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.

輔助齒輪箱
21．在發動機和飛機機體的布局沒有足夠的空間在一個外部齒輪箱上安裝所有的附件裝置的情況下。輔助齒輪箱便是提供外加附件裝置傳動的一種方便的方法。
20．正如第7段所述，傳動來自兩個壓氣機軸的時候，就要求有兩個獨立的齒輪箱。它們分別裝在壓氣機機匣的兩側，通常稱為“低速”和“高速”外部齒輪箱。
19．由于起動機發出的扭矩是傳動系統所遇到的最大扭矩，所以它是設計的基礎。通常起動機的位置安排要使其與發動機的核心的傳動線路最短。這就不必要去加強整個齒輪系，否則會增加齒輪箱的重量。當裝有輔助齒輪箱時(第21段)。起動機要順著齒輪系移開，讓載荷很大的輔助齒輪箱傳動裝置能通過外部齒輪箱引出。這就要求在起動機和起動機／傳動齒輪軸之間的正齒輪應有更大的齒面寬度來承受起動機施加的載荷（圖7-5）。
22．輔助齒輪箱由外部齒輪箱(圖7-5)來驅動，它為附件分配相應的齒輪傳動比，其情況類似于外部齒輪箱。
結構和材料
齒輪
23.外部齒輪箱或輔助齒輪箱齒輪系(圖7-4和圖7-5)的正齒輪裝在前后機匣支承的軸承之間。前后機匣則由螺栓連接在一起。這些正齒輪將傳動傳遞到每一個附件裝置。附件裝置的轉速通常在5000到6000轉／分之間，而離心通風機則為20000轉／分左右。
Accessory drives

25．螺旋傘齒輪被用來連接那些彼此呈一定角度，但位于同平面內的那些軸。在一個齒輪系之中的多數齒輪是直齒正齒輪，齒面最寬的那些正齒輪可用來承受最大的載荷。對于要求較為平穩的運轉來說，通常使用螺旋齒輪，但由這種齒輪的齒型造成的端面合成推力應當由齒輪的安裝方式加以解決。

圖7-4 一種外部齒輪箱和一些附件裝置
Fig. 7-4 An external gearbox and accessory units.
低壓燃油泵
液壓泵
前機匣
滑油泵
后機匣
起動機
燃油流量
調節器
　
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