LUBRICATING OILS AND GREASES

wear products, so they are grease lubricated (like automobile steering joints). These bearings are usually fitted with a zerk fitting (grease fitting), but some may have grease cups installed. FUNCTIONS OF LUBRICATION Lubrication reduces friction between moving parts by substituting fluid friction for sliding friction. Most lubricants are oils or greases; but other units, such as water, can be used for lubrication. When a rotating journal is set in motion, a wedge of oil is formed. This wedge (layer of oil) supports the rotor and substitutes fluid friction for sliding friction. The views shown in figure 10-41 represent a rotor (journal) rotating in a solid sleeve-type bearing. The clearances are exaggerated in the drawing so you can see the formation of the oil film. The shaded portion represents the clearance filled with oil. While the journal is stopped, the oil is squeezed from between the rotor and the bearing, As the rotor starts to turn, oil adhering to the rotor surfaces is carried into the area between the rotor and the bearing. This oil increases the thickness of the oil film, tending to raise and support the rotor. Thus, sliding friction has been replaced by fluid friction. LUBRICATING OILS AND GREASES Many different kinds of lubricating materials are in use, each of them filling the requirements of a particular set of conditions. Animal and vegetable oils and even water have good lubricating qualities, but they cannot withstand high temperatures. Mineral oils, similar to the oils used in an automobile engine, are the best type of Figure 10-41.—Rotating journals in sleeve-type bearings. lubricant for modern machinery operating at high speeds and high temperatures. Mineral lubricating oils are derived from crude oil in the same process that produces gasoline, kerosene, and fuel oil. They vary according to the type of crude oil and the refining methods used. The same type of oil is usually made in several grades or weights. These grades correspond to the different weights of oil for an automobile, varying from light to heavy. Oils used in the Navy are divided into nine classes, or series, depending on their use. Each type of oil has a symbol number that indicates its class and viscosity. For example, symbol 2190 oil is a number 2 class of oil with a viscosity of 190 SSU. The viscosity number represents the time in seconds that is required for 60 cubic centimeters (cc) of oil, at a temperature of 130°F, to flow through a standard size opening in a Saybolt viscosimeter (fig. 10-42). A 2190TEP oil is used for all propulsion turbines and reduction gears. The letters TEP indicate that the oil contains additive materials that increase its ability to displace water from steel and inhibit oxidation. Internal combustion engines (gasoline and diesel) use symbol 9110, 9170, 9250, or 9500 lubricating oils. Figure 10-42.—Viscosimeter tube. 10-49