Powerful gear sets, usually consisting of large spur and helical gears, drive rugged machines in a variety of heavyduty applications. In the construction industry, for example, they are typically used in drag lines, power cranes, and shovels. Applications in the mining industry include large grinding mills plus stationary crushing and pulverizing equipment. And, steel companies use them to drive rolling mills.
These large gears can be manufactured by three methods — forging, fabricating, or casting. Each method has certain advantages and limitations that make one more appropriate than another in a given application. For example, casting methods produce gearing from 2-ft diameter to 40 ft. But, fabricated and forged gears are generally difficult to manufacture in sizes over 18 ft because of design and manufacturing constraints (discussed later). Also, each method affects the shape, size, and metal composition differently.
But, which of these manufacturing methods is best suited to your design criteria and application requirements? A basic understanding of the three processes will help answer this question.
When the gear design has a relatively simple configuration, forging is a viable process. To make forged gears, steel ingots are cast, reduced in size, and forged into the desired shape. The forging process mechanically works the steel, thereby enhancing its fatigue properties. Forging dies are generally required, especially if the entire gear is forged, not just the rim and hub.
Depending on size, a gear is formed either by welding two large halves together, or by piercing a hole through a solid billet to form the bore. To do the latter requires a separate heat treatment to strengthen the billet for piercing (to prevent tearing). In some cases, hardness and material specifications may require pre-machining and welding the gear blank before the teeth are finish-cut.
Because it requires tremendous force to shape metal by forging, size and section thickness are limited. For this reason, forged gears usually fall in the 6 to 10-ft diameter range. Also, obtaining steels with special chemistries may be difficult because of heat sizes required by the mill.
Generally, the shape and metal composition of a casting can be customized for the application. The casting process uses the ability of molten steel to flow into complex shapes — including those with internal pockets (cavities) and external projections. As a result, castings often require less machining than forgings because they are closer to the desired shape as cast. Smaller gears, less than 36,000 lb, are cast in one piece, eliminating the need to weld or assemble components. Others are cast in halves or quarters and bolted together.
Engineers can specify different alloys (such as manganese, chrome, molybdenum, and nickel) to provide mechanical properties that meet application requirements. Thus, cast gears for applications in the construction industry (swing ring gears, walking gears, reducer gears, and hoist and drag drum gears) are produced from materials that give different metallurgical and mechanical properties. In the mining industry, cast gears accommodate special designs and are available in high-strength steel alloys.
Cast gears must be produced in sufficient quantities, especially in sizes from 2 to 5-ft diameter, to amortize the cost of pattern equipment. However, for “one time” or prototype samples, inexpensive Styrofoam patterns can be used. Limited only by foundry capacity and experience, cast gears can range up to 40-ft diameter and weigh up to 100 tons.
Another option, fabricated gears, can reduce costs in some cases because no pattern is required. Typically, a fabricated gear consists of forged rims and hubs connected by welded, steel-plate web sections. Forged rims are often formed by a ring-rolling process, which requires no forging dies. Rims made from steel plate are also available.
The maximum size of fabricated gears ranges from 18 to 24 ft, depending on rim thickness, face height, and material requirements. As gear diameters get larger, it becomes more difficult to maintain rim stiffness with a “T” section design and high face height. And, gears with large box sections can be difficult to weld.
The ease with which steel components can be welded depends on their thickness, design complexity, and chemical composition. Plain carbon steel with a low hardness is typically easiest to weld, whereas AISI 4140 and 8640 steels are more difficult. Heat-treatable electrodes are often used to ensure that the weld hardness matches that of the base metal. This requires heat treating and stress-relieving facilities.
Fabricated gears are typically used in dryers, kilns, and small mills, which operate at up to 1,000 hp, as well as large rolling mills and grinding mills.
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Special design requirements must be considered, especially when gears require precision machining later in production. For example, gear manufacturers must anticipate shrinkage and distortion in order to meet critical dimensions. Tolerances are particularly important in areas that require finish machining. A stress-free (and therefore, distortion-free) part helps to achieve these critical dimensions.
Though gears can be made efficiently by all three methods — forging, fabrication, and castings — some defects still occur, usually because of inadequate design or poor manufacturing practices.
Types of forging defects include internal bursts, poor grain structure, laps of folded-over metal, and cracking. Fabrication defects include improper welding, lack of weld penetration, stress cracks, and distortion. And, castings exhibit defects such as shrinkage, trapped nonmetallic elements, distortion, lack of hardness on finished machined surfaces, and other solidification defects.
In some cases, the components can be repaired; in others, they must be scrapped, depending on the specifications. Consult the manufacturer for information about the potential influence of defects on your application.
How can you avoid surprises and ensure your gears are delivered on time? Choose a manufacturer based on experience, state of-the-art technology, and service reputation. Some manufacturers can produce all sizes of gears; and they have capabilities ranging from computeraided design to welding, machining, and testing. Companies with facilities for all three manufacturing methods — forging, fabricating, and casting — can give advice on the pros on cons of each method as they relate to your application.
By providing accurate specifications and explaining the application, you can help the manufacturer produce a gear that is delivered on time and performs as expected.
William Rhody is the manager of steel casting & contract manufacturing sales for The Falk Corp., Milwaukee.