Page 235 - Tribology in Machine Design

220 Tribology in machine design

vertical clearance, but a high horizontal clearance, in an attempt to induce a
cooling oil flow through the bearing, so-called eccentric wall suction
cavitation erosion has occurred in the centres of the bearing lands in the
high clearance regions. Theoretically, low level of clearance is required for
good load-carrying capacity and generation of the hydrodynamic oil film,
but the oil flow through the bearing is restricted, leading to increased
temperature of operation, and consequently a reduced viscosity within the
oil film which in turn results in a thinner film. Thus, clearance has to be a
compromise of several factors. Theoretical calculations, based on a heat
balance across steadily loaded bearings, point to a standardized minimum
clearance of 0.00075 x journal diameter which has been verified by
satisfactory hydrodynamic, cavitation-free operation in a whole range of
different engine types under normal, varied, service conditions. The same
order of clearance has also been confirmed theoretically in computer
simulation of minimum film thickness with due allowance for the variation
in viscosity as a consequence of a clearance change.

5.9.4. Bearing materials
The modern medium-speed diesel engine, especially at the higher outputs at
increased running speeds, uses either tin-aluminium or copper-lead lining
materials in various compositions. With increasing requirements for high
load carrying capacity in a multitude of operating conditions there is a
general tendency for both types of lining material to be given a thin
galvanically-plated overlay of soft lead-tin or lead-tin-copper. This layer
derives its strength from the underlying lining, and becomes weaker with
increasing thickness. The basic advantage of an overlay is that it will
accommodate significant levels of built-in dirt particles, oil-borne con-
taminants, misalignment and distortion, together with minor manufactur-
ing inaccuracies of all the relevant parts.
With these inherent advantages, it is usual to design bearings for the
modern engine with a view to retaining the overlay, but this must be
considered semisacrificial in allowing for the above mentioned defects.
With copper-lead lined bearings, the overlay provides a much more
important service, that of corrosion protection. If the lubricating oil
contains organic acids and peroxides, for example, from leakage of sulphur-
containing fuel oils or blow-by of exhaust gases, the lead phase in a
copper-lead matrix can be leached out leaving an extremely weak porous
copper matrix which is easily fatigued by the dynamic loads applied to the
surface. Tin-aluminium is not subject to the same type of damage, and only
suffers corrosion if directly contacted by water, in the absence of oil.
Relative to the stronger copper-lead materials, the tin-aluminium ma-
terials are weaker, but if the fatigue limit of the composition lining and
overlay are taken into consideration, the levels are the same, being
dependent upon the fatigue strength of the overlay. As it is the intention to
retain this overlay, this becomes the design criterion. If the overlay is worn
away, for example, to accommodate misalignment then compatibility