Case Drain

Could Mr. Cosford perhaps be wrong about using case flow testing to monitor pump wear? I realize he is not the only one in the industry who recommends case flow testing: people write articles about it, we teach it, it's in service manuals, and most of us do it. But does case flow testing accurately account for all the leakage in a pump or motor?

I personally believe that case flow testing epitomizes the term “false positive? Here’s why: I was fortunate enough to have spent a decade analyzing hydraulic pump failures. The more I studied wear patterns in external drain pumps and motors (mostly due to contamination) the more obvious it became that whomever came up with the notion that ALL leakage in an internal drain pump and motor MUST discharge through the case drain line was apparently wrong.

All one has to do to figure out why a case flow test does not, and cannot, account for all the leakage in a pump is to look at the leakage pathways in an internal drain pump. Internal drain pumps also leak, but without case drain lines where does the leakage go to? A simple in-line flow test will show that the leakage is being forced “backwards” through the pump – from the outlet port back, through the clearances, to the inlet port. So why couldn’t the same thing be happening with an external drain pump: with at least some of the flow? It does.

I conducted many a pump test wherein I had, for example, a 20-GPM pump with a pressure line flow loss of 5-GPM. However, I had only 2-GPM flowing through the case drain line. The other 3-GPM was “slipping” inside the pump in exactly the same manner “slippage” occurs in an internal drain pump.

One only has to look inside a worn axial piston pump to see why this can happen. Wear typically occurs between the barrel and the wear-plate. The contaminants “mine” a groove between the elongated slots. This groove becomes a “channel” that connects the outlet side of the pump directly with the inlet side: and, therein lies the reason why a case flow does not, and cannot, accurately reflect pump leakage. Other than the issue of accuracy, there are other reasons why I always recommend a pressure line test:

• There is no need to know the pump flow
• It will show problems associated with pump speed.
• It will show problems at the inlet side of a pump.

One other thing to remember: case flow testing only applies to open loop pumps. A case flow test is only valid for over-center pumps if the primary objective is to monitor charge pump flow at full load.

If it isn’t safe don’t do it!

Respectfully,
Rory S. McLaren

Good article, Josh.
Like any tool in one's toolbox, Case Drain is valuable if analyzed alongside other factors (heat, noise, performance, sampling, filter cutting) that make a complete toolkit.
Many mobile applications specify case PRESSURE instead of flow. As long as the return path is unfiltered, headed for a vented tank, and all original fittings and lines, a pressure reading log can be indicative as well (and simpler to test once fittings are installed).
One word of caution to readers: Many piston pumps also have a check valve in the head that will direct case flow to the suction port in the event of a restricted case drain line (several Rexroth and Vickers pumps I see a lot have this).
If the check valve ever fails to seat properly, or the line is restricted, very low or no flow can be indicated in error on the external line.
Another feature on many mobile applications is case drain filters. Looking in the pleats of every filter removed gives you an idea of what is normal and what is not. Opening EVERY filter is a practice we preach at the school here.