Mazda Training manual - part 235

 

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Mazda Training manual - part 235

 

 

 
3 – SHORT BLOCK 

 

25

Piston Engine Fundamentals

TC010-05-01S

CRANKSHAFT

 

 
The crankshaft is the component that changes the up-and-down motion of 
the pistons into rotational motion that drives the wheels of the vehicle. The 
crankshaft attaches to the cylinder block supports with main bearing caps, 
as shown in Figure 14. When the bearing caps are bolted on, they hold 
the crankshaft in place. When the block is machined, main bearing 
surfaces are aligned and bored on the crankshaft’s center line (line-bored), 
so bearing caps must never be interchanged. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 

FIGURE 14. Main 
bearing caps are 
used to attach 
the crankshaft to 
the cylinder 
block.

 

 

Cylinder 
block

 

Crankshaft 

Main bearing 

Thrust 
bearing 

Main bearing 
cap 

Main bearing 
cap mounting 
bolt

 

 

 

 

 
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26

Piston Engine Fundamentals

TC010-05-01S

 
 
 
 
 

FIGURE 15. Main 
bearing journals 
are used to 
mount the 
crankshaft. 
Connecting rod 
journals connect 
the pistons to 
the crankshaft.

 

 

Front end

 

Oil passage

 

Connecting 
rod journal 
(crankpin)

 

Flywheel end

 

Main bearing 
Journal 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Features

 

 

Figure 15 shows the important features of a crankshaft. The dashed line 
represents the center of the shaft. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
journal is a smooth, round bearing surface. A main bearing journal is a 
surface that mounts on the supports in the crankcase, with an insert 
bearing between the journal and the support. The main bearing journals 
actually support the crankshaft in the crankcase. In Figure 15, the main 
bearing journals are all in a line defined by the center line of the shaft. 

 

The other journals — called connecting rod journals or crankpins — are 
offset from the center line of the crankshaft. These journals are used to 
attach the connecting rods from the pistons. 

 

The connecting rod journals are positioned on the crankshaft so that the 
pistons can be at different points in the combustion cycle as the crankshaft 
turns. This spaces out the power strokes from the cylinders as they fire in 
turn. The result is a smooth flow of power, rather than a jerky, pulsing 
motion. 
 

 

 

 
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27

Piston Engine Fundamentals

TC010-05-01S

Construction

 

 

Because the crankshaft must handle the tremendous forces of the pistons’ 
power strokes, it is usually made of heavy, high-strength cast iron or 
forged steel for high performance or heavy-duty applications. Some 
crankshafts have counter- weights cast opposite the crankpins, as shown 
in Figure 16. The counterweights help balance the crankshaft and prevent 
vibration during high-speed rotation. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

The main bearing journals on a crankshaft are highly polished and are 
manufactured to exact roundness so they will rotate properly in the 
bearing inserts. Oil passages drilled into the main journals receive oil flow 
from the supports in the cylinder block. 

 

Slanted oil passages are drilled from the main journals to the crankpin 
journals to lubricate the connecting rod bearings. In addition, one of the 
main journals — usually in the middle — is machined with a thrust surface
This surface rides against a special thrust bearing that controls 
front-to-rear movement of the crankshaft. 

 
 
 
 
 
 
 
 
 

FIGURE 16. 
Counterweights 
on the crankshaft 
improve balance 
and prevent 
vibrations. 
 

Crankshaft 
counter- 
weights 

Main bearing 
journals

 

 

 

 

 
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28

Piston Engine Fundamentals

TC010-05-01S

Number of Journals

 

 

The crankshaft shown in Figure 16 has five main bearing journals and four crankpin 
journals. This is a common design for a 4-cylinder engine. One piston is connected to each 
crankpin journal with a connecting rod. 

 

You might expect V-6 and V-8 engines to use longer crankshafts, with more crankpin 
journals to connect the additional pistons. However, most V-6 and V-8 engines actually 
have shorter crankshafts because they connect two pistons to each crankpin journal. 

 

For example, a V-6 engine might use a crankshaft with three crankpin journals and four 
main bearing journals. When the cylinder banks are arranged at 90 degrees from each 
other, two pistons on the same crankpin journal will always be a half-stroke apart in the 
cycle. 

 
 
 

Vibration Damper

 

 

Even though the crankshaft is very strong, it must have a certain amount of “give” that 
allows it to flex slightly. The downward thrust against the crankpin during the power stroke 
actually twists the crankpin slightly. This force is immediately relieved, and the crankpin 
returns to its original shape. 

 

At a normal hot idle, this twisting and untwisting may repeat five times every second. When 
the vehicle is accelerating under load, the cycle may occur 25 or 30 times per second. The 
result of this constant twisting and untwisting is torsional vibration, or flexing of the 
crankshaft. 

 

To control torsional vibration, a vibration damper (or harmonic balancer) is usually mounted 
on the front end of the crankshaft, often as part of the crankshaft pulley. Figure 17 shows a 
typical torsional damper. 
 
 
 

 

 

 

 

 

 

 

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