**2. Lubrication conditions**

**Figure 1** shows the optical microscopic image of the oil-impregnated sintered bearing surface. Many pores and gaps among the grains could be found in sintered metal body. Lubricating oil is impregnated in these pores and gaps usually using vacuum condition. As described above, lubricating oil exudes from the porous body to the sliding surface in some ways during sliding and lubricates the shaft and bearing surface. The schematic view of the shaft and bearing is shown in **Figure 2**. As can be considered from these oil supply mechanisms, oil amount in the bearing gap is usually smaller than fully lubricated bearings and sometimes insufficient for good lubrication.

In addition to oil supply problem, there are some features that make low friction difficult to realize. Some exuded oil returns into the pores of the bearing body by capillary force and also leading to insufficient oil on the sliding surface [4, 6]. The porous surface of the bearing means a reduction in the load area, which lowers the loading capacity of the bearing. Although fluid lubrication was reported to be possible for oil-impregnated sintered bearings especially under large sliding speed conditions [7], the sliding area of the bearing is often under a boundary or mixed

#### **Figure 1.**

*Optical microscopic image of oil-impregnated sintered bearing surface. Arrows indicate the example of a grain and a pore.* 

**Figure 2.**  *Schematic view of shaft and bearing.* 

*On the Friction of Oil-Impregnated Sintered Bearings DOI: http://dx.doi.org/10.5772/intechopen.81757* 

lubrication condition [6] due to the reasons mentioned above, resulting in large bearing friction.

To improve the bearing friction under such lubrication conditions, solid lubricants are sometimes added to the bearing materials, or an oiliness agent (e.g., stearic acid) is added to the impregnated oil [2, 4, 8]. However, the friction of oil-impregnated sintered bearings is generally higher than that of fully lubricated bearings [6].
