One of the benefits of plain linear bearings is their simplicity. Especially when compared to recirculating bearings, plain bearings are easy to specify, install, maintain, and replace. But don’t let that simplicity fool you. Choosing the wrong bearing, or more specifically, the wrong bearing material, can result in poor performance, degradation of the bearing, or premature failure. I recently spoke with Mark Huebner, Market Development Manager at PBC Linear and Nicole Lang, Bearings Product Manager at igus, about bearing materials and how they affect the performance and life of plain linear bearings.
Collins | Linear Motion Tips: Material choices for plain linear bearings include metals, plastics, and composites. What are the primary benefits and drawbacks to each type of material?
Huebner | PBC Linear: A majority of metal plain bearings are made from bronze. They have the highest load capacities of all three types, and they are corrosion-resistant. But, they introduce metal-to-metal contact with the shaft, which can result in galling if proper lubrication is not maintained. And lubrication for bronze bearings must be supplied externally.
Plastic bearings, on the other hand, are self-lubricating. And since they do not contain metal surfaces, they are very resistant to corrosion. However, plastic bearings act as a thermal insulator because they don’t readily dissipate heat, and they can experience deformation. They may also swell in the presence of moisture, which can result in higher friction than anticipated.
Composite bearings are typically characterized by a metal backing with either a spray-on or bonded PTFE (Teflon®) liner. Aluminum-backed bearings with a PTFE liner, such as the Simplicity® line from PBC Linear, are ideally suited to linear motion applications. Because the bearing surface is PTFE, there is no metal-to-metal contact, and the self-lubricating property of the PTFE eliminates the need for external lubrication. The presence of a metal backing also allows them to dissipate heat, unlike plastic bearings. The drawbacks of composite bearings are their limited speeds and reduced ability to handle cantilevered loads.
Lang | igus: The most important benefits of metal bearings are their reduced clearance (i.e. tighter tolerances) and a low initial coefficient of friction (COF). However, they require hardened shafting, which can be a significant expense. Also, some types of metal bearings are subject to corrosion.
Plastic bearings, such as the iglide series, are lighter in weight than other bearing types. They require no external lubrication and are completely maintenance-free. Another key point about plastic bearings is that they can be used on softer shaft materials, which means lower weight and lower-cost assemblies. And while the general perception is that plastic bearings are not as strong as their metal counterparts, the wide range of plastic materials that bearing manufacturers offer means that there’s a plastic bearing option for just about any load or application. Case in point: plastic bearings can actually perform better when exposed to edge loading or shock loads, which could cause metal bearings to break and fail.
Collins | Linear Motion Tips: What effect does bearing material have on service life?
Huebner | PBC Linear: Material selection is most important when considering the environment in which a plain bearing will need to perform. If the material will not stand up to its surroundings, then the bearing is guaranteed to fail prematurely.
For example, in extremely high load situations, bronze bearings are a good choice as long as external lubrication can be maintained. On the other hand, you typically don’t want to use a metal bearing in contaminated environments where there is sand, dirt, weld slag, or other particulate that could affect the bearing. With metal bearings that require lubrication, the added oil or grease will attract particulate that will inevitably enter the bearing, resulting in increased galling and wear to both the bearing and the shaft.
In the case of plastics, they will either clog and friction will increase, causing failure to other components, or the material will tear away at the plastic itself, destroying the material and resulting in required maintenance. Composite bearings, such as Simplicity with the FrelonGOLD® liner material, have an inherent wiping action that seals most contaminants from the bearing. Small particulate that does migrate into the inner diameter will be embedded into the material, allowing the bearing to continue operation without degrading performance.
Lang | igus: Choosing the correct bearing material is very important and can have a significant effect on the lifetime of the bearing. Oftentimes, customers think that the most expensive solution is the one that will offer the longest service life. This is not always the case, as there are very inexpensive materials that are appropriate for different environmental conditions. For example, there are over 40 different iglide bearing materials.
To choose the best material, igus has developed the online iglide expert system, which can provide a bearing’s lifetime (in hours) based on the input of application information. And for more complex applications or special conditions, igus has experts in bearing materials and selection who can help with selection.
Collins | Linear Motion Tips: How does bearing material affect performance?
Huebner | PBC Linear: One of the biggest factors that affect a plain bearing is heat generation. In linear applications, the longer the stroke, the less of a factor this becomes, but in short stroke situations, it will affect performance. Plain bearings will generate more heat than ball bearings due to the amount of surface contact with the shaft and the material’s coefficient of friction. The higher a bearing’s COF, the more heat will be generated. As heat goes up, it diminishes the speed and load carrying capabilities, so that the bearing’s actual PV (pressure x velocity) performance will decrease.
