16.
The overall layout of an external gearbox is dictated by a number of factors. To reduce drag whilst the aircraft is flying it is important to present a low frontal area to the airflow. Therefore the gearbox is 'wrapped' around the engine and may look, from the front, similar to a banana in shape. 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.
17.
The starter/driven gearshaft (fig. 7-4) 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.
18.
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
pumps, because these units are vital to the running of the engine and any failure would necessitate immediate shutdown of the engine.
19.
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 (para, 21) 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. 7-5).
20.
When a drive is taken from two compressor shafts, as discussed in para. 7, 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.
Auxiliary gearbox
21.
An auxiliary gearbox is a convenient method of providing additional accessory drives when the con-figuration of an engine and airframe does not allow enough space to mount all of the accessory units on a single external gearbox.
22.
A drive is taken from the external gearbox (fig. 7-5) to power the auxiliary gearbox which distributes the appropriate gear ratio drive to the accessories in the same manner as the external gearbox.
CONSTRUCTION AND MATERIALS
Gears
23.
The spur gears of the external or auxiliary gearbox gear train (fig. 7-4 and 7-5) 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,
24.
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.
辅助齿轮箱
21．在发动机和飞机机体的布局没有足够的空间在一个外部齿轮箱上安装所有的附件装置的情况下。辅助齿轮箱便是提供外加附件装置传动的一种方便的方法。
20．正如第7段所述，传动来自两个压气机轴的时候，就要求有两个独立的齿轮箱。它们分别装在压气机机匣的两侧，通常称为“低速”和“高速”外部齿轮箱。
19．由于起动机发出的扭矩是传动系统所遇到的最大扭矩，所以它是设计的基础。通常起动机的位置安排要使其与发动机的核心的传动线路最短。这就不必要去加强整个齿轮系，否则会增加齿轮箱的重量。当装有辅助齿轮箱时(第21段)。起动机要顺着齿轮系移开，让载荷很大的辅助齿轮箱传动装置能通过外部齿轮箱引出。这就要求在起动机和起动机／传动齿轮轴之间的正齿轮应有更大的齿面宽度来承受起动机施加的载荷（图7-5）。
22．辅助齿轮箱由外部齿轮箱(图7-5)来驱动，它为附件分配相应的齿轮传动比，其情况类似于外部齿轮箱。
结构和材料
齿轮
23.外部齿轮箱或辅助齿轮箱齿轮系(图7-4和图7-5)的正齿轮装在前后机匣支承的轴承之间。前后机匣则由螺栓连接在一起。这些正齿轮将传动传递到每一个附件装置。附件装置的转速通常在5000到6000转／分之间，而离心通风机则为20000转／分左右。
Accessory drives
25．螺旋伞齿轮被用来连接那些彼此呈一定角度，但位于同平面内的那些轴。在一个齿轮系之中的多数齿轮是直齿正齿轮，齿面最宽的那些正齿轮可用来承受最大的载荷。对于要求较为平稳的运转来说，通常使用螺旋齿轮，但由这种齿轮的齿型造成的端面合成推力应当由齿轮的安装方式加以解决。
图7-4 一种外部齿轮箱和一些附件装置
Fig. 7-4 An external gearbox and accessory units.
低压燃油泵
液压泵
前机匣
滑油泵
后机匣
起动机
燃油流量
调节器
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