Tribology and Vibration

Tuesday, May 05, 2009

Tags :  crankshafts

Crankshafts - Crank simple torsionThe crankshaft is perhaps one of the most critical components in the modern Formula One engine; its design is full of compromises and its manufacture is complicated and time-consuming.The ongoing fight to reduce friction leads the designer to ever smaller bearings, but for each engine there is a point at which friction begins to increase again as crankshaft flexibility causes edge loading on bearings. There are a couple of options for bearings, namely the almost ubiquitous plain journal bearing, and the less common roller bearing. Both plain and roller main bearings have been used in Formula One in recent years, although some manufacturers did not find the expected gains with the roller bearing when they tested it.

The banning of ceramic bearing elements threatened to deal the roller bearing a fatal blow in Formula One, but there are new steels available today which mean that this remains a viable proposition.Torsional vibration is another problem to be dealt with by the racing engine designer. This can be calculated in the first instance by hand calculations as can be found in the early books on the subject published decades ago. Ker Wilson’s five-volume epic on the subject has some good material for the modern engineer to digest. Many years ago detailed calculations for torsional fundamental frequencies were a legal requirement for the many people involved in engine design for marine applications, and it is thanks to this that literature is available to help the engineer who does not have expensive finite-element methods to provide his answers.The question of lubrication can be dealt with by either feeding the crankshaft journals from the main oil gallery, and subsequently to the crankpins via angled drillings. The alternative, which is a favourite in Formula One, is to use a nose-fed lubrication system. This method was used on the Second World War Merlin aircraft engine, and has the advantage of requiring lower oil pressure than the gallery fed system as the oil does not have to overcome centrifugal forces in order to travel radially inwards to longitudinal galleries on the crankshaft. A V10 Formula One engine manufacturer achieved good results from a gallery feed crankshaft by employing some clever design features which allowed them to run quite a low main gallery pressure, however, this engine was never raced. The choice of location, size and shape of lubrication drillings has a profound effect on crankshaft stress, and failing to take this into account leads to many failures in service.The vast majority of crankshafts are made of nitriding steel grades of varying quality, and then nitride hardened to give a hard, fatigue resistant surface. Opinions vary as to the optimum depth of the nitrided case, and the range of depths employed in Formula One varies from around 0.4mm to 0.9mm. The specified hardness of the core also varies widely. Nitride hardening is felt to be most effective on steels of higher core hardness, but many engine manufacturers feel that core hardness does not need to be very high. One final note about material choice concerns the quality of the steel, or its ‘cleanliness’. Fatigue studies by a well-known steel manufacturer show that the fatigue strength of a nitrided steel is strongly influenced by how ‘clean’ it is, with the more expensive double-remelted grades showing a very clear advantage.Owing to the restrictive nature of the Formula One regulations, engines had begun to converge toward today’s maximum allowed bore of 98mm, especially before the decreasing rev limits recently introduced. The 98mm bore gives a very short stroke of less than 40mm, and it is thought that the majority of manufacturers use this bore size or something very close to it.The matter of counter weighting in Formula One is dealt with using tungsten alloy pieces which are fastened to the crankshaft using threaded fasteners. Some previous designs had inserts which were inserted into the steel crankshaft, but this does not give the same benefit in terms of reducing dimensions and inertia. Depleted uranium has been considered but is now outlawed under the maximum density regulation: “No material with a density exceeding 19,000kg/m3 may be assembled to the crankshaft.”Coatings are sometimes used on Formula One crankshafts with DLC being popular. This is not universal however, as some manufacturers had not developed this technology before the engine homologation regulations came into force.The challenges for the crankshaft designer are many and varied, but also interesting. He must provide a part which is sufficiently strong and stiff whilst keeping mass, inertia and friction in mind.

Leave your comment