Types of Bearings—Structure, Load-Bearing Capacity, Use

For cost reasons, one strives to satisfy the requirements for the particular application with the simplest possible bearing design. But contradictory demands for strength, tight seating, and good running properties ultimately lead to a “division of labor” and to a multilayer structure for the bearing.

The use properties of the bearings and above all their dynamic load-bearing capacities are influenced not only with the selection of materials but also with the structure and thickness of the layers and other engineering measures. Thus, there are, beyond classical multilayer concepts, newer types that optimize the bearing’s utility with a closely defined sequence of layers and/or design of the running surface.

The fundamental advantages and disadvantages have already been mentioned in the discussion of the materials. Figure 7-269 provides a survey of the types of bearings most commonly used for a particular application range.

Fig. 7-269. Most important types of bearings and application ranges

Solid Bearings. Solid material is used primarily in large, industrial engines in the form of hard bronzes for thick-walled bushings and AlSn6 for thrust washers (axial bearings). The advantage is simple manufacture and in thrust rings the additional option of enabling use at both ends with proper engineering.

In passenger car engines the slow-running camshafts are borne directly in the aluminum cylinder head. Although these alloys are not bearing metals, proper functioning is reliable because of the low energy density in the bearings.

Two-Material Bearing (Fig. 7-270). Here there are two essentially different application areas:

- Suitable for use at wristpin and rocker arm bearings are rolled bushings made of leaded bronzes, since the high specific loading of up to 120 N/mm² requires endurance strength and the disadvantage of low running capacity, because of the low sliding speed, is of little significance. When there is an insufficient oil supply, these bushings tend to liberate lead from the material and cause oil carbonization.

Fig. 7-270. Material structure (examples)

- Bearings based on AlSn, because of their excellent ratio of performance to cost, are the preferred solution for moderately loaded applications involving rotational movement and thus primarily the main and conrod bearings in gasoline engines and industrial diesels.
Their wear is low but there are limits to their adaptability. The low wear also harbors a risk: the appearance of the bearings changes hardly at all. Consequently, evaluating their condition with a visual evaluation is difficult. This makes necessary adequate statistical confidence for service life during the testing stage.

The continuously rising loads found in new developments and advances in engines have resulted in the development of two-material bearings using higher-strength AlSn alloys. The above applies to them in principle, but these bearings, because of the smaller lubrication gap and the greater energy density, are at greater risk of friction and wear damage. The increase of strength achieved by reducing the tin content is thus not an option, which helps meet development objectives. And the bonding layer made of pure aluminum can become a weak point.

Three-Material Bearing (Fig. 7-270). Three-material bearings with an overlay applied by electroplating and above all on a leaded bronze basis are the type used predominantly for crankshaft bearings. They represent a fully mature technology, are available worldwide, and offer a good ratio of cost to benefit. They are distinguished by good adaptability and are tolerant of grime and error for as long as the soft overlay is present. In larger engines three-material bearings based on aluminum are also used.

Three-material bearings are suited only with some limitations where high loading situations are encountered, above all in the conrod bearings for modern direct- injection engines (both gasoline and diesel). Their weak point is faster wear at the overlay as loading increases. Corrosion resistance, too, which becomes more important at longer oil change intervals, is not high. Wear at the overlay, from 15 to 30 μm thick, has in and of itself only an insignificant effect on the bearing function; exposure of the substrate, however, leads to a drastic increase in sensitivity to disturbances.

The classical three-material bearing with a PbSnCu overlay is therefore supplanted to an even greater extent by higher-strength, two-material aluminum bearings in the lower load range and by the true high-performance concepts—grooved bearings for industrial engines and sputter bearings for passenger car and utility vehicle engines.

Miba™ Grooved Bearings(Figs. 7-270 and 7-271). The grooved bearings developed by Miba™ almost 20 years ago and shown in Fig. 7-271 delay the degradation of the running layer with a special geometry for the surface. The overlay is embedded in very fine grooves in the running direction; between them are lands made of the harder bearing material.

Fig. 7-271. Miba™ grooved bearings

The ratio of materials at the running surface is about 75% overlay to 25% bearing metal. With this geometry, it is possible to continue to determine the tribologic properties by selecting the overlay material but to protect that layer against wear with the harder lands. Thus the good running properties are retained for a much longer time than in three-material bearings.

The grooved bearing today finds its primary application in diesel engines with greater specific power and is used to drive locomotives and ships; in the passenger car and utility vehicle engine segment it has been supplanted in recent years to an increasing extent by the sputter bearing, because of the continuously rising loads.

Sputter Bearing (Fig. 7-270). The bearings that can stand the most extreme loading and are produced in large number today are three-material, leaded bronze bearings with a sputter overlay. Because of their greater load-handling ability, up to more than 100 N/mm², and with good running properties at the same time, they are installed in engines with high power density and used for passenger cars, utility vehicles, and drives for fast ships. Today, hardly any other type of bearing can be considered, above all for conrod bearings in direct- injection diesel engines for passenger cars.

Fig. 7-272. Guideline values for various bearing designs

The only major drawback to the sputter bearing is its price. Because of the complex vacuum coating process, a sputter bearing is five to eight times as expensive as a three-material bearing. Thus, in the conrod and main bearings a sputter shell on the side subjected to heavy loads is combined with a three-material or grooved bearing shell on the side with less loading. This combination offers the additional advantage that tiny particles of grime cannot become embedded in the soft overlay.

The application limits and costs for the various bearing designs are shown in Fig. 7-272.