At a Glance:
- In the production of mechanical components, materials selection pinpoints the appropriate trade-offs related to cost and availability of materials.
- Turning to a range of subject matter experts from Misumi, Timken, igus, Festo and Sumitomo, the author examines how specific applications influence material selection and emphasizes the critical mechanical requirements for components such as gears and rolling element bearings.
The most critical aspect of machine design? Material selection, according to Cameron Washak, engineering standards and products general manager at Sumitomo Machinery. “It’s directly linked to performance, reliability and efficiency,” he says. “A finished product is only as good as the materials selected.”
For insights into current best practices in materials decisions—and some interesting case studies where a swap in materials brought unprecedented benefits—Machine Design reached out to industry experts. Here’s a close look at how material selection currently factors into component reliability and more, all at an affordable cost and with an acceptable lead time.
Different Measures of Quality
First, Washak explains that the critical factors in selecting the right material for any component relate directly to quality: attributes that affect performance such as durability, stability and strength. In Sumitomo gear boxes, for example, materials with outstanding wear properties like raw metals with high tensile and absolute strength are often top choice.
“Other strength factors gear manufacturers consider when designing their equipment include compressive strength and shearing strength—critical for gearing where tooth engagement is the primary form of operation,” Washak notes. “For applications where the unit will be operating intermittently or is subjected to high shock loading, impact strength is a critical feature. Engineers will also factor in fatigue strength and potentially also corrosion resistance for special environments.”
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In selecting material for the optimal performance of rolling element bearings, the specifics of the application (and again, performance in that application) dictate the correct choice, says Scott Hyde, senior scientist at Timken. “Important application requirements are lifespan, load capacity, operating speed, environmental conditions and lubrication,” he explains.
But in addition to mechanical demands, there are also requirements in many applications for a material to withstand or not react with specific chemicals, explains Stefan Emberger, director of engineering at Festo’s Technical Engineering Center in Boston. In some applications, components must not only be made of materials that offer absolute chemical inertness to avoid contamination or have a reaction with fluids or gases, but they also must be able to withstand operational pressures, temperatures and millions of actuation cycles in these conditions.
Emberger explains that especially in life science applications, materials must resist harsh cleaning solvents and exposure to reactive or toxic fluids without degrading or leaching any substances into samples. In these scenarios, he notes that “materials must not interact with or contaminate customer fluids. This drives the choice of high-purity stainless steels, fluoropolymers and specialty elastomers designed for aggressive chemical environments.”
It’s all About Trade-offs
Because customer funds are not unlimited and components are needed quickly, a huge part of materials selection is typically to nail down the appropriate trade-offs related to cost and availability of materials. Hyde points to steel cleanness as a good example (where cleanness is a measure of inclusion concentration, and higher concentrations increase bearing fatigue).
He explains that steel made using consumable electrode remelt processes is cleaner and thus offers longer bearing life, but is very costly and have long lead times. But if an application doesn’t require longer lifespan, less costly and more easily available “air melt” steels are a better solution.
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Hyde notes that narrowing down what cleanness level equates to what lifespan is a challenge. To help customers with this—in an era where demand for cleaner steel/higher-performance bearings has ramped up—Timken has gone beyond standard inspection methods for inclusion analysis such as metallography and fracture testing.
Instead, Timken often uses ultrasonic testing and automated scanning electron microcopy analysis. “These advanced techniques allow for more precise measurements and inclusion source determination,” says Hyde, “[as well as] supplier finger-printing and improved application assignment. This engineered analysis allows us to appropriately specify and purchase the best steel for any application while balancing cost, availability and performance.”
igus has taken another approach—although it has invested heavily in materials testing, as well. That is, to help customers choose the material that provides the required lifespan at the lowest cost, igus has expanded its range of materials options. Hope Mammone, the company’s iglide bearings product manager, explains that igus offers more than 60 proprietary plastic blends in these bearings. These blends are also available in other products, such as barstock, linear guides, spherical bearings, cable carriers and components for automation assemblies.
Misumi takes yet another approach, offering a model where machine builders choose a standard part and select a specific material, which is much faster and cost-effective than ordering a custom part. “Misumi has also performed in-house wear testing to compare different materials available on the market for specific products,” says Brian Hettinger, product engineer at Misumi USA. For example, the Misumi engineering team has compared toughness, cost and wear resistance of common tool steels and alloy steel used for locating pins, using that data to help customers.
Mechanical Performance—More Trade-offs
Narrowing in on the application requirements for strength-to-wear ratio in gearing and bearings, Washak says the formula most commonly used is mechanical strength (tensile) divided by wear rate. Higher ratios indicate higher durability.
Hyde notes that strength-to-weight ratio, wear resistance and also thermal stability are primarily determined by chemical composition and the microstructure that forms during heat treatment. Component size is also a factor. As an example, Hyde points to rolling element bearings, which tend to be made from low alloy steels (less than 5% alloy).
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“The amount and type of alloy typically needed depends on the size of product and heat treat process,” he explains. “As a product becomes larger, the amount of alloy needed to produce the correct microstructure, hardness and strength increases. At the same time, minimizing the amount of alloy needed and selecting certain elements will reduce cost and overall impact on the environment.”
In terms of chemistry, carbon and alloys must be present to create the strength and hardness needed in bearings. Carbon is either inherent to the steel chemistry, such as a through hardened AISI 52100 steel, says Hyde, or carbon can be introduced to the steel, such as a case hardening AISI 8620 through the heat treatment process like carburizing.
Either way, he notes that high carbon content at the surface of a bearing gives it a microstructural layer with the wear resistance and fatigue properties needed in roller bearing applications. Again, trade-offs with cost and manufacturability come into play when looking at which material and heat treatment process is best. Hyde explains that if thermal stability is required, additional heat treatment processes may be necessary. These stabilize the bearing microstructure to manage either elevated or sub-zero conditions.
Looking at the wear resistance needed for valve components and seals that undergo millions of cycles, Emberger says the materials choice range is large. FFKM elastomers maintain flexibility and resist degradation from cleaning solvents, for example, while stainless steel or coated surfaces reduce wear in moving metal parts. In addition, he says “engineered polymers such as Ultem and PEEK offer high strength-to-weight ratios, reducing system mass—important in compact automation systems—while maintaining chemical resistance and dimensional stability.”
“In practice,” he concludes, “we find that an integrated approach considering both mechanical and chemical requirements, alongside manufacturability, leads to the most reliable and cost-effective solutions.”
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Swapping Materials: Cases From Industry
While changing a component’s size, shape or other design attributes can provide simpler manufacturing, lower costs and other benefits, swapping the materials in a component is often an innovative move. Here are several real-life examples of how a materials switch led to unprecedented benefits, typically in more than one area of benefits.
Case Study: Permanently Reducing Risk
The problem: A food poisoning outbreak was traced back to a food processing plant. There, contamination of vegetables had resulted from the continual use of additional lubricant, required for the proper operation of metal bearings in various pieces of processing equipment.
The solution: Metal bearings across the plant were quickly replaced by Igus with its iglide FDA-compliant bearings, which do not need additional lubrication. This swap has reduced costs, the environmental impact of plant operation and also maintenance labor, while permanently removing a serious ongoing risk to food safety.
Case Study: Harsh Operational Conditions
The problem: EPDM seals in valve diaphragms used in automated systems that handle aggressive fluids are negatively affected by harsh cleaning solvents and elevated temperatures.
The solution: In this case, replacing standard EPDM seals with Festo FFKM elastomers dramatically increased resistance to these factors, enabling valves to reliably flex millions of cycles without chemical contamination. This improved durability and reduced downtime.
Case Study: More Sustainable Alloys
The problem: Timken manufactures very large (2.5m in diameter) mainshaft bearings for wind turbines. Case hardening through carburizing is the preferred traditional heat treatment process for the alloy used, but carburizing requires long processing times, high energy use, a complex press bench process and other challenges.
The solution: Timken engineers have developed new alloys—and a new heat treatment through hardening process called austempering. It offers short processing time and there is also no requirement for using large volumes of heat treatment atmosphere, which emit CO2. Austempering is also a batch process with no tooling involved, with the capacity to process multiple parts at the same time. The new process and alloys save costs, improve sustainability and shorten lead times while meeting the required application requirements.
Case Study: Addressing Unnecessary Weight and Size
The problem: Sumitomo’s Bevel Buddy Box conventional gear unit was larger and heavier than required.
The solution: Sumitomo changed the housing to a cast-iron version, which (with other design changes) has enabled a more compact and lightweight box that still offers the required shock loading capability. Performance of the hew housing was validated through strength calculations and physical testing.
Case Study: Better Wear Resistance Without Extra Weight
The problem: Longer lifespan was desired for a CNC-machined aluminum part being used in demanding conditions.
The solution: Misumi helped the customer switch from 6061 aluminum to 7075 aluminum, which has higher tensile strength and wear resistance. By staying with aluminum, the customer gained significant improvement in durability without adding extra weight.
