3.12.
Engine Bearings
Engine bearings support the operating loads of the engine at all engine speeds and along
with lubricant, minimize friction. Most engine bearings are plain or sleeve bearing, in contrast
to roller, ball and needle bearings, called anti-friction bearings, which are used where minimum
lubrication is available. The lubricating system in automotive engines continuously supplies
lubricant to each bearing so that the shaft actually rolls on a film of lubricant in plain bearings.
The friction caused, in this case, is almost same as in antifriction bearings.
Fig. 3.75. Typical rod and main bearing load diagram and polar diagram.
It is important that the bearing surface must be large enough so that the bearing unit load
is within safe limits. Bearing load capacity is the bearing load per unit of the bearing projected
area. The projected area is the bearing length multiplied by bearing diameter. As greater bearing
loads are applied, bearing life is reduced, unless a higher quality, more expensive bearing is
installed. The load on engine bearings is determined by developing a polar bearing load diagram
(Fig. 3.75), which indicates the magnitude and direction of the instantaneous bearing loads.
Circles in the diagram provide the amount of force.
The ability of bearing materials to creep or flow slightly to match shaft variations is called
conformability. The bearing conforms to the shaft during the engine break-in period. In
modern automobile engines, there is very little need for bearing conformability as accurate
machining with close tolerance is possible.
The bearings must be capable of allowing foreign particles, which invariably pass through
oil filters, air filters and closed crank case ventilation systems, to embed in the bearing surface
so they do not score the shaft. To embed the particles, the bearing material gradually works
across the particles, completely covering it. This bearing properly is called embedability.
Bearing have a characteristic’. lied score resistance that prevents the bearing materials from
seizing on to the shaft during oil film break-down, allowing the shaft to come in contact with
the bearing causing localized hot spots of bearing material. Score resistance property is usually
the result of the relatively low melting temperature of the bearing material. Lubricating oil
contains a number of additives, to provide oil of the required characteristics. The additives
break-down under high engine temperature and high bearing loads, and from acids combining
with the by-products of combustion. The bearing resists attack form these acids and this ability
is called corrosion resistance. Corrosion of the bearing surface reduces bearing life.
Three bearing materials used for automobile engine bearings are babbitt, copper-lead and
aluminium. A 0.25 mm to 0.50 mm thick layer of the bearing materials is applied over a low
carbon steel backing of about 0.025 mm thickness. The steel provides adequate support for the
shaft load. The bearing material meets the rest of the bearing requirements. Copper-lead is a
stronger and more expensive bearing material than babbitt. This bearing material is most
readily damaged by corrosion. Babbitt is the oldest automobile bearing material and is still in
use where soft material is required for soft shafts running under moderate loads and speeds.
Copper-lead is used for intermediate and high speed applications. Aluminium is the latest one
and is well suited to high speeds, high loads conditions.
The bearing-to-journal clearance is generally from 0.027 mm to 0.0625 mm depending on
the engine. Doubling the journal clearance causes more than four times flow of oil from the edges
of the bearings. A large oil flow at one of the bearings can cause starvation on other bearings in
the oil system, resulting in the failure of the oil-starved bearing. The bearing has a slightly
larger arc than the bearing housing. This is called bearing spread and is from 0.127 mm to
0.508 mm to 0.508 mm wider than the housing bore. During installation, the bearing half
protrudes slightly above the parting surface. When the cap is assembled, ends of the two half
bearing shells touch and are forced together, which is called bearing crush. Crush holds the
bearing in place without turning when the engine runs. Crush must exert a force at least 82737
kPa at 394 K and the maximum considered is 275790 kPa.
3.12.1.
Crankshaft Main-journal Bearings
The crankshaft, underslung in the crankcase, is supported by main bearings (Fig. 3.68)
housed in the cross-webs forming the bulkhead of the cylinder block. The intersection of the
Fig. 3.76. Main bearing in pictorial view.
A. Win integral thrust flange. ‘ B. With separate thrust washers.
main-bearing axis is perpendicular to the cylinder axis. Half of the bearing housing bore is
machined out of the bulkhead web and the other half is formed in the bearing cap (Fig. 3.76A
and B). The main-bearing caps are aligned to the bulkhead half bore, and the most common
alignment device used in the dowel, collar, or stepped location joints. Liner half shell bearing
are used in between both halves of the housing bore. The main bearing normally contains a
central circumferential lubricating groove on the working face.
3.12.2.
Crankshaft Thrust Bearings
The crankshaft is subjected to end-thrust, which is an axial load in addition to that
experienced by the big-end journals and bearings. This axial load is transferred from the
crankshaft to the main bearing housing. This thrust may be generated intermittently or
continuously due to several factors which include :
(a) Intermittent disengagement of the clutch causes the flywheel to push against the
crankcase.
(6) Continuous torque conversion in some automatic transmissions causes the flywheel
to pull away from the crankcase.
(c) Either acceleration or deceleration of the helical valve-timing gear train during
operation pushes or pulls the crankshaft axially one way or the other.
(d) The helical-gear primary drive on front-wheel-drive cars with integral engine and
transmission creates an almost continuous load on the thrust washers.
The crankshaft is subjected to bending loads under certain operating conditions. This
imposes strains on the cylinder block and crankcase disturbing the axial alignment. Under this
situation, only the crankcase half of he thrust washer controls axial movement effectively.
Thrust washers are provided on each side of only one main-bearing housing bore, which takes
total axial crankshaft thrust in both directions. Since the thrust washers and the crankshaft
web have a parallel-ring face contact, no wedge-shaped oil film can develop to separate these
rubbing pairs, so very marginal lubrication takes place under continuous axial loading.
