Crankshaft

As the pistons collectively might be regarded as the heart of the engine, so the crankshaft (fig. 3-40) may be considered its backbone. The crankshaft is the part of the engine that transforms the reciprocating motion of the piston to rotary motion. It transmits power through the flywheel, the clutch, the transmission, and the differential to drive your vehicle.

Crankshafts are made from forged or cast steel. Forged steel is the stronger of the two and is used in commercial and military engines. The cast unit is primarily used in light- and regular-duty gasoline engines. After the rough forging or casting is produced, it becomes a finished product by going through the following steps:

1. Each hole is located and drilled.
2. Each surface is rough machined
3. The crankshaft, with the exception of the bearing journals, is plated with alight coating of copper.
4. The bearing journals are case-hardened.
5. The bearing journals are ground to size.
6. Threads are cut into necessary bolt holes.

Crank throw arrangements for four-, six-, and eight-cylinder engines are shown in figure 3-41. The arrangements of throws determine the firing order of the engine. The position of the throws for each cylinder arrangement is paramount to the overall smoothness of operation. For the various engine configurations, typical throws are arranged as follows:

• In-line four-cylinder engines have throws one and four offset 180 degrees from throws two and three.

• V-type engines have two cylinders operating off each throw. The two end throws are on one plane offset 180 degrees apart. The two center throws are on another common plane, which is also 180 degrees apart. The two planes are offset 90 degrees from each other.
• In-line six-cylinder engines have throws a-ranged on three planes. There are two throws on each plane that are in line with each other. The three planes are arranged 120 degrees apart.
• V-type twelve-cylinder engines have throw arrangements like the in-line six-cylinder engine. The difference is that each throw accepts two-engine cylinders.
• V-type six-cylinder engines have three throws at 120- degree intervals. Each throw accepts two-engine cylinders.

The crankshaft is supported in the crankcase and rotates in the main bearings (fig. 3-42). The connecting rods are supported on the crankshaft by the rod bearings. Crankshaft bearings are made as precision inserts that consist of a hard shell of steel or bronze with a thin lining of antifrictional metal or bearing alloy.

Bearings must be able to support the crankshaft rotation and deliver power stroke thrust under the most adverse conditions.

The crankshaft rotates in the main bearings located at both ends of the crankshaft and at certain intermediate points. The upper halves of the bearing fit right into the crankcase and the lower halves fit into the caps that hold the crankshaft in place (fig. 3-43). These bearings often are channeled for oil distribution and may be lubricated with crankcase oil by pressure through drilled passages or by splash. Some main bearings have an integral thrust face that eliminates crankshaft end play. To prevent the loss of oil, place the seals at both ends of the crankshaft where it extends through the crankcase. When main bearings are replaced, tighten the bearing cap to the proper tension with a torque wrench and lock them in place with a cotter pin or safety wire after they are in place.

VIBRATION DUE TO IMBALANCE is an inherent problem with a crankshaft that is made with offset throws. The weight of the throws tend to make the crankshaft rotate elliptically. This is aggravated further by the weight of the piston and the connecting rod. To eliminate the problem, position the weights along the crankshaft. One weight is placed 180 degrees away from each throw. They are called counterweights and are usually part of the crankshaft but may be a separate bolt on items on small engines.

The crankshaft has a tendency to bend slightly when subjected to tremendous thrust from the piston. This deflection of the rotating member causes vibration. This vibration due to deflection is minimized by heavy crankshaft construction and sufficient support along its length by bearings.

TORSIONAL VIBRATION occurs when the crankshaft twists because of the power stroke thrusts. It is caused by the cylinders furthest away from the crankshaft output. As these cylinders apply thrust to the crankshaft, it twists and the thrust decreases. The twisting and unwinding of the crankshaft produces a vibration. The use of a vibration damper at the end of the crankshaft opposite the output acts to absorb torsional vibration.

Figure 3-40.—Crankshaft construction.

Figure 3-41.—Crankshaft throw arrangements.

Figure 3-42.—Crankshaft bearings.