Common Gear Manufacturing Processes

The concept of gears has been in existence for a very long time, as they are among the oldest mechanical components known today. Whether in the automotive industry, aerospace, various industrial machinery, or even in everyday items like clocks, gears are ubiquitous. Perhaps you are interested in understanding the manufacturing of gears.

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Gear manufacturing is not a singular process, as it involves various techniques that depend on the type of gear and its intended applicationt.

Casting

Casting is a simpler process primarily used to prepare blanks or cylindrical forms for gears, with the teeth subsequently machined. Due to its mass production capabilities and relative ease, casting is a viable gear manufacturing process for many applications.

However, in one particular area, casting stands out as the preferred manufacturing method. That is for producing exceptionally large gears. In cases of large diameters, machining methods and other gear-forming techniques may not be practical. Typically, larger gears are almost always of the spur gear type. Hence, their relative simplicity makes casting a very favorable choice.

Among the most common types of casting methods, shell molding, die casting, sand casting, and permanent mold casting are the most prevalent in gear production. Other methods have limited use in certain applications. However, the aforementioned ones are the most common in the industry.

Forging

This is another forming process that can provide you with blanks and ready-to-go gears as per your requirements. For relatively straightforward gears, forging is highly viable.

In theory, forging stands out as an excellent gear manufacturing process for heavy-duty applications, and the reason is simple. Forging involves heat treatment, implying that the final gear will have superior fatigue performance. However, the massive forces required in the forging process limit its dimensions and thickness.

In general, forging is suitable for gears with diameters ranging from 6 to 10 feet. Depending on the type of forging, such as precision forging, final machining of the gears may or may not be necessary.

Extrusion and Cold Drawing

This is another versatile and straightforward gear forming process. In fact, extrusion has lower tooling costs, but that doesn't necessarily mean it is the most economical process.

As the name suggests, extrusion involves pushing heated metal stock through a smaller, pre-determined shape. As a result, you obtain a bar of the desired shape, with its outer surface hardened and smooth.

The cold drawing process is very similar to extrusion with two differences. Extrusion pushes the stock through the die, while cold drawing pulls it through. Another distinction lies in temperature. Cold drawing does not heat the steel stock, thus improving mechanical properties at the cost of increased production expenses.

Powder Metallurgy

Powder metallurgy is an advanced process that has made significant strides in recent years. Today, it is employed in various manufacturing processes, including gear production.

So, how does powder metallurgy work? It may seem simple from an outward perspective, but it involves many intricacies.

It all begins with metal powder. The first step is to give all the powder the final shape you desire. Once that's achieved, the next step ensures that the entire setup is extremely compact, as this results in better mechanical performance. Heat the entire setup carefully, and you're done.

Powder metallurgy is highly efficient, simple, and suitable for large-scale production. You don't need to worry about any post-processing; the product is ready for use. However, gears produced through this method may not withstand very heavy loads, and there are also size limitations.

Moreover, the initial cost of any powder metallurgy setup is quite high, making it unsuitable for small-batch production.

Gear Machining

Due to its versatility, machining is one of the most common gear manufacturing processes. Traditional machining is prevalent in gear cutting and manufacturing, but the advancements in CNC machining have propelled its widespread adoption.

The following are the four most common gear cutting methods across the industry:

Gear Hobbing

Gear hobbing utilizes a conical cutting device known as a hob. As the hob rotates around the gear blank, both the hob and the workpiece are in motion. Currently, external spur gears and helical gears can be manufactured using gear hobbing.

The process is highly flexible and fast. You can also enhance productivity by simultaneously processing multiple blanks. However, it requires more skill and precision.

Gear Forming

Gear forming can produce gears that gear hobbing cannot handle. The tool can take any shape, such as small gears, rack shapes, or single-point shapes. It looks very much like a gear and works by cutting the blank into the desired shape. You can use the gear forming process to create internal or cluster gears, but specialty means individual pieces and higher costs.

Gear Broaching

Gear broaching may be the fastest method of gear forming cutting. It relies on a multi-tooth tool with embedded cutters that are deeper than their predecessors. This results in smaller, more manageable incremental cuts, providing you with the desired shape quickly without compromising accuracy.

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This gear manufacturing process is typical for internal gears, but you can also use it for external gears. For this, you would need a specialized shaving tool that allows you to replicate the same precision and efficiency.

CNC Milling

This is a fundamental method of gear cutting where you can sequentially produce individual gear teeth. However, it is highly versatile, especially when using CNC milling machines. While you can manufacture any type of gear on a milling machine, this method has its limitations in terms of precision.

Nevertheless, recent developments in the CNC and multi-axis domains have changed the landscape. Manufacturing gears on milling machines is becoming increasingly common. So, over time, the scenario may change.

Post-Processing in Gear Manufacturing

Depending on the gear manufacturing method you employ, your gears may require some post-processing to be 100% ready. The range of post-processing can vary from heat treatment to improve fatigue characteristics to dimension correction and surface finishing.

Here are the four most common surface finishing processes in gear production:

• Grinding: As the name suggests, grinding is a common surface finishing process that provides a smooth surface for the entire gear. It can be performed intermittently or continuously without affecting the outcome. An increasing number of gear manufacturers are adopting the grinding process.
• Honing: This process is suitable for gears that require extremely high precision. Honing uses small abrasives to grind the surface at low or moderate speeds.
• Lapping: Another common process for polishing surfaces and making them smooth. Additionally, it can correct minor errors in the tooth profile geometry.
• Gear Shaving: This process removes extremely thin layers from the surface to achieve a smooth contour. Gear shaving is typically expensive and less commonly used in gear production.

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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.

Forging

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.

Casting

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.

Fabrication

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 and 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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Design considerations