Engine Components : Crankshaft assemblies

 Crankshaft assemblies:

• Crankshaft
• Engine bearings
•  Flywheel
•  Reciprocating action
• Crankshaft & bearings
•  Valve train overview

Crankshaft

The crankshaft is attached to the connecting rod in offset areas called throws - where the downward power pulses change into rotating motion.
Crankshafts must be strong enough to do this without bending or twisting. They are a one piece casting, or forging, of heat-treated alloy steel of great mechanical strength. Counterweights are formed to balance the throws, and also the big end of the connecting rod.
Fine balancing is done by drilling out or adding small weights.
The crankshaft rotates in the engine on journals which run in bearings called the main bearings.
The connecting rod with its big end bearings are attached to the crankshaft crank pins located on the throws.
Drilled passages carry lubricant under pressure from the main bearings to the adjacent big end bearings.
The rear of the crankshaft is drilled and tapped for flywheel attachment. Near the front of the crankshaft, a timing gear or sprocket is attached to drive the camshafts.
A drive pulley is mounted on front of the crankshaft. It drives many components including the alternator, the fan and water pump, and power steering.
Many in-line and V engines have a harmonic balancer attached to the crankshaft. The harmonic balancer is more correctly called the crankshaft torsional vibration damper. It prevents crankshaft vibration. In most cases the harmonic balancer incorporates the drive pulley.

Engine bearings

No engine can run without bearings. Bearings are used in engines to support and protect rotating parts and allow them to turn freely. The connecting rod must be able to spin freely on the crankshaft. The crankshaft must be able to spin freely in the engine block.
Connecting rod bearings and the crankshaft main bearings are called split-sleeve types, which means they are in two halves, called inserts, slippers or shells.
These precision-inserts have a steel back with a very thin layer of bearing material bonded to it. The bearing material is an alloy that can include metals such as tin, lead, aluminium and copper.
Bearings designed for light duty may be made of white metal. It’s an alloy of tin and lead, with small amounts of copper and antimony.
Alloys of tin and aluminium improve the load-carrying capacity for intermediate applications.
Copper-lead alloys give even more improvements. They’re used in applications such as diesel engines, and high-performance vehicles.
Bearings need a difficult mix of properties. They must be hard enough to resist wear, but soft enough not to damage the shaft.
The soft bearing surface also allows any hard abrasive particles to become embedded in the surface. They can become so deeply embedded, they are prevented from touching the rotating shaft by the film of oil.
It is the mix of metals, tin, lead, copper and others, into an alloy that makes this combination of hardness and softness.
In a main bearing, the upper half of the bearing fits into a machined section of a crankcase web. The lower half is carried in the bearing cap which bolts onto the crankcase web.
In a connecting rod bearing, its upper half is carried in the big end of the connecting rod. The lower half is in the connecting rod cap.
One main bearing has thrust faces which accept the end movement of the crankshaft. These can be in the form of flanges that are part of the beanng. Alternatively, a separate thrust washer can be fitted into a machined recess in each side of the bearing cap. Sometimes a mating recess for each side is machined into the cylinder block and mating halves fitted to both.
Under normal running conditions, spinning shafts ride on a microscopic wedge of oil.
Oil flows through a long gallery in the cylinder block. Each main bearing has its own oil supply passageway from this gallery. Passageways drilled in the crankshaft carry oil from the main bearing journals to rod journals.
Oil flow maintains the oil wedge between the shaft and bearing, and carries away particles that could cause wear.
Engine manufacturers specify the clearance required between the bearing material and the crankshaft. This clearance gives the best combination of oil pressure and flow.
As clearance increases with wear, oil flow increases, causing oil pressure to drop. Then the shaft may rub against the bearing surface and wear even faster.

Flywheel

A flywheel is a large rotating mass mounted on the rear of the crankshaft.
On a car with manual transmission, the flywheel is very heavy, and its momentum helps smooth out engine operation.
The flywheel links the crankshaft to the transmission, through the clutch. The flywheel has a machined rear surface. It is the clutch’s main driving member. Holes are drilled and tapped into the flywheel for attaching the clutch pressure plate.
On a car with automatic transmission, the flywheel is usually called a drive or flex plate. The drive plate is lighter than a conventional flywheel because of the weight provided by the torque converter.
The outer edge of the flywheel or drive plate has a gear called a ring gear. The electric starter pinion engages on this gear to rotate the engine for starting.

Reciprocating action

Reciprocating means going back and forth over the same path. A piston travelling back and forth in a cylinder is a reciprocating action. The connecting rod fastened at one end to the piston and at the other end to the crankshaft turns that reciprocating action into rotary action. Using reciprocating motion to make rotary motion and vice versa has many applications, large and small.

Crankshaft & bearings

The crankshaft turns because of the forces transmitted through the connecting rods. At the same time, it must be held in place. That's done by bearings. They reduce friction, and allow free movement.
The Crankshaft is held in place in the engine block by main bearings at points called journals. Different bearings do different jobs These bearings support the crankshaft in place but let it turn freely.
The crankshaft also needs to be located to stop lateral movement. This is done by using flanges between the connecting rod and the crankshaft are connecting rod bearings. They protect the spinning crankshaft at points also called journals.
On the rear of the crankshaft is a heavy flywheel. It stores up energy from the power stroke and helps keep the crankshaft turning.

Valve train overview

In the 4-stroke gasoline engine, the inlet and exhaust ports are opened and closed by valves. These valves need a system to control how they work. The valve is held in place by a valve guide, with a spring on its stem. A rocker arm is attached to a stationary shaft that allows the arm to pivot. The rocker arm compresses and releases the valve spring, so that the valve opens and closes.
The valves need a system to control how they work. This is done by using cams.
A cam is a lobe, on a shaft. It is specially shaped to open the valve, hold it open, and let it close. The cams control the valve action, but what drives the cams?
Cams are attached to a camshaft. In modern vehicles it's usually mounted over the cylinder head and is called an overhead camshaft.
Intake and exhaust lobes can be on the same shaft. Or there can be a shaft each for intake and exhaust.
Notice that in this design (right), there is no need for rocker arms. It's a much simpler arrangement. A camshaft can also be mounted in the engine block near the crankshaft. As the camshaft rotates motion is transferred through the pushrod to the rocker arm. It pivots and opens the valve.