Gear Couplings

Gear couplings are no longer the primary coupling of choice, having been superseded by the flexible element coupling. When used for critical equipment, the gear coupling should be specified as the high speed type regardless of speed for most applications. This class of coupling can be balanced and has the highest grade material. Plain hub style gear couplings are usually called a marine type. When the application permits, this is the preferred type. Since first preference is integral hubs, it follows that the marine type must then be used. Another advantage to this type is that all wearing parts are removable with the center section. The disadvantage of the marine type coupling is that it has a higher overhung moment because the teeth are on the spacer and the weight is further removed from the bearing centerline. This may cause a problem with lateral criticáis. When this is true, teeth on the hub must be considered. This requires a removable hub design because the hub itself now becomes a wearing part. Teeth on the hub can be furnished as a standard arrangement with teeth near the outboard end or a low moment arrangement with the teeth on the inboard end of the hub. The latter, which is attractive in reducing overhung weight, tends to have problems with clearance for sleeve withdrawal and, therefore, should only be used if rotor dynamics problems cannot be solved in another manner.

It should be noted that a mix and match has been used with a marine type plain hub on one end of the coupling and a toothed hub on the other. This, however, is most extraordinary and is mentioned only to indicate a possibility for problem solving.

Gear teeth hardness should be a minimum of 45 on the Rockwell C, scale. Hardness of the teeth having the greater face width (generally the sleeve teeth) must be equal to, or preferably greater than, the hardness of the mating teeth.

Lubrication may be continuous from the lube oil system or grease packed. For longer continuous operation and where the coupling speeds are high, continuous lubrication is preferred. Where maintenance intervals permit and the separation forces on grease are not too high, grease is a good solution. There are newer coupling greases on the market whose oil and thickener are the same density and, therefore, are not subject to centrifugal separation. Run times on conventional grease lubricated couplings are 8-18 months.

When continuous lubrication is called for, the couplings should be sup plied with oil filtered to a minimum of 2 microns. Experience shows that filtering to Yi micron is possible and desirable. Usually this is done with a separate filter; however, the whole system may be filtered to this level. The oil quantity furnished, per gear mesh, should be 3 gpm minimum.

Alignment

The absolute angular misalignment capability of a gear coupling is a function of the tooth form and backlash. The total angular misalignment at each mesh will be the total of the angular component and the angular result of parallel offset, both of which are the vector sums of horizontal and vertical misalignments. Values from !<47mesh to 6°/mesh may be found in the catalogs. However, this higher number is the value at which the teeth exceed the clearance and the coupling truly locks up.

Equation 8.1 is based on a maximum sliding velocity during misalignment of 5 ips and Equation 8.2 is based on 8 ips. Research by the Naval Boiler and Turbine Laboratory [7] developed these values. Experience indicates that the more conservative range of 1-3lA ips is most desirable.

Figure 8-22. Cutaway ot a multiple diaphragm flexible element coupling. {Courtesy

338 Compressors: Selection and Sizing where a = angular misalignment per mesh, deg d ~ pitch diameter of the teeth, in.

N - shaft speed, rpm

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