High-Strength Tungsten Alloys
In a recent article in the crankshaft section of the website, I discussed very briefly the methods by which heavy metal counterweighting can be added to crankshafts. Heavy metal is the common term, but a more technically correct description would be dense metal, and these are generally tungsten alloys.
The picture which accompanied the aforementioned article showed additional counterweighting mass in the form of cylinders or slugs of tungsten pressed into place in each counterweight. There are some advantages to this method, one of which is that the strength of the tungsten material is not too critical. There are several disadvantages of this method, probably chief among them being the small amount of tungsten which can thus be added. Also, the optimum place to add such heavy metal is at the very outside radius of the counterweight. Clearly, in order to contain these inserts, we require an amount of steel to surround in order to retain these dangerous inserts in place. There are serious consequences if one of these parts should become detached from the crankshaft - this generally happens at high engine speeds, and the high speed tungsten is easily capable of punching a hole through the side of the engine and any other objects blocking its bid for freedom.
To place the tungsten at the very outermost position, we must look to fasten these to the crankshaft, generally using threaded fasteners. If one works through the necessary calculations to determine the size and the strength of the fasteners required for this critical job, you soon come to realise that the strength of the tungsten itself is also critical. There are a number of material stockholders who supply tungsten alloys to the racing industry; it is used extensively as chassis ballast in Formula One. Owing to the fact that the loads due to these ballast parts is small in comparison with their dimensions owing to low levels of acceleration, there is scope to use a larger number of less critical fasteners and therefore the design engineer can tolerate a low strength tungsten material. Popular materials for chassis ballast typically have a yield strength of 550MPa.
For optimal counterweighting however, we need strong tungsten materials, because we naturally want to use the minimum number of fasteners for retention. The purpose-designed fasteners have a much lower density (approximately 50% lower) than the tungsten alloy, and each counterbore or clearance hole provided for these fasteners has a density approximately one fifteen-thousandth of the density of the tungsten, because it contains only air. Therefore any extra fasteners taking up volume which could be occupied by tungsten is diluting the effectiveness of the counterweighting.
In order to produce high-strength dense metal materials we must look to work-hardening methods. The main application for high-strength heavy metal materials is armour-piercing ballistics, and it is therefore to companies supplying the military that we must turn to gain a supply of this specialist material. In Europe there are a small number of suppliers who can supply material with a yield strength exceeding 1000MPa, and one in particular has two ranges of round bar which are termed ‘medium-calibre and large-calibre’, in reference to their normal application.
Written by Wayne Ward.