Thrust ball bearings are used where purely axial loads have to be accom-modated. The single direction (二single row) design is designed for loadsfrom one direction， the double direction one (二double row) for reversingloads. Besides the design with flat washers， designs with spherical housing washers and seating washers are also available that can compensate for misalignment.
Spherical roller thrust bearings can accommodate high axial loads. They aresuitable for relatively high speeds. Theraceways， which are inclinedtoward the bearingaxis， allow the bearings to accommodate radial loads as well. The radial load must not exceed 55% of the axial load.
The bearings have asymmetrical barrel rollers and compensate for mis-alignment. As arule， spherical roller thrust bearings have to be lubricated with oil.
Wear.The life of rolling bearings can be terminated， apart fromfatigue， as a result of wear. The clearance of a worn bearing gets too large.
One frequent cause of wear is that foreign particles penetrate into a bearing due to insufficient sealing and have an abrasive effect. Wear is also caused by starved lubrication and when the lubricant is used up.
Therefore， wear can be considerably reduced by providing good lubrica-tion conditions (viscosity ratio x > 2 if possible) and a good degree of cleanliness in the rolling bearing. Where x三0.4 wear will dominate in the bearing if it is not prevented by suitable additives (EP additives).
The kinematically permissible speed may be higher or lower than the thermal reference speed. The basis of the thermal reference speed for cases where the operatingconditions(load， oil viscosity， or permissible temperature) deviate from the reference conditions. .ecisive criteria for the kinematically permissible speed are the strength limit of the bearing parts and the permissible sliding velocity of rubbing seals. Kinematically permis-siblespeeds， which arehigher than the thermal reference speeds， can bereached， for example， with specially designed lubrication， bearing clearanceadapted to the operating conditions， and accurate machining of the bearing seats with special regard to heat dissipation.
The thermal reference speed is a new index of the speed suitability of rolling bearings. It is defined as the speed at which the reference temperature of 160 oF (70 o.) is established. For high temperature rolling bearings， the steel used for bearing rings and rolling elements is generally heat-treated so that it can be used at operating temperatures of up to 300 oF (150 o.). Athigher temperatures， dimensional changes and hardness reductions result.Therefore， operating temperatures over 300 oF (150 o.) require special heat treatment.
Journal Bearings
The heavy frame type gas turbines use journal bearings. Journal bearings may be either full round or split; the lining may be heavy as used in large-sizebearings for heavy machinery， orthin， as used in precision insert-type bearings in internal combustion engines. Most sleeve bearings are of the split type for convenience in servicing and replacement. Often in split bear-ings， where the load is entirely downward， the top half of the bearing acts only as a cover to protect the bearing and to hold the oil fittings. Figure 13-6 shows a number of different types of journal bearings. A description of a few of the pertinent types of journal bearings is given here:
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本文鏈接地址：燃?xì)鉁u輪工程手冊(cè) Gas Turbine Engineering Handbook 2(81)
