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Finish lapping of high-speed gears requires that the gear teeth be carefully hobbed to 40-50 microinches (1016-1270 μmm) surface finish with good lead profile and tooth spacing. Lapping is required for surface finish andprofile improvement only. When required， a surface finish of 8-15 micro-inches (200-380 μmm) can be obtainedbut， as a general rule， 20-30 micro-inches (508-762 μmm) is acceptable.
Grinding
Grinding produces the best absolute values of lead and involute profile. Asa generalrule， tooth spacing is not as good as gearing produced on a preci-sion hobbing machine because of the smaller index wheel and the single-space indexing procedure. The disadvantage of the grinding process is the skill and patience required of the operator. Gears too large to be checked depend ona roll-in blue checkfor verification of lead and involute. Also， the grinding machines in current use have a reciprocating motion of the grinding head and require greater maintenance to produce good gears.
Gear .ating
TheAPI， with help from gear manufacturers， has a 1977 standard (API 613) for rating gears. (See Chapter 4.) A common procedure for comparingand sizing gear is based on the tooth pittingindex， the K factor
Material index number
Allowable K二 Service factor (14-1)
The material index is based on hardness and geometry parameters. The service factor takes into account the driving and the driven equipment characteristics. The AGMA 211 standard defines K as
K二  126， 000 xPSc Npxd2 xF  xMg +1 Mg  (14-2) 
where: 

PSc二transmitted horsepower Mg二gear ratio Np二pinion rpm
d二pinion pitch diameter in inches F二net face width of narrowest matinggears， or the sum of the face width of each helix of double-helical (inches)
API 613 is consistent with AGMA 211.However， API 613 is more con-servative than the AGMA procedure， using service factors of 1.5 and 2.0. The strength rating based on API 613 is
St二.t xPn x(SF)x 1.8 cos e (14-3)
FJ
where:
St二bending stress number e二helix angle J二geometry factor
SF二service factor Pn二normal diameter pitch
Gear Noise
Noise from an operating gear set is a function of roundness and concen-tricity of operating elements (both gearing and shafting)， accurate balanceand， in particular， control of tooth spacing errors and uniformity of mesh stiffness to reduce meshing frequency excitation. It is significant that for submarinegears， where the ultimate in quietness is essential， the hallmarksare moderate tooth loading， finepitch， high-helixangle， and low-pressure angle.all diametrically opposite from the usual and necessary practicefor single-helical， hardened and ground gearing， which have low-helix angle(for minimumthrust)， very coarse pitch teeth (to get adequate strength)， and high loading (because of the carburized hardening).
Some factors causing gear noise can be attributedto， but not limitedto， the following:
1. Tooth spacing or involute error

2. Contact ratio

3. Surface finish

4. Wear on tooth flanks or pitting

5. Excessive or too little backlash

6.Gear，
shaft， or housing resonance

7. Tooth deflections

8. Pitch-line runout

9. Load intensity on gearing

10. Clutches and couplings

11. Lube oil pump and piping

12. Transmitted noise from driven or driving machinery

.nstallation and .nitial .peration
The mounting of a gearbox into trains is a precision job and should be done carefully. Gear unit installation is one of the most important factors tobe considered forlong， trouble-free operation. No matter how accurately thegear unit is manufactured， it can be destroyed in a few hours of operation when improperly installed. The same care should be taken when installing a gearbox as with any high-speed machinery. The mounting surface should beaflat，level， single-plane surface of finished steel at a height that will permit the shimming necessary to align the gear unit properly to connecting shafts. The shims should be of a size at least equal to the width of the unit foot pad. Then the gear unit should be placed on the foundation in the approximate required position. Uneven supports can distort the gearbox and adversely affect the gear tooth contact.
　
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