FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1957
1957 - 1483.PDF
FLIGHT, 11 October 1957 ROLLS-ROYCE AVON 200 SERIES . . . 573 Underside view of a 200 series Avon, showing disposition of accessory drive, ignition units, fuel- cooled oil cooler and fuel control unit. through the ring of swirler vanes and is mixed with atomized fuelwhich leaves the burner at this point. Some air also escapes into the double-walled flare section to cool the flame-can before thefirst secondary air enters the combustion area. The secondary air, passing round the can, is led back into the tube by a series ofholes and flutes. Some 25 per cent of the total airflow reaches the primary combustion zone, the remainder being drawn in later sothat it first insulates the inner and outer air casings. As stated earlier, the cans are interconnected in order to pass the startingflame round from Nos. 3 and 6 cans, which hold the igniters, and in order to balance the pressures in the cans. The ends of the flame-tubes and the nozzles, which have asegmental shape, are located by straight double flanges which pick up neighbouring pairs of tubes. Flutes allow a controlled leakageof secondary air between the nozzles for cooling purposes. Nozzle guide vanes, made in C.242 steel, are located ahead ofeach stage of the two-stage turbine, the high-pressure guide vanes being mounted between slotted flanges in the outer casing andinner ring. Each vane carries tongues which engage with slots for circumferential location. The low-pressure guide vanes aresupported in a similar manner at their outer ends; and the inner ends are located in the ring which carries the interstage seals. The exhaust unit, which is fabricated in Nimonic 75, is boltedto the nozzle casing and carries the exhaust cone on four radial struts, these being faired to allow a smooth airflow through theexhaust unit. The front face of the exhaust cone forms a sealing plate close against the rear face of the low-pressure turbine, high-pressure sealing air flowing radially between the two to seal the rear of the blade roots. Heat-insulating material is packed betweenthe double skins of the exhaust-unit outer wall. As outlined above, the main rotating assembly consists of a shaftbuilt up in three sections and supported on three bearings. The compressor shaft is a hollow, conical stainless-steel unit whichlocates and supports the fifteen separate compressor discs on fourteen splines and the fifteenth-stage disc on a flange. In earlyAvons the splines only transmitted torque to the discs, which were themselves located on flat diameters. All the 200-series discs areforged steel and—except the fifteenth—have a hairpin cross- section at the root to allow expansion without disturbance of themounting spline. During assembly these discs are simply placed on the shaft in order and a large nut at die front is tightened toretain the stack in position. The resulting end-loads are trans- ferred to the periphery of the discs, spacing rings being inter-posed to maintain correct separation. The web of the forward discs is considerably enlarged to accommodate the transfer of theseloads from hub to disc rim. Blades in the first to eighth compressor stages are of aluminium alloy. From the ninth stage aft they areof stainless steel. In the very latest engines the last four stages are of titanium. The fifteenth compressor stage blades are located on a discwhich is bolted to a flange on the shaft, the whole being further supported in the fore-and-aft sense by another disc which isintegral with the cone which picks up the flexible coupling between rear and forward shaft sections, and also carries the centre bearing.Flanges thrown up on the first and fifteenth disc carry balance plugs inserted as required when the rotating assembly is firstbalance-tested. Further balance plugs can be inserted in a similar flange on the forward face of the high-pressure turbine. All compressor rotor-blade roots are pin-jointed between flangesin the disc peripheries, the pins having a clearance in the roots so that each blade can rock considerably. Roots on the first fourstages are bushed for extra strength and, for the same purpose, the first-stage blades have double fork fittings. In the first fivestages, the blade-retaining pins carry heads which prevent forward movement. Rearward movement is precluded by flanges, over-lapping the pins, on the spacer rings. The spacer rings in all other stages carry two flanges which positively locate pins without heads. Air for cooling and sealing is bled through holes in the spacerring between fifth and sixth compressor stages and passed through a double sheet-steel diaphragm, which prevents vortices, to thecompressor shaft. It there passes through holes into the inside of the shaft and provides the low-pressure cooling and sealing airfor the whole shaft assembly. Thereafter it travels forward to a series of labyrinth seals round the forward bearing, where it bothcools the bearing and prevents oil leaking to the rest of the engine. It does the same at the centre bearing by passing boththrough the shaft and out to the rear face of the bearing, and also through holes straight to the front of the bearing housing. Fromhere it flows both inside and outside the shaft, within the "milk churn," and does similar work at the rear bearing before passingthrough the radial nozzle box struts, as described above. The seals themselves consist of male and female members. Thelatter are a large number of closely pitched fine grooves in the bore; and the male member has a smaller number of large grooves.When the engine is hot they run with a fine clearance between them. The centre bearing is actually carried by the cone which formsthe support for the fifteenth-stage disc. A ball coupling accom- Rear bearing and turbine assembly (dark arrows indicate h-p. sealing air and light arrows l-p. sealing air): A, one of six h-p. sealing-air tubes; B, low-pressure air outlet through spoke; C, l-p turbine disc; D, insulation plate; E, inter-disc space; F. intermediate casing.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
