Design of Mechanical Bearings in Cardiac Assist Devices
3. Lubrication of Concentrated Contacts
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The types of bearings considered in the previous section possess “geometric conformity” between the two surfaces, e.g., two relatively flat surfaces in case of thrust bearings. Many machine elements, however, have curved surfaces, such as contacts between a cylinder or a sphere and a flat surface. In such contacts the pressure generated at the contact can be quite high and produce elastic (or reversible) deformation of the contacting bodies. For example, when an elastic spherical body is pressed against a rigid flat surface, the sphere will be deformed to form a flat contact between the two surfaces, as shown in Figure 3-1a. For a reverse case of a rigid sphere against an elastic flat surface, it is the flat surface that deforms to conform to the rigid sphere, Figure 3-1b. The dimensions of the deformed regions are highly exaggerated in the figure for visualization. In most cases however, elastic deformation takes place in both components. Such contacts between non-conformal surfaces are often referred to as concentrated contacts, due to the large contact pressures on the order of GPa. The contacts in pivot bearings discussed in the Introduction (Section 1) fall in the category of concentrated contacts, and therefore this section is applicable to lubrication of such bearings.