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Crankshaft balance factors

If you look at an old crankshaft from a large engine, such as a steam engine that might be found on a ship, you may notice that it has no counterweighting. The engine speeds that were common at that time were so slow that it seems wrong to use the word ‘speed’ at all. However, with low engine speeds, it was not necessary to provide any balance weighting to counter the rotating mass of the crankpin and the part of the con rod mass that can be considered to rotate along with the crankpin.

The forces exerted by these rotating masses are proportional to the stroke of the crankshaft and the square of engine speed. As engine speeds have risen, so has the need for crankshaft counterbalancing. Although we see many crankshafts with a pair of counterweights opposite each crankpin, this is not actually necessary, and there are crankshafts with four pins that have four counterweights, and in some cases only two.

There is always some discussion about the proper level of balance to be provided by any crankshaft counterweighting. When engineers refer to a percentage balance factor, this is to do with to the percentage of the reciprocating mass that we aim to balance. A more accurate way to describe it would be 100% of rotating mass and 50% of reciprocating mass – 50% is often the ‘standard’ that people aim for, and this can give a reasonable compromise between crankshaft mass/inertia, peak bearing loads and vibration. However, balance factors of between 20% and 80% are commonly used for various engine architectures, various running speeds and types of use, from light use on the roads to much higher duty cycles in motorsport. Sticking to a favourite value or something that is recommended for a particular brand of engine is often not the best case for engine performance.

Knowing the masses involved, speeds of rotation, cylinder pressures and so on, a good estimate can be made of the bearing loads throughout the engine cycle and at various relevant operating speeds, and if you have been involved in the design of a bespoke race engine then this may be a process you have worked through (unless you have a kind bearing supplier who will do the work on your behalf).

Depending on operating conditions, it might be found that a different balance factor would be a performance advantage, and it is possibly at this stage that the bearing supplier will want to be recompensed for its extra input if you want them to work through all the possible permutations. While the peak bearing load might increase, for example by changing to a lower balance factor, the mean bearing load through the operating cycle in the engine’s operating speed range may fall significantly. By comparing the expected frictional losses for various levels of balance factor, it might be decided to look at a crankshaft with a balance factor of less than 50%.

Like so many other aspects of race engine design, the choice of balance factor is a matter of compromise. In order to run the balance factor that is most advantageous for engine performance, the engine supplier may have to accept lower mileages between engine rebuilds (if the bearings are the limiting factor) or having to use more expensive bearings with a high load rating. 

Written by Wayne Ward

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