Bushings, also known as plain bearings or sleeve bearings, are used in just about every industry, and have been for many, many years. In this post we are going to look at the life expectancy of polymer bearings for a qualitative viewpoint, as opposed to a quantitative, calculation-based look.
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Primary Design Variables
Load, speed, and temperature are the primary design variables constraining the projected life of a bushing regardless of whether it is metal bushing or a plastic bushing. The life expectancy of a polymer bushing is also highly dependent on the shaft itself, especially its hardness and finish. Since the customer, and not the manufacturer, sets the shaft requirements, it is difficult for manufacturers to predict ahead of time the expected life of a bushing-shaft combination.
The life expectancy of a bushing is usually defined in terms of wear. Wear in bushings is measured by how much of the surface has worn away from the ID. There are two mechanisms for wear: adhesion and abrasion.
- Adhesion occurs when two surfaces contact and particles of one surface adheres to the other. In polymers, this is primary wear mechanism.
- Abrasive wear takes place when the harder surface gouges into the softer surface. Debris that gets between the bushing and the shaft can become embedded and result in scoring and abrasive wear.
Self-lubricating plastics, however, dont have the oil or grease that tends to trap the debris.
For any type of bushing, temperature can be a factor in wear and, ultimately, life expectancy. At higher temperatures, metal bearings may expand, causing a reduction in clearance and the potential for additional wear. If high temperatures are combined with high speeds and high pressure, wear will be greatly accelerated.
With polymers, it seems that temperature near the upper limits of the operating temperature result in the most significant wear. This is due to changes in the polymers structural characteristics when loaded at extreme temperatures. For example, if the temperature is close to the polymers melting point or if it has a low glass transition temperature, the polymer will be more likely to soften or deform. It is worthy of note, though, that there are high temperature polymers that are well adapted for use a bushings and bearings.
Intermittent operation can be very detrimental to most bushings because lubricant will tend to settle in one area. This can make start up very difficult until the lubrication is again distributed. However, self-lubricating polymer bearings avoid this problem.
Friction is another contributing factor to bushing wear, but most polymers have a very low coefficient of friction and, again, many are self-lubricating. Because polymers have less stiffness than metals, they do not follow the same rules when it comes to friction. For example, the dynamic coefficient of friction for a polymer is greater than its static coefficient of friction. As a result, frictional forces in polymer bushings are actually dependent more on speed than on the loading.
The life expectancy of polymer bearings is highly dependent on the design of the shaft it will be working with. However, looking at the design conditions that the bearing will be operating in, such as temperature and intermittent operation, can reveal critical factors that can severely influence the life of a polymer bushing.
For more on polymer bushings and bearings download the “7 Step Process in Polymer Bearing Design Consideration.”