Crankshaft main features

The crankshaft converts the reciprocating motion of the pistons into a rotational movement, which is then used to power, in the case of a motor vehicle, the transmission and wheels. The large forces caused by the combustion pressures are transferred to the pistons and the connecting rods through to the crankshaft, which then rotates at very high speeds. On multi-cylinder engines the crankshaft harnesses the power from individual cylinders and converts it into a single rotational force. The length of the crankshaft dictates the length of the cylinder block.

The crankshaft of an engine is formed of a number of ‘sections’ such as that illustrated in Figure 2.26.

The main journals (a part of a shaft which rotates in a bearing is called a journal), which are located through the centre line of the crankshaft, rotate in the main bearings.

The crankpin or crank journal, to which the connecting rod is fitted, is offset from the main journals by a distance called the crank radius.

The webs of the crankshaft connect the main journals to the crankpin. Where the journals join the webs a fillet or radius is formed to avoid a sharp corner, which would be a source of weakness. This is of vital importance in crankshafts that are subjected to particularly heavy loads.

Balance masses

In some cases the webs are extended to form balance masses, which are used in certain types of engine. These masses help to ensure that the rotating parts are balanced as effectively as possible and so cause as little vibration as possible while the engine is running. It is not possible to balance completely the rotating parts of all types of engine; this is, for instance, one of the objections to the use of single-cylinder engines, which are impossible to balance perfectly, though they can be made satisfactory for certain purposes.

Figure 2.26 Example of a crankshaft fitted to a multi-cylinder engine

In some engines in which the rotating parts are perfectly balanced, masses similar to balance masses may still be used: centrifugal force acting on the crankpins causes heavy loading of the adjacent main bearings at high speeds, and by extending the webs to form balance masses a counter-centrifugal force is applied in opposition to that on the crankpin, thus reducing the bearing loading. Masses used in this way are more properly known as counterbalance masses.

Throw

There is some confusion as to the meaning of this term.

It is often used as an alternative to crank radius, but old books on steam engines define the throw as the diameter of the crankpin circle, i.e. the throw is equal to the stroke of the piston. It might, therefore, be better to avoid the use of the word ‘throw’ in this sense and clarify what is intended by using the terms ‘crank radius’ or ‘crank circle diameter’ as appropriate.

The same word ‘throw’ is also used in a slightly different sense: it is the name given to a crankpin together with its adjacent webs and main journals.

Figure 2.27 illustrates a single-throw crankshaft.

Crankpins do not always have a main journal on both sides. Figure 2.28 illustrates a two-throw crankshaft having only two main journals, the crankpins being connected together by a flying web. However, on most modern engines, a crankpin usually has a main journal at each side. Figure 2.29 shows a four-cylinder engine crankshaft with five main bearing journals.

Flywheel attachment

The flywheel is usually attached to the rear end of the crankshaft. The attachment must be perfectly secure, and it should be possible to assemble the flywheel in one position only. This is because the crankshaft and flywheel, besides being balanced separately, are also balanced as an assembly, and if the flywheel is not fitted in the same position as that in which the assembly was balanced, some imperfection of balance may arise that can cause vibration.

Note: It is possible that the crankshaft itself may be balanced within the limits permitted, but not perfectly;

the flywheel may also have a small but ‘tolerable’

imbalance. Should the crankshaft and flywheel be assembled so that the imbalance of the crankshaft and the imbalance of the flywheel are ‘added’ this would cause severe vibration. The problem could, however, be solved by turning the flywheel through 180° on the crankshaft.

Another reason to only allow securing the flywheel to the crankshaft in one position is that a reference point on the flywheel may also be used as an indication of TDC for number l cylinder (the reference point is detected by a sensor). This is common practice when an engine management system is used that requires an accurate reference point to TDC of number 1 cylinder.

Figure 2.27 A single-throw crankshaft

Figure 2.28 A two-throw crankshaft

Figure 2.29 Four-cylinder crankshaft with five main bearing journals

Figures 2.30 and 2.31 show a common method of attaching the flywheel to the end of the crankshaft. A flange is formed on the end of the crankshaft which is a close fit into a counter-bored hole in the centre of the flywheel. The flywheel is secured by a number of bolts, which pass through holes drilled axially through the flywheel face, and screwed into threaded holes in the crankshaft flange. One or more dowels are usually fitted to relieve the screws of shearing loads, and the dowels, or bolts, are often unevenly spaced to permit assembly in one position only. A suitable locking arrangement is provided for the nuts or screws.

Oil-ways

Engine lubrication systems are dealt with in section 2.13 in this chapter, but it is necessary to mention the method by which oil is supplied to the crankpin journals and main bearing journals. Oil is delivered under pressure to the main bearings (from an oil pump), where it is fed into a groove running around the bearing at about the middle of its length.

A hole is drilled through the crankshaft (as shown in Figure 2.32) running from the main journal, through the web to the surface of the crankpin. The main journal end of the drilled hole is positioned so that it ends at the groove in the main bearing; therefore, as the shaft rotates, a continuous supply of oil is able to pass from the groove through the drilled hole and along to the crankpin.

Some large crankshafts have their crankpins and journal bored out to reduce weight and therefore reduce

Figure 2.30 Flywheel bolted at rear of crankshaft

Figure 2.31 Crankshaft; four-cylinder, five-bearing and flywheel mounting

Figure 2.32 Oil-way drilled through crank web

Oil retainers

The crankshaft projects from the rear end of the crankcase, and some of the oil which is pumped into the rear bearing to lubricate it will escape from the rear end of the bearing to the outside of the crankcase. The oil, besides making a mess of the engine, will invariably contaminate other components (e.g. the clutch plate) which would be undesirable.

