The effects of “hammer blow” are discussed in the response to the FAQ relating to perceived benefits of a 3-cylinder arrangement. However they are covered in more detail in the Notes on FDC 8 where Wardale’s conclusions are presented as follows:
The 5AT locomotive can be satisfactorily balanced for 200 km/h operation by all criteria given in these calculations. The key to this is the utmost lightness of the reciprocating parts.
For 200 km/h the maximum permissible dynamic augment limits the amount of the reciprocating mass which can be balanced to some 19.7%, this fraction being quite small even for the lightweight components concerned. Conversely, according to the criteria of FDC 8 line items / and /, the reciprocating parts need no balancing at all. However the present recommendation is that the reciprocating balance be set at FDC 8 line items  – , which will give the least unbalanced forces on the locomotive’s structure at a dynamic augment no greater than that of the present BR 5MT at speed. If a lower maximum speed, V, is mandated than 200 km/h, the reciprocating balance can be increased accordingly, in the ratio of (200/V)2.
It should be noted that these calculations have mostly been based on ‘worst case’ conditions, e.g. design speed (200 km/h) which is greater than the maximum continuous operating speed (180 km/h), an engine-tender mass which is less than the minimum which will occur in practice, due to the necessity of always running with a reserve of fuel and water in the tender, and so on. Therefore, even under the most extreme conditions likely to be found in service, the unbalanced forces acting on the locomotive, the dynamic augments on the track, and the amplitudes of vibrations will all be less than given by these calculations, i.e. the riding of the locomotive will be smoother and its impact on the track less severe.