The proper selection and use of lubricants, as well as the care and operation of lubricating systems, is an essential part of any powerplant maintenance program. Any piece of equipment with moving parts depends on some type of lubricant to reduce friction and wear and to extend its life. To choose an appropriate lubricant for a particular application and to maintain the lubricant’s effectiveness, a basic understanding of lubrication theory and the characteristics of lubricants can be very beneficial. This document will discuss lubrication fundamentals, lubricant characteristics, additives, maintenance of lubrication systems, and the selection of lubricants for common powerplant equipment.

1. INTRODUCTION

The proper selection and use of lubricants, as well as the care and operation of lubricating systems, is an essential part of any powerplant maintenance program. Any piece of equipment with moving parts depends on some type of lubricant to reduce friction and wear and to extend its life. To choose an appropriate lubricant for a particular application and to maintain the lubricant’s effectiveness, a basic understanding of lubrication theory and the characteristics of lubricants can be very beneficial. This document will discuss lubrication fundamentals, lubricant characteristics, additives, maintenance of lubrication systems, and the selection of lubricants for common powerplant equipment.

2. FUNDAMENTALS OF LUBRICATION

The basic purpose of a lubricant is to reduce friction and wear between two surfaces moving relative to one another. In most cases, a lubricant also dissipates heat, prevents rust or corrosion, acts as a seal to outside contaminants, and flushes contaminants away from bearing surfaces. For the lubricant to accomplish these functions, a fluid lubricant film must be maintained between the moving surfaces. This condition is known as fluid film lubrication.

2.1 Fluid Film Lubrication

Fluid film lubrication reduces friction between moving surfaces by substituting fluid friction for mechanical friction. Fluid film lubrication is illustrated in figure 1. Surface 1 moves over surface 2 at velocity V, separated by a film of fluid with a thickness h. The oil film can be considered to be made up of many layers. The layer in contact with moving surface 1 clings to that surface and moves at the same velocity. Similarly, the layer in contact with surface 2 is stationary. The layers in between move at velocities directly proportional to their distance from the moving surface. For example, at a distance of ½ h from surface 1, the velocity would be ½ V. The force F, required to move surface 1 across surface 2 is simply the force required to overcome the friction between the layers of fluid. This internal friction, or resistance to flow, is defined as the viscosity of the fluid. Viscosity will be discussed in more detail later.

To keep the surfaces separated, the fluid pressure must be high enough to support the load. In highly loaded bearings, like thrust bearings and horizontal journal bearings, relatively high fluid pressures are required to support the load. If this pressure is supplied by an outside source, it is called hydrostatic lubrication. If the pressure is generated internally (i.e., within the bearing by dynamic action), it is referred to as hydrodynamic lubrication. In hydrodynamic lubrication, a fluid wedge is formed by the relative surface motion of the bearing journals or the thrust runners over their respective bearing surfaces. This wedge is similar to the fluid wedge that forms under a speeding boat, pushing the bow out of the water, or under water skis, allowing the skier to skim across the water. Figure 2 illustrates the wedge action in a pivoting shoe thrust bearing. As the thrust runner moves over the thrust shoe, fluid adhering to the runner is drawn in between the runner and the shoe, causing the shoe to pivot and forming a wedge of oil. As the speed of the runner increases, the pressure of this wedge increases, the runner is lifted vertically, and full fluid film lubrication takes place.

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