The Crankshaft - the Hardest Working Part

Of all the parts within our motorcycle engines the crankshaft is surely one of the hardest working. It's function is to convert linear piston motion into rotary motion at the main shafts. However, the big end is without doubt the most highly stressed component.

The big end baring is subject to inertia loadings created by combustion chamber pressures. These forces in single engines are extremely high. For the crankshaft to produce the maximum engine performance, it must be a rigid as possible, yet capable of absorbing moments of compressive, tensile and torsional stress without breaking.

For simplicity, we'll concentrate on single cylinder crankshafts, using the term flywheel in respect of the large wheel into which the related components attach. Early flywheels were cast iron with detachable steel main shafts and big end assemblies, which performed satisfactory in engines of limited horsepower.

As engine speed and performance increased, cast iron wheels became vulnerable to fatigue cracking. The answer was flywheels manufactured from steel using drop forging. Drop forging is where the hot steel billet (flywheel) is forged between a stationary die and the moving die (hammer), this procedure improves the strength of the steel by aligning the grain structure along the lines of potential stress, therefore minimizing the chance of fatigue cracks. These stamping also provided the capability to stamp pockets into the forging ensuring the part of the wheel below the center line and a right angles to the big end hole is heaver than the top half of the wheel.
The heavy section is commonly referred to as the bob or balance weight. Without going into complex balance factors, the bob weight are required to cancel out some of the out of balance weight created by the big end assembly, con-rod, piston and gudgeon pin. Forged flywheels are preferable when taper fit big end assemblies are utilized as these impose high spreading loads on the wheel. A modern approach is the manufacture the wheels from billet (steel blanks).

Two large holes positioned on, or near, the center line of the main-shaft and two located each side of the big end hole take care of the bob weight requirement, ensuring the bottom half of the flywheel is heaviest. These holes are also carefully positioned to ensure the crankshaft has the intended peripheral weight. If the holes on the centerline of the wheel are moved inboard or decreased in size, this would have the effect of increasing the outer peripheral weight of the assembly (and vice versa), the small holes are used for final balancing.

Quite often old two-stroke flywheel halves are manufactured with integral main shafts and these perform with outstanding reliability, however this process has major drawbacks with large capacity single flywheels. Firstly, the material suitable for the flywheels is not necessarily compatible for the main-shafts. Normally the two main-shafts are manufactured from different materials as their mechanical loadings differ considerably. Secondly, the size of the billet required to manufacture one flywheel half with integral shaft for say a 350 or 500cc classic single-cylinder motorcycle would be just over 203mm long, which would incur enormous stock removal. Also, if one shaft fails or wears, then the flywheel assembly is scrap or at least requires special machining to install another shaft.

Early big ends featured crowded rollers, i.e. no cage, and while this configuration provides excellent dynamic load capabilities, it has limitations in higher revving engines. The roller assembly does not rotate round the pin at a constant speed instead it has to speed up and slow down, particularly at TDC, as the swing of the con-rod is in opposition to the motion of the big end pin, whereas at BDC, the two motions are the same but at different speeds. At high speed these uneven centrifugal forces inhibit the natural rolling motion.

A similar problem is encountered by the rollers being in contact with each other, as a roller turning clockwise tries to reverse the rotation of its corresponding roller causing it to slow down and rub against the two bearing surfaces. To overcome this, later big end assemblies incorporated a brass or alloy cage, which ensures separation of the rollers and therefore reduces friction and wear associated with the crowded type. Although the caged roller big end was a major improvement it still had limitations.

Large diameter rollers and their cage are heavy and as the inertia loads increase with engine speed relatively high loads are applied to the rollers and the spacing bars of the cage, causing the rollers to slow and limit the normal rolling process. It's not unusual for the cage, although made from a softer material, to wear away the crankpin shoulders and shards of debris can also hinder roller motion. Modern pressed up crankshafts overcome the above problems by utilizing larger diameter crankpins, smaller diameter rollers and silver plated alloy cages, the lighter cage and rollers ensure minimal inertia.

In any bearing there is inevitably a certain amount of rubbing encountered at high speed and without lubrication a seizure would result. Contrary to opinion, there is little to choose between plain big ends and roller big ends with regard to friction. Although high engine revs can be achieved with a plain big end, it requires a copious flow of filtered oil at high pressure.

The drive side main shaft of the older single-cylinder engines are mostly manufactured from EN32 (32 ton tensile) case hardened, carbon steel, while the timing side shaft is manufactured from the stronger EN24 nickel chrome steel (60 ton tensile). This is because the inertia loading on the drive side shaft is straightforward and although its effective diameter is quite large, it is theoretically increased by the shock absorber sleeve, which is pulled up against the outer bearing. The stronger steel is used for the timing side because while it does not carry high loads, it is subject to high reversal loads, imparted by the camshafts. Also because of design constraints, it is smaller in diameter and steps down further beyond the bearing. Keyway slots and the central oil-way drilling also reduces it potential strength.

One piece crankpins are normally case hardened to a depth of about 1.02mm and hardened to about 50/60 Rockwell. Although the pin must be hard on the roller surface, the core and threads must remain soft. If the core was hardened, the pin would break in service. Taper fit crankpins with shoulders and securing nuts, such as we see on most classic motorcycles, required very close tolerance control. A slight gap between the flywheel face and crankpin shoulders (about 0.254mm) is required prior to tightening, this enables the crankpin shoulders to be pulled up tight against the flywheel faces and at the same time governs the interference fit on the tapers, providing a high degree of rigidity.

Parallel crankpins with shoulders, present minimal problems providing the pin to flywheel hole interference is satisfactory (normally about 0.0254mm). The shoulders of the pins are pressed up hard against the flywheel faces, then the nuts are fully tightened to provide a rigid assembly. Modern large diameter parallel crankpins rely on very tight interference fits. If the flywheels are of the nickel steel case hardened type these fits can be extremely tight, which maintains maximum rigidity of the assembly. It must be noted that although case hardened nickel chrome steel has an extremely hard outer case the material retains a high core strength. One train of thought is that flywheels made from the softer EN24, in its T condition, can absorb inertia loadings from the crankpin, it cannot achieve the same level of interference fit on the crankpin and is prone to fretting in service.

Breakage of crankpins of the shoulder type invariably occur at the corner of the shoulder, so it is imperative that the correct corner radius is achieved during manufacture. Other failures include poor design, incorrect heat treatment or simply incorrect fitment. It is important that crankpins with shoulders (plain or taper) are pulled up hard against the flywheel faces, any gap no matter how small at this point will compromise the rigidity of the assembly allowing it to flex and increasing the possibility of breakage.

Classic four-stroke small ends are usually phosphor bronze, which retains its strength at high temperatures. Some two-stroke use needle rollers in a hardened and ground bore. Needle rollers have been used in four-strokes but, due to higher combustion pressure, are not as reliable.

Improvements to con-rod reliability has been the result of improved materials, heat treatment and stress relieving processes, and advanced machining techniques. Modern rods are supplied in many sections with strengthening ribs around big and small end sections and a shot-peened surface, which prevents fatigue cracks.
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