Airline lubricators have been used in factor pneumatic systems for decades. Lubrication helps reduce friction between sliding surfaces to improve efficiency and increase cycling speed of a component. It also reduces wear, which ultimately means longer component life and less maintenance.
In a pneumatic system, lubrication can reduce both internal and external leakage around valve spools, cylinder rods and pistons, air motor and rotary-actuator vanes, rotors, and housings, among 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 some roughness exists even on polished surfaces. 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 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 and seals, but is rough enough to allow oil to collect in the microscopic valleys of the surface and provide lubrication between sliding components.
As always, the advantages of using lubricated compressed air can be outweighed by disadvantages. Having to monitor lubrication rates and replenish oil supplies keeps operators from spending time on more productive endeavors. Without careful control of lubrication rate, excess oil can create a messy and unsafe workplace. Also, the cost of oil used by a lubricated system must be factored into the operating cost. This cost may seem negligible, but it can add up over time. It may be offset, though, by longer component life attributed to the lubrication.
Lubricators can be installed in different locations depending on the intent. In the upper image, mounting the lubricator upstream of the valve lubricates the cylinder and valve every time they cycle. In the lower image, the lubricator bypasses the control valve and lubricates the cylinder only while its rod retracts. When the rod extends, exhaust air from the rod end of the cylinder lubricates the directional control valve.
Manufacturers began introducing pneumatic components that could operate with non-lubricated air long before legislation for oil-free or nearly oil-free exhaust air began. One reason why oil-free pneumatics came about was because designers and operators viewed lubricators as temperamental components that put out too much or not enough oil or required regular maintenance.
Manufacturers point out, though, 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.
Dealing with Exhaust Air
Deciding whether specifying a lubricator is practical for a given application is one issue. Another, which falls under OSHA (Occupational Safety and Health Administration) requirements, deals with air that exhausts from a compressed-air system. OSHA Standard 1910.1000, in part, mandates that compressed air exhaust may not exceed 4.32 ppm of oil mist contamination in any 8-hr work shift of a 40-hr work week.
In addition, noise may not exceed 90 dBA during an 8-hr day of a 40-hr week. This means exhaust mufflers (silencers) have become an important consideration for pneumatic systems. Although silencers have been used primarily for keeping noise from exhausting air within OSHA requirements, coalescing mufflers can also reduce oil emissions. Internal geometry to reduce air velocity and baffles for audio damping take care of noise; coalescing filtration takes care of the oil.
A coalescing muffler operates on the same principles as a coalescing filter. As air flows through the coalescing element, oil particles are captured by three different mechanisms: direct interception, inertial impaction, and diffusion. In direct interception, oil particles simply collide with and are trapped by filter fibers. With inertial impaction, the element's turbulent air stream throws oil particles against fibers, which trap the oil. Diffusion causes the smallest particles to vibrate and collide with each other, and eventually the element’s fibers, which trap the oil.
Most coalescing mufflers have a port for draining collected oil from the element. The majority also have replaceable elements that can be changed without having to replace the entire muffler. However, coalescing mufflers create a higher pressure drop than standard filter-mufflers do. Therefore, compressors may have to generate slightly higher pressure to compensate for the drop. This may not be a factor, though, if lubrication increases efficiency of components enough to offset the pressure drop.
Standard filter-mufflers on the other hand, have a porous element to trap any solids that may be carried in the compressed air stream. Porous elements cannot trap vapors or liquids, such as atomized oil. So unless the pneumatic system uses an oil-free air compressor and no lubricators, exhaust air should be routed through a coalescing muffler. And even if an “oil-fee” pneumatic system has been specified, you still might want to specify coalescing mufflers to ensure that you’re keeping emissions within OSHA requirements.
Coalescing mufflers come in various sizes according to flow to remove contaminants from the exhaust air stream and reduce noise. The units shown have 99% oil-removal efficiency, 1 µm filtration, reduce noise by up to 25 dBA, and produce backpressure of about 5-psid at flows to nearly 150 scfm (left) and 250 scfm (right). (Courtesy: Emerson/ASCO)
Non-Lubricated or Self-Lubricated?
Two types of non-lubricated components exist: those that can use non-lubricated air and those that must use it. Self-lubricated components (primarily valves and cylinders) have a built-in lubrication system that relies on long-lasting solid lubricants or have an internal supply of oil or grease that resists being washed away by the compressed air.
Components that can operate on non-lubricated air often are referred to as “lubricated for life.” Lubricated-for-life generally means that the component is expected to perform to specifications for at least a certain number of cycles or accumulated travel. Providing lubrication may allow the component to far exceed this projected life by reducing wear of sliding components. Of course, the manufacturer should be consulted to determine components best suited to an application.
On the other hand, lubricated air can have a detrimental effect on components designed to operate only with non-lubricated air. Cylinders, for example, may have a plastic or composite barrel with a super-smooth inner wall. With non-lubricated air, there is no reason, theoretically, to maintain a surface finish rough enough to retain oil. Therefore, cylinder walls may be finished to 10 µin. RMS or better. The ultra-smooth surface minimizes wear of elastomers and low-friction materials sliding across it. To remain effective, though, these surfaces must maintain their mirror-like smoothness.
Contamination can damage the smooth surface, so non-lubricated pneumatic systems demand reliable filtration. For example, residual compressor oil entrained in the compressed air could form a varnish-like coating on the smooth cylinder wall. Being rougher than the original surface, this coating could cause excessive heat buildup and seal wear—ultimately shortening the cylinder’s service life. These components often use proprietary elastomers or low-friction materials incompatible with certain oils or have coatings that would be dissolved and washed away by oil.
Coming Down to Cost
Cost, of course, also enters the picture. Components requiring smoother surface finishes and exotic materials to allow long-life operation only with non-lubricated air usually cost more than conventional components. Whether they can or must operate from non-lubricated air, these components eliminate the cost, maintenance, and attention of lubricators.
The question then becomes, which type of systems will provide the lower total cost of ownership? If components using lubricated air cost less, and if they achieve longer service life, then from a system standpoint they can be more economical by reducing machine downtime because they need not be replaced as often. Their replacement cost could also be lower. On the other hand, the cost and responsibility of maintaining a lubricated compressed air system may outweigh the hard costs of components.
From a performance perspective, the designer must decide whether it is easier and more economical to provide consistently fine filtration to components in a non-lubricated system or to provide lubricated air. This can be a complicated evaluation that takes into account maintenance, downtime, and component costs.
Whatever the choice, mufflers that remove oil from the exhaust air should be considered—especially if air exhausts near workers. It is not uncommon for coalescing mufflers to remove 99.97% of the entrained oil. Even if oil-free compressors are used, exhaust mufflers can reduce noise to OSHA-acceptable levels and ensure that exhausted air is not only nearly oil-free, but clean.
What Does OSHA Say?
As might be expected, compressed air exhaust mufflers and other products may be promoted as complying with new OSHA noise and oil-emissions regulations. This statement, in itself, reveals little unless it specifies a duration. OSHA regulations that spell out specific noise and emissions limits specify time periods in which these limits are allowed. For example, exhaust noise from a pneumatic system as high as 115 dBA is acceptable. However, workers may only be exposed to this noise for 15 min. within an 8-hr shift. So if workers will be near the exhaust of a pneumatic system for an entire 8-hr shift, noise to only 90 dBA is allowed.
The same holds true for emissions. A compressed air muffler may reduce oil emissions from a pneumatic system to an acceptable OSHA level, but that level may be unacceptable if workers will be near the exhaust for more than a small fraction of an 8-hr shift. OSHA allows up to 4.32 ppm of oil aerosol in a plant’s atmosphere if workers will be near the exhaust for a full shift. Manufacturers state that this level can be achieved easily with a coalescing muffler, which reduces emissions to 0.015 ppm. Without coalescing mufflers, contamination as high as 50 ppm or more may exist in a plant that is tightly closed for operation in cold weather.