Two basic types of main-journal radial and end-thrust bearings are :
(i) Plain thin-wall bearings with integral thrust flanges (Figs. 3.68 and 3.76A).
Hi) Plain thin-wall bearings with integral thrust washers (Fig. 3.76B).
The integral flanged bearings use steel backing, bent at right angles on each outer edge to
form a flanged thrust face. Theses bearings are simply pressed into position, usually near the
flywheel end of the crankshaft. They are slightly more expensive than other types. The separate
thrust-washer bearings are split rings, placed in grooves machined on both sides of a main-bear-
ing bore housing. To prevent them from spinning, tags on each side of the cap half-washers align
with slots on both sides of the main bearing cap. No tags are provided on the crankcase half
washers. After installing the bearings and thrust washers, the end-float is checked by inserting
a feeler blade between the thrust washer and the crankshaft web. End-float should be normally
within 0.08 and 0.30 mm.
3.12.3.
Bearing Materials
Selection of Bearing Materials.
The properties required in a bearing material include the following.
High Fatigue Strength.
This permits the bearing to resist the high fluctuating pressure
in the lubricant film due to the periodic reciprocating-inertia and gas loads.
High Melting Point and Hot Strength.
This resists damage by high temperature
lubricant films and the reduction of yield strength of bearing alloys at elevated temperatures.
The oil temperatures in big-end bearings can reach around 423 K.
High Resistance to Corrosion.
This permits the bearing surface to resist attack from
degraded acidic lubricants at elevated temperatures.
Adequate Hardness.
This allows the relatively soft bearing surface to resist abrasive wear
and cavitation erosion caused by high-velocity oil and to sustain static and dynamic loads, but
without sacrificing conformability and embeddability.
Good Conformability.
This is the ability of the bearing surface to tolerate misalignment
between the bearing and the crankshaft. In general, conformability is inversely related to
bearing hardness.
Good Embeddability.
Due to this property the bearing surface absorbs dirt particles being
carried round by the lubricant and prevents scoring of the journal under high loads.
Good Compatibility.
This property provides resistance to steel journal against local
welding or pick-up from the bearing when loaded under boundary-lubrication conditions, but
with a rotational speed insufficient to provide a thick hydrodynamic oil film.
Classification of Plain-journal-bearing Materials.
Journal bearing material can be categorized into three broad groups :
(i) Lead- or tin-based white metal (Babbitt metal),
(ii) Copper-based alloys, and
(tit) Aluminium-based alloys.
White (Babbitt)-metal Bearings. White metals are basically either antimony-tin or
antimony-lead alloys having excellent conformability, embeddability, compatibility, and cor-
rosion resistance. However their fatigue strength is not sufficient for the loads experienced by
main and big-end bearings of today’s engines. Although the load-carrying capacity is slightly
improved by reducing the thickness of these bearing materials, still not high enough to meet
the requirements of today’s high-compression-ratio engines. White metal is mostly used for
camshaft and thrust bearings. The lead-based alloy has slightly superior hot strength to the
tin-based alloy. Typical compositions are as follows :
| Lead-based | White metal | Tin-based | White metal |
| Antimony | 15.0% | Antimony | 7.5% |
| Tin | 1.0% | Lead | 0.2% |
| Arsenic | 1.0% | Arsenic | 0.1% |
| Copper | 0.5% | Copper | 3.0% |
| Lead | remainder | Tin | remainder |
Copper-based Bearing Alloys. These alloys exhibit higher hardness and have higher
fatigue strength than the white-metal alloys, but have comparatively less conformability,
embeddability, compatibility, and corrosion resistance.
The copper-based bearing alloys fall into three main groups :
{i) Copper-lead 70% : 30% alloys, used for low to moderate-duty petrol engines.
Hi) Lead bronze with 24% lead, 1.5% tin, and 74.5% copper, used for heavy-duty petrol
engines.
(Hi) Lead bronze with 8% lead, 5% tin, and 87% copper, used for heavy-duty naturally
aspirated and turbo-charged diesel engines.
To improve the poor conformability, embeddability, compatibility, and corrosion resistance,
a soft overlay of either lead-tin-copper or lead-indium alloys is used. Since the latter overlay has
better fatigue strength than the former, slightly thicker lead-idium overlays is used, to improve
the properties of the complete bearing. Typical overlays compositions are 10% tin, 1% copper,
and 89% lead; or 5 to 10% indium and the remainder lead. An average overlay thickness is 0.02
mm, which can vary depending upon the applications.
Aluminium-based Bearing Alloys.
These bearing alloys cover a similar application range
to the copper-based alloys and are suitable for medium- to heavy-duty operating conditions with
both petrol and diesel engines.
These alloys fall into three main groups :
(i) 20% tin, 1% copper, and 79% aluminium. These alloys are used un-plated for moderate-
duty petrol engines.
(«) 6% tin, 1% copper, 1% nickel, and 92% aluminium with a lead-tin overlay. These alloys
are used for moderate-to high duty petrol and diesel engines.
(Hi) 11% silicon, 1% copper, and 88% aluminium with a lead-tin overlay of 0.025 mm
standard thickness. These alloys are used in turbo-charged heavy-duty diesel engines.
The tin-aluminium alloys are roll-bonded to a steel backing using a pure-aluminium-foil
bonding layer. The silicon-aluminium alloys are directly cold-roll-bonded on to their steel liners.
The high-tin-content aluminium alloy exhibit desirable bearing properties, hence does not
require an overlay. The low-tin and silicon alloys are harder and hence are overlaid. Aluminium-
based alloys do not suffer from corrosion attack unlike the copper-based bearing alloys.