For bronze bearings, the COF is dependent on the lubrication used and whether it can be maintained effectively over the entire linear stroke. In the case of plastic bearings, the COF will vary greatly as heat is generated, but plastics act as an insulator for heat. There may be a lower COF at install, but in operation, as the heat rises, it is held inside the bearing. This causes thermal expansion, resulting in the bearing liner tightening on the shaft, which leads to more heat, resulting in an increased COF and decreased life for the bearing. The tradeoff for plastics is to increase clearances in order to deal with heat and thermal expansion and to keep a reasonable COF. Composite-lined PTFE bearings, however, act almost like a heat sink and transfer heat away from the bearing liner, thus maintaining a consistent COF and increasing the life of the bearing.
Lang | igus: There are different bearing materials that can handle different maximum loads, temperatures, and speeds, and there are many factors that will determine the best material: load, speed, shaft material, housing material, temperature, water contact, chemical contact, and, in some industries, regulatory requirements.
But a primary factor for wear in all polymer bearings is heat buildup. The more quickly the heat can be removed, the longer the lifetime of the bearing. This is the reason that manufacturers ask for the load, speed, and shaft material when selecting the best bearing material. The heat dissipation factor is often why iglide bearings are made up of a thinner wall. The thicker the wall of a plastic bearing, the more likely it is trap heat. Also, using a plastic housing or shaft can have this impact as well.
The PV relationship, which can be considered a measure of frictional heat, is very critical to the suitability of a linear plain bearing. iglide plastic bearings are typically meant for use in applications where there is a high load and low speed, or a low load and high speed.
Collins | Linear Motion Tips: What effect does shafting selection have on the performance of plain linear bearings?
Huebner | PBC Linear: The bearing material chosen needs to be a fit for the application in terms of basic performance characteristics, such as load and speed. But overall, there are many factors that contribute to the actual service life that any bearing can achieve. The bearing material is only one factor and it will be affected by all of the factors around it. The most important and direct factor will be the shaft, which acts as the inner race of the bearing. If the shaft is not suited for the bearing material, it will increase wear and shorten the bearing’s service life. It is critical to have the right match so that together they extend the life of the assembly.
For example, with a PTFE composite bearing, the ideal surface finish is 0.2 to 0.4 µm (8-16 Ra). If the surface finish is rougher, there can be premature wear of the liner as the shaft surface begins to tear away at the material. However, if the shaft is too smooth, the bearing will also show greater wear, since transferred material will not stick to create the self-lubricating condition. The same will hold true for a bronze bearing, but instead of it being self-lubricating, it needs the surface of the shaft to have a slight roughness to hold the lubrication. If too smooth, it will reach a metal-to-metal running condition sooner and shorten life.
Lang | igus: Shaft selection is very important factor in choosing the best bearing material. One of the benefits about iglide bearings is that there is a material that can be used with almost every type of shaft, even with less expensive, softer shafting.
Collins | Linear Motion Tips: Which of these materials is better suited for applications that require washdown, such as food and beverage, and why?
Huebner | PBC Linear: In washdown applications where no lubricant can be present, a composite plain bearing will be most suited due to its self-lubrication capability and one-piece design. A two-piece plastic bearing would not be suited as bacteria build-up can occur inside the bearing assembly, and the plastic is susceptible to moisture absorption and swelling that could bind the assembly.
Lang | igus: The iglide A181 line of bearings is FDA compliant and suitable for most washdown applications. But if the application will see higher temperatures or more aggressive chemicals, the iglide A350 and A500 series may be more appropriate for the application.
Collins | Linear Motion Tips: Which of these materials is better suited for cleanroom environments, and why?
Huebner | PBC Linear: In most clean room situations, adding standard lubricants is not acceptable, so bronze or impregnated metal bearings are not often used effectively. But both plastic and composite bearings can be applied because of the self-lubricating capability and low particulate generation. However, added seals or shielding may be required, depending on the class level of clean room.
Lang | igus: A critical aspect of a bearing’s suitability for cleanroom environments is outgassing, and the iglide T500 series has very low levels of outgassing. If other bearing materials are used, they should first be dehumidified.
As Lang points out, “With material selection being such an influential factor in the performance and life of a plain bearing, it’s always a good idea to consult the manufacturer to ensure you’re making the best selection for your application.”
Feature image credit: igus