Figure 2.34 shows a section of a ‘lip-type seal’. The seal consists of a specially shaped synthetic rubber ring supported by a steel shell and fitting into a recess in the crankcase. The ring has a shaped lip which is held lightly in contact with the shaft by a ‘garter spring’.

the groove and directed towards the drain hole, thus allowing the oil to pass back to the crankcase. In most cases both the methods described are used on the crankshaft, although sometimes only one method may be considered sufficient.

Clutch shaft spigot bearing

The power developed by the engine is transmitted through the clutch to the gearbox. The gearbox shaft, onto which the driven part of the clutch is fitted, is usually supported in the gearbox and also at its forward end in a bearing in the rear end of the crankshaft. The bearing in the rear of the crankshaft (referred to as a

‘spigot bearing’) is often a roller-type bearing. Other spigot bearings may be simple bronze bushes pressed into axial holes in the rear end of the crankshaft (see Figure 2.36).

Figure 2.33 Method of sealing hollow journals

Figure 2.34 Section of a lip-type seal

Figure 2.35 shows two other methods of preventing oil escape from the crankshaft. Immediately behind the rear journal a thin ring or fin of metal can be formed around the circumference of the shaft. Oil reaching this ring from the rear bearing is flung off by centrifugal force as the shaft rotates, and is caught in a cavity from which a drain hole leads the oil back inside the crankcase.

Behind this ‘flinger ring’ a square-section, helical groove or scroll, rather like a coarse screw thread, is machined onto the surface of the shaft. The outer surface of the shaft has a small clearance inside an extension of the rear bearing housing, and any oil that reaches this part of the shaft will tend to be dragged round with the shaft but at the same time held back by the stationary housing. As a result the oil is drawn along

Figure 2.35 Scroll-type oil retainer

Figure 2.36 Clutch shaft spigot bearing

Front end or nose of the crankshaft

For most engines, the front of the crankshaft is used to drive other components and mechanisms. Importantly, the camshaft (which is the mechanism used for opening and closing the inlet and exhaust valves) is usually driven from the front end of the crankshaft, although in a few cases it is taken from the flywheel end. The front end of the shaft is extended beyond the front main journal, the extension being parallel or sometimes stepped, i.e. the forward part is of slightly smaller diameter than the rear part.

Onto this extension is pressed the timing gear or sprocket (chain-driven system) or timing pulley (belt-driven system). The timing sprocket or timing pulley then drives a timing chain or timing belt, which in turn the forces created when the crankshaft is rotating. The

ends of these hollow pins and journals are usually closed by caps or plugs held in place by bolts (see Figure 2.33) to prevent the escape of oil.

drives the camshaft. An additional pulley (or pulleys) are then attached in front of the timing gear or timing pulley. The additional pulleys are used to drive other belts that in turn may drive the cooling fan, the water pump and alternator or other devices such as a power steering pump.

A common method for locating the sprocket and pulleys on the crankshaft is with a ‘Woodruff key’. The key locates in a slot in the crankshaft nose and also in a slot in the gear or pulleys. The key transfers the drive from the crankshaft to the sprocket and pulleys. The sprocket and pulleys are then secured by a nut or screw threaded into the end of the shaft.

If a timing chain is used to operate the valve mechanism, the timing sprocket is enclosed inside a cover called the timing case, with the additional pulleys being outside of the timing cover. Oil retaining arrangements on the timing cover are similar to those at the rear of the crankshaft, although the oil flinger is usually a separate part, gripped between the timing gear and pulley bosses.

Note that some engines may use a train of gears to drive the camshafts. The first gear in the train will be the crankshaft timing gear, located on the nose of the crankshaft. All of the gears will be contained within the timing cover.

If a timing belt is used instead of a timing chain, the timing pulley will also be external (not enclosed within an oil-tight timing cover). A simple cover will, however, be provided to protect the belt.

Figure 2.37 shows the arrangement for locating a timing sprocket and the additional pulley, as well as showing the timing cover and a lip-type oil seal retained in the timing cover. The lip of the seal should be smeared with oil before fitting the pulley onto the crankshaft.

Crankshaft materials

Crankshafts are made of high quality steel or iron. These materials are processed by either forging or casting.

Forging gives greater strength, but stiffness is of more importance than strength alone. Most modern shafts have large journals and relatively short throws that allows the crankpins to overlap the journals when viewed from the end of the shaft. This feature allows the shaft to be cast, which results in a lighter shaft and is cheaper to manufacture because it requires less machining. In addition modem nodular irons are exceptionally strong and, when ground to a fine finish and then surface-hardened, provide an excellent bearing surface. In this case the term ‘nodular’ applies to the inclusion of minute rounded lumps of graphite, (carbon). These particles are not combined with the base material so their existence in a free state classifies the material as iron and not steel.

Following the initial forming, the shaft is ground to give a very smooth and accurately dimensioned surface of the journals and crankpins. Often the webs are not machined, but in the case of a high-performance engine the shaft would be machined all over to reduce oil drag.

Unless special precautions are taken, the repeated load applied to the shaft and the continual torsional flexing of the shaft while the engine is running, especially in the region of the rear journal fillet, causes the shaft to fracture. This fatigue can be minimized by

‘fillet rolling’ (Figure 2.38).

Figure 2.37 Mounting of crankshaft pulley and timing chain sprocket

Figure 2.38 Fillet rolling

For long life there must be a large difference in hardness between the shaft and the bearing surfaces. Many modern engines use a comparatively hard bearing, so the journals and crankpins must be hardened. The hardening of the bearing surfaces is achieved by either nitriding or an induction hardening process. The former process involves heating the complete shaft to a temperature of about 500°C in an atmosphere of ammonia gas for several hours. During this period the steel absorbs nitrogen from the ammonia and forms a hard surface of iron nitride. Induction hardening is an electrical process.