Air lubricators have been an important part of pneumatic systems for decades. Lubrication helps reduce friction between sliding surfaces to not only improve efficiency and increase cycling speed of a component, but reduces wear, which ultimately means longer component life and less maintenance. Moreover, in a pneumatic system, lubrication can reduce both internal and external leakage around valve spools, cylinder rods and pistons, and air motor and rotary actuator vanes, rotors, and housings, as well as other components. This goes for conventional pneumatic components as well as those that can operate with non-lubricated air. Ultimately, the total savings from using lubricated air can exceed the cost of installing and maintaining the lubricators.

A small amount of oil in compressed air works with elastomeric seals to form a tighter seal than if no lubricant were available. This is because there is some roughness even on polished surfaces; manufacturing processes do not produce a perfectly smooth surface. Peaks and valleys appear on any surface if viewed at high enough magnification. A typical pneumatic cylinder wall, for example, is finished to 20 µin. RMS. Fundamentally, this means that the average distance between peaks and valleys of the surface is 20 µin.

One reason for the popularity of the 20-µin. finish is that it provides a good compromise of properties. For cylinders, 20 µin. is smooth enough to prevent excessive wear of piston rings, seals, O-rings, etc. It is also rough enough to allow oil to collect in the microscopic valleys of the surface and provide lubrication between sliding components.

Lubricated air systems
The advent of pneumatic components that could operate with non-lubricated air started long before the rumblings of legislation for nearly oil-free exhaust air began. One reason for their development was because lubricators were viewed by designers and operators as temperamental components that put out too much or not enough oil or required regular maintenance.

Manufacturers point out, however, that these problems are not inherent to lubricators, but are evidence that application and operation of lubricators are misunderstood. This misunderstanding is a double-edged sword: not only must the designer specify the right type of lubricator for the application, but the operator must set the lubricator to dispense the right amount of lubricant.

Air flow has a big influence on the type of lubricator that should be specified. For example, regardless of size, some types of lubricators may not be able to dispense enough lubricant for a given air flow, while others may always dispense too much.

Dispensing too much lubricant can create a mess and is wasteful. On the other hand, if a pneumatic system is allowed to run dry for an extended period after receiving too much lubricant, residual oil may form a baked-on varnish on internal surfaces, hindering proper operation, performance, and service life.