Reference cited in this section

2. C.J. Thwaites, "Practical Hot Tinning," Publication No. 575, International Tin Research Institute Centrifugal Casting

Centrifugal casting is the preferred method for babbitting medium- and thick-wall, half-shell or full-round (nonsplit) journal bearings because it virtually eliminates porosity and allows close control of the cooling process to promote a strong bond. Disadvantages are the need for more extensive equipment and tooling than static casting requires and minor segregation of the intermetallics in the babbitt across its thickness. (It should be noted, however, that segregation along the axial length of a statically cast bearing can be more serious, and is more difficult to detect). The spinning axis is usually horizontal, but vertical orientation is sometimes employed for unusual sizes (e.g., large diameters or short lengths). The descriptions of equipment and procedures below apply only to horizontal casting.

Equipment. Centrifugal babbitting requires a machine expressly designed or modified for this purpose (Fig. 2). No mandrel is used in horizontal applications. A variable-speed centrifugal casting (spinning) machine is fitted with a safe means for supporting and rotating a reasonably well-balanced workpiece clamped between recessed sealing plates. With the shell rotating, molten babbitt is fed to the inside of the prepared bearing through a hole in the outboard spinning plate, then solidified by air and/or water sprayed on the outside diameter of the shell while it is spinning. A speed is selected that produces a centrifugal force high enough to eliminate porosity but low enough to minimize metal segregation. Too low a speed causes metal "tumbling," while too high a speed causes segregation. Table 5 shows a range of spinning speeds for various bearing sizes (inside shell diameter), based on minimum centrifugal forces of 20 g for tin-base alloys. The minimum for lead-base babbitts is 16 g. To promote directional solidification after the babbitt is poured, the tooling plates must be preheated to 200 °C (390 °F) minimum and faced with gaskets cut from sheets of environmentally approved insulating materials, such as Inswool, to seal and insulate both ends of the bearing when it is clamped between the support plates by hydraulic pressure. The centrifugal casting machine must be of fail-safe design in the event of a loss of power and/or clamping pressure.

Table 5 Centrifugal casting speeds for tin-base babbitt bearings

Optimum speed is slowest speed that will produce sound (porosity-free) castings with minimum segregation.

Table 5 Centrifugal casting speeds for tin-base babbitt bearings

Optimum speed is slowest speed that will produce sound (porosity-free) castings with minimum segregation.

Inside diameter of bearing shell

Centrifugal casting speed, rev/min

mm

in.

51

2

850-975

76

3

700-800

102

4

600-700

127

5

535-610

152

6

485-550

178

7

450-525

203

8

420-500

229

9

395-470

254

10

375-450

279

11

360-430

305

12

345-410

330

13

330-395

356

14

320-370

381

15

305-360

406

16

295-350

432

17

285-340

457

18

280-330

483

19

275-325

508

20

265-315

533

21

260-310

559

22

250-300

584

23

245-295

610

24

240-290

635

25

235-285

660

26

230-280

686

27

230-275

711

28

225-270

737

29

220-265

762

30

215-260

787

31

215-255

813

32

210-250

838

33

205-245

864

34

200-245

889

35

200-240

914

36

195-235

965

38

190-230

Fig. 2 Loading of a large steam turbine generator bearing into a centrifugal babbitting machine. Courtesy of Pioneer Motor Bearing Company

Procedure. After tinning, full-round (nonsplit) shells require no further preparation and can be immediately loaded into the casting machine. Split-type bearings (in halves) or segmented (tilting pad) style journal bearings require fixturing after the pretinning of each component to make up an assembly ready for spinning. The fixturing consists of laminated spacers for each parting face. These spacers must prevent both radial and axial leakage without unduly restricting the smooth flow of the molten rotating metal.

Spacers can be cut from steel sheet with a minimum thickness of 1.0 mm (0.04 in.) and faced on both sides with 3 mm (0.12 in.) gaskets that are cut to fit. Total spacer thickness should be at least 7 mm (0.28 in.) before assembly to allow for the later separation of the halves or segments by saw cutting through the babbitt without damage to the steel parting faces.

When the spacers are in place and the halves or segments are retained by bolting or other means of safe clamping, the assembly must be reheated in the tinning bath, if necessary, to ensure a minimum temperature of 300 °C (570 °F) before loading in the casting machine. Alternatively, the cold assembly can be mounted in the machine and slowly rotated while gas torches are applied along the outside of the bearing until it reaches tinning temperatures. This latter method, however, does not allow visual in-process inspection to ensure complete wetting (tinning) before casting commences. In any case, there must be little time lost after tinning temperatures are reached, and before babbitt metal is poured, to achieve a satisfactory bond; tinning must be molten when the babbitt is poured.

The babbitt alloy should be melted in a temperature-controlled cast iron pot that is held at its recommended pouring temperature (Tables 1 and 2) and stirred with a vertical motion to promote uniform metal temperature and to avoid metal segregation. Dross on the surface of the pot is skimmed aside while a preheated ladle, preferably of the bottom-pour style, is filled with a predetermined volume of metal. The volume of metal poured should be sufficient to provide a minimum of 4 mm (0.16 in.) machining stock per side after babbitting so that impurities, which float to the inside during centrifugal casting, can be removed.

With the bearing shell rotating at a preselected speed (Table 4), the metal is ladled in one continuous pour directly, or through a preheated trough, into the spinning shell. Water cooling should be started immediately after pouring is completed, and the bearing rotation should continue until the assembly temperature drops to around 150 °C (300 °F). At this point, the water should be shut off and the bearing allowed to air cool more slowly while spinning. This cooling regime minimizes the stress imposed on the bond by the difference in thermal coefficients between typical babbitts and backing materials. (The ratio is almost 2:1 between babbitt and steel, and 1.5:1 between babbitt and bronze.) A controlled flow of coolant along the length of the bearing ensures directional solidification. That is, the babbitt freezes (solidifies) uniformly along the bond line first, then progressively toward the inside of the casting. This ensures that molten metal under pressure is available to feed the shrinkage and to squeeze out trapped gases and dross. When a shell has a nonuniform cross section, the coolant flow should be adjusted to promote uniform solidification from end to end.

Nonsplit journal bearings follow the same basic procedures as above without the need for assembling half shells or tilting pad segments and reheating them before they are loaded into the casting machine.

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