Insane Hydraulics

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"Five Plus Five Rule" for Drain Flow Rate Evaluation

In this article I will reveal my "five plus five rule", which I apply when I have to perform a "quick and dirty" test of a pump through evaluating the flow rate of its drain.

The purpose of this guide 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, then? 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 pressure 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.

A note - the "five plus five rule" can be 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 built in flushing valve, or a pilot pressure pump that is venting into its 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 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 sheet of paper that you just created:

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

Flow rate between the Good and 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. The 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 here is for the 620 series. I strongly advise studying it in detail and using as universal reference. Parker is close behind, and Danfoss is in the 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:

The "five plus five" rule is not the best method to detect a problematic pump - it's a method. Another tool in your tool box. 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?