Methods of fitting heavy metal to counterweightsTags : crankshafts
In the article on crankshafts which will appear in Race Engine Technology (issue 50, November 2010) there is a brief discussion on the use of heavy metals for crankshaft counterweighting purposes. There are a number of reasons why it might be deemed desirable to use a high-density material for a crankshaft but, for a given level of counterweight moment, it will lead to a lower inertia crankshaft assembly.
The advantages of adding a high-density material to a crankshaft - or rather, replacing a given volume of the base crankshaft material with something more dense - are not lost on production engine manufacturers, with at least one company having offered production vehicles with tungsten alloy fitted to the crankshafts of some premium models for a number of years.
The material most commonly used for high-density counterweights is tungsten - or, to be more precise, one of a number of tungsten-based alloys. These generally contain nickel and other elements such as copper or iron. The density of such alloys lies in the 17-18g/cm3 range, more than twice that of steels.
A number of companies we questioned for the RET article said they have produced crankshafts incorporating high-density alloys, fitting them using a number of methods. The most commonly preferred method is based on cylindrical tungsten inserts press-fitted into holes machined into the steel counterweights of the crankshaft; these holes run parallel with the crankshaft axis.
This method has several points which make it a favourite:
1) The machining required to accept the inserts is made up of a number of simple cylindrical bores, and so can be done economically.
2) The inserts themselves are simple cylindrical pieces, easily made on conventional lathes
3) The method of attachment creates a high frictional force between the insert and the crankshaft, and in the direction that the insert is installed the forces which would act to overcome this friction are small. A large factor of safety against loss of the material is therefore easily achieved.
Where crank suppliers are making crankshafts that incorporate high-density materials, this method is least likely to cause problems. One company questioned said that, when fitting tungsten to a crankshaft by any other method, this is always done to the customer's design and specifications, and clearly at the customer's own risk. The photo here shows a crankshaft with tungsten added by this method.
The question of the material being able to break when put under the combined action of centripetal and press-fitting forces is reasonably easily dealt with by carrying out some basic calculations. The method of fitting places no great demands on the tungsten insert itself, so the alloy chosen for the inserts in this method does not need especially high tensile strength, thus giving another cost saving. High-strength material, which is generally used for military purposes as armour penetrators, commands a premium over normal ballast materials.
There are a number of other methods which are used for adding tungsten, and these will be examined at a later date.
Fig. 1 - This Peugeot Formula One crankshaft has tungsten inserted parallel with the crankshaft axis (Courtesy of Vitesse Engineering Services)
Written by Wayne Ward