Gears have been around for a very long time. It is generally understood that gears were invented by the Greek mechanics of Alexandria in the third century B.C., were further developed by Archimedes, and saw wide use in the Roman empire.
Gears and gearbox configurations come in many shapes and sizes, not to mention materials, tooth profiles and accuracy standards. Accordingly, there is a range of different materials and manufacturing techniques that can be used. Selecting the correct materials, gear designs and accuracy standards are not only critical to gear performance, but also can significantly affect cost. Over specifying a gear will incur unnecessary cost, under specifying a gear will result in under performance in the form of inadequate gear life or catastrophic failure.
The gear configuration itself, primarily relies on the specific application in which the gear is to be used. For the sake of brevity, I will focus on gear materials, relative performance, associated manufacturing process, accuracy standards and their collective impact on cost.
Plastic Gears
Plastic gears are widely used in low cost, low performance applications. However, material choices and manufacturing processes can enhance performance, but it comes at a price.
Plastic is inherently softer than metal, so plastic gears are subject to greater tooth wear over time. Plastic gears have the benefit of being corrosion resistant, quiet and light weight but, if subjected to high forces, are not as strong as metal gears and can warp, soften, deform and even melt if they get too hot.
One large advantage is that they can be produced at significantly lower cost if designed to be injection molded. Yes, tooling can be expensive but, the high production rate, and low overall manufacturing cost, makes this a very cost-effective solution for a gear in higher volumes. However, materials are limited to Thermoplastics or Thermoset Resins, both materials that can be melted and reformed without affecting their mechanical properties. Unfortunately, these plastics are relatively soft, so Injection molded gears are best suited to lower force applications.
In general, due to the poor wear characteristics of the softer plastics, along with dimensional issues associated with shrinkage or sink during cooling, injection molded plastic gears are most typically associated with small, light duty gears.
If greater strength and accuracy is required, machined plastic gears are a better solution. With much longer, more complex production cycles, their costs are significantly higher. However, the wider range of high-performance engineering plastics available, along with the higher accuracy attainable from CNC machining or hobbing, will result in stronger, wear resistant gear. Gear machining is also better suited to low volume gears as the amortization costs of an injection tool can be significant over a small production quantity.
Metal Gears
Brass has a high tensile strength and is relatively easy to machine. It is therefore ideal for intricate low-friction machined parts, such as gears. Its high corrosion resistance and inherent low friction properties produce a resilient, efficient gear, well suited to outdoor applications. Pound for pound, brass is more expensive than steel but, given its better machinability, it can be machined or hobbed faster than steel gears.
Steel gears are ideal for high-load, high-speed applications because of their exceptional strength and durability. Unlike plastic or other materials, steel gears can withstand high torque and horsepower without breaking or deforming. Steel is a hard material and therefore, relatively time consuming and expensive to machine. Pound for pound, steel is relatively low cost, and so a good solution for larger gears. With that said, I believe a big advantage of steel gears is their ability to have their material properties enhanced to improve overall performance. “Steel” is a very general term as there are many different steel grades. Alloying steel with other elements can significantly affect both performance and cost.
Selecting the correct steel grade for the application is critical in very high-performance applications. Its ability to be forged, hardened or annealed (softened) through heat treatment processes adds yet another facet to its performance. The “ideal” gear would have a softer, ductile inner core to make it less susceptible to shock and resistant crack propagation. However, the same “ideal” gear would have a hard, low friction surface to minimize tooth wear and extend gear life. By incorporating “case carburising” into the heat treatment process, where carbon is infused into the outer surface of a more ductile low carbon steel body, a gear can be produced with a shock and crack resistant core, along with a hardened, wear resistant surface.
Regardless of the heat treatment process used (if any), steel gears are usually machined or hobbed. Steel, especially hardened steel, lends itself well to being ground and so, for very high accuracy gears, machining or hobbing can be followed by a gear grinding process to produce gears of the highest accuracy. However, gear grinding is an expensive process and so care should be taken not to over specify gear accuracy unless it is really needed for the application.
Material Selection
In summary, along with the mechanical design considerations relating to the configuration of gears and gearboxes, there are many other factors that should be considered, especially for mid and high volume manufacture. Selecting the correct material to facilitate the most cost-effective manufacturing processes is key, and is fundamental to the mechanical design of the gear. Understanding the relative manufacturing costs of a gear is not necessarily obvious so, working with a company, with wide ranging capability and an ability to supply a variety of different options, can be key to minimizing the overall cost of the product while, at the same time, fully meeting all performance requirements.
Manufacturing Process Matters Too
The above is generally true, but, in some instances, the design requirements mean that custom process flows need to be developed. An instance that comes to mind is when a Sure Source client needed plastic (PEEK) gear to be end user interchangeable. In its normal operating conditions, the gear would be exposed to an abrasive environment which would accelerate wear or even cause significant damage.
The client had designed an outer PEEK gear ring (keyed bore, toothed outer diameter) which keyed onto a metal hub, mounted to the output shaft of the gearbox, and mechanically secured in place. PEEK was selected in order to minimize noise, give good corrosion resistance, reduce weight and keep the cost low.
Unfortunately, the injection moulding tolerances, were too large to reliably form a snug fit of every gear ring onto any hub. This variation was primarily due to batch to batch variation of the raw material along with shrinkage variation from the injection moulding process itself. Manufacturing the hub to tight tolerances was relatively straightforward given the stable nature of metal. To ensure a tight fit of the gear ring to the hub, a post injection machining process was added to take a light finishing cut around the bore of the gear. As the machining tolerance was much tighter than the injection tolerance, it therefore ensured a snug fit of the gear to the hub.
Modifying the bore, however, might adversely affect the runout (concentricity) of the bore to the outer toothed diameter of the gear ring. This was prevented by adding a light finish hobbing process to improve the tooth form and significantly reduce any runout. Although more expensive than an injection moulded (only) gear, the resulting part was still a lot less expensive that a metal gear, as well as being lighter, quieter and corrosion resistant.
Finding a company that fully understands the application, has a broad knowledge of a wide range of manufacturing processes, and has the in-house engineering experience and determination to find a solution is critical, not only to cost and performance, but to the development lead time and ultimately, time to market.
