Methods of fitting heavy metal to counterweights, part 3Tags : crankshafts
The reasons why we might want to augment the moment caused by the counterweight by using a dense material are well understood and, in previous RET-Monitor articles, some of the methods by which we can add 'heavy metal' to crankshaft counterweights have been discussed. In this article I want to discuss the method that is generally held to be the most effective - adding tungsten. Happily, it can also prove to be one of the cheaper methods, especially when judged by the criterion of most additional moment per unit of expenditure.
If we are to take fullest advantage of the technique of replacing steel with tungsten, we need to replace the maximum mass - or, more accurately, reproduce the optimised counterweight moment by using tungsten. Tungsten achieves the same moment with lower mass and inertia. To summarise this 'simple' method, we bolt a big chunk of tungsten onto the crankshaft, with the bolts pointing radially inward, or more commonly parallel to each other and in the same direction as the mass centre of the counterweight acts.
The point about this method that makes it less widely used than we might expect is that it is one of the more risky methods by which the material can be attached. However, in light of the more positive reasons above, we should not be surprised to find that, in those rare production engines that use tungsten counterweighting, this is the chosen method - precisely for reasons of effectiveness and economy.
The reason that this method is risky is due to the high engine speeds in many motor racing series. The force acting on the dense metal mass at any given engine speed is proportional to its mass, the distance between the crankshaft axis and the centre of mass of the dense metal part, and the square of the rotational speed of the crankshaft.
Such are the forces involved that it can be necessary to use some pretty special fasteners. It is not possible to measure the stretch on these critical fasteners as we often prefer to with con rod bolts, whether or not the bolts are 'off-the-shelf' or a bespoke creation. Therefore we need to be sure that, taking into account the significant uncertainties involved in bolt tightening using conventional tools, we can preload the bolts to the required amount, giving a sufficient factor of safety against joint separation. For example, it may be necessary to commit to testing a number of instrumented representative assemblies in order to understand the torque-preload relationship, if this is the method to be used.
The risks of a mass becoming loose are significant - it will almost certainly puncture the engine, and possibly the car too, and land somewhere it was not intended to. If you are unlucky enough for this to happen, the damage to the engine can be extreme, ranging from comparatively 'mild' punctured casings to serious damage to many engine internals and castings.
This method should be used only where the forces are well understood, safety factors are proven and confidence exists in the design and manufacture of the fasteners and in their tightening.
Fig. 1 - This production engine from VW uses heavy metal counterweights on its crankshaft
Written by Wayne Ward