Forged Gear Blanks: Truly Time-Tested
by Jack McGuinn, Senior Editor
A forged rolled ring from Ajax Rolled Rings.
It has been said that clichés become clichés because they typically contain a kernel of everyday truth. One of the more common ones in manufacturing is "garbage in, garbage out"—and with good reason. In many ways, the process path to quality, cost-effective and profit-laden manufacturing begins at the receiving dock, where incoming materials—raw or otherwise—are inspected to ensure they meet agreed-upon specifications.
The gear industry is certainly no different. If on any given day what comes through your receiving area and OK'd by your quality team is later found to be wanting, various personnel will have some explaining to do.
Gear blanks, especially those intended for gears used in tight-tolerance, heavy-duty applications, are components that call out for extremely rigorous and vigilant inspection. And, there exist more than a few metal-forming processes for making blanks (more on that to follow). But arguably the oldest known metalworking process—forging—with its DNA linked way back to those smarty-pants Egyptian engineers—is our focus here.
We asked some suppliers for their take on the differences between forgings and, for example, castings—the two predominant processes for producing metal gear blanks.
Weighing in with a response—but necessarily wishing to remain anonymous—a vice president of sales for a Midwest-based forger says "The main advantage of a forged gear is that, due to the mechanical hot-working of the piece, the part will have higher inherent strength and cleanliness levels."
What other inherent advantages exist in forging over other metal-forming processes for manufacturing gear blanks? According to www.forging.org:
Forging of gear blanks compared with:
- Forgings are stronger. Casting cannot obtain the strengthening effects of hot and cold working. Forging surpasses casting in predictable strength properties, producing superior strength that is assured, part to part.
- Forging refines defects from cast ingots or continuous cast bar. A casting has neither grain flow nor directional strength, and the process cannot prevent formation of certain metallurgical defects. Pre-working forge stock produces a grain flow oriented in directions requiring maximum strength. Dendritic structures, alloy segregations and like-imperfections are refined in forging.
- Forgings are more reliable, less costly. Casting defects occur in a variety of forms. Because hot-working refines grain pattern and imparts high strength, ductility and resistance properties, forged products are more reliable. And, they are manufactured without the added costs for tighter process controls and inspection that are required for casting.
- Forgings offer better response to heat treatment. Castings require close control of melting and cooling processes because alloy segregation may occur. This results in non-uniform heat treatment response that can affect straightness of finished parts. Forgings respond more predictably to heat treatment and offer better dimensional stability.
- Forgings' flexible, cost-effective production adapts to demand. Some castings, such as special-performance castings, require expensive materials and process controls, and longer lead times. Open-die and ring rolling are examples of forging processes that adapt to various production-run lengths and enable shortened lead times.
- Forgings offer production economies, material savings. Welded fabrications are more costly in high volume production runs. In fact, fabricated parts are a traditional source of forging conversions as production volume increases. Initial tooling costs for forging can be absorbed by production volume and material savings and forging's intrinsic production economics lower labor costs, scrap and rework reductions and reduced inspection costs.
- Forgings are stronger. Welded structures are not usually free of porosity. Any strength benefit gained from welding or fastening standard rolled products can be lost by poor welding or joining practice. The grain orientation achieved in forging makes stronger parts.
- Forgings offer cost-effective designs/inspection. A multiple-component welded assembly cannot match the cost-savings gained form a properly designed, one-piece forging. Such part consolidations can result in considerable cost savings. In addition, weldments require costly inspection procedures, especially for highly stressed components. Forgings do not.
- Forgings offer more consistent, better metallurgical properties. Selective heating and non-uniform cooling that occur in welding can yield undesirable metallurgical properties such as inconsistent grain structure. In use, a welded seam may act as a metallurgical notch that can lead to part failure. Forgings have no internal voids that cause unexpected failure under stress or impact.
- Forgings offer simplified production. Welding and mechanical fastening require careful selection of joining materials, fastening types and sizes, and close monitoring of tightening practice, both of which increase production costs. Forging simplifies production and ensures better quality and consistency.
- Forgings offer broader size range of desired material grades. Sizes and shapes of products made from steel bar and plate are limited to the dimensions in which these materials are supplied. Often, forging may be the only metalworking process available having certain grades in desired sizes. Forgings can be economically produced in a wide range of sizes, from parts whose largest dimension is less than 1 inch to parts weighing more than 450,000 lbs.
