I would like to hear some expert opinion on crankshafts. A lot gets bandied about, but experience is extremely valuable. I have some basic engineering understanding, but would like to hear what has worked and under what conditions.
The choices are cast, forged, and billet.
Cast is the manufacturers choice. It is the least expensive to mass produce. Castings come in a variety of types. Investment, sand, die cast. The auto industry uses sand castings most of the time for engine components. A mold is made, then sand and resin is used to make the sand mold. Molten metal is poured into the mold and cured. Relatively inexpensive and gives adequate results. A casting is close to net shape and requires minimal machining. Castings have a fair amount of variability from part to part. Inclusions are common in castings which cause variation in life and strength.
Forgings are much better than castings, but the tooling has to be good to get the proper net shape(and the tooling is expensive). A forging can yield excellent material properties, much better than a casting. A one off is not going to happen with a forging. Forgings are also near net shape and reduce the amount of required machining.
Billet starts from a forged bar, and the final shape is achieved through machining, lots of machining. In most cases, the billet probably assures the best material properties, and certainly will provide a near perfect final shape and dimensions.
As to whether a forging or billet is better, stuff made in china is questionable. I would certainly be OK with a Chinese forging because I don’t make big power or spin high RPM. But for big power race applications, US manufactured billets, and US manufactured machined parts would be the way to go in my humble opinion(professional racing applications).
Achieving the right properties is either achieved through the casting/forging process or after. I am not a metallurgist nor am I going to play one, but heat treating is not a simple process and it effects the microstructure of the material. Materials have been engineered to achieve the strength and fatigue properties required for the application. The right properties resist cracks and crack growth, which is what will produce long life in a component.
Ability to with stand high cycle fatigue is as important as overall strength. A stock crank under stock conditions can probably last a million miles. The ability to handle higher cyclic loads decreases exponentially. Understanding this will help to decide what is best for a given application. I know the stock crank has been pushed fairly hard, but it would be good to know the condition prior to pushing it. Also, anybody know if the stock crank can be reconditioned to a “zero time” state? For mild applications this would great when it came time to rebuild.
The choices are cast, forged, and billet.
Cast is the manufacturers choice. It is the least expensive to mass produce. Castings come in a variety of types. Investment, sand, die cast. The auto industry uses sand castings most of the time for engine components. A mold is made, then sand and resin is used to make the sand mold. Molten metal is poured into the mold and cured. Relatively inexpensive and gives adequate results. A casting is close to net shape and requires minimal machining. Castings have a fair amount of variability from part to part. Inclusions are common in castings which cause variation in life and strength.
Forgings are much better than castings, but the tooling has to be good to get the proper net shape(and the tooling is expensive). A forging can yield excellent material properties, much better than a casting. A one off is not going to happen with a forging. Forgings are also near net shape and reduce the amount of required machining.
Billet starts from a forged bar, and the final shape is achieved through machining, lots of machining. In most cases, the billet probably assures the best material properties, and certainly will provide a near perfect final shape and dimensions.
As to whether a forging or billet is better, stuff made in china is questionable. I would certainly be OK with a Chinese forging because I don’t make big power or spin high RPM. But for big power race applications, US manufactured billets, and US manufactured machined parts would be the way to go in my humble opinion(professional racing applications).
Achieving the right properties is either achieved through the casting/forging process or after. I am not a metallurgist nor am I going to play one, but heat treating is not a simple process and it effects the microstructure of the material. Materials have been engineered to achieve the strength and fatigue properties required for the application. The right properties resist cracks and crack growth, which is what will produce long life in a component.
Ability to with stand high cycle fatigue is as important as overall strength. A stock crank under stock conditions can probably last a million miles. The ability to handle higher cyclic loads decreases exponentially. Understanding this will help to decide what is best for a given application. I know the stock crank has been pushed fairly hard, but it would be good to know the condition prior to pushing it. Also, anybody know if the stock crank can be reconditioned to a “zero time” state? For mild applications this would great when it came time to rebuild.
