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Gear Housings
Gear housings are made from materials such as castiron， steel， or alum-inum. Before final matching， the gear housing must be stress-relieved for dimensional stability. Housing should also be rigid enough to resist mis-alignment. A sufficient clearance should be provided around the gears toprevent oil choking. To prevent thermal distortion， the design should be able to maintain uniform case temperatures. Gear housings can cause alignment problems from thermal distortion.
Lubrication
The oil furnished to high-speed gears has a dual purpose: tooth and bearing lubrication and cooling.Usually， only 10-30% of the oil is forlubrication， and 70-90% is for cooling.
A turbine oil with rust and oxidation inhibitors is preferred. This oil mustbe keptclean， cooled， and have the correct viscosity. Synthetic oils should not be used without the manufacturer.s approval.
When selecting a high-speed gearunit， the possibility of using an AGMANo. 2 oil should be considered. In mostcases， the sleeve bearings in thesystem can use this oiland， ifnot， a compromise 200 SSU at 100 0F (380C) oil should be considered.
When 150 SSU at 1000F (380C) oil isnecessary， inlet temperatures should be limited to 110-1200F (43-490C) to maintain an acceptable viscosity. Oil should be supplied in the temperature and pressure range specified by the manufacturer. Up to a pitch-line speed of approximately15，000 feet per minute (4572 mpm) the oil should be sprayed into the out-mesh. Spraying allows maximum cooling time for the gear blanks andapplies the oil at the highest temperature area of the gears. Also， a negative pressure is formed when the teeth come out of themesh， pulling the oil into the tooth spaces.
At more than approximately15，000 feet per minute，(4，572 mpm) 90% of the oil should be sprayed into the outmesh and 10% into the inmesh. This procedure is a safety precaution to assure the amount of oil required forlubrication is available at the mesh. When the speed ranges from25，000 to40，000 feet per minute(7，670-12，192 mpm)， oil should be sprayed on the sides and gap area (on double-helical) of the gears to minimize thermal distortion.
.anufacturing .rocesses
Gear manufacturers use several methods for manufacturing good high-speed gearing. The most common arehobbing， hobbing andshaving，hobbing and lapping， and grinding.
High-speed gearing requires a finishing operation after cutting thatinvolves shaving， lapping， or grinding. Shaved or lapped gears have beenmore widely used than ground gears. However， there are some advantages and disadvantages to each manufacturing process.
Hobbing
This process produces good tooth spacing and accurate lead. It cannot economically achieve a surface finish better than 40 μin (1016 μmm). Thecuttingtool， called the ..hob gear，.. is basically a worm gear that has been fluted and has form-relieved teeth. These flutes provide the cutting edges and can be sharpened to retain the original tooth profile. As the workpiecemeshes with thehob， the teeth are formed by a series of cuts known as thegenerating process. To cut the helixangle， the rotation of the workis slightlyretarded or advanced in relationship to the hobrotation， and the feed is held in a definite relationship with the workand hob. Very accurate gearing can be produced by this process.
Hobbing and Shaving
The shaving process improves surface finish， involuteprofile， andlead， and can be used to crown the teeth. Shaving with inaccurate cutters willreduce hobbed accuracy， and it cannot improve spacing or pitch-line runout.The shaving cutter has involute teeth and meshes with the part beingshaved， thus improving the finish.
Hobbing and Lapping
The lapping process improves surfacefinish， involute profile，lead， andpitch-line runout. Absolute accuracy is not as good on lappedgears， but the mismatch error is generally as good as other production methods. Lapping is performed with the gears on an accurate mounting stand running at zero backlash with a cutting compound mixed with oil or grease performing the finishing operation. Lapping is also sometimes performed at the site.If site lapping is done， care must be taken to thoroughly clean the lubrication system before the system is put on-line. If the lubrication systemis notclean， scoring of the bearing can take place.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(102)