- Forgings have grain oriented to-shape for greater strength. Machined bar and plate may be more susceptible to fatigue and stress corrosion because machining cuts material grain pattern. In most cases, forging yields a grain structure oriented to the part shape, resulting in optimum strength, ductility and resistance to impact and fatigue.
- Forgings make better, more economical use of materials. Flame-cutting plate is a wasteful process, one of several fabricating steps that consumes more material than needed to make such parts as rings or hubs. Even more is lost in subsequent machining.
- Forgings yield lower scrap; greater, more cost-effective production. Forgings, especially near-net shapes, make better use of material and generate little scrap. In high-volume production runs, forgings have the decisive cost advantage.
- Forgings require fewer secondary operations. As supplied, some grades of bar and plate require additional operations such as turning, grinding and polishing to remove surface irregularities and to achieve desired finish, dimensional accuracy, machine-ability and strength. Often, forgings can be put into service without expensive secondary operations.
- Forgings are stronger. Low-standard mechanical properties (e.g., tensile strength) are typical of P/M parts. The grain flow of a forging ensures strength at critical stress points.
- Forgings offer higher integrity. Costly part-density modification or infiltration is required to prevent P/M defects. Both processes add costs. The grain refinement of forged parts assures metal soundness and absence of defects.
- Forgings require fewer secondary operations. Special P/M shapes, threads and holes and precision tolerances may require extensive machining. Secondary forging operations can often be reduced to finish machining, hole-drilling and other simple steps. The inherent soundness of forgings leads to consistent, excellent machined surface finishes.
- Forgings offer greater design flexibility. P/M shapes are limited to those that can be ejected in the pressing direction. Forging allows part designs that are not restricted to shapes in this direction.
- Forgings use less costly materials. The starting materials for high-quality P/M parts are usually water atomized, pre-alloyed and annealed powders that cost significantly more per-pound than bar steels.
- Forgings offer greater productivity. New advanced-composite part designs may often require long lead times and substantial development costs. The high production rates possible in forging cannot yet be achieved in reinforced plastics and composites.
- Forgings have established documentation. RP/C physical property data are scarce and data from material suppliers lack consistency. Even advanced aerospace forgings are established products with well-documented physical, mechanical and performance data.
- Forgings offer broader service temperature range. RP/C service temperatures are limited and the effects of temperature are often complex. Forgings maintain performance over a wider temperature range.
- Forgings offer more reliable service performance. Deterioration and unpredictable service performance can result from damage to continuous, reinforcing RP/C fibers. Forging materials out-perform composites in almost all physical and mechanical property areas, especially in impact resistance and compression strength.
Metallurgy is indeed a complex, obscure science to most of us, which can perhaps make deciding on a supplier a bit tricky. But in reality the process is quite similar to choosing other vendors.
As for our mystery guest: "We believe that reliability and on-time delivery should play a key role in the decision making process. Quality is pretty much a given if you want to participate in the market long-term, and it costs our customers money on their bottom line when the suppliers are late."
Also responding is Bob Pancoast, sales and marketing for Ajax Rolled Ring & Machine in York, South Carolina: "Quality, on-time deliveries, price—in that order," are the criteria for choosing a blanks supplier.
One might reasonably wonder what level of sophistication is required of equipment operators at forging plants. Is, for example, gear design knowledge imperative for forgers of gear blanks?
"Gear design is not really of importance to forgers and casters" says Ajax's Pancoast. "The important thing is for the manufacturer of the gear blank to make the part to the metallurgical and dimensional requirements of the customer."
On the other hand, our Midwest-based forger believes that "Anytime a supplier can be more informed about their customer's products, the better the business relationship and opportunity to look for ways to add value to the customer's process. While that knowledge by itself doesn't win orders, the supplier does have to fully understand the specification requirements to ensure making something that will meet the intended design criteria. More knowledge of the customer's design can help facilitate more interaction and foster a long-term relationship. To that end, we have held forging seminars at some of our gear customers' locations to help them design better products." (Ed.'s Note: Have a case study, technical paper, etc., for us on any of the other gear blank processes mentioned here? We'd love to hear from you!)