Page 365 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer
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Be st Practice 6 .5           Gas Turbine Best Practices






































                        Fig 6.5.2   Shaft bearing dynamics (Courtesy of Bently Nevada Corp.)

       tin and lead. This material is known as Babbitt. Its melting
       temperature is above 204 C (400 F), but under load will begin



       to deform at approximately 160 C (320 F). Typical thickness

       of Babbitt over steel is 1.5mm (0.060"). Bearing embedded
       temperature probes are a most effective means of measuring
       bearing load point temperature and are inserted just below
       the Babbitt surface. RTDs or thermocouples can be used.
       There are many modifications available to increase the load ef-
       fectiveness of hydrodynamic bearings. Among the methods
       available are:
         Copper backed Babbitt or ‘Trimetal’ e to aid in heat removal
         Back pad cooling e used on tilt pad bearings to remove heat
         Direct cooling e directing cool oil to maximum load points
          A typical straight sleeve hydrodynamic journal bearing is
       shown in Figure 6.5.3.
          Straight sleeve bearings are used for low shaft speeds (less
       than 5,000 rpm) or for older turbo-compressor designs. Fre-  Fig 6.5.3   Straight sleeve bearing liner (Courtesy of Elliott Co.)
       quently, they are modified to incorporate a pressure dam, in the
       direction of rotation. The pressure dam must be positioned in  passes through the major axis of the bearing. These types of
       the top half of the bearing to increase the load vector (see  bearings may have to be rotated in the bearing brackets to pre-
       Figure 6.5.2). This action ensures that the tangential force vector  vent this occurrence.
       will be small relative to the load vector, thus preventing shaft  The most common hydrodynamic bearing for higher
       instability. It should be noted that incorrectly assembling the  speed applications is the tilt pad journal bearing shown in
       pressure dam in the lower half of the bearing would render this  Figure 6.5.5.
       type of bearing unstable. When shaft speed is high, other alter-  A tilting pad bearing offers the advantage of increased contact
       natives to prevent rotor instabilities are noted in Figure 6.5.4.  area since the individual pads conform to the shaft orbit. In
          Shown are examples of anti-whirl bearings. The most  addition, this type is also a highly effective anti-whirl bearing
       common types of these bearings are the three and four lobe  since the spaces between the pads prevent oil whirl.
       design. Elliptical and offset bearing designs do prevent in-  Most end users specify tilt pad radial and thrust bearings for
       stabilities but tend to increase shaft vibration if the load vector  turbo-compressor applications.

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