Which Are Better—Ball Bearings or Sleeve Bearings?
Bearings reduce friction for moving or rotating parts—they allow the part to move or rotate smoothly. Whether it is a pump, fan, conveyor, winch, crane, automobile, hard disk, or anything that has a moving or rotating part, usually a shaft, it will most likely have bearings to allow it to move or rotate freely. Bearings can be broadly categorized as being based on round balls or based on cylindrical sleeves. The former is called as ball bearing and the latter as sleeve or journal bearing.
During its operation, the rotating shaft faces two types of forces or thrust loads. These are the axial thrust loads acting along the axis of the shaft, and the radial thrust loads acting perpendicular to the axis of the shaft. For instance, the major thrust on a shaft operating a corkscrew is axial, while that acting on the shaft of a centrifugal pump is predominantly radial. Gravity also plays a part here, depending on whether the shaft is oriented vertically or horizontally. A vertical orientation gives a greater axial thrust, while a horizontal shaft faces higher radial thrust. Whether axial or radial, bearings carry the thrust on the shaft while allowing smooth operation. While ball bearings can carry both radial and axial thrust loads, sleeve bearings are specifically meant to carry radial thrust loads only, as they have no capacity to carry axial thrust loads.
Construction of Bearings
A ball bearing is made of an inner and outer race, with a series of balls between them, and a cage to hold the balls in place. Sleeve bearings, as the name suggests, are shaped as a sleeve on the shaft. A small clearance exists between the two, with a lubricant filling the space.
Most parts of a ball bearing are made from hardened steel and finished with precision. Sleeve bearings are typically made from porous, powdered metal using a sintering process. While a ball bearing is compact in width, comparatively, the sleeve bearing is longer.
Advantages and Disadvantages of Ball Bearings
Generating less heat due to lower contact area, ball bearings require simple and inexpensive lubrication such as by oil bath, oil mist, or oil rings. Their narrow width allows ball bearings to fit in compact housings, making the span of the shaft and bearing combination shorter, and resulting in lower shaft deflections. As most ball bearings can carry both thrust and radial loads, they are suitable for use in angular contacts. It is easy to increase the thrust load carrying capacity of ball bearings, simply by stacking them in tandem or in a back-to-back arrangement.
There are certain disadvantages of ball bearing usage. As ball bearings have limited operating speed and thrust capacity, centrifugal pumps running above 3600 RPM seldom use them. Chances of failure are greater as ball bearings have higher number of moving parts, although ball bearings are selected and sized to provide an operating life of more than 25,000 operating hours under specified conditions. Components of a ball bearing can develop microscopic defects or fail prematurely. This can result in high and destructive rotor vibrations—especially so if the running frequency of the balls coincides with the frequency of the impeller mounted on the shaft.
Advantages and Disadvantages of Sleeve Bearings
More expensive than ball bearings, sleeve bearings have practically unlimited radial thrust capacity. However, they are suitable for use in moderate to high-speed applications alone. Sleeve bearings offer better stiffness than ball bearings do, and as they have split halves, are easier to inspect or replace. Worn-out sleeve bearings can be reused after being rebabbitted.
However, because of their construction, sleeve bearings do not have the capacity to carry axial thrust load, unless used in conjunction with another axial thrust bearing. As oil rings used for sleeve bearings are good only for high-speed operation, sleeve bearings are not effective for very low-speed applications. Being wider and having a higher contact area compared to ball bearings, sleeve bearings have higher friction losses and therefore, generate more heat and require more lubrication.
Selection Criteria for Bearings
Engineers have to consider several factors when deciding on whether to use ball bearings or sleeve bearings for their application. In other words, to decide on the type of motor or fan to use, based on whether it employs ball or sleeve bearings, engineers must consider several factors including:
Operating Life
Mounting Method
Noise Generated
Parts Precision
Lubrication Issues
Effects of Friction
Cost Considerations
Operating Life
As reliability is a critical factor in performance, engineers look for consistent performance during the operating life of a motor or fan. For this, they use the L10 method to gauge the operating life in hours at various operating temperatures. The L10 method estimates the operating life of 90% of fans in a large sample.
According to the L10 method, fans lasted equally long at low temperatures, irrespective of whether they used ball bearings or sleeve bearings. Between 25-60°C, ball bearing fans lasted nearly twice as long as the sleeve bearing fans did. Beyond 70°C, sleeve bearing fans stopped operating, while ball bearing fans soldiered on.
This indicates if the application must operate in high ambient temperatures, engineers would prefer to use a fan with ball bearings.
Mounting Method
One can operate a fan or motor mounted horizontally with its shaft parallel to the ground, or mounted vertically with its shaft perpendicular to the ground. For fans or motors using sleeve bearings, the method of mounting affects the operating life. For instance, vertically mounted motors or fans with sleeve bearings last as long as do ball bearing fans or motors. However, the life span decreases noticeably for a sleeve bearing fan or motor when it is operated while mounted in a horizontal position. The mounting position does not affect the life span of a fan or motor, provided it is using ball bearings.
This indicates the engineer could choose to use a sleeve bearing fan if it was meant to be operated in a vertically mounted position, and select a ball bearing far or motor for any other mounting.
Noise Generated
Noise generated by electrical equipment is a matter of concern if many situations. For instance, the ICU in a hospital demands all equipment must operate below a specified noise level. Noise generated by fans or motors is speed dependent—noise generated rises with increase in speed. Although in the past, sleeve bearing fans generated much lower noise compared to that generated by ball bearing fans; engineers have overcome this by using preloaded ball bearings. Therefore, special low noise fans can have either ball bearings or sleeve bearings. In addition, sleeve bearings are generally quieter in their early life, but generate increasing levels of noise with passage of time.
Parts Precision
As most parts of ball bearings are made from precision ground steel with super finish, they are more exact in fitting together and perform better. Sleeve bearings on the other hand, are made by sintering porous, powdered metal, which can deteriorate under high operating temperatures. Therefore, ball bearings would be the right choice for applications requiring precision parts. However, if the engineer does not need precision, sleeve bearings could be the alternative.
Lubrication Issues
All bearings need to be lubricated for consistent performance. Improper lubrication may cause a fan or motor to stop rotating, ultimately leading to a major systemic failure. Ball bearings can be sealed-for-life with thick lubricants with more additives, which have a lower tendency to evaporate. Sleeve bearings, with their more open construction, can hold only a fixed amount of lubricant and this is more likely to evaporate.
Effects of Friction
Lack of lubricants in bearings causes greater friction, ultimately leading to bearing failure. Friction also depends on the level of contact within the bearing. Ball bearings, being point contact bearings, generate minimal friction. Sleeve bearings, on the other hand, are line contact presenting a broad contact area between the shaft and the bearing, which generates a considerable amount of friction. Although sleeve bearings can be reused after machining, they may also undergo high micro-hardness, making it more difficult to machine them for reuse.
Although indentations in the raceway or brinelling may damage a ball bearing, this is less likely for a well-assembled bearing with each component fitting together.
Cost Considerations
Although sleeve bearings may be less expensive per unit when purchased in substantial amounts, ball bearings offer larger life spans. When considered over extended periods, ball bearings offer greater value per dollar.
Conclusion
Although at a glance, it may seem ball bearings are better than sleeve bearings, there are applications where the sleeve bearing is a better fit. For instance, an application generating considerable heat in a densely packed system, using a motor or fan with ball bearings will offer a greater life span, and a better long-term investment. On the other hand, a fan or motor with sleeve bearings may be adequate for a system with a short life span, and not generating copious amounts of heat.
