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Small end lubrication

con-rodsThe lubrication of the big end of the con rod is generally looked after very well, especially in the four-stroke racing engine as it is normally positively lubricated, i.e. continuously fed by high pressure oil issuing from the oil holes thoughtfully provided by the designer in the crankpin. The lubrication of the small end of the con rod is, by comparison, not so well provided for in terms of lubrication. Owing to the low surface speeds and the oscillating nature of the contact, we cannot expect to provide a continuous hydrodynamic lubrication regime in this contact, and therefore the requirement for a continuous flow of oil is not so important here.

As was mentioned in the early RET Monitor article on the subject of the little end and its design features, it is thought possible by some experts even to dispense with the bronze bush provided in the small end in some circumstances. It should be noted that this is not possible without taking other measures to ensure that the bare small end bore is suitably finished and of the correct mechanical properties, and that the piston pin is correctly designed and specified.

However, we must provide some lubrication in this area, and it is certainly possible to supply too little lubrication. It has been traditionally the case in the past to design con rods to have a single hole through the top of the small end bore into which oil, being carried around in the turbulent motion of the air / oil / blow-by mist in the crankcase could find its way. As engines have become more powerful and loads and pressures have increased, we haven't really seen an equivalent increase in the dimension of the piston pin, either in diameter or supported length in the con rod or piston. The same is true also of the series production vehicle engines, where 'power density' (the ratio of power to engine mass) is increasing as we seek to provide sufficient performance to our cars with a lower engine and vehicle mass. As we have increased the load applied to the small end contact and the frequency of oscillation we have seen a trend to increasing the number of holes from one to two (or perhaps more in some cases) and this has necessarily meant that these are no longer on the centre-line of the con rod. At this point we should take note of the con rod designs from two-stroke engines. The accompanying picture shows the small end of con rod from a production Honda motocross engine. The pattern of the two holes shown here is very similar to that now used on four-stroke competition con rods in many racing categories.

Referring to our mechanical engineering text books and taking to pencil and paper calculations can point us in the correct direction for the best (or indeed worst) positions for these holes in terms of stress. Clearly, finite-element stress analysis would be a further step toward accurate calculation of the effect of these holes on the stress field around the small end, if used correctly.

Fig. 1 - This Honda CR250 con rod has two holes allowing oil to enter the small end

Written by Wayne Ward

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