The main characteristics of the function of a plain bearing are the friction losses that occur in the bearing. They are recorded using the coefficient of friction μ and are usually represented as a function of the sliding speed. Due to the complexity of the shaft – lubricant – bearing system, it is strictly speaking not possible to specify generally applicable operating characteristics for sintered plain bearings. For example, just selecting the appropriate lubricant requires a high degree of experience. The choice of the wrong impregnating oil can lead to premature bearing failure. Similarly, the geometric installation conditions and the bearing clearance have a significant influence on the bearing characteristic curve. Nevertheless, Figure 1 shows the quantitative relationship between the coefficient of friction μ and the sliding speed v

Figure 1: Characteristic curves for sintered plain bearings

Figure 2: Load field for self-lubricating sintered plain bearings made of Ferro-Porit A00 and Bronze-Porit B50

The limits of the load capacity for sintered plain bearings made of standard materials are shown in Figure 2. From a certain minimum speed v, which is between 0.02 and 0.05 m/s, the load capacity p is at its highest and decreases with increasing speed. At high speeds of around 5 m/s and above, the oil film begins to tear, causing wear on the bearing. Maximum P·V-values ​​are reached at about 1 m/s.

The service life of a sintered plain bearing of several thousand hours is determined under optimal operating conditions, i.e. in continuous operation at a constant speed in a hydrodynamic lubrication state, among other things by the amount and type of oil available. Oil losses can occur during operation due to evaporation, aging or oil leaking from the side of the bearing. In such cases, appropriate design measures are required. Since there is no direct contact between the shaft and the bearing, no wear can occur as long as there is enough lubricant. In practice, however, a shaft is repeatedly stopped and the bearing inevitably passes through the mixed friction area in which wear can occur. Experience has shown that the wear in mixed friction is less for sintered plain bearings than for solid plain bearings. Apart from the amount of lubricant, its chemical resistance and the operating temperature also have a significant influence on the service life of the bearing.

Figure 3: Bearings for increased operating temperatures

Materials with embedded solid lubricants for use in conditions of insufficient lubrication and at elevated temperatures

Plain bearings with embedded solid lubricants ensure particularly smooth running at low speeds and in the event of insufficient lubrication. These materials can also be used at elevated temperatures. Graphite, molybdenum disulfide MoS2, tungsten disulfide WS2 or hexagonal boron nitride BN can be used as solid lubricants.

Figure 3 shows how the operating temperature of a sintered bearing affects the expected service life and which materials should be used at higher operating temperatures. At room temperature and with sufficient lubrication, a sintered bearing can easily achieve a service life of 100,000 hours. The maximum operating temperature up to which the bearing can be used is determined by the properties of the lubricant. Commercially available lubricating oils can be used up to 120 °C, special oils even up to 250 °C. If even higher operating temperatures occur, a material with embedded solid lubricant must be used. Sintered iron, sintered bronze and graphite, MoS2 or WS2 can be used in continuous operation up to a maximum of 300 °C. In the temperature range between 300 and 800 °C, plain bearings made of Cr-Ni-Porit BN, a scale-resistant chromium-nickel steel with embedded boron nitride, should be used.