Focused Practice

I. Answer the following questions:

1. What does the structural integrity of turbine disc components depend on?

2. What knowledge is required to calculate the service life of each component of the turbine disc?

3. What can photoelasticity be used for?

4. Why have techniques that predict the directions of crack growth been developed?

5. Where does crack initiation occur?

6. What is the photoelastic prediction of the fatigue crack path constructed from?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Fatigue cracks in turbine discs.

Unit 18 Grammar: The Passive Voice Word List:

1. tip seal arrangement	герметичное устройство, кожух
2. forward and rear assemblies	передний и задний блоки (агрегаты)
3. blades	лопатки, лопасти (fixed – неподвижные, moving – движущиеся)
4. bearing housing bearing wall	установочный узел с подшипниками несущая стена
5. casing	кожух, каркас, рама
6. clearance	зазор
7. plenum chamber	нагнетательная камера высокого давления
8. cold setting	холодная обмуровка (на холоде)
9. downstream	нисходящий поток
10. gland	сальник, уплотнитель
11. traverse /traversable probe instruments	зонды (приборы для определения поперечных потоков в лопатках турбины)
12. carrier rings	несущие кольца
13. rack and pinion arrangement	зубчато-реечная передача
14. trailing edge stiffness	жесткость задней кромки
15. split shaft 16. thrust	разъёмный вал осевое давление
17. fitting and removal	сборка и разборка
18. friction and torque load	нагрузка, обусловленная трением и моментом вращения
19. spacing ring	шайба
20. pitch and lean angle	продольный и поперечный крен

The Split Shaft Design

The turbine casing is divided into structurally independent forward and rear assemblies to suit the split shaft design. Each of the two bladed discs is mounted on its own separate cantilevered shaft system in a manner which allows rapid fitting and removal. The first stage diaphragm and rotor are housed in the forward assembly which is bolted onto the inlet plenum chamber. This assembly also carries the second stage diaphragm since the diaphragm glands of both stages seal against the first stage shaft. The rear assembly, which is mounted on its own foundations, carries the bearing housings and tip seal arrangement for the test stage. This avoids any problem with setting and maintaining correct radial clearances which might otherwise arise due to the split shaft arrangement. The abutment between the two assemblies lies between the test stage fixed and moving rows and the rear section can slide axially on its mountings to allow variation of the interspace gap and to provide access for traversable probe instruments. Spacing rings are inserted between the casing sections to form the end wall profile.

The rear assembly can also be removed as a unit to improve access during strip and rebuild operations.

Special moulding techniques have been developed to produce low cost plastic fixed blades with steel reinforcements to provide adequate trailing edge stiffness and overall diaphragm strength. Cold setting plastics are also used in the construction of model diaphragms to retain the fixed blades at the precise chosen settings of stagger, pitch and lean angle. After test the diaphragms can be readily dismantled and the blades reused in later test configurations.

Radially adjustable rollers support the test stage diaphragm in order to allow the fixed blades to be indexed past the traverse probe instruments at the moving blade inlet and outlet planes. Traverse probe instruments at the stage inlet plane are held in the rotatable diaphragm and are thus traversed circumferentially through the wakes from the first stage fixed blades. The diaphragm rotational movement is effected through a rack and pinion arrangement.

An air bearing on the downstream face of the diaphragm carrier ring is pressurized during rotation to lift the diaphragm axially against the aerodynamic thrust to reduce friction and torque load.