Load Capacity Simplicity ® Plain Bearings
Load Capacity (Pressure) Depending upon the material used, a plain bearing’s load capacity can greatly exceed a rolling element bearing. There are three basic reasons for this: 1. The area of surface contact with the shaft is far greater than rolling element bearings, which have point-to-point contact with a given number of balls. 2. A rolling element bearing must be oriented properly for the ball tracks to carry the load adequately, while a Plain bearing can be mounted in any orientation. 3. Only one or two of the tracks in a rolling element bearing will actually carry any of the load applied. Simplicity bearings have a thin liner that is bonded to a metal shell at the molecular level, allowing the load to be transferred throughout the bearing. This gives it an advantage over other Plain bearings of solid plastic or polymer materials. These other materials will tend to “cold flow” under pressure. “Cold flow” means to deform or lose shape. The idea is similar to pressing your finger into a bar of soap – material will move or deform as pressure is applied. Linear Surface Speeds (Velocity) In typical applications, speed is a known quantity and easily converted. Typically feet per minute or meters per minute are used. The most important factor that speed (along with friction) produces is heat buildup. This is not a critical factor in most linear applications because the heat is dissipated over the length of travel, and it does not affect the bearing. Short stroke or extremely high speed applications may see the effects of heat buildup in thermal expansion and the bearing ID locking on the shaft. A compensated ID bearing (FLC) is recommended in these applications.
Factors that Contribute to Wear Life Plain bearings are rated by the wear rate of the bearing material. Wear is greatly dependent upon the proper application of the bearing and material used. • Proper mating of shaft and liner materials. • Surface finish 8-16 Ra (.20-.40 mm) is required. Peaks in the surface that are polished to a radius provide the best running surface. Sharp peaks in the finish will be like a fine lapping compound wearing the I.D. of the bearing. Note: Shafting damaged by use with ball bearings can be salvaged and used with Simplicity bearings. Spin in a lathe and polish with sand papers in this order: 120 grit, 180 grit, and 300 grit. This will also remove sharp peaks in the surface finish. • Surface speed - at high speeds, heat buildup will affect liner wear. • Break-in transfer - proper transfer process of the liner to the shaft. • Lubrication - proper lubrication can greatly improve the wear rate of a bearing. At the same time, improper lubrication can increase wear and failure. • Load & Wear Relationship - wear rate is proportional to load to the third power: wear rate x (load) 3 . If load is reduced to 1/2, wear will be reduced to (1/2) 3 . • Contamination - while migrating into the bearing and embedding into the liner, certain types of contamination may, over time, cause increased wear to the liner. Note: This is not an all inclusive list. There are many more factors within an application that can affect wear to different degrees. These are the major issues and the first things to address in a design.
140 Round Shaft Technology • pbclinear.com
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