A PV limit of
15000 ordinarily can be used for dry operation of carbon bearings. This should be reduced
for Continuous running with a steady load over a long period of time to avoid excessive
wear. When operating with liquids which permit the development of a supporting fluid film,
much higher PV values can be used.
A hard, rust-resistant shaft with at least a 10-µin,
finish should be used. Hardened toot steel or chrome plate is recommended for heavy loads
and high-speed applications. Steal having a hardness over Rockwell C50, bronzes, 18-8
stainless steels, and various carbides and ceramics also can be used.
Certain precautions should be observed in applying
carbon-graphite. Since this material is brittle, It Is chipped or cracked easily if struck
on an edge or a corner, or if subjected to high thermal, tensile, or bending stresses.
Edges should be relieved with a chamfer. Sharp corners, thin sections, keyways, and blind
holes should be avoided wherever possible. Because of brittleness and low coefficient of
expansion (about ¼ that of steel), carbon-graphite bearings are often shrunk into a steel
sleeve. This minimizes changes in shaft clearance with temperature variations and provides
mechanical support for the carbon-graphite elements.”
The comparative properties of three proprietary materials are summarized In Table 1.
TABLE 1
COMPARATIVE PROPERTIES OF GRAPHITAR,(i) OIL.ON PV® _80(ii))
AND RULON(iii)
| Property | Graphitar (Carbon-Graphite) |
Oilon PV®-80 (TFE) |
Rulon® (TFE) |
| Coefficient of
friction Temperature range Approx. max. PV (unlubricated) Max. P Max. V Recommended shaft surface finish Recommended shaft clearance Typical elastic modulus Tensile strength *Consult manufacturer |
0.04 to 0.25 Cryogenic to 1000ºF in some grades 15,000 * * +=30rms 0.003 nun for most unlubricated applications (0.5 to 3.5)106 psi 1000 — 9500 psi, depending on grade |
0.05 to 0.10 -40ºF to +250ºF 18,000 3000 pal 1700 ft/mm. * (tw)10-4 +0.004" t = temp. °F w = bearing wall thickness (in.) (3.5 = 3.8)106 psi 7200 psi |
0.15 to 0.20 -400ºF to+550ºF 10,000 (Sleeve bearing) 1000 psi 400 ft/mm. 8-32 rms * * * |
Data reprinted with the
permission of the following manufacturers:
(I) “Graphitar” Wickes, 1621 Holland Ave., Saginaw, Mich. 48601;
(ii) “Oilon PV®_80 Design Guide”, TFE lndustries, 148 Parkway
Kalamazoo, Mich. 49006;
(iii) “Rulon® Standard Stock Bearings, Engineering Manual, Cat. 75”,
Dixon Corp.. Div. of Dixon Industries, Bristol,
RI., 02839.