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Use the "Five Plus Five Rule" for Drain Flow Rate Evaluation

In this article, I will be revealing the "formula" that I apply whenever I need to perform a "quick and dirty" test of a pump by evaluating its drain flow rate. It's called the "five plus five rule".

Please note that the purpose of this formula is to establish a ballpark figure that can be used to sound an alarm if the drain flow rate is above it, but in no case to affirm that a given pump is "safe and sound" if the drain flow is below it. Not all pump malfunctions are reflected in the increase of the drain flow, you know.

You may ask - why bother measuring drain flows at all? For convenience! When you troubleshoot a hydraulic system with unknown service history and want to do a quick check of the pump, measuring its drain flow is often much easier to perform than measuring its output flow simply because the drain lines have better access. Also, since measured flow rates are smaller, alternative flow measuring methods that don't require special gear can be used.

On another note - the "five plus five rule" can only be applied to "common" medium-sized axial-piston variable displacement open-loop hydraulic pumps, in which the only two sources of the drain flow are the rotary group internal leakage, and the leakage associated with the displacement control. Obviously, if a hydraulic pump has a built-in flushing valve, or a pilot pressure pump that is venting into the case, or an internal connection between the case and the suction line, this rule can't be applied.

The rule goes as follows (remember - we're establishing a ballpark figure here, and we only do this because we have no manufacturer recommended base-line available to us, or we simply don't feel like looking it up):

Step 1 - take the displacement of your pump and multiply it by the rated rpm - the result is the theoretical nominal flow. If you're not sure what the rated speed of your pump is, look up any similarly sized unit in a catalog of one of the Big Four (Rexroth, Parker, Eaton, or Danfoss) - pumps are pumps, and nominal values always boil down to roughly the same figure across similarly sized models. For example - in the case of a 100 cc open loop pump of an unknown brand with what seems like a PC and LS control I would estimate 100 x 2200 = 220 l/min.

Step 2 - Calculate five percent (this is the first "five" of the rule) of the figure from step 1 - this is the ballpark figure for your drain flow at nominal pressure and nominal speed. In our case - 220 x 0,05 = 11 l/min.

Step 3 - Consider the pump control and adjust your "ballpark" drain value - if the pump has a mechanical torque limiter control, you can add 2 l/min to the drain flow - torque controls always add to internal leakage, if there's a bleed orifice in the LS control - maybe add another liter. Take a sheet of paper and write the value down with the word "Good" to the left of it. In our example there's no torque control, we're not sure about the orifice, so we're keeping the 11 l/min. Our sheet of paper reads: "Good: 11 l/min"

Step 4 - Take the figure from step 3 and add the five percent from step 2 (this is the "plus five" part of the rule). In our example - 11 + 11 = 22. Write the result on the same sheet of paper with the word "Bad" to the left of it. So, now our paper goes like this: "Good: 11 l/min ... Bad: 22 l/min".

Step 5 - Find a way to load the pump to its nominal pressure without de-stroking (reaching the PC setting value) and then measure the drain flow rate. Draw conclusions by using the chart you just created:

Flow rate below or equal to the "Good" flow - the volumetric efficiency is OK.

Flow rate between the "Good" and the "Bad" - the pump has wear, but can probably still operate for some time. The closer to "Bad" the more wear the pump has (In our case - between 11 - 22 l/min). Schedule an overhaul.

Flow rate close to or above the "Bad" - the pump should be overhauled immediately (In our case I'd estimate anything above 18 l/m as very concerning, and above 22 l/min as unacceptable).

Here's an example sheet that I would use for our imaginary 100 cc hydraulic pump (smiley faces are optional):

If you want a more scientific way to guesstimate a baseline for the drain flow rate evaluation - I suggest consulting, once again the big four. Well, not four - three - Eaton, Parker, and then Danfoss. Rexroth is kind of way behind and I'll explain why.

Eaton does a great job presenting drain flow graphs in their open-loop pump catalogs. This one is for the 620 series. I strongly advise studying it in detail and using it as a universal reference. Parker is close behind, and Danfoss is in third place, with mere volumetric efficiency graphs. Rexroth, unfortunately, is looking at the podium wishing one day he'd be there... You'll have to look really hard to find this information in their catalogs. Just use the Eaton figures, these charts are beautiful.

Some important/interesting points to remember:

• Drain flow evaluation should be performed with the oil at rated viscosity, so make sure you know which oil the system is using and that the temperature is right. I like 50C (about 120F) - a nice round number that works for most hydraulic systems with mineral oil.
• Drain flow primarily depends on the system pressure.
• Drain flow does increase with speed, but only to a small degree (in fact - for some pumps it can be almost constant above a certain speed) so the volumetric efficiency of a hydraulic pump increases with speed.
• The typical volumetric efficiency of a new axial piston pump is about 95% at rated speed and pressure. Don't forget that the key word here is "rated". If we ran our pump at 1000 rpm, the 11 l/min drain flow rate would mean volumetric efficiency of less than 90%, which could be falsely interpreted as a sign of a worn-out pump.
• When the pressure compensator is de-stroking the pump - the drain flow increases, sometimes by another 2,5 percent of the rated full flow, it's important to know and expect that. Again - Eaton's graphs illustrate this perfectly.
• When an LS pump is at stand-by - drain flow can increase too - this is normal.
• When a pump is equipped with a mechanical torque limiter control - the drain flow will be higher.

The "five plus five" rule is not the best method to detect a problematic pump - it's a method. Another tool in your toolbox. You should decide when and if to use it. If I gave you a knife to cut bread, don't blame me if you accidentally chop off one of your "appendages".

High Five anyone?