Whether it is a pump, fan, automobile, hard disk, conveyor, or anything with a moving part like a shaft, you are likely to find that a bearing is used as a part of the rotating mechanism. Bearings reduce friction for rotating or moving parts by allowing the application to operate smoothly. Bearings can commonly be defined as one of two types: ball bearings and sleeve bearings.
Bearings can be used in a number of applications, but especially in a rotating shaft. During its operation, a rotating shaft faces two types of forces. The first is an axial thrust load along the axis of the shaft, and a radial thrust load acting perpendicular to the axis. Moreover, the orientation of the shaft plays a major role. If the shaft is oriented vertically, there is greater axial thrust whereas a horizontal orientation produces a higher radial thrust. Bearings allow for a smooth operation in either instance, through sleeve bearings cannot be used for applications that carry axial thrust loads being that they have no capacity for it.
A major factor in a bearing’s efficiency is their construction. Beginning with ball bearings, these compact mechanical elements made from hardened steel contain an inner and outer race that is lined with a series of balls, and a cage that keeps the balls in place. Sleeve bearings, or journal bearings as they are often called, are made from a porous, powdered metal using a sintering process. Sintering is a heat treatment often used to increase the strength and durability of a material. Sleeve bearings have a long cylindrical shape that resembles a sleeve on a shaft, and a small gap between the shaft and the bearing is filled with a lubricant for ease of operation.
In order to understand what kind of bearing you need for your application, it is important to define some of the advantages and disadvantages of each.
Since ball bearings typically generate less heat due to their lower contact area, they require simple and inexpensive lubrication like an oil bath, oil mist, or oil rings. Moreover, their compact construction can allow them to be housed in tightly-packed applications, making the length of the shaft and bearing shorter, which results in lower shaft deflections. This kind of bearing is suitable for angular contacts because of its capacity to carry both thrust and radial loads, unlike sleeve bearings. Additionally, by stacking ball bearings together, you can increase the thrust load carrying capacity. With regard to the disadvantages, ball bearings have a limited operating speed and thrust capacity, which means they are rarely used in high capacity applications. The number of moving parts within this bearing increases their chance of failure and the likelihood that the bearing develops abrasions or defects. This wear, over time, can result in destructive rotor vibrations.
One of the biggest advantages of sleeve bearings is their unlimited radial thrust capacity. More than that, they are easy to inspect, replace, and can even be reused after being rebabbitted. As previously mentioned, these bearings do not have the capacity to carry axial thrust loads unless paired with another axial thrust bearing. Additionally, the lubrication process they require makes them ineffective for very low-speed applications. Lastly, sleeve bearings have a much greater contact area so they have higher friction losses, and as a result, generate more heat and need additional lubrication.
When it comes to selecting which bearing is the best fit for your application, you must consider what mounting method you will utilize and what lubrication issues you may face. For instance, the life span for ball bearings and sleeve bearings are typically the same for certain applications. In some cases, however, the lifespan of a sleeve bearing may significantly decrease for a horizontally mounted fan or motor. So if an engineer is trying to select between the two bearings, a sleeve bearing could be used for an application with a vertically mounted position, and a ball bearing fan or motor for any other mounting. With regard to lubrication issues, improper lubrication can result in systemic failure. Ball bearings require thick lubricants, which are less likely to evaporate over time, while sleeve bearings’ open construction result in the lubricant evaporating at a quick rate.
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