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The most common type used are plain journal bearings. They have goodload-carrying capacity， but they can also have oil whirl problems. To pre-vent oil whirl problems， pressure dam or tilting pad journal bearings are used. Gear motors have imposed operating loads and do not require thesame degree of no-load bearing stability as compressorturbines， which have only the rotor weight applied to the bearings.
The exclusion of rolling-type bearings from drives of this class may beunwarranted. In the lower horsepowerranges， bearing ratings can be easily provided so that race and roller fatigue can be ignored as a source of failure.
.rives using rolling element bearings may sometimes provide additional design latitude for the gear manufacturer. The extensive use of rolling element bearings in comtemporary light-weight gas turbine designs bears strong testimony for this point.
Thrust bearings vary from the ball bearing to the self-equalizing tiltingpad-type. The most common type is the babbitt-lined， flat-face thrust bear-ing. The flat-face bearing is sometimes modified to add tapered lands， which double the load-carrying capacity. Tilting pad bearings are becoming more popular because of their high-thrust capacity and misalignment capabilities.Also， the tilting pad thrust bearing is more efficient because of the higher allowable loading and lower rubbing speeds.
Service Factor
When selectinggears， two major areas to consider are the service factor and type of drive to be employed. The service factor is defined as the minimum ratio between calculated capacity and average transmitted loadfor any component of the system. In general， one of three criteria will be the controlling influence in gear drives. These are failures due to tooth surfacepitting，wear， or physical loss of teeth from breakage. Consequences of thethree failure modesdiffer， particularly in regard to the time length involved. Wear can continue for a long period of time without affecting machineryserviceability or reliability. Pitting， if progressive， will eventually destroy theworking profile of the teeth， altering their thermal characteristics， and often rendering the drive unsuitable because of high-vibration levels long before the teeth are incapable of carrying load. Loss of a portion of a tooth by breakage has immediate consequences. The balance is immediately anddrastically affected and， with major toothbreakage， the gear will be incap-able of further operation. Any evaluation of a service factor should deter-mine which of the three modes is involved.
Current practice includes the automatic provision of an additional 50% margin when designing for gear tooth bending. This margin has the effect of eliminating gear tooth breakage as a primary cause of failure except with severe and unforeseen overloads.
.esign against failure by wear under heavy tooth loads will result in theselection of heavy-bodied lubricants， generally 150 SSU or more at supply temperature. Pitting failures are the most difficult to provide a margin against as increasing gear size or hardness are the only means of improvingcapacity， and both entail an increase in cost.
The service factor itself is not an overload capacity perse， as it includes either empirical or theoretical estimates of the effect of such factors as length of servicelife， torque fluctuations， and reliability level required. The service factors as established by the American Gear Manufacturers Association and published in their standards are intended for application to transmitted load requirements. if substantial overload capacity is planned or allowed(an oversizedriver)， additional gear rating must be included to provide foroperation at those levels. Similarly， torque loads resulting from torsional oscillations or faulty operation are outside the scope of normally applied service factors and must be evaluated and provided for separately. Any torque fluctuations that result in a separation of gear teeth at speed will bemost difficult to provide for. Impact loads occur during re-engagement， and very short service life is a frequent result of operation under these conditions.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(101)
