Purpose of a Case Drain Filter on a Final Drive or Travel Motor
About 90% of all final drives have a return line to the tank, and the case drain filter (aka the in-line filter) is located on that return line. Many drives have only one case drain filter for both motors, but some will have a separate case drain filter for each. The case drain filter is typically a silver canister-style filter with a golden-colored element on the inside. Its purpose is to filter out contaminants in the hydraulic fluid. Contaminants include things like metal shavings, flakes, sand, dust, ash, rubber, etc.
Dangers of a Dirty Case Drain Filter
Let's talk about what happens when the case drain filter on your final drive or travel motor gets clogged. First, the hydraulic fluid pressure begins to build up higher and higher. Next, that highly pressurized hydraulic fluid blows the lower shaft seal (aka, oil seal), allowing the fluid to now flow into the gear section – still under tremendous pressure. With nowhere else to go, that high pressure fluid will force its way through the weakest link: the cover plate. The cover plate will either crack, blow off, or even skyrocket off. The figure below shows just such a damaged cover plate, which looks like it was torn apart.
You can imagine the kind of pressure it took to tear that cover plate apart — and you can also probably imagine the damage that occurred to the expensive seals and bearings inside that hub.
Preventing Catastrophic Failure
To prevent this type of failure, you need to make sure your case drain filter is clean. To locate the case drain filter, first look for the case drain line – it's the small line next to your two in and out lines. As you follow the line, you'll encounter the filter itself. The filter looks like an aluminum canister that is about 1 ¼" in diameter and between 3" and 3 ½" long. Remove the filter from the drain lines and be sure to plug the drain lines to avoid losing hydraulic fluid. A fully assembled case drain filter is shown below for reference.
To access the filter element, simply unscrew the hex nut. It contains a tapered sintered-bronze filter element and two springs holding the filter element in place. If the filter is no longer a bronze color, it needs to be replaced. Otherwise, you should be able to clean it and put it back in. The two figures below show a dirty filter that needs to be replaced, and a clean filter for comparison.
Just taking the time to check the case drain filter on a regular basis can save you thousands of dollars in repairs and alert you when something serious is going wrong inside your final drive.