily dependent upon the ease of engine starting. This is achieved by rotating the compressor shaft, usually via an input torque from the external gearbox. In practice the high pressure system is invariably rotated in order to generate an airflow through the engine and the high pressure compressor shaft is therefore coupled to the internal gearbox.

To minimise unwanted movement between the compressor shaft bevel gear and radial shaft bevel gear is mounted as close to the compressor shaft location bearing as possible, but a small amount of movement movement without affecting the bevel gear mesh. A more complex system utilises an idler gear that meshes with the compressor shaft via straight spur gears, accommodating the axial movement, and drives the radial driveshaft via a bevel gear arrangement. The latter method was widely employed on early engines to overcome gear engagement difficulties at high speed.

Types of Internal Gearbox

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To spread the load of driving accessory units, some engines take a second drive from the slower rotating low pressure shaft to a second external gearbox (fig.8.8.).

This also has the advantage of locating the accessory units in two groups, thus overcoming the possibility of limited external space on the engine. When this method is used, an attempt is made to group the accessory units specific to the engine onto the high pressure system, since that is the first shaft to rotate, and the aircraft accessory units are driven by the low pressure system. A typical internal gearbox showing how both drives are taken is shown in fig.8.10. This method may also be used to drive speed sensors and governors for the low pressure shaft.

An Internal Gearbox With an LP and HP Output.

Figure 8.10.

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The purpose of a radial driveshaft is to transmit the drive from the internal gearbox to an accessory unit or the external gearbox. It also serves to transmit the high torque from the starter to rotate the high pressure system for engine starting purposes. The driveshaft may be direct drive or via an intermediate gearbox.

To minimise the effect of the driveshaft passing through the compressor duct and disrupting the airflow, it is housed within the compressor support structure. On by-pass engines, the driveshaft is either housed in the outlet guide vanes or in a hollow streamlined radial fairing across the low pressure compressor duct.

To reduce airflow disruption it is desirable to have the smallest driveshaft diameter as possible. The smaller the diameter, the faster the shaft must rotate to provide the same power. However, this raises the internal stress and gives greater dynamic problems, which result in vibration. A long radial driveshaft usually requires a roller bearing situated halfway along its length to give smooth running. This allows a rotational speed of approximately 25,000 r.p.m. to be achieved with a shaft diameter of less than 1.5 inch without encountering serious vibration problems.

8.3.4 DIRECT DRIVE

In some early engines, a radial driveshaft was used to drive each, or in some instances a pair, of accessory units. Although this allowed each accessory unit to be located in any desirable location around the engine and decreased the power transmitted through individual gears, it necessitated a large internal gearbox.

Additionally, numerous radial driveshafts had to be incorporated within the design.

This led to an excessive amount of time required for disassembly and assembly of the engine for maintenance purposes.

In some instances the direct drive method may be used in conjunction with the external gearbox system when it is impractical to take a drive from a particular area of the engine to the external gearbox. For example, figure8.8. shows a turbo-propeller engine which requires accessories specific to the turbo-propeller reduction drive, but has the external gearbox located away from this area to receive the drive from the compressor shaft.

8.3.5 GEAR TRAIN DRIVE

When space permits, the drive may be taken to the external gearbox via a gear train (fig.8.8). This involves the use of spur gears, sometimes incorporating a centrifugal breather. However, it is rare to find this type of drive system in current use.

8.3.6 INTERMEDIATE GEARBOX

Intermediate gearboxes are employed when it is not possible to directly align the radial driveshaft with the external gearbox. To overcome this problem an intermediate gearbox is mounted on the high pressure compressor case and

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8.3.7 EXTERNAL GEARBOX

The external gearbox contains the drives for the accessories, the drive from the starter and provides a mounting face for each accessory unit. Provision is also made for hand turning the engine, via the gearbox, for maintenance purposes.

Fig.8.11. shows the accessory units that are typically found on an external gearbox.

An External Gearbox.

Figure 8.11.

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The overall layout of an external gearbox is dictated by a number of factors. To reduce drag it is important to present a low frontal area to the airflow. Therefore the gearbox is 'wrapped' around the engine. For maintenance purposes the gearbox is generally located on the underside of the engine to allow ground crew to gain access. However, helicopter installation design usually requires the gearbox to be located on the top of the engine for ease of access.

The starter/driven gearshaft (fig.8.11.) roughly divides the external gearbox into two sections. One section provides the drive for the accessories which require low power whilst the other drives the high power accessories. This allows the small and large gears to be grouped together independently and is an efficient method of distributing the drive for the minimum weight.

If any accessory unit fails, and is prevented from rotating, it could cause further failure in the external gearbox by shearing the teeth of the gear train. To prevent secondary failure occurring a weak section is machined into the driveshafts, known as a ‘shear-neck', which is designed to fail and thus protect the other drives. This feature is not included for primary engine accessory units, such as the oil pumps, because these units are vital to the running of the engine and any failure would necessitate immediate shutdown of the engine.

Since the starter provides the highest torque that the drive system encounters, it is the basis of design. The starter is usually positioned to give the shortest drive line to the engine core. This eliminates the necessity of strengthening the entire gear train, which would increase the gearbox weight. However, when an auxiliary gearbox is fitted the starter is moved along the gear train to allow the heavily loaded auxiliary gearbox drive to pass through the external gearbox. This requires the spur gears between the starter and starter/driven gearshaft to have a larger face width to carry the load applied by the starter (fig.8.12.).

When drive is taken from two compressor shafts, two separate gearboxes are required. These are mounted either side of the compressor case and are generally known as the 'low speed' and 'high speed' external gearboxes.

8.3.8 AUXILIARY GEARBOX

An auxiliary gearbox is a convenient method of providing additional accessory drives when the configuration of an engine and airframe does not allow enough space to mount all of the accessory units on a single external gearbox.

A drive is taken from the external gearbox (fig.8.12.) to power the auxiliary gearbox, which distributes the appropriate gear ratio drive to the accessories in the same manner as the external gearbox.

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An External Gearbox with an Auxiliary Gearbox Drive.

Figure 8.12.

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The spur gears of the external or auxiliary gearbox gear train (figs.8.11. and 8.12.) are mounted between bearings supported by the front and rear casings which are bolted together. They transmit the drive to each accessory unit, which is normally between 5000 and 6000 r.p.m. for the accessory units and approximately 20,000 r.p.m. for the centrifugal breather.

All gear meshes are designed with 'hunting tooth' ratios which ensure that each tooth of a gear does not engage between the same set of opposing teeth on each revolution. This spreads any wear evenly across all teeth.

Spiral (helical) bevel gears are used for the connection of shafts whose axes are at an angle to one another but in the same plane. The majority of gears within a gear train are of the straight spur gear type, those with the widest face carry the greatest loads. For smoother running, helical gears are used but the resultant end thrust failure. The use of an air blown seal results in a gearbox pressure of about 3 lbs.

per sq. in. above atmospheric pressure. To supplement a labyrinth seal, an 'oil thrower ring' may be used. This involves the leakage oil running down the driving shaft and being flung outwards by a flange on the rotating shaft. The oil is then collected and returned to the gearbox.

Materials

To reduce weight, the lightest materials possible are used. The internal gearbox casing is cast from aluminium but the low environmental temperatures that an external gearbox is subjected to allows the use of magnesium castings which are lighter still, The gears are manufactured from non-corrosion resistant steels for strength and toughness. They are case hardened to give a very hard wear resistant skin and feature accurately ground teeth for smooth gear meshing.

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