Page 84 - The Tribology Handbook

Oscillatory journal bearings A13
-~
ENGINE CROSSHEAD BEARINGS Dimtral
Bearing allowable clemmce
Oil pressure as high as possible, materials * peak (in am/ Remarks
but at least greater than presswe mmfl
0.2 MN/m2 JOIbf/in" pin dia.)
\
White metal JMN/m2 A, Excellent resistance
(tin base) 1000 0.5-0.7 against scoring.
Ibf/in2 Corrosion
resistant. Low
fatigue strength
Copper-led 14 MN/m2 2A2 High-strength
I
2000 =1 bearing metal
lbf/in2 sensitive to local
high pressure.
Liable to
corrosion by
acidic oil unless
an overlay of
lead-tin or lead-
indium is used
25
(e pm)
Tri-netal 14MN/m2 2A2 Same as above, but
SECTION 6-6 e.g. steel 2000 11 better resistance
Bad design axial oil copper-lead lbf/in2 to corrosion,
groove. Edge may act white metal wear and
as an oil scraper (lead) scoring. Installed
as bearing shells,
Good design axial oil precision
groove, with well machined
rounded edges
CROSSHEAD ENGINE * Tin-aluminium is also a possible alloy for crosshead bearings,
amax% t14~ and spherical roller bearings have been used experimentally.
EXAMPLE OF OIL GROOVE DESIGN IN A CROSSHEAD
BEARING Of: CA 300mm DIAMETER. ALL MEASUREMENTS
ARE IN MILL.IMETRES (SECTION 6-6)

Old (and still common) practice: New practice:
Bearing metal scraped to con- Bearing precision machined to
formity with wristpin over an an exact cylinder with radius
arc of 120-1 50". Mostly used slightly greater than wristpin
for large, two-stroke marine radius. MainIy hydrodynamic
diesel engines. Works mainly lubrication. In common use for
R =0.5 -2 with boundary friction. 4-stroke engines, and becoming
Lo-' common on 2-stroke engines.
Local high pressures and thermal instability

Oscillating bearings in
W
general, and crosshead Cent rat
bearings in particular,
have a tendency to be-
come thermally unstable
at a certairi load level.
It is therefore of great
importance to avoid
local high pressures due
to wear, misaIignments Elastic
or deflections such as supports A-A
shown in the figure at (a) (b)
RESULTING PRESSURE left.
Two possible solutions used
DISTRIBUTION ON In critical machine corn- in crosshead bearings:
WRISTPIN
w ponents, such as (a) Elastic bearing supwrts
crosshead bearings,
(b) Upper end of connecting
NOMINAL SHAPE DEFLECTED SHAPE temperature warning rod acts as a partial
equipment should be bearing [central loading)
installed
A13.4