Fluid power filters

Moving parts in fluid power systems are subject to wear from contamination. Neither atmospheric air nor hydraulic fluids are clean enough as supplied to avoid this and they both become more contaminated with use. Therefore all fluid power systems require filters to remove contamination and thereby increase component life.

Pneumatic filters and lubricators

A pneumatic filter should be the first component at the inlet of most air circuits. This unit usually is one part of a combination of components that filters the air, regulates its pressure, and adds lubricants for moving parts in the circuit. The air filter and lubricator are covered in this section. (An air line regulator performs the same function as a hydraulic reducing valve and is covered in that section.)

Air from the compressor contains dust from the ambient atmosphere, condensed water, and rust and oil sludge that bypass the compressor rings. These by-products of compressing and transmitting air must be removed to keep moving parts of the machine working properly. Most filters clean the air and separate condensed water from it before the air enters the circuit.

Figure 7-1 shows a simple air filter (and the ISO symbols that represent it). These units usually require little attention if the compressor has an air dryer at its outlet. Air enters at the left and is channeled into the bowl with a downward circular motion. The centrifugal force of this swirling action slings water droplets outward. They collect and fall to the bottom of the bowl below the baffle into a quiet zone for draining either manually or automatically. The air then flows through a porous filter element to the outlet. These units typically remove particles of 40-micron (40-µ) size or larger but they also are available for particles as small as 5 µ if required.

The coalescing filter shown in Figure 7-2 removes water and oil vapors as well as condensed moisture from an air line. To accomplish this, coalescing filters are not only designed differently but they have reverse flow in relation to a standard filter. These filters will remove particles as small as 0.3 to 0.6 µ. They use a coarse mat of very fine fibers that are small enough to catch and hold these very fine particles. As the vapor collects into droplets, they are channeled to the quiet zone and drained.

Coalescing filters must be applied according to manufacturers’ specification to keep collected liquid from being re-entrained into the air stream. Also, always make sure the flow direction is according to information on the filter housing. Several companies make these high-efficiency air filters though they are not often applied to every day circuits.

Figure 7-3 shows a cutaway view of an air-line lubricator. After the combination unit filters and regulates air pressure, some downstream system components may require a small amount of lubrication. (For example: air motors are one item that needs a constant supply of oil to extend their life and maintain torque.) Some cylinders are pre-lubed and most valves require little if any lubrication, so keep oil supply to these units at minimum. A general rule: a ½-pint bowl of oil in a lubricator should last three weeks to a month in most situations.

When air passes through the lubricator’s venturi section, pressure drop across it gives a negative pressure in the area below the adjustable orifice. Vacuum in this area draws oil from the bowl as fast as the adjustable orifice will allow. These droplets then mix with the air as it passes through. This arrangement means that oil flows only when there is air flow and only as fast as the adjusting screw allows.

Air line lubricators are designed to send a mist of oil to the parts in the downstream circuit. However, the physical size of some circuits makes it impossible for the mist to stay in suspension long enough to reach some parts. In this case -- and for some air motor applications -- it is necessary to inject oil at the components inlet. There are electric and air-driven lubrication units to meet the needs of these applications.