Military hydraulic applications range from relatively simple locomotion to precise alignments needed to position a gun barrel. As precision increases, the need to maintain a high level of operating integrity grows as well. The reliability of military hydraulic systems is vital not only to soldiers’ ability to perform their missions but also to the very survival. Reducing the rate of contaminant ingression into a hydraulic system is, therefore, of paramount importance.

One piece of military hardware that figures prominently in the readiness of both the U.S. Army and NATO Alliance forces is the M109 mobile howitzer. In appearance, the M109 resembles a conventional military tank, but its firepower extends over a greater range. A hydraulic system positions and aims the howitzer, and like most other hydraulic systems, this one breathes — as the hydraulic fluid level in the resrvoir rises and falls, air is expelled from or drawn into the reservoir. During each breathing cycle, the system can ingest large amounts of solid contamination and water vapor from the ambient air. Unchecked, this will ultimately lead to a contamination that can severely impair and damage the howitzer's hydraulic system.

Studies by the U.S. Army have shown that, depending on ambient conditions, the amount of solid contamination in the air can range between 1000 and 24,000 particles larger than 10mm/ft3 of air. Ingression of contaminant-laden air could jeopardize the ability of the M109 to operate reliably throughout an extended mission.

Water can also be a damaging contaminant. Water vapor enters a hydraulic reservoir when fluid level changes during normal operation. As warm oil heats the air in the reservoir, the air is able to contain more water vapor than it could at lower ambient temperatures. When the machine shuts down, the saturated air cools, and the water vapor condenses. Condensate that gathers on the inside of the reservoir falls or drips into the hydraulic fluid.

To resolve these contaminant ingression problems, contractor Barnes & Reinecke Inc., Elk Grove, Illinois, installed a special reservoir breather for the M109. The breather mounts inside the engine compartment near the reservoir to which it is connected through the fill port. The breather is constructed of Nylon and polycarbonate plastics that allow it to withstand rugged operating conditions.

When fluid level in the M109’s reservoir lowers, air enters the breather through a flapper valve which seals the unit from the atmosphere to prolong the life of a desiccant cartridge. The air next passes through a bed of silica gel which removes water vapor. The dried air then passes through a filter — with a Beta rating greater than 75 — which effectively removes the particles that could damage the system. The clean air finally enters the reservoir. When the system exhales (the oil level in the reservoir rises), the air is vented to atmosphere through a check valve which also helps isolate the unit from the environment.

Click here to watch a video of the M109 howitzer in action.