The Crucial Role Of Material Selection in Machine Design

Among the many factors to consider in machine and equipment design, few are more critical than the material performance features.  This short article will explore some of the aspects of machine performance that a designer must consider in selecting the proper component materials for use in the end product.  A chart is provided herewith to assist in the material hardness conversions often needed when specifying components from various suppliers, who use differing methodologies to determine their hardness ratings. 

Click the above image to view the Material Selection Chart

When choosing materials for the structures and especially the motion-related components on a machine or a piece of assembly equipment, the physical properties of those materials are vitally important to the overall life of the end product.  The primary considerations when selecting materials must include the following: 

• Strength-Will the material selected withstand the impact, torque and friction forces placed on it?
• Brittleness-If the part is to be impacted on a repetitive scale, will it crack/shatter?
• Hardness-If pressure is applied to the surface, how much will it deform?
• Weight-How much will the part weigh, if made from a certain material, and will that weight create problems in other areas of the machine’s design?
• Machinability-How difficult is the material under consideration to machine, as this will effect manufacturing and build times?
• Weldability-How well does the material react to various types of welding and what impact will welding have on the resulting structure?
• Price-How expensive is the material under consideration and is it available from local sources?
• Corrosion Resistance-Will the part corrode when subjected to the ambient conditions in use?
• Ferrous-Is the material magnetic?
• Conductivity-Does the material conduct electricity or is it subject to static electricity?
• Wear Resistance-When subjected to repeated forces over time, how well does the material under consideration resist wear and what surface finishes might be considered?
• Temperature?

A reliable component supplier will advise the designer in all the above areas, once the specifics of the application are known and examined.  This checklist may need to be expanded and modified by the application environment, the particular restrictions on materials used, e.g. cleanroom applications, and even the local operating standards at the location of use. 

When consulting printed or online catalogs from component suppliers, always look for each component’s hardness ratings and available surface finishes.  Generally, this information will be included or available from the manufacturer.  Have such information on hand before making the materials selections of the components used.  For example, an engineer is seeking a precision linear shaft for use on an assembly.  The literature from a potential supplier indicates their shaft is available in 52100 bearing steel hardened to 58HRC or 440C stainless steel hardened to 56HRC.  This part is also available with hard chrome plating.  The engineer would know that the 52100 bearing steel shaft would be somewhat harder than the 440C stainless steel shaft and would also be less expensive.  However, the 52100 bearing steel would be much more susceptible to rust.  In many applications, this might not be a serious problem, if the part is to be covered with grease or oil, thus preventing severe exposure to the atmosphere and slowing down the corrosion process considerably.  In other applications, adding the hard chrome plating would be sufficient to retard the rust process, while simultaneously increasing the surface hardness of the shaft without excessive additional cost.  In certain cases, even this combination will not be sufficient to assure quality performance and the best selection is the more expensive, but considerably more rustproof 440C stainless.  Whatever the application, the catalog information should be sufficient to assist the engineer in making this determination and thus drive the buying decision. 

When the materials listed in a catalog are not familiar to the engineer, that catalog should have material property tables included.  These are also readily available online nowadays, with a simple word search.  These material property tables are especially useful when sourcing such materials as thermoset rubbers, urethanes, thermoplastics and especially the emerging thermoplastic elastomers for end products such as washers, bumpers, gel sheets, anti-vibration pads and others.  If the information included in the catalog is not sufficient, a quick call to the supplier’s customer service department is possible.  Sometimes, such calls will require application engineering assistance and it is always advisable to take this additional step, if there are any questions about a material’s suitability for use in the application under design.  As more exotic metal and composite  materials come into use, this service is increasingly necessary, but will benefit the designer in many ways.  With cost issues always a factor, as well, making such calls can often lead to alternatives the engineer might not be considering during the design phase. 

A good supplier will generally provide as much information as needed for the engineer to reach a functional, cost-effective and deliverable solution to the design challenge in materials.  Occasionally, material availability will not effect the pricing on a component, but it might effect the delivery and therefore should be investigated prior to determining a final specification. 

An engineer who calls a supplier with a predetermined material might want to conduct a quick review of the specifications to be certain of that material’s viability in application.  For example, there is a commonly held notion in many engineering departments that stainless steel does not rust.  This is, of course, untrue.  Stainless does indeed rust, but at a much slower rate than carbon steel, which begins to corrode literally as it is produced at the mill.  How much slower the stainless rusts depends on its material grade (303, 304, 440 etc.) and such information should be readily available from the component supplier. 

When assessing the hardness of a material for an application, it may not always seem apparent where this factor enters the equation.  There are two general application areas where the material hardness is critical.  The first is dampening materials such as urethane bumpers, washers and pads.  The second is in the area of wear resistance. 

Hardness is the measure of any material’s resistance to deformation under pressure, that is, its resistance to denting.  Pressure is force divided by area.  This becomes important for designers, especially when a much higher force is applied to a very small area.  In such cases, the pressure becomes substantial and can cause accelerated damage and effect machine performance rather quickly.  To illustrate this example, consider bearings.  There are basically two types of bearings:  rotary bearings that spin around an axis, as in a wheel bearing, and linear bearings where they travel in a linear direction, often in combination with precision linear shaft.  These linear bearings have ball bearings that make direct with the linear shaft.  Because each ball bearing contacts the shaft at one very small point, there is a substantial pressure created at that point under load.  In such cases, the surface of the shaft material must be very hard to resist such pressure and thus allow the reliable and accurate operation of the linear bearing.  If the surface was not hardened properly in its manufacture, the ball bearings would quickly dent the surface of that material, effectively creating grooves along its surface of operation and range of motion.  Among the many shaft manufacturers’ catalogs, an engineer should be able to easily locate soft (unhardened) shaft in 1045 carbon or 304 stainless with a hard chrome plating surface treatment.  Such shaft will not function properly with ball bearings.  Although there is a hard chrome layer with a surface hardness exceeding that of hardened steels, the chrome layer is not sufficiently thick to function with heat-treated ball bearings.  Essentially, the chrome plating layer can handle the pressure, but the material underneath will remain too soft to support it.  These shafts will, however, function quite well with the composite bushings made from a much softer material than the hardened steel ball bearings.  Such bushings are usually supplied with a solid lubricant, as well.  Since they do not utilize ball bearings, the composite bushings exert far less pressure on the surface of the shaft. 

Surface treatments are many today.  All have their function and a price point to consider, when designing.  Two major factors to assess in choosing the proper surface treatment are corrosion resistance and cosmetic improvement.  For example, black oxide is chosen to slow down the corrosion process when the part is in storage.  It is not particularly durable and will usually wear off in use.  It is also utilized to give a consistent appearance to the final structure by providing a uniform coloration.  Nickel plating assists in corrosion resistance and is more durable than black oxide.  However, it will also wear over time in use.  It is very effective in improving cosmetic appearance on a part, as well.  Hard chrome plating will increase the surface hardness of the part and improve its corrosion resistance.  Clear, black and the newer color anodizing are often used for aluminum parts as a primarily cosmetic improvement, while zinc galvanizing is used quite effectively for corrosion resistance. 

The most comprehensive component catalogs will generally include material hardness conversion tables as well as material variety/application and surface treatment option/performance charts.  The hardness chart here details and compares the various scales used in industry today, namely, Rockwell C, Vickers, Brinell, Shore). 

Click the above image to view the Material Selection Chart

Mr. Chris Blaszczyk and Mr. Mike Melone of MISUMI contributed to this article.  They may be contacted for further information at 1-800-681-7475.