Pump Flow Tests and Life Spans

    Many equipment owners believe that they can evaluate the condition of their hydraulic pumps through flow testing without removing them from their machinery, and they often ask for our help in this matter. I perfectly understand the desire of a machine owner to know how long his pump will last, but the truth about standard pump flow tests (monitoring the flow while inducing system pressure with a restrictor valve, and comparing the results against nominal values for the given pump model) is - such tests give but the information about the volumetric efficiency of the pump, which can label a pump "bad" when the efficiency is low, but can not serve as a guarantee of the longevity of the unit when the efficiency is high or acceptable! Now this is something many people find hard to believe in - how can it be that a perfectly working pump can have an unpredictable life span?

    Well, the reason for this is simple - aside form "pressure making" components pumps have other parts inside that don't influence directly the volumetric efficiency while wearing out. I am referring to stuff like shaft bearings, swashplate bearings, piston shoes (which can gain enormous play and be virtually slipping off the pistons without lowering the volumetric efficiency much), servo-pistons and the respective links, feedback links, etc...

    They say that a picture is worth a thousand words, that is why I am putting these pics here (and not in the Kaboom section) - to serve as a perfect illustration to my point. These all are parts from a A7V355 Hydromatik open circuit pump just four days after I performed exhaustive tests on the unit without opening it or dismounting it from the powerpack. Under all situations and at maximum oil temperatures the pump showed excellent volumetric efficiency (above 94) and controllability, the surface temperature of the pump never rose over 70 C during the tests, and yet four days later it suffered bearing failure which led to the complete destruction of the unit. This pump model has suction line connected internally to the casing, therefore all the contamination from the breaking bearing was being immediately sucked into the inlet and then pumped through the system, so when the pump went down, it Kamikazeed along two recently rebuilt A6VM200 motors and a huge Rexroth distributor valve. The operators did notice a change in the noise of the pump in the last two days of operation, but the change of noise was attributed to the already loud gearbox, which was scheduled to be overhauled in a couple of weeks. The breakdown was virtually instantaneous - according to the operator the machine did "f-z-z-z-z-z-i-i-u -k-h-k-h" and stopped...

    So, on-site pump flow tests are good, they provide valuable information about volumetric efficiency and controllability of the pump, and they can tell you if the pump needs repairs, BUT - they can not predict lifespan of the pump when they return positive results.

    History, my friends, what you need is detailed history - hours and work conditions since the last overhaul. If you don't have this information - your best bet to evaluate your pump's condition and to predict how long it will last is to open it and look inside.

     And to conclude - a very important tip to all, both hydraulic equipment owners and mechanics:

    In case of hydraulic pumps, especially large ones, abrupt change of normal noise when not caused by air or bad adjustments, is a serious "heads up" which means "trouble and soon". Normally the only person that can confirm you that the noise is changing is the usual operator of the machine, so give him a chance to speak up. Continuing operation with the changed noise can easily end up in a nasty major failure and the following "whose fault was it?" witch-hunt nobody wants to be a part of.