Most owners and operators don’t realize how important case drain flow is when evaluating and diagnosing a final drive motor. This test can tell you a great deal about what’s going on inside your final drive without having to open it up.
In this blog post, we take a look at case drain flow testing, including how to perform the test and how to interpret the results.
Internal leakage is designed to support lubrication and cooling of dynamic components (e.g., bearings, pistons, etc.). Now, this leakage never leaves the system. It will always be returned to the hydraulic reservoir, often via a low-pressure drain line. Internal leakage does not contribute to the work being done by the final drive motor, though.
There is a baseline case for the hydraulic motor drain flow on your machine. However, as wear increases internal clearances and internal seals wear out, the case drain flow increases. This directly reduces the final drive's volumetric efficiency.
The goal of a case drain flow test is to measure the amount of hydraulic fluid bypassing the motor's working components. Baseline operation refers to the normal case drain flow that prevents excessive pressure buildup inside the motor casing, which could otherwise cause the main shaft seal to blow out. Failure indication is an excessively high case drain flow. This indicates that high-pressure fluid is bypassing the motor's internal clearances (e.g., worn piston shoes, a scored valve plate, or worn gear teeth) instead of performing mechanical work.
Before we dive into performing the test, let’s focus on safety for a minute. We know that hydraulic systems operate under extreme pressure, and fluid injection injuries are a serious risk. So make sure the system is powered down, and all residual pressure is relieved before you break any connections.
Next, the system must be at its normal operating temperature. Hydraulic fluid viscosity changes drastically with temperature. If you test cold oil, you’ll get artificially low leakage rates and false diagnostics. Also, when disconnecting the case drain line, remember to cap the fitting on the reservoir side to prevent environmental contamination and fluid loss.
There are two primary methods for testing case drain flow: using an inline flow meter or performing a timed manual measurement (best known as the "bucket test"). Note: Testing a motor without a load is useless. High system pressure is required to force fluid across worn internal clearances.
Now that you’ve got the data, compare your recorded flow rate against the Original Equipment Manufacturer (OEM) specifications. The OEM standard is the only definitive metric. Manufacturers will provide a maximum allowable case drain flow for a specific pressure and temperature. Now, if the OEM data is completely unavailable, a common industry heuristic is that case drain flow should typically not exceed 1% to 5% of the motor's total theoretical input flow.
If you're seeing more than that, it's likely that your final drive motor is wearing out. Also, if you’re experiencing repeated shaft seal failures along with high case drain flow, the internal leakage is likely overwhelming the case drain line's capacity to return fluid to the tank. This can cause the case pressure to exceed the shaft seal's maximum pressure rating, resulting in a leak.
If you need some help diagnosing your final drive motor, contact the experts here at Texas Final Drive. And if you need a new final drive motor, check out our Final Drive Motor Shop.