For most applications, conventional manufacturing methods are capable of processing gears with the sufficient strength, fatigue resistance, tolerances, and friction coefficients needed to fulfill their intended purposes. For critical applications, however, manufacturers must overcome a variety of design, material, and production-related challenges to produce components capable of withstanding these demanding environments.
For the purpose of this article, we will define “critical” as an environment or industry requiring its parts to have one or more of the following attributes:
Extreme temperature flux resistance.
- High tensile strength.
- High fatigue resistance.
- Extreme corrosion resistance.
In contrast to conventional gear applications, critical applications require detailed engineering analysis, specialty materials, and unique designs — all of which can challenge conventional manufacturing methods.
The purpose of this article is to explain the various design, material, and production-related challenges affecting critical gear manufacturing, while also addressing potential solutions to improve production efficiency.
Design Challenges: Miniaturization and Thin Walls
Gear manufacturers are seeking new ways to reduce the size of critical components, while simultaneously maintaining (or potentially improving) the part’s original functionality in a process called part miniaturization. While this practice is not unique to gear manufacturing, part miniaturization is among the biggest hurdles for manufacturers in producing next-gen critical gears. The benefits of miniaturization include:
- Reducing manufacturing time.
- Reducing material usage.
- Installation in smaller environments.
Furthermore, the reduction in gear sizes can often be correlated with a broader manufacturing objective. For example, aerospace gear miniaturization often plays a role in a broader goal of “lightweighting” — reducing the weight to improve fuel efficiency.
Despite these varied benefits, a reduction in part sizes creates additional engineering challenges. Specifically, machining critical tolerances on gear features — such as low-backlash teeth — becomes increasingly difficult as part size decreases. Manufacturing processes that use heat or friction, such as milling, hobbing, or extrusion, can have particular challenges such as:
• Distorting the workpiece from tool vibration and/or heat.
• Develop re-cast layers or heat affected zones which degrade mechanical or fatigue properties.
• Create burrs and other surface irregularities.
One specific example of the challenges of conventional machining is creating thin-walled features. These features are often found in strain wave gears, also known as harmonic drives. A strain wave gear profile is comprised of an elliptically-shaped plug rotating a flexible spline along the inner teeth of another spline. These unique gear profiles have minimal backlash, high contact ratios, and are capable of operating with minimal lubricant, making them popular in critical industries such as aerospace and robotics.
However, strain wave gears are challenging to machine as the thin-walled features of the flex spline cup are easily distorted by thermal-based or contact-based processes. This issue worsens when the gear is made of a tough-to-machine material, such as bulk metallic glasses or hardened gear steels, which can increase machining forces, heat, and distortion.
Material Challenges: Advanced Materials & Surface Quality
While the design of a given part can determine a part’s ability to function within critical environments, its material can be equally as important; environments such as aircraft engines or in space require materials that fulfill unique prerequisites, including high resistances to temperature flux and corrosion, high tensile strength, and extreme fatigue resistance. Insufficient material types in these environments can lead to gears melting, corroding, freezing, or breaking.
For example, bevel gears within rotorcraft engines can only be comprised of specific temperature-resistant materials such as Pyrowear 53, as only these materials are capable of the high torque, speed, and corrosion resistance required to power the aircraft’s blades.
Bulk metallic glasses are another particularly unique material for gears. NASA has chosen these materials to comprise strain wave gears used in space-based robotic systems. Only this unique design and material combination can withstand the temperature extremes in space, have little-to-no backlash, and operate entirely without lubrication for prolonged periods of time.
The unique properties of advanced materials can be a “double-edged sword” for manufacturers, as the same material properties that appeal to design and application engineers are also the source of manufacturing challenges.
For instance, as these materials are specifically designed to withstand extreme temperatures and high loads, they are therefore difficult to machine with contact and/or heat-based machinating processes. Hardened materials, which can reduce friction in gears, can create microcracks, recast layers, and other surface irregularities when machined with conventional processes.
Furthermore, when materials such as some stainless steels are machined, they tend to work-harden quickly, leading to increased tool wear and breakage (also in turn reducing machining accuracy). Additionally, these materials also tend to have relatively low thermal conductivity, retaining heat during machining that can cause localized melting and other types of damage, affecting machining accuracy — especially important when machining smaller, tight-tolerance gears.
High surface quality is another important prerequisite for functional critical gears, and poor surface quality can lead to a variety of negative impacts including:
• Poor contact stress distribution, as a rougher surface can increase the risk of localized wear or failure, decreasing the lifespan of the gear.
• Rough or uneven gear surfaces can create noise and vibration during operation, which can further reduce efficiency or distract the human operator (for example, within robotic surgery equipment).
• Increased friction can decrease the efficiency of the gear, leading to higher energy consumption, heat generation, and lower productivity.
Even relatively minute changes to a gear’s surface quality can have an enormous impact on its functionality. In a study from 2019, researchers determined that decreasing the surface roughness from 0.5um to 0.2um Ra reduced the risk of fatigue failure by 37 percent.
Production Challenges
In addition to the challenges related to the design or material of the gears, gear manufacturers are also facing difficulties in affordably producing critical gear profiles in higher part volumes.
There are a few reasons why demand is increasing for unique, tough-to-machine gear components (such as strain wave gears) in critical industries in the first place. One reason is that aerospace engineering is increasingly incentivizing fuel efficiency by using lightweight, advanced materials in gear profiles (among many other parts in the aircraft).
Another example of demand growth can be found in the surgical robotics industry, expected to grow at an average of 16 percent CAGR over the next decade. This industry prioritizes precision, repeatability, and the reduction of unwanted noise/haptic feedback via small, tight-tolerance gears in the robotic joints, warranting higher part volumes in the future.
The central issue for gear manufacturers to accommodate these increasing part volumes is cost, and at the center of these costs is tool wear associated with difficult materials. As most conventional processes are contact and/or heat based (including hobbing, griding, and milling), tool wear is extremely common, exacerbated by the increased presence of brittle, tough materials in the gear industry mentioned previously. Tool wear significantly increases costs for high-volume gear manufacturers for a few reasons:
• Tool wear can lead to lower-resolution machining, affecting production efficiency by machining gears with increasingly inferior quality as the tool becomes less accurate with wear.
• Replacing a tool is usually a costly and sometimes lengthy endeavor, affecting production schedules.
• More energy may be required to achieve the desired gear geometries via higher cutting forces or speeds, resulting in increased energy consumption.
One other production issue can sometimes be found in the design of the gear profile, as gear hobbing or grinding often requires significant access to the gear teeth, which could be problematic for certain part designs, such as a spiral bevel gear and mating overhung pinion with an integral shaft. To make this gear as a single piece, the manufacturer had to redesign the shaft that goes through it for access of the gear-manufacturing process.
At this point, advanced gear manufacturers must choose from one of two broad options. One option is they may modify their existing machining technology to produce parts more efficiently, but this is often at the detriment of part tolerances, surface quality, or machining accuracy. The other option for manufacturers is to consider another machining process entirely.
Exploring Alternative Processes
To alleviate issues related to part design, material, and especially part volumes, gear manufacturers are often incentivized to explore alternative technologies capable of affordably and efficiently producing critical gears with the desired resolution, friction coefficients, and strength.
Some high-volume manufacturing processes may theoretically be capable of affordably producing high volumes of gears, (such as MIM), but may require a trade-off in the form of reduced resolution or machining accuracy. On the contrary, other processes can alleviate design or material-based challenges associated with gear manufacturing, such as wire EDM, but may not be capable of affordably producing these quality gears in high part volumes.
One alternative technology being explored is metal additive manufacturing (AM), as the technology’s benefits include allowing quicker design iteration, significantly reducing material and energy usage, and even potentially consolidating an assembly of gear parts into a single part. These advantages may be useful in alleviating many of the issues related to advanced materials, complex design profiles, and other costs.
However, AM is often not optimized for higher part volumes, and when it is, additional sacrifices to resolution and surface quality must be made — often in the form of thicker layer lines, larger powder sizes, and faster laser scan strategies. Furthermore, this technology is usually not applicable to most advanced gear applications, as its inherent limitations in resolution and surface quality are inferior to even most conventional processes. Lastly, some of the specialty gear materials are not easily processed with additive manufacturing techniques.
One example of an advanced technology with particularly unique benefits is pulsed electrochemical machining (PECM), a non-contact, non-thermal process capable of producing small features and high surface quality in advanced materials, as well as repeatedly machining identical features in thousands, or tens of thousands, of parts.
PECM works by flushing a charged electrolytic fluid in the microscopic gap between the tool (cathode) and the workpiece (anode). The custom-made cathode is shaped as the inverse of the desired geometry of the workpiece, which can be used to reduce the manufacturing and finishing steps necessary for complex components (such as the aforementioned bevel gear with the internal shaft).
The unique properties of this process afford it unique advantages for gear manufacturers:
• As it does not rely on friction or heat, PECM can machine otherwise tough materials with relative ease and speed, including tough gear steels or bulk metallic glass.
• PECM can achieve surface quality of 0.005-0.4um Ra (0.2-8 uin Ra) on a range of materials.
• As there is no contact with the workpiece, the tool is never worn during PECM, allowing a near-infinite lifespan and high repeatability for thousands, or tens of thousands, of identical parts.
• PECM is also capable of “parallel processing” for production applications, as a single machine can potentially create multiple features, or multiple parts, in tandem.
However, this machining technology’s advantages come with some tradeoffs:
• PECM is limited to conductive materials only; the electrochemical machining process will not occur on plastics, polymers, or other non-metal components.
• PECM has higher NRE costs than other advanced manufacturing processes to develop the custom cathode. PECM’s economic feasibility is largely limited to high volumes of parts and would not be ideal for machining low-volume or prototype gears.
Conclusion
Ultimately, gear manufacturers must consider a multitude of issues for gears capable of withstanding critical environments — be it miniaturization, thin walls, or the increased usage of advanced materials. The extreme temperatures in jet engines, oil refinery equipment, surgical robotic arms, or other critical applications warrant unique gear profiles, and therefore may require the use of unique, advanced material removal processes.
Rather than downgrading existing manufacturing methods to accommodate higher part volumes and increased use of critical gears, manufacturers should instead consider implementing alternative manufacturing processes entirely, as the distinctive properties of advanced manufacturing (including AM, PECM, or EDM) can improve design iteration of unique gear profiles, reduce production costs, or machine advanced materials with ease.
