Mazda Training manual — part 236

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Piston Engine Fundamentals

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MAIN BEARINGS

The crankshaft main bearings are split circular sleeves that wrap around
the crankshaft main journals. The upper half of the bearing has one or
more oil holes to allow lubricant to coat the inside surface of the bearing,
as shown in Figure 18. The upper bearing fits into a main support on the
bottom of the cylinder block. The lower half of the bearing fits into the
bearing cap.

FIGURE 17. The
torsional damper
controls twisting
of the crankshaft.

Torsional
damper
(crankshaft
pulley)

Rubber

Timing belt
pulley

Crankshaft

FIGURE 18.
Crankshaft main
bearings wrap
around the main
journals.

Upper main
bearing

Oil hole

Lower main
bearing

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FIGURE 19.
Bearing
clearance allows
oil to circulate
between the
bearing and
journal
surfaces.

Oil passage

Upper main
bearing

Oil film

Lower main
bearing

Crankshaft
main journal

Bearing
clearance

The wear surfaces of the bearings are made of softer material than the
crankshaft. The softer material reduces friction, and it tends to mold itself
around any uneven areas on the main journal. In addition, if wear does
occur, it will affect the bearing, which is cheaper to replace than the
crankshaft.

Lubrication

In most engines, the upper and lower bearings are not interchangeable
because the upper bearings have an oil hole. Pressurized oil is forced
through the hole, allowing lubricant to flow around the bearing as the
journal turns.

To prevent the bearing from seizing on the crankshaft journal, the inside
diameter of the bearing is slightly larger than the journal diameter, usually
by a few thousandths of an inch. The gap between the bearing and the
journal allows oil to circulate freely between the two surfaces, as shown in
Figure 19.

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Clearance

The gap between the bearings and the main journal is called the bearing clearance.
Clearance is one of the most critical measurements in the engine.

The oil that lubricates the bearings does not actually stay in a continuous film. As the
crankshaft turns, the oil works its way to the outer edges of the bearings, where it is thrown
off into the crankcase. New oil constantly feeds in through the oil hole to replace the oil
thrown off.

The constant flow of oil over the bearings helps cool them and flushes away grit and dirt
from the bearing surface. If the clearance is too small, not enough oil will be allowed in to
lubricate the bearings. The resulting friction will wear out the bearings quickly. On the other
hand, if the clearance is too large, too much oil will flow through the bearings. Oil pressure
will drop and the crankshaft journal may start to pound against the bearing rather than spin
inside it.

The oil pump in an engine has a limited capacity, and if all the oil is being pumped through
a few bearings close to the pump, then other engine parts will be robbed of lubrication. For
example, bearings further away from the oil pump will not get an adequate supply of oil, so
they will wear out quickly. (The major early symptom for loose or failing bearings is low oil
pressure.)

To prevent damage to the bearings — and possibly to the crankshaft — the bearing
clearances must be set precisely whenever the bearings or crankshaft are repaired.

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FIGURE 20. The
thrust bearing
controls the
front- to-rear
movement of
the crankshaft.

Upper thrust
bearing

Oil grooves

Lower thrust
bearing

Thrust Bearings

In addition to rotating, the crankshaft also tends to move back and forth.
This movement is called end play. To limit end play, one of the main
journals on the crankshaft is machined to accept a thrust bearing. This
bearing keeps the crank- shaft from moving back and forth. The upper and
lower thrust bearings have oil grooves that allow oil to flow around the
journal, as shown in Figure 20.

















REVIEW EXERCISE 3

Fill in the words that correctly complete these sentences. Check your
answers with the answer key on page 35.

1. The ___________________ is the main supporting member of the

engine.

2. The crankshaft is attached to the cylinder block with U-shaped parts

called ___________________ .

3. The term main refers to parts used to mount the

___________________ .

4. The bearing that controls crankshaft end play is called the

_____________________ .