Reply to "NEW AXLES FROM WILKINSON"

Well, I will post my opinion on this topic.

I have received a few calls from my DeTomaso buddies asking about forged axles versus billet axles because of this recent Wilkinson offer. So, to go on record I will post a response here.

I am a profession Engineer with a Masters Degree. My area of specialization is in aeronautical Structures - with over 26 years of experience. Material properties are an integral part of this area of engineering. I am Director of Engineering for the Canadian Government Civilian Airline.

Forging is the process by which metal is heated and is shaped by plastic deformation by suitably applying compressive force. Usually the compressive force is in the form of hammer blows using a power hammer or a press to form a part close to its final appearance. Forgings yield parts that have high strength to weight ratio-thus are often used in the design of aircraft frame members.

Forging refines the grain structure and improves physical properties of the metal. With proper design, the grain flow can be oriented in the direction of principal stresses encountered in actual use. Grain flow is the direction of the pattern that the crystals take during plastic deformation. Physical properties (such as strength, ductility and toughness) are much better in a forging than a billet piece machined to the same geometry (read shape)), which has, crystals randomly oriented.

The principle stresses, in the case of the axles result from the transmission of torsional loads. These are primarily resolved as principle shear stresses. The shear stresses are highest at the surface of the shaft proportionally reducing to zero at the neutral axis or shear centre at the centre of the shaft. There is relatively little strength added with a solid centre. Alternatively, weight is added by having material in the centre of the shaft which does proportionally little good. That is why good designs will hollow out the centres – saves weight. But, there is a compromise. There is a weight saving - but it costs more to machine the centre out.

Cost is an important factor in the design of a part as well. If few parts are required, it is much better to machine the part out of a billet of material. The amount of machining required is costly. Imagine taking a block of wood and wittling out a candlestick. You would have a lot of work to do and you would have created a lot of wasted shavings. The relative cost is acceptable because only a low number of parts are required. But each part costs about the same to make. If a large number of parts are required, the higher tooling costs to prepare a forging is offset and shared over a large number of pieces. A good forged part will be pretty close to the final shape so that there is minimal machining - minimal relative machining cost. Which help explains why, as claimed earlier, Wilkinson made over 1000 axles.

A good design will cater for the material, geometry and how the part is made - whether machined or forging.

One more thing: All things being equal, Forgings normally provide better durability – meaning fatigue performance. Mechanical fatigue performance is also a design consideration for a shaft.

Lets put it this way … how many of us always look for drop forged tools. We insist on this is because this tells us something about the strength and durability of the tool. Early blacksmiths hand forged swords and other items where the best material properties were important.

Personally, given the nature of the part, and the great price, it is an offer hard to pass up. I placed my order.

B. Goyaniuk