Evaluating Casting VS. Forging VS. Fabricating Technology
Which process is better for your application
Forged, fabricated, and custom cast steel gears are critical to highly engineered mechanical power transmission applications. It is evident in the meticulous way they're produced: they must be engineered for reliability, manufactured for durability, and machined for precision performance. Only then can they meet the rigorous demands of heavy-duty applications.
But which process is better for your application--forging, fabrication, or casting? In many cases, it depends upon the requirements of the design engineer. But, there are certain limitations that make one technology more appropriate than the other. For example, all methods can produce gearing up to 18 ft. in diameter. But, beyond 18 ft, fabricated and forged gears are difficult to manufacture due to design and various other manufacturing constraints.
More specifically, which will more cost-effectively satisfy your design criteria for gears greater than 2 feet in diameter? To answer, a basic understanding of the three processes is necessary since each affects the shape, size, and metallic alloy
composition of the component differently.
Forging
Forging is a viable processing solution when the following
criteria are met: first, the gear's specified steel composition must be obtainable from a mill; second, the design should be relatively simple; and third, there must be sufficient lead time. In general, foundries can accommodate special material requests easier than mills.
In the case of a forged gear, steel ingots are cast at a mill. The ingots are then reduced and forged into the desired shape.
Depending on the size of the forging, the gear's diameter is either formed by welding two large halves together, or by piercing a hole through a solid billet. To do so, requires two separate heat treatments: one to strengthen the billet for piercing (to prevent tearing); the other to meet the gear's hardness specifications. In some cases, certain hardness and/or material specifications may require pre-machining and welding (to complete the design), before the gear can be finished cut.
Because it requires tremendous force to shape metal in this manner, there is a practical limitation on achievable size and section thickness of forged gears. For this reason, forged ring gears usually fall in the 6- to 10-foot diameter range.
Casting
Generally speaking, castings are versatile and economical. Shape, size, and metal composition can be customized to
accommodate various applications without many of the limitations inherent in the forging process.
The casting process utilizes the liquid metal's ability to flow into extremely complex shapes--even those with internal
pockets and external projections. As a result, castings require less machining than forgings because they are closer to the desired shape from the onset. Castings also produce a seamless,
one-piece component that offers uniform strength and toughness. Any parting lines are removed during turning, boring and facing operations.
Custom cast gears are found in a wide variety of applications and industries. In the construction industry, for
example, they are typically used in drag lines, power cranes or shovels. This involves the careful design and production of a variety of swing ring gears, walking gears, reducer gears, and hoist and drag drum gears--each of which may require widely varying metallurgical and mechanical properties.
Types of castings used in the mining industry (especially for stationary crushing and pulverizing equipment) require special engineering considerations as well. Adjustment rings, top and bottom shells, wedges, and bevel gears all need the strength that only specially-cast steel alloys can provide.
Sizes for custom cast gears are limited only by the foundry's capacity and experience. Large gears 40 feet in
diameter and weighing up to 100 tons have been successfully manufactured. Such large gearing is expensive.
To effectively product gearing from 2 to 5 feet in diameter some quantities must be ordered, in order to amortize the cost of pattern equipment. Although, for "one time" or prototype samples, inexpensive, styrofoam patterns can be used. Material and hardness specifications can also add to the gear's cost.
Fabrication
Fabricated gears offer another option which, in some
applications, can reduce costs since no pattern is required while still maintaining high quality. Manufacturers are again limited by the ability to obtain a rolled, forged ring for the rim of the gear as well as oven or furnace capacity to stress-relieve after welding.
Size is also limited to about 18 feet. Generally, as gear diameters get larger, it becomes more difficult to maintain rim stiffness with a "T" section design and high face heights (common on fabricated gears). Also, the "box section" design can be difficult to weld.
The ease with which steel components can be welded together depends on their thickness, complexity of design, chemical composition and the manufacturer's experience. Plain carbon steel with a low hardness is typically easiest to weld. Type 4140 and 8640 steels are more difficult weld.
Since the weld is critical, heat-treatable electrodes are
often used to ensure that the hardness of the weld matches that of the base metal. This will require heat treating and/or stress relieving facilities.
Such fabricated gears are typically used in low horsepower applications like dryers, kilns, and small mills. Individual applications may "redefine" low horsepower limits.
Customization
As for customized metal composition, castings give engineers far more flexibility in specifying the types of metals which will ensure the gear's design integrity. Steel is essentially an alloy of iron and carbon, but through alloy additions (such as manganese, chrome, molybdenum, nickel, etc.), its characteristics can be radically altered.
Although steel forgings are mechanically worked, to enhance the steel's properties, obtaining special chemistries are difficult due to heat sizes required by the mill.
Design Considerations
Beyond dealing with steel's innate metallurgical characteristics, there are other design requirements and variabilities that must be considered. This is especially true
since gears require precision machining later in production.
For example, gear manufacturers must anticipate overall shrinkage and distortion in order to meet these critical dimensions. Tolerances are necessary, particularly on certain critical areas which require finish machining. A stable and
stress free part help to ensure that these critical dimensions are obtained.
As efficient as forging, fabricating, and casting have become, some defects can still occur, and are usually the result of inadequate design or poor manufacturing practices.
Types of forging defects include: internal bursts, poor grain structure and die design, laps of folded over metal, and cracking. Fabrication defects include improper welding techniques, lack of penetration, stress cracks, and distortion. Castings exhibit defects like shrinkage, trapped nonmetallics and gases, distortion, lack of hardness on finished machined surfaces, and heat tears. In some cases, the components can be repaired; in others, they have to be scrapped, depending on the specifications.
Relying On Manufacturer Support
How can you avoid surprises and make sure your forged or
custom designed gear is delivered on time? Choose a qualified manufacturer based on:
- experienced sales representatives and engineers
- state-of-the-art technology
- single source facilities for forging, fabricating, casting and machining
- reputation for excellent service
The most experienced manufacturers can produce both small and large gears; and their capabilities run the full gamut--
from computer-aided design and pattern making to in-house testing, welding and machining. Finally, look for a custom service-oriented supplier that gives you accurate quotations, offers cost-saving design suggestions, and provides realistic delivery dates.
Today's gears must meet the heavy-duty demands of industries' toughest jobs with precision, versatility and reliability. But as with any other specialized piece of machinery, they must be designed, engineered and manufactured with careful attention to detail. By providing accurate specifications and explaining your application, you can help the manufacturer produce an efficient forged, fabricated, or custom cast steel gear that's delivered on time and within budget.
Types of Steels
Plain Carbon Steel--(Less than .20% carbon) Softest and most ductile of the carbon grades. Heat treatment does not greatly influence properties. Typically used in the railroad industry, for automotive and steel industry castings, and in electrical and magnetic applications.
Medium Carbon Steel--(.20% to .35% carbon) Various heat treatments improve the strength, ductility and impact resistance of these steels. Applications include: machinery and tools in the transportation industry, rolling mill equipment, road and building machinery, etc.
High Carbon Steel--(.40 to .50% carbon) Higher hardness and strength make these steels ideal for applications where increased wear-resistance is necessary.
Low Alloy Steels--(Less than 8% alloy content) Can be heat treated to high levels of hardness and strength at specific cross sections. Generally feature better mechanical properties than plain carbon steel, as well.
High Alloy Steels--(Greater than 8% alloy content) Offer superior corrosion, heat and wear resistance.
Most commonly used elements for improving hardenability:
- Manganese
- Molybdenum
- Chromium
- Silicon
- Nickel
- Vanadium
- Copper
|
