CARSONS

Reconditioning Service for Motor Vehicle Engine Components

 

 

 

Regrinding crankshaft

Crankshaft Regrinding


The crankshaft is probably the most highly stressed component of the internal combustion engine as it has to harness and transmit the entire motive power generated by it. Crankshafts are usually made of high strength steel alloys by a forging process but they are occasionally made of a special grade of cast iron. The crankpins are encircled by the plain bearings in the connecting rods and these are subject to constant wear and tear due to the high speed of rotation, the heavy loads acting on them and the friction generated. The wear is not uniform however and is most pronounced on one side causing the crankpins to lose their perfectly cylindrical form and adopt a slightly oval cross section after long service. These changes may not be perceptible to the casual observer but can be detected by a trained eye and by measuring with a micrometer, which will give an accurate indication of the wear.

A decrease in diameter of the crankpins will naturally result in an increase in the clearance between the working surfaces. The wear on the connecting rod bearings too can be appreciable and this will contribute to a further increase in the clearance, causing a drop in oil pressure, possibly leading to bearing seizure! Therefore, any decrease in engine oil pressure could be taken as an indication of worn bearings but there may also be other reasons. Usually, the wear on the big end bearings and crankpins would be rather more than the wear on the main bearing shells and journals.

The remedy for wear on these components and the resulting ills is to regrind the journals and crankpins and fit new sets of plain bearings. These bearings are of semicircular form, having a soft layer of metal alloy deposited on a steel backing. It is the soft inner layer, commonly made of Babbitt metal that functions as the bearing surface. Babbitt metal is an alloy of lead, tin, antimony and copper of highly variable composition. Other materials that find extensive use in plain bearings are copper-lead alloys and tin-aluminium compositions.

New engines are always shipped with standard size bearings. As regrinding decreases the diameter of journals, undersize bearings should be fitted. Undersize bearings are available in various thicknesses from 0.25mm U/S to 1.5mm U/S, in increments of 0.25mm. The thickness of the Babbitt layer remains about the same for all undersize bearings, while the thickness of the steel backing is adjusted as necessary to make up for the increased gap between the journal and bearing housing. The reason for doing it this way is because bearing alloys are only strong when deposited in thin layers, with thick layers being susceptible to creep and eventual disintegration when subjected to heavy loads and rigorous service. Copper-lead based bearings are also deposited on a steel backing but are overcoated with a thin layer of tin-lead alloy to provide a low friction surface. The tin-aluminium composition is strong enough that the entire bearing can be made out of this material. In all cases, the thinnest possible undersize bearings should be chosen, to avoid grinding off more material from the crankshaft than is strictly necessary.

 

 



The configuration of crankshafts


In modern engines there is a main journal on either side of each crankpin to provide for smoother running. On older engines a pair of crankpins used to be joined together directly by a web without a main journal being interposed between them. As sharp corners are notorious for giving rise to stress concentrations that can result in the formation of cracks leading to eventual mechanical failure, the places where the journals join the webs are radiused distinctly to minimize such occurrences.

Crankshafts are usually designed and made for 1, 2, 3, 4, 5, 6, 8, 10, 12 and 16 cylinder engines. 7 cylinder engines are not unknown though! The crankpins of multicylinder engines are usually spaced out at 180, 120, 90, 72 or 60 degrees to ensure equal firing intervals, i.e. there is a power stroke every time the crankshaft rotates through a specified number of degrees. There are however, quite a number of engines that have unequal firing intervals. Shown below are photos of various types of crankshaft.

1-cyl-crank

Single Throw Crankshaft

2-cyl-crank

Two Throw Crankshaft

3-cyl-crank

Crankshaft of Three Cylinder Engine


4-cyl-crank

Crankshaft of Four Cylinder Engine

5-cyl-crank

Crankshaft of Five Cylinder Engine

Built up crank

Assembled crankshaft of 2-stroke engine
uses needle roller bearings throughout


The camshaft


Camshaft

A camshaft from an OHV V8 engine


The lobes of the camshaft operate the valves of the engine. The main journals of the camshaft can be reground if necessary on a crankshaft regrinding machine but not the lobes as they have complex profiles. But even if this proved technically feasible in a standard workshop environment, the case hardened lobes should not be machined as that would remove the thin, hard, wear resistant surface layer while also reducing the lift and affecting the timing of the valves.

 

 



More about crankshafts


truck crank

Ferrari crank

Both crankshafts depicted above share the same basic configuration; i.e. each has 7 main journals and 6 crankpins that are disposed with the identical angular spacing. The crankshaft at the top is from an inline 6 cylinder truck engine and the one below from the V12 engine of a Ferrari. The crucial difference is that a pair of connecting rods are fitted to each crankpin of the latter. Note that two oil holes are drilled through every crankpin.

Odd crankshaft delivers even firing


Buick V6 crank


Engine makers sometimes use their V8 engine block production lines to make their V6 blocks too, in order to cut tooling costs. When they compromise like this, the result is a 90° V6 that fires at uneven intervals of 90°, 150°, 90°, 150° and so on with a standard three throw crankshaft. It is quite possible to transform this into a regular firing engine by resorting to a staggered crankpin arrangement, spaced 30° apart, such as is evident in this Buick crankshaft.


Lubrication


oil passages

oil holes

All crankshafts have oil passages drilled through them to feed engine oil under pressure from the main journals to the crankpins and the big end bearings. Lubricating oil is supplied under high pressure by the oil pump to the main bearings first through oil galleries formed in the engine block. Oil continuously passes through the crankshaft as it rotates at high speed. Needless to say, the recommended grade of oil must be used as it is the lifeblood of the engine.

There has to be a clearance between the rubbing surfaces of the crankshaft and the plain bearings to permit engine oil to circulate between them and prevent metal to metal contact. The clearance is critical for ensuring long and reliable performance and service. The only way to achieve this is by regrinding the crankshaft accurately to the specified dimensions, taking care to obtain a good surface finish. This naturally calls for the use of a purpose built machine and a skilled operator.


Smoothness of operation


Whereas the smoothness of an engine is primarily determined by the number and configuration of cylinders, certain design features of the crankshaft can also play a part in this. Separating adjacent crankpins by interposing main journals between them helps, as does the use of integral or separate balance weights on each crank web to minimize vibration. Additional machining may be required to ensure perfect mechanical (static) balance.

 

 

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