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About The Shackle Forging Vs Casting : Which is Better
Time: 2018-11-26
This question, “Forging vs. Casting: Which is better?” is one that I have been asked many times. To properly explore the answer, let’s first consider the process of each
Forging and casting are two very different manufacturing methods. When something is cast the material is heated above its melting temperature and poured into a mold where it solidifies. When something is forged it is physically forced into shape while remaining in a solid state – although it is frequently heated.
According to engineer, I have always known that forgings normally have less surface porosity, finer grain structure, higher tensile strength, better fatigue life/strength, and greater ductility than castings. In other words, forgings are generally better for shackles. The basics of why are pretty simple. When you melt metal to cast it, the grain size is free to expand. When it cools back to a solid, the grain structure is courser and more random, decreasing its strength.
But just how much better is a forging than a casting ?
I did some research on the internet and found an excellent research paper* written by members of the Industrial & Manufacturing Engineering Department at the University of Toledo, shared by the Forging Industry Association. This paper compares a single type of product made both ways.
How’s this for hard numbers? No pun intended.
Based on this paper:
1.Forged parts had a 26% higher tensile strength than the cast parts. This means you can have stronger shackles at a lower part weight.
2.Forged parts have a 37% higher fatigue strength resulting in a factor of six longer fatigue life. This means that a forged shackle is going to last longer.
3.Cast iron only has 66% of the yield strength of forged steel. Yield strength is an indicator of what load a shackle will hold before starting to deform.
4.The forged parts had a 58% reduction in area when pulled to failure. The cast parts only had a 6% reduction in area. That means there would be much greater deformation before failure in a forged part.
If you were hanging a load overhead from a shackle, wouldn’t you want that shackle to warn you before it failed? Or do you like surprises?
“Why or when would a rigging equipment manufacturer choose to use the forging method versus the casting method?”
The forging method is appropriate when a manufacturer is:
- (To a large extent) making solid parts. Forged parts can have open sides, through-cavities and pierced holes; however, certain design considerations, such as a draft angle for mold release, have to be taken into account.
- Producing high quantities of parts when an investment in tooling can be justified.
- Needing smaller, lighter parts. Parts can be smaller and lighter due to increased strength, toughness and ductility.
The casting method is appropriate when a manufacturer is:
- Concerned about high tooling costs or a large mold inventory. Cast tooling is less expensive and disposable.
- Looking to eliminate draft angles. Draft angles are incorporated in the mold to allow the part to be removed from the mold. With casting, the mold can be considered disposable or sacrificial. So, because the mold will be destroyed, it eliminates the need for draft angles. Cast tooling is also cheaper, but it only makes one part before being sacrificed.
- Minimizing required secondary operations because casting allows manufacturers to start closer to the finished shape.
Conclusion
Forged parts are always better for rigging equipment because of their part size (same strength from a smaller part), weight, strength, toughness and ductility properties. Cast parts are larger, heavier, weaker, more brittle and require more expensive inspection techniques due to the probability of internal defects. Unfortunately, many rigging manufacturers have resorted to cast rigging hardware because they have hammer size limitations.
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