Sleeve Bearing VS. Ball Bearing
By Application Engineer (Comair Rotron)

Introduction

Quiet often in low-flow electronic cooling fans for computers, where the aerodynamic noise is low, the predominant noise emitted by the fan is generated in the bearing system. This type of fan usually has either an oil-impregnated sleeve bearing or ball bearings. Both bearing systems create noise that can be very different in both frequency content and amplitude. it is generally accepted in the computer and business machine industries that ball bearing fans are noisier than sleeve bearing fans. Furthermore, ball bearing fans can become noisier by mishandling or long term running. This paper will present a brief discussion of the various noise-source mechanisms in each bearing system and will show acoustical data, comparing the two systems against various factors such as life, shock., vibration, etc.

Discussion

Before discussing the acoustical aspects of sleeve and ball bearings, a brief discussion on the basic operation of each bearing system is in order. Figure 1-A shows a typical sleeve bearing assembly for a small cooling fan. In this particular case the shaft rotates while the bearing is stationary. It is also possible to use a stationary shaft with the bearing rotating. In either case, lubricating oil is impregnated into the porous bronze bearing and is fed to the shaft via the small porosity openings in the bearing pore. Rotation causes a wedge or film of oil to build up on which the shaft rides. If a perfect bearing system could be built, this oil film would prevent metal-to-metal contact and thereby eliminate almost all-bearing noise. Because the shaft and bearing have rough surfaces, at least on a microscopic level, the bearing can create a scraping or grinding sound. Also due to forces such as unbalance and motor driving frequencies, vibration can cause the shaft to rock in the bearing and make contact at the bearing ends. This type of contact causes a knocking or rattling sound. Another source of noise is from the thrust washers which must slide relative to each other thereby creating a rubbing sound. At any rate, noise from a sleeve bearing is usually broad band in nature and somewhat intermittent. However, most good sleeve bearing designs are very quiet and stay quiet until they begin to run out of oil.