Figure 14-.. .ace overlap (contact) ratio variation with helix angle. (Courtesyof LufkinIndustries， Inc.)
Figure 14-.. Thrust and separating load as a percentage of transmitted tangentialload. (Courtesy of LufkinIndustries， Inc.)
sensitivity tocoupling thrust， no measurable apexrunout， and it is less expensive to cut teeth. The disadvantages of single-helical gears are thatthey require expensive thrust bearing and thrust faces， and are less efficient due to heat load on the thrust bearing.
Advantages of double-helical gears are that they are very simple to designand manufacture because of the absence of thrust faces and thrust bearings，very little thrust is produced by gearing， and they are more efficient thansingle-helicalgears， which are subject to thrust bearing losses. Some dis-advantages of double-helical gears: they are sensitive to couplinglockup，it is hard to modify the tooth longitudinally， and it is slightly more expensive to cut teeth because of setup and tool change.
Tooth Hardness
Gearsavailable today have varying hardness， ranging from 225 BHNto 60 Rc. Each has advantages and disadvantages， so factors determining hardness must be carefully evaluated.
Medium-range gears are not too sensitive to operational errors and wearslightly before failing. Also， very hard gears are more susceptible to scoring due to high-lead intensity and sliding velocities. The noise levels on medium-hard gears increase with wear and give warning of gear failure. Heat treatment on medium-hard gears is simple compared to surface-hardened gearing.
Hardened gearing is used mainly when weight and space are at a premium.When carbonized， case-hardened gearing must be subjected to a grindingoperation to provide the finish. When nitriding isused， finish lapping orhoning can be used. Hardened gears result in larger tooth deflections， thus resulting in noisier gears.
Scoring
For high-speedor high-load intensity， scoring must be evaluated. The probability of scoring is predicted using the flash temperature index. If theindex value is below275， it is considered to be a low-scoring risk. Values between 275 and 335 are considered high risks. Copper or silver plating is sometimes used on gear teeth to reduce failure during startups when the scoring probability increases. This plating acts as extreme-pressure lubri-cant to separate the tooth surface asperities until the teeth breakin.
Figure 14-8 shows the effect of speed and load intensity on the flashtemperature index. These curves are general innature， since scoring is afunction of pressure angle， lubrication， and tooth size.
Gear Accuracy
There are no accuracy standards for high-speed enclosed gear drives. The AGMA 390 is the standard for gear accuracy and has tabulated allowable errors based on gear size for the different elements of gear teeth. Thisstandard holds only for loose gearingand， if used for high-speed， wide-facewidthgears， will lead to early failure. As a generalrule， manufacturers ofhigh-speed gearing monitor the gears and pinions for involute，lead， runout，spacing， and surface finish. Light-load blue transfer checks are conducted to prove the accuracy of the system.
Types of .earings
Bearings of all types can be used to support gears. Normally， gear drivesproposed for turbine-driven applications， whether they are single-or double-helical， will be supported in sleeve-type bearings.